CA1247439A - Photographic element exhibiting reduced sensitizing dye stain - Google Patents

Abstract

A PHOTOGRAPHIC ELEMENT EXHIBITING
REDUCED SENSITIZING DYE STAIN
Abstract of the Disclosure A spectrally sensitized silver halide photographic element capable of producing a stable, viewable silver image on development and fixing out is disclosed. The latent image forming silver halide grains in the image recording emulsion layer or layers of the photographic element are silver bromide, chloride, or chlorobromide grains. At least one of the image recording emulsion layers contains spectrally sensitized tabular grains, Located in proximity to the spectrally sensitized tabular grains are relatively fine high iodide silver halide grains capable of being dissolved during fixing out.

Description

4~t A PHOTOGRAPHIC ELEMENT EXHIBITING
REDUCED SENSITI~ING DYE STAIN
Field of the Invention This inventîon relates to ~ilver halide photogr~phic elements capable of producing view~ble silver images. The invention relates more ~pecifi-c~lly to an improvement in photogrnphic element~
containing 6pec~rally sen6itlxed tabular graln sllver halide emulsions.
Back~round of the Invention St~ble, viewable black and whlte photograph~
c~n be produced by im~gewise expo~ing A photographic element containing one or more radiAtion ~ensltive silver hallde emulsion l~yers c~pable of producing a develop~ble latent image. To extend the response of the silver halide into the green and/or red regions of the visible spec~rum and thereby better approxi-mate the image seen by the human eye it i8 ~ommon practic~ to adsorb a spectral 8en~itizing dye to the surfaces of the silver halide grains in the emulsion layers. Following im~gewise exposure a viewable lmagP can be produced by development in an ~queous alkaline processing solution. The imsgewise conver-sion of silver halide to metallic silver provides the viewable image. To avoid an eventual increase in density attrlbutable to residu~l silver halide it is common practice to fix out ~dissolve ~nd remove by washlng) the residual, undeveloped silver halide grains. This lesves a stable, v~ewable sllver image in the photographic element.
In silver h~l~de photography a cholce of three halides, chloride, bromide, and iodide, and com~inations thereof are available. Silver lodide i8 known to be the most dificult silver halide to employ for producing a latent im~ge and developing and is seldom used alone in emulsions intended to be processed by develop~ent in aqueous alkaline ~olu-f~

tions followed by fixing out. When present in aphotographic element silver iodide i8 often releg~ted to performing functions which do not require the formation of a develop~ble la~:ent image in silver 5 iodide grains. The following are illustrative of known uses of silver iodlde grains and ~oluble iodide s~lts:
P-l Carroll U.S. Patent :2,327,764 disclo~efi the use o silver iodide as an ul;:rQviolet filter for a 10 color photo~rap~ic element;
P-2 V~n Pee et al U.S. Patent 3,745,015 disclo6-es the incorpor~tion of a silver lodide sol in a direct print radiation ~ensitive silver halide emulsion;
P-3 M~skAsky U.S. Patent 4,094,684 discloses radiation sensitive silver iodide grains onto which have been epitaxially grown silver chloride, P~4 M~ternagh~n U.S. Patent 4,184,878 di~clo6es the use of high iodide silver halide grain~ a6 seed ~ grains in prep~ring tabulAr grain silver bromoiodide emulsions;
P-5 U.K. Speciflc~tion 1,413,826 discloses the use of 0.01 to 1.0 mole percent OEoluble iodide to assis~ in the spectral æensitization of silver 25 bromoiodide;
P-6 Maskasky U.K. Specification 2,132,373 discloses gamma phase tabular grain silver iodide emul 6 ions; and P-7 Jap~nese Kokai Sho 52[1977]-130639 diæcloses 30 the use of potassium iodide in a fixing solution to lncrease fixing speed.
The highest speed silver hallde emulsions are silver bromoiodide emul6ions, which sre most frequently employed for camera speed imaging. These 35 emulsions contAin bromide as the predomin~nt h~lide.
Silver iodide can be present up to i~s solubility limit in silver bromide, about 40 mole percent, but is seldom employed in concentratio~s above 20 mole percent ~nd i8 usually employed in concentratione below 10 mole percenk.
For a number of photogr~phic a~plicatlons processing speecl And convenience are of parRmount import~nce. Silver chlorlde, silver bromide, and silver chlorobromide emulslonls are outst~ndingly suited for these applications, since they c~n be more r~pidly proces~ed than silver iodide or sllver bromoiodide emulsions. Further, acceptable proces~-lng of these emulsions c~n be obtained wlth greaterv~riance~ in the time and temperature of proce~lng.
Interest in silver halide photography h~
recently focused on tAbular gr~in emul~lons, pArtiCu~
larly inter~ediate and high aspect ratio tabulsr grain emulsions. It has been 6hown that the latter emulsions can produce incressed image sharpness~
When eficiently chemically and spectrally sen~i-~ized, these emulsions exhibit outstanding speed-granularlty relationsh$ps. Higher ~ilver covering power has been observed in fully forehardened photo-graphic elements. In radiographic elements with emulslon coatings on each of the two opposite face~
of the ~upport msrked reduction6 in crossover have been observed using hlgh aspect ratlo tabular grain emul~ions, and improvements in epeed ~t compar~ble crossover levels have been demonstrated u6ing thin, ~ntermediate a6pect ratio t~bular grain emul6ions, Photographic element6 Containing tabular grain silver bromide, silver chloride, and fiilver chlorobromide emuls~on~ as well ~s their ~ensitiza-tion, use, ~nd sdvant~ges are illu~trated by the following:
P-8 Mi;gnot U.S. Patent 4,386,156 di6closes a tabular grain silver bromide emulsion wherein tabular silver bromide ~rains bounded by {100) ma~or crystal faces ~nd having an average aspect rs~io of at least 8~5.1, QCCount for ~t least 50 percent of the total pro~ected area of the silver bromlde gralns present in the emulsion;
P-9 Wey U.S. Patent 4,399,215 dlsclo6es n tabular gr~in silver chloride emulsion where~n the S tabular grsins have an average a6pect ratlo greater than 8:1;
P-10 M~skAsky U.S. Patent 4,400,463 di~closes a tabular 8r~in emulsion the grain6 of wh~ch are at least 50 mole percent chloride and have one or more 1~ edges of a particular crystnllogr~phic orient~tion;
P-ll Dickerson U.S. E'atent 4,414,304 discloses fully forehardened photographlc elements capNble of producing a stable, viewable 6ilver image of increased covering power by reason o contain~ng a high aspect ratio tabular grain sllver halide emulsion;
P-12 Wey et al U.S. Pat~nt 4,414,306 discloses tabular grflin silver halide emulsions wherein the halide is fl combinatlon of chloride and bromide;
P-13 rnd P-14 Abbott et ~1 U.S. Patents 4,425,425 a~d 4,4259426 disclose radiographic elements contain~ng silver halide emulsion layers on opposite ma~or faces of a ~upport. Hlgh and lnter-mediate aspect ratio tabul~r gr~in silver bromide emulsions ~re ~pecifically dlsclosed;
P-15 Maskasky U.S. Patent 4,435,501 dis 108e6 the selective site epitaxlal sen3itizat~on of hi8h a~pec~
ratio tabular grain ællver halide emulsions;
P-16 Kofron et al U~S. Paten~ 4,439,520 disclo~e6 eficiently chemically ~nd ~pectrally 6ensitized high aspect ratio ~abular gr~in ~ilver halide emul3ions;
and P-17 Daubendiek et al U.K. Spec~ficatlon

2,110,831A ~dlscloses direct po~itive ~ilver halide emulslons containing internal latent image forming high aspect ratio tabular grain emul6ions.

A disadvantage that has been discovered with the use of spectrally sensitized tabular grnin ilver bromide, silver chloride, and sil~er chlorobromlde emulsions ~n producing stable, viewable sil~er imageB
is dye stAin. In contrast to spectrally sensitize-l silver halide emulsions o slmil~r halide content which are not tabular grain emulsions, sufficient residual spectral sensitizing dye rem~ins in the photogr~phic element at the conclu~ion oE proce~eing to lncrease the density in the low ~nd lntermediate denslty regions of the ima8e bearlng photographic element. Dye stain can be undesirable in alterlng image tone. Variations in image tone ~re part$cu-larly undesirable in radiography, slnce this c~n complicate proper interpret~tion of x-ray images.
Further, residual dye stain is ob~ctionable in that it does not affect all wavelengths equally. Rather, it is particularly large at wavelengths at or near the absorption peak of the dye. Residual dye ~ain is highly objectionable where it is desired to scan the photographic image with a laser of a wavelength approximating the absorption peak of the spectral sensitizing dye.
Summary of the Invention In one aspect this invention is directed to a photographic element capsble of producing a stable, viewable silver image on development in ~n ~queous alkaline processing ~olution and fixing out compris-ing a support end one or more image recording sllver halide emulsion layers each comprised o ~ dispersing medium and latent image forming silver halide grains, the halide consisting essentially of chloride~
bromide, or mixtures thereof, at least one of the image recording ~ilver halide emulsion l-~yers being comprised of spectral sensitizing dye adsorbed to the surace of tabular latent image forming silver halide grains having A thickness of less than 0.5 ~m and an average aspect ratio of at least 5:1 accountlng for at least 35 percent of the total pro~ected ~re~
of said latent lmage forming silver halide grains present ~n sa;d silver halide emul~ion lay~r, the improvement comprising high iodide silver halide ~rains o less than 0.25 ~m in mean diameter located in proximity to said tabular ~ilver hallde grsins ~nd limited to 8 concentration cflpable of being dissolved on fixing out.
It has been discovered that the introduction o the relatlvely ine hlgh iodide 6ilver halide gr~ins dram~tlcally reduces dye s~ain in the photo-gr~phic elements cont~ining t~bular grain silver chlorlde, silver bromide, and sllver chlorobromide emul9ions~ Thus, the advantages o lntermediate and hi8h aspect ratio silver halide emulsions and the procefising advAntages of silver chloride, silver bromide, and silver chlorobromide emulsions ~re both realized whil~ reduclng dye stain attributable to the presence of spectral sensitizing dye.
Description of Preferred Embodlments This invention relates to an improvement in photographic element~ in~ended to produce stable, viewable silver images as a result of imagewise exposure, development in an a~ueous alkaline process-ing solution, and fixing out to remove residual silver halide. The photographic elements are comprised of a support and one or more image record-ing silver halide emulsion layers con~aining tabular latent image forming sllver halide grains. In addition~ relatively fine high iodlde ~ilver halide grains are present in at least one image recordlng tabular grAin emulsion layer or in proximity thereto.
The high iodide 6ilver halide grains can consi6~ e6sentially of silver iodide or can contain other halides--i.e.~ bromide or chloride--in minor amounts. It: is generally preferred ~o limit the i;Z~74~9 other halides to those concentration capable of exis~ing in ~ or y phase silver iodide without phase separation. Typically the high iodide silver halide grains contain at least g~ mole percent iodide, based on silver.
Relatively fine high iodide silYer halide grains are employed. The grains are less than 0.25 ~m in mean diameter, preferably less than 0.10 ~m in mean diameter. The above maximum mean diameters are based on the assumption that relatively regular gr~ins will be employed, such as regul~r ~ ph~6e (cubic) or regular ~ phase (hexa~onal pyramidal) grains. In substituting high iodide silver halide grains of lrregular configuration, BUCh 8S tabular lS grains, equivalent results can be obtained with larger mean diameter grains. The minimum mean diameters of the hi8h iodide silver halide grains are limited only by synthetic convenience. Typically grains of at least about 0.01 ~m in mean diameter are employed.
The high iodide silver halide grains are preferably relatively monodispersed. It is preferred to employ high iodide silver halide grains hsving a coefficient of variation of less than 20. As employ-ed herein the coefficient of varistion is defined as100 times the standard deviation of the grain diameter divided by the average grain diameter.
The concentration of the high iodide silver halide grains is limited to a level that can be removed during fixing out. This is inversely related to both mean grain diameter and the coefficient of variation of the grains. In general the silver iodide provided by the high iodide silver halide grains is limited to less ~han S mole percent of the total silver halide present in the photographic element, preferably less than 3 mole percent, ~nd optimally less than 1 mole percent. Very small 4~3 concentra~ions of high iodide silver halide gralns are effective. Silver iodide concentr~tions of at leact 0.1 mole percent are effective to produc~
ob~ervable reductions in dye stain.
High iodide silver h~lide grains csn be prepared in the form of emulsions according to procedures generally known in the art~ Such emul-sions and their prepAratlon ~re disclosed by M~ternaghan U.S. Patent 4,184,,878 ~nd Daubendiek et al V.S. Paterlt 4,414,310.
Once prepared the high iodide sllver halide grains can be placed in proximity with the latent image forming ~pectrally sen61tized tabular grains o the photogràphic elements of this invention by blending the emulsions containing the respective grain populations. Blending can be undertaken at any stage of element preparation following precipitation of the emulsîons, but is preferably delayed until just before coating to minimize the risk o halide migration between the separate grain populations.
Preferably, the high iodide silYer halide gr~in6 ~re located in a separate layer of the pho~ographic element located to permit ionic transport between the image recording emulsion layer or layers conealnlng the spectrally sensitized tabular grains and the high lodide silver halide grain6 during processing. For example, a high iodlde ~ilver halide emulsion, as precipita~ed or supplemented ~y additional vehicle and addenda augmenting the dispersing medium, can be coated between the æpectr~lly sensitized tabular grain emul~ion l~yer and the support or can form ~n overcoat positioned to receive processing solutions before the apectrally sensitized tabular gr~ln emulsion layer. Where ~ultiple image rècording 35 layers are present, interlayer location for the high iodide silver halLde grains is advantageous. It ls not essentLal that the high iodide silver hal$de grains be in a l~yer contiguou~ to the imAge record-ing layer containing spectrally sen6itized tabular gr~ins, although this is usually preferred.
Each of the image recording emuls~on layers is comprised of a dispersing medium and radiAtion sensitive, latent image forming silver halide grains. The latent lmage orming ~ilver hallde grains of at least one of the image recording emul-sion l~yers ~re spectrally fiensitlzed by having a spectral sensltizing dye adsorbed to the grain surfaces, and the spectr~lly ~:ensi~iæed grains together with the disperslng medium form a t~bular grain emulsion. The l~tent image forming 6ilver halide gr~ins present in the photographic element are in each instance substanti~lly free of iodide, although small amounts o~E iodide can be adsorbed to the grain surfaces to promote aggregation and ~dsorp-tion of the spectral sensitizing dye. The ~ilver halide present in the latent im~ge forming 8ilver halide grains consists es~entially of sil~er chloride, silver bromide, or silver chlorobromide.
Tabular grains are herein defined a~ those having two substRntially parallel crystal aces~ esch of which is substantially larger than any other single crystal face of the gr~in. The term "tabul~r grain emulsion" is hereln defined as requir~ng that the ~abular silver halide grains having ~ thickness of less than 0.5 ~m have ~n average aspect ratio of at least 5:1 and account for at least 35 percent of the total pro~ected area of the silver halide grains present in the emulsion.
Preferred tabular grain emulsions are lntermediate and hlgh aspect ratio tabular grain emulsions. As ~pplied to tMbul~r 8rain emulsions the term "~igh aspec~ ratlo" i~ hereined d~fined as requiring that the silver halide grains hav~ng a thicknesg o~E less than 0.3 ~m and a di~meter of at least 0.6 ~m have an average aspect ratio of greater than 8:1 and account for at least 50 percent of the t~al pro~ected area of ~he silver halide grains present in the emulsion. The ~erm is ~hue defined in conformity with ehe usage of ~his term in the patents relating to t~bular gra~n emul6ions cited above.
The term "intermedia~e a6pect ratio" as applied to tabular grain emulsions i8 deflned a8 requlring that thc tabular ~ilver halide grains having a thickness of less than 0.3 ~m and an average aspect ratio in the range of from 5:1 to 8:1 account for at least 50 percent of the total pro~ect-ed area of'the silver halide grains present in the emulsion. The term "thin, intermedinte aspect ratio"
ls similarly defined, except that the reference thickness of 0.3 ~m notad above is replaced by a reference thlckness of 0.2 ~m. This is the defini-tion of "thin, intermediate aspect ratio" tabular grain emulsions employed by Abbott et al U.S. Patent 4,425,426.
In general tabular grains are preferred having a thickness of less than 0.3 ~m, optlmally less than 0.2 ~m. For some applications, aB where R photographic image ~s to be viewed wi~hout enlarge-ment or in applications where granularity is of little importance, t~bular ~r~in thicknesses of up to 0.5 ~m are acceptable. Such tabular 8rain thick-nesses are illustrated by Jones et al U~Ko Specifica-tion 2,111,705A. The impro~ement of the pre6entinvention can, for example, be applied to r duclng dye ~tsin in a retained ~ilver image produced aecord-ing to the ~eachlngs of Jones et al. Intermediate aspect ratio tabulsr grain emulsions, p`articularly thin, intermediate aspect ratio tabular grain emul-sions, have partieular applicability to radiographic imaging, as taught by Abbott et al U.S. P~tent a;~3 4,425,426, but can be applied generally to black and white photography. However~ ln general, the prefer-red tsbular grain emulsion6 are hi8h aspect r~tio tabuler grain emulsions. While the ensuing de~crip-tion is for convenience specifically direcSed to highaspect ratio tabular grain emulsions, lt 6hould be appreciated nevertheless that the teachings are generally spplicable to tabular grain emulsions as herein defined.
The preferred high aspect ratio t~bular 8rain ~ilver halide emulsions are those wherein the silver halide grains having Q thickness of les6 than 0.3 ~m (optimally le6s than 0.2 ~m) and a dlame-ter of at least 0.6 ~m have an average aspect ratio lS of at least 12:1 and optimally at leas~ 20:1. In a preferred form of the lnvention these silver halide grains satisfying ~he ~bove thickness and diameter criteria account for at least 70 percent and optimal-ly at least 90 percent of the total projected area of the silver halide grains.
It ls appreci~ted that the thinner the tabular grains accounting for a given percentage of the projected area, the higher the average aspect ratio of the emulsion. Typically the tabulAr grains have an average thickness of at least 0.03 ~m, although even thinner tabular grains can in prlncipal be employed.
High aspect ratio tabular grain emul6ions useful in the practice of this invention can have extremely high average aspect ratios. Tabular grain average aspect ratios can be increased by increa6ing average grain diameters. This can produce sharpne~s advantages, but maximum ~verage grain diameters ~re generally limited by granularity requ~rements for a specific photographic application. Tabul~r grain ~verage aspect ratios can also or alternatively be increased by decreasing average grain thicknesses.

~ J

When silver coverages ~re held con~tan~, decreasing the thickness of tabular grains generally improves granularity as a direct function of increasing ~spect ratio. Hence the maximum ~verage aspect ratios of the tabular grain emulsions of ~his lnvention ~re function of the maximum aver~ge gr~in diameters acceptable for the specific photographic ~pplication and the minimum attainable tabular grain thicknesses which can be produced. Maximum av~rage ~spect r~tlos have been observed to vary, dependin~ upon ~he preclpitation technlque employed ~nd the tabular gra~n halide composition. The highest observed ~ver~ge ~spect ratios, 500:1, for tabular graln6 with photographieally useful average gr~in d~ameter~, h~ve been achieved by Ostwald ripenin~ preparations of silver bromlde grains, wi~h Aspect ratios of 100:1, 200:1, or even higher being obtainable by double-~et precipita~ion procedures. Average aspect ratios as high as 50:1 or even 100:1 for silver chloride tRbular grain6, optionally containing bromide, can be prepared as ~augh~ by Maskasky U.S. Pstent 4,400,463, cited above.
The latent lmage forming grains can consiOEt essentially of silver chloride or silver bromide as the sole silver halide. Alternatively, silver chloride or silver bromide can both be present within the same grains or in different gr~ins of the same emulsion in ~ny desired proportions, and the term "~ilver chlorobromide" is to be understood as embrac-~ng all such Pmulsions. The latent image forming fiilver halide grains are ~ubstantially free of iodide. Tha~ iB, iodide concentrations are less than 0.5 mole percent, based on total silver. Typically iodide is present only in impurity concentra~ions.
Sub~ect to the requirement that the latent image forming grains be subs~anti~lly free of lodide, the tabular grain emulsions c~n be chosen from ~ny of ~ ;3~

the variou6 orms of tabul~r gr~in emulsions describ-ed in the patents cited ~bove nnd in Rese~rch Disclo-sure, Vol. 225J January 1983, Item 22534, and any emulsions other than t~bul~r Brain emulsions present (e.g., octahedral, cubic, or complex grain emulsions) can take conventional forms, such as illustrated by Research Disclosure, Vol. 176, December 1978, Item 17643. Research Disclosure iB published by Kenneth Mason PubiicAtions, Ltd., The Old Harbourmaster'~, 8 North Street, Emsworth, Hampshire P010 7DD, England.
In ~ 6pecifically preiEerred orm one or more high aspect ratio t~bulAr graill silver bromide emulsions are included ln the photogr~phic element~
of this invention. According to one preferred procedure these emulsions can be ormed by a double ~et precipitation process similsr to ~hat taught by Wilgus et al U.S. P~tent 4,434,226, except th~t the emulsions are substantially free of iodide. Into conventional react~on vessel for ~ilver halide precipitation equipped with an efficient stirrin~
mechanism is introduced A di~persing medium. Typi-cally ~he dlsperslng medium initially introduced into the reaction vesel is ~t le~st ~bout 10 percent, preferably 20 ~o 80 percent, by weight based on tot~l weight of the dispersing medium present in the silver bromide emulsion ~t the conclusion of grain precipl-tation. Since dispersing medium cRn be removed from the reaction vessel by ultrafiltration durlng silver bromide grain precipi~ation~ as taught by Mignot U.S~
Patent 4,334,012, lt is appreciated that the volume of dispersing medium initi~lly present in ~he rea~
tîon vessel can equal or even exceed the volume of the silver bromide emulsion present in the reaction vessel ~t the conclusion of grain precipitation. The dispersing medium initially introduced into the reaction vessel i preferably water or a dispersion of peptizer in water, option~lly containing other ingredients, such a~ one or more F,ilver hal~de ripening agents and/or metal dopants, more ~pecifi-cally described below. Where a peptizer i6 inleially present, lt is preferably employed in a concentration S of at least 10 percent, most preferably a~ least 20 percent, of the total peptizer pre~ent at the comple~
tion of silver bromide precipitatlon. Additional dispersing medium ls added to the reaction v~s~ 1 wlth the silver and bromide ~alt~ nnd can al80 be lntroduced through a separate ~et. It i~ com~on practice ~o ad~ust the proportion of disper6ing medium, partlcularly to increase the proportion of peptizer, after the completion of the ~alt introductio~s.
A minor portion, typic~lly less than 10 percent, of the bromide salt employed in forming the silver bromide grains iB initially present in the reaction vessel to adjust the bromide ion concentra-tion of the disperæin~ medium at the outset of silver bromide precipitation. It is contemplated to ma~n-tain the pBr of the reaction ve6sel initially st or below 1.6, preferably below 1.5. On the other hand, if the pBr is too low, the formation of nontabular silver bromide grains is favored. Therefore ~ it i~
contemplated to maintain the pBr of the reaction vessel at or above 0.6, preferably above 1~1. (As herein employed, pBr is defined as the negative logarithm of bromide ion concentration~
During precipltation ~ilver and bromide salts are added to the reaction vessel by techniques well known in the precipitAtlon of ~ilver bromide grains. Typically an aqueous ~olution of a soluble silver ealt, such as s~lver nitrate, is introduced into the reaction ~es~el concurrently with the introduc~ion of the bromide salt. The bromi~e salt is al~o typically introduced as an aqueous ~alt solution, ~uch as an aqueou~ solution of one or more ~ 3 soluble alkali metal (e.g., sodium or potas~lum), or ~lkaline earth metal (e.g., magnesium or calcium) bromide salts.
With the introduction o silver aalt into the reaction vessel the nucleation ~t~ge of graln formation i6 initiated. A populat~on of grain nuclei i9 formed which i6 capable of serving a8 precipita-tion sltes for silver bromide as the lntroduction of silver ~nd bromide salts contLnues. The preci~itA-~ion of silv~r bromide onto existlng grain nuclelconstitutes the growth stage o graln formation. The aspect ratlos of the tabular grains formed according to this invention are less affected by bromide concentr~tions during the growth stage than dur~ng the nucleatlon Btage- It is therefore possible during the growth stage to increase the permissible latitude of pBr during concurrent introduc~ion of silver and bromide salts above 0.6~ preferably in the range of from abou~ 9.6 to 2.2, most preferably from about 0-8 to about 1.6, the latter bein8 particularly preferred where n substantisl rate of grain nuclei formation continues throughout the introduction of silver and bromide sAlts, such as in the preparatlon of highly polydispersed emul6ions. Raising pBr values above 2~2 during tabular grain grow~h results in thickening of the gralnB ~ but can be tolerated in many instances while still realiz~ng an average aspect ratio of greater than 8:1.
As an ~lternative to the introduction of silver and bromide salts as aqueous solution~, it i8 specifically contemplated to introduce the silver and bromide salts; in~tially or in the growth stage, ln the form o fine silver bromide grains 6uspended in dispersing medium. The grain size is such that they are readily Ostwald ripened onto l~r~er grain nuclei, if any are present, once lntroduced into the reaction vessel. Th,e maximum useful gr~in slæes will depend :~'.1'7~ g on the specific conditions within the resction vessel, auch ns temperature and the presence of solubilizing and ripening agents. (Since bromide iB
precipitated in preference to chlorlde, it i8 al~o possible to employ silver chlorobromide grains.) The silver halide gralns are preferably very flne--e.g., less than 0.1 ~m in mean diameter.
Sub~ec~ to the pBr requiremen~ 6et ~orth above, ~he concen~rations and rates o~ sllver and bromide salt lntroductions can take any convenlent conventional form. The sllver and halide ~alts are preferably introduced in concentration~ of from 0.1 to 5 mole6 per liter, altho1Jgh broader conventi~nal concentration ranges, such as from 0.01 mole per lS liter to saturAtion, for example, Qre con~emplated.
Speciflcally préferred precipitation technlqueæ are those whlch achieve ~hortened precipitstion tlmes by increasing the rate of silver and halide ~alt intro-duction during ~he run. The rates of silver and bromide 8~1t introduction can be increased elther by increasing the rate at which the dispersing medium and the silver and bromide ~alts are introduced or by increasing the concentrations of the sllv4r and bromide salts within the dispersing medium being ~ntroduced. It is specifically preferred to increase the rate of ilver ~nd bromide ~alt ineroduction1 but to maintain the rate of introduction below the threshold level at which the ormation of new grain nuclei is favored--i.e., to avoid renucleation, AS
taught by Irie U.S. Patent 3,650,757, Kurz U.S.
Patent 3,672,900, Snito U.S. Patent 4,242,445, Wilgus German OLS 2,107,118~ Teitscheid et al European Patent Application 89102242, and Wey "Grow~h Mechanism of AgBr Crys~als in Gelatln Solution", Photographic Scienre and En&~n~ , Vol. 21, No. 1, January/February 1977, p. 14, et. ~. By a~oiding the formation of addi~ional grain nuclei after passing into the growth stage of precipitation, relatively monodisperse tabular silver bromlde gr~ln populations can be obtained. Emul~ions having coefficients of variation of less ~h~n about 39 percent can be prepared. By intentionally iavoring renucleation during the ~rowth stage of precipit~-tion, it is, of course, possible to produce polydis-perse emulsions of ~ub~tantially higher co~fflcient6 of variation.
Modifying compound~ can be pre~ent durlng t~bul~r silver bromide graln precipitation. 5uch compounds c~n be lnitially in the reaction ves~el or can be added along with one or more of the ~alts according to conventional procedures. Modifying compounds, such ss compounds of copper, ~hallium, lead, bismuth, cadmium, zinc, middle chalcogens (~.e. 9 sulfur, selenium, and tellurium), gold, and Group VIII noble metals, can be present during ~ilver halide precipitation, as illustrated by Arnold et al U.S. Patent 1,195,432, Hoch6te~ter U.S. Pa~ent 1,951,933, Trive~ll et al U.S. Patent 2,448,060, Overman U.S. Patent 2,628,167, Mueller et al U.S.
Patent 2,9S0,972, Sidebotham V.S. Patenk 3,483,709, Rosecrants et al U.S. Patent 3,737,313, Berry et ~1 U.S. Patent 3,772,031, Atwell U.S. Patent No.
4,269,927, and Researc_ Disclosure, Vol. 134, June 1975, Item 13452. The tabular grain ~llver ~romide emulslons can be internally reduction ~ensitized during precipitation, aB illustrated by Moissr et al, Journal of Pho~o~raphic SciencP~ Vol. 25, 1977, pp.
19-27.
The individual silver and bromide 6&1t6 can be added to the reac~ion ves~el ~hrough surface or sub~urface delivery tubes by gravity feed or by delivery apparatus for main~aining control of the ra~e of delivery and ~he pH, pBr, and~or pAg of the reaction ve!ssel contents, as illustrated by Culhan~

-18~
et al U.S. Patent 3,821,002, Oliver U.S. Patent

3,031,304 ~nd Claes et al, Photo~raphische ~ n-denz, 102 Band, Number 10, 1967, p. 162. In order to obtain rapid distribution of ~he react~nts wlthin the reaction vessel, specially construc~ed mlxing devices can be employed9 as illustrated by Audran U.S. Pstent 2,996,287, Mc CrosBen et al U.S. Patent 3,342,605, Frame et al U.S. Patent 3,415,650, Port~r et ~1 U.S.
Patent 3,785,777, Finnicum et al U.S. P~tent

4,147,551, Verhille et al U~S. Patent 4,171,224, calAmur U-R- Patent Application 2,02Zj431A, Sai~o et ~1 German OLS 2,555,364 and 2,556,885, ~nd Research Disc_osure, Volume 166, February 1978, Item 16662.
In formlng the tabular gr~in ~llver bromide emulslons ~ di6persing medlum i~ initi~lly contained in ~he re~ction vessel. In a preferred form the dispersing medium i~ comprised on an a~ueous peptizer suspension. Peptizer concentrations of from 0.2 to abou~ 10 percent by weight, based on the total weight of emulsion component~ in the reaction ves6el, can be employed. It is common practice to main~ain the concen~ration of the peptizer in the reactlon ves~el in the range of below about 6 percent, based on ~he total weight, prior to and durlng silver bromide formstion and to adjust the emul~ion vehicle concen-tration upwardly for optimum coatlng characteri~tics by delayed, supplemental vehicle additions. It i6 contempla~ed that the emulsion as initially formed will contain from about 5 to 50 grams oi peptizer per mole of silver bromide, preferably about 10 to 30 grams of peptizer per mole of silver bromide.
Additional vehicle can be added later to bring the concentration up to ~8 high as 1000 gram~ per mole of silver bromide. Preferably the concentration of vehicle in the fini~hed emulsion i8 above 50 grams per mole of silver bromide. When coated ~nd dried in forming a photographlc element the vehicle preferably ;3'3 forms about 30 to 70 percent by weight of the emul-8 ion layer.
It is specifically contempla~ed that grain ripening can occur during thle preparatlon of hLgh aspect ratio tabular grain silver bromide emulsions, and it is preferred that grain ripening occur within the reaction vessel during At lea8t silYer bromlde grain formation. Known silver halide solvent~ sre useful in promoting ripening. For example, an exce~s of bromid~ lons, when present in the reactlon vessel, ~s known to promote rlpenlng. It i8 therefore apparent that ~he bromide salt solution run lnto the reaction vessel can l~self promote ripening. Other ripening agents can also be employed and can be entirely contained with~n the di6persing medium in the reaction vessél before silvPr and bromide aalt addition, or they can be introduced into the reaction vess~l along with one or more of the h~lide salt, silver salt, or peptizer. In still ~nother variant the ripening agent can be lntroduced independently during bromide and silver salt ndditions. The preferred high aspect ratio tabulsr silYer bromide emulsions are non-ammoniacal or neutral emul6ions.
Among pre~erred ripening agents are those containing sulfur. Thiocyanate selt6 can ~e u~ed, ~uch _s alkali metal, most commonly sodium and potassium, ~nd ammonium thiocyanate salt6. Whil2 any convent~onal quantity of the ~hiocysnate salts can be introduced, preferred roncentr_tions are ~ener~lly from about 0.1 to 20 grams of thiocyanate 6alt per mole of ~ilver halide. IllustratiYe prior te_chings of employing thiocyanate ripen~ng agen~ Are found in Niet~ et al U.5. Patent 2~222~264) Lowe et al U.S.
Patent 2,448,534, and Illingswor~h U.S. Patent 3,320,069. Alternatively, conventional thioether r~pening agen~s, such as those dlsclo6ed in McBr~de U.S. Patent 3,271,157, Jones U.S. Patent 3,574,62B, ~nd Rosecrants et al U.S. Patent 3,737,313, c~n be employed.
The hlgh aspect ratio tabular graln silver bromide emulsions are preferably washed ~o remoYe soluble salts. The soluble 6alts can be removed by decantfl~ion, filtration, and/or chill setting and leaching, as illustrated by Cra~ U.S. P~t~nt 2,316,845 and McF~ll et ~1 U.S. Patent 3,396,027; by coagulation washing, as illustrated by Hewitson et al U.S. Patent 2,618,556, Yutzy ~et al U.S. P~tent 2,614,928, Y~ckel U.S. Patent 2,565,418, H~rt et al U.S. Patent 3,2~1,969, Waller et al U.S. Patent 2,~89,341, Klinger U.K. Patent 1,305,409 and Dersch et al U.K. Patent 1,167,159; by c~ntrifugation snd decantation of a co~gulated emulslon, as illustrated by Murray U.S. iatent 2,463,794, U~ihara et al U.S.
Patent 3,707,378, Audran V.S. Patent 2,996,287 and Timson U.S. Pa~ent 3,498,454; by employing hydro-cyclones alone or in combination with centrifuges, as illustrated by U.K. Patent 1,336,692, Clae6 V.K.
Patent 1,356 J 573 and U6homirskii et al Soviet Chemical Industry, Vol. 6, No. 3, 19749 pp. 181-185 by diAfil~ration with a semipermeable membrane, as illustrated by Research Disclosure, Vol. 102, 9ctober 1972, Item 10208, Hagemaier et al Research Disclo-sure, Yol. 131, March 1975~ Item 13122, Bonnet Research Disclosure, Vol. 135, July 1975, Item 13577, , Berg et al German OLS 2,436 J 461, Bolton U.S. Patent 2,495,918, and Mignot U.S. Patent 4,334,012, ci~ed above, or by employing an ion exchange resin, as illustrated by Maley U.S. Patent 3,78?,953 ~nd Noble U.S. Patent 2,827,428. The emulsions, with or without 6ensitizers, can be dried and stored prior to use as illu~rated by Research DiBCloBUre J Vol. 101, Septem~er 1972, Item 10152. In the present invention washing is particularly advan~ geous in terminating rlpening of the tabular ~rains af~er ~he completion of precipitHtion to avoid increa~ing their thickness and reducing their nspect ratio.
While the foregoing procedure constitu~es a preferred double ~et preCipitAtion proce6s for preparin~ high aspect ratio t~bular grain silver bromide emulsions, it i~ recognized that departures therefrom can also produce high ~spect rat$o tabular grain silver bromide emulsions. For example, the preferred pBr ranges will ~h~ft under varied precipi-tation conditions, 6uch in prep~ring ammoniacAlemulsiona or ln varying concentratlons or modifler~
within the reaction vessel. High aspect ratio tabulnr grain silver bromide emulsion6 can alterna-tively be prepared following a procedure 61milar to that employed by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Cry6tals During Physic~l Ripening", Science et Industries Photo-raphiq_es, Vol. 33, No. 29 ~1962), pp. 121-125.
High aspect ratio silver bromide emuls~ons cont~in~ng square and rectangular tabular grains ran be prepared as taught by Mignot U.S. Paten~ 4,386,156, noted above .
Although the procedures for preparing high aspect rstio ~abulAr silver bromide grain6 de6cribed above will produce high aspec~ ratio t~bular grain silver bromlde emul6ions ~n which tabular grains satisfyin~ the thickness and diameter criteri~ for aspect ratio accoun~ for at least 50 percent of the total projected area of the total silver halide gr~in population, it ls recognized that further ad~antsge6 can be realized by ~ncreasing the proportion of such tabular grains presen~. Preferably at least 70 percent (opt$m~11y at least 90 percent3 o the total projected area is provided by tabular silYer ~romide gr~ins meeting the thickness and diameter criteria.
While minor amoun~s of nont~bular gra~ns are fully compatible with many photographic applicstions, to achieve the full advantages of tabular grains the proportion of tabulAr grains can be increased.
Larger tabular silver bromide grains can be mechani-cally separated from ~maller, nontabular ~ilver bromlde grains in a mixed popul~tlon of grain6 u~i~g convent~onal separation techn.Lques--e.g., by u~lng ~
centrifuge or hydrocyclone. An illu~rative t~aching of hydrocyclone ~epnration iB provided by Audran et al U.S. P~tent 3,326,641.
Vehicles (including both binder6 and peptizers) which form the disper~ing media of the emulsions can be chosen from among tho~e convention-ally employed in silver halide emulsions. Preferred peptizers ~re hydrophilic colloids, which can be employed alone or in combination with hydrophobic materials. Suitable hydrophilic materials include substances such as proteins, protein derivative~, cellulose derivative~-^e.g., cellulose e6ter6, gelatin- e.g., alkali-treated gelatin (cattle bone or hide gelatin), acid-treated gelatin ~pigskin gela-tln), or oxidizing agent treated gelati~, gelatin derivatives--e.g., acetylated gelatin, phthalated gelatin, and the like, polysaccharides ~uch AS
dextran, gum arabic, zein, casein, pectin, collagen derivatives, agar-agar, arrowroot, albumin and the like as described in Yutzy et al U.S. Patents 2,614,928 and '929, Lowe et al U.S. Patents 2,691,582, 2,614,930, '931, 2,327,808 ~nd 2,448,534, Gates et al U.S. Patents 2,787,545 and 2,956,880, Corben et ~1 U.S. Pa~ent 2,890,215) Himmelmann et al U.S. Patent 3,061,436, F~rrell et al U.S. Patent 2,816,027, Ryan U.S. Patents 3,132,945, 3,138,461 and 3,186,846, Dersch et al U.K. Patent 1,167,159 and U.S. Patents 2,960,405 and 3,436,220, Geary U.S.
Pa~ent 3,486,896, Gazzard U.K. Patent 793,549, Gate~
et al U.S. Patents 2,99Z,213, 3,157,506, 3,184,312 and 3~539~3535 Miller et ~1 U.S. Patent 3,227,571, ~ .3 Boyer et al ~l.S. Patent 3,532,502, M~lan U.S. Patent 3,551,151, Lohmer et al U.S. Patent 4,018, sos, Luciani et al U.K. Patent 1,186,790, ~Horl et al U.K.
Patent 1,489,0B0 and Belgian Patent 456,631, U.K.
Patent 1,490,644, U.K. Patent 1,483,551, Ar~se et al U.K. Patent 1,459,906, Salo U.S. P~tent6 2~110J4~1 and 2,311,086, Komats~ et al Japane8e Kokai P~tent No. Sho 5~[1983~-70221, Falle~sen V~S. Pstent 2,343,650, Yutzy V.S. Pa~ent 2,322,~B5, Lowe U.S.
Patent 2,563,791, Talbot et al U.S. Patent 2,725,293, Hilborn U.S. Pa~ent 2,748,022, DePauw et al U.5.
P~tent 2,956,883, Ritchie U.K. Patent 2,~95, DeStubner U.S. Patent 1,752,069, Sheppard et ~1 U.S.
Patent 2,127,573, Lierg U.S. Paten~ 2,256,720, Gaspar U-S- Patent 2,361,936, Farmer U.R. Patent 15,727, Stevens U.K. ~atent 1,062,116 and Yamamoto et al U.S.
Patent 3,923,517.
Other ma~erlals commonly employed in combi-nation with hydrophilic colloid peptizers as vehicle6 (including vehicle extenders--e.g., material6 in the form of latic s) include synthetic polymeric peptiz-ers, carrier6 and/or binders such as poly(vinyl l~ctams), acrylamide polymers, polyvinyl alcohol and its deriva~ives, polyvinyl acetal6, polymers oi ~lkyl and sulfoalkyl acrylates and methacryl~tes, hydro-lyzed polyvinyl acetates, polyamides, polyvinyl pyridine, acrylic acid polymers, maleic anhydride copolymers, polyalkylene oxides, methacrylamide copolymers, polyvinyl oxazolidinones, maleic acid copolymers, vinylamine copolymer6, methacrylic acid copolymers, acryloyloxyalkyl~ulfonic acid copolymers, ~ulfoalkylacrylamide copolymers; poly~lkyleneimlne copolymers, polyamines, N,N-dialkylaminoalkyl ~cryl-ates, vinyl imida~ole copolymers, vinyl fiulflde ~opolymers, halogenated styrene polymers, amineacryl-amide polymers~ polypeptides and the like as describ-ed in Hollister et al U.S. Patents 3,679,425, ~ t 3,706,564 And 3,813,251, Lowe U.S. PAtents 2,253,078, 2,2763322, '323, 2,281,703, 2,311,058 and 2,414,207, Lowe et al U.S. Patents 2,484,456, 2,541~474 and 2,632,704, Perry et al U.S. Patent 3,425,836, Smith et al U.S. P~tents 3,415,65:3 and 3,615,624, Smith U.S. Patent 3,488,708, Whiteley et al U.S. P~tent~
3,392,025 ~nd 3,511,818, Fitzgerald U.S. Pa~ents 3,681,07g, 3,721,5659 3,852,073, 3,861,918 ~nd 3,925,083, Fitzger~ld et ~1 U.S. P~tent 3,879 9 205, Nottorf U.S. Pstent 3,142,568, Houck et ~1 U.S.
P~tents 3,062,674 end 3,220~844, D~nn et ~1 U.S.
Patent 2,882,161, Schupp U.S. Patent 2,579,016, Weaver U.S. Patent 2,829,053, Alles e~ al U.S. PAtent 2,698,240,`.Priest et al U.S. Patent 3,003,879, Merrill et 81 U.s- Patent 3,419,397, Stonham U.S.
Patent 3,284920i, Lohmer et al U.S. PAtent 3,167,430, Williams U.S. Patent 2,957,767, D~wson e~ al U.S.
Patent 2,893,867, Smith et al U~S. Patentæ 2,860,986 and 2,904,539, Pont~cello et al U.S. Patents 3,929,482 and 33860,428, Ponticello U.S. Patent 3,939,130, Dykstra U.S. Patent 3,411,911 ~nd Dykstra et al Canadian Patent 774,054, Ream et al U.S. Patent 3,287,289, Smith U.K. Patent 1,466,600, Stevens U.K.
P~tent 1,062,116, Fordyce U.S. Patent 2,211~323, Martinez U.S. Patent 2,2849877, W~tkins U.S. Patent 2,420,455, Jones U.S. P~tent 2,533,166, Bolton U.S.
Patent 2,495,918, Graves U.S. P~tent 2,289,775, Yackel U.S. Patent 2,565,4189 Unruh et al U.S.
Patents 2,865,893 ~nd 2,875,059, Rees et al U.S.
3~ Patent 3~536,491~ Broadhead et al U.K. Patent 1,348,815, Taylor et el U.S. Pstent 3,479,186, ~errill et al U.S. Patent 3,520,857, Bacon et Al U.S.
Patent 3,690,888, Bowman U.S. Patent 3,748,143, Dickinson et al U.~. Patents 808,227 ~nd '228, Wood U.K. Patent 822,192 and Iguchi et al U.K. Pa~ent 1~398~055O These additional materi~ls need not be present in the reaction ves6el during silver bromide ~ 3~3 precipitation, but rather are convention~lly added ~o the emulsion prior ts co~ting.
The vehicle materials, including p~rticul~r-ly the hydrophilic colloids, ~8 well ~s the hydro-phobic materials useful in combination therewith canbe employed not only in the e~lulsion layers of the photographic elements of this inven~ion, hut ~160 ln other layers, ~uch as overeoat: layers, ~nterlayer~
and layers positioned beneath the emul~ion layers.
The layers of the photographic elementfi containing crosslinkable colloid0, particularly gelat~n-contaln-in~ layer~, can be hardened by various organic or inorganic hardeners, ~uch ~s those descrlbed by Research Disclosure, Item 17643, cited above, Section X- The tabular grain emulsion layer6 are preferably fully forehardened7 AS taught by Dlckereon U J S
Patent 4,414,304.
Although not e6sential to the practice of the invention, AS a practical matter the latent lmage forming grains of the im~ge rec~rding emul~ion layer~
~re chemically sen~itized. Chemical sensitiz~tion can occur either before or after spectr~l ~en~itlza-tion. Techniques for chemically ~ensitizing latent image forming silver halide grains are generally known to those skilled in the Qrt and are summarized in Research Disclosure, Item 17643; clted above, Section III. The tabular grain latent ~mage forming emulsions ran be chemically sensitized a~ taught by Maskasky U.S. Patent 4,435,501 or Koron et al U.S.
P~tent 4,4397529, both cited above.
It is specifically contemplated ~o employ in combination with the tabular grain emul&ions and, preferably, other latent image forming emulsion6, lf any~ forming a part of the photographic elements spectral sensitizing dye6 that exhibit absorption maxima in the visible spectrum. In addition~ ~or ~pecialized application~, ~pe~trfll ~ensitizing dyes . -26-can be employed which improve spectrAl respon~e beyond the visible spectrum. For example, the use of lnfrared absorbing spectral sen6itlzers is sp~cifi-cally contemplated.
The latent image forming silver hnlide emulsions can be ~pectrally ~ensitized with dyes from ~ variety of cla~ses, lncluding the polymethlne dye class, wh~ch classe~ include the cy~nines, mero-cyanines, complex cyanines ~nd merocy~nines ~.e., tri-, tetra-, and poly-nucl~ar cyflnines and mero~
cyanines), oxonols, hemioxonols, styryl~, mero-styryls, ~nd streptocyanlne5.
The cy~nlne spectral sen~itizlng dyes include, J~ined by a methine link~ge, two ba6$c lS heterocyclic nuclei, such as those derlved from quinollnlum, pyridinium, isoquinolinium, 3H-indolium, benz[e]indolium, oxazolium, ox~zolinium, thi~zolium, thiazolinium, æelenazolium~ selenazolinium, imida-zol~um, lmidazolinium3 benzoxazolium, benzothla-zolium, benzoselenazolium, benzimidazolium, naphthox-~zolium, naphthothiazolium, nsphtho~elen~zolium, dihydronaphtho~hiazolium, pyrylium, ~nd im$dszopyra-zinium quateroary ~alts.
The merocyanlne ~pectral ~en6itizing dyes include, 30ined by ~ methine linkage, a basic hetero-cyclic nucleus of the cyanine dye type and an acidic nucleus, such as c~n be derived from barbituric acid, 2 thiobarbituric acid, rhodanine, hydantoin, 2-thio-hydantoin, 4-thiohydantoin, 2-pyrazolin-5-one, 2-isoxaæolin-5-one, lnd~n-1,3-dione, cyclohex~ne-1,3-dione, 1~3-dioxane-4 9 6-dione, pyraæolin-3,5 dione, pentane-2,4-dione, alkylsulfonylacetonitrile, malononitrile, isoquinolin-4-one, and chromsn-2,4-dione.
One or more æpectral senæitizing dyes may be used. Dyes with sensitizlng m~xima at wavelengths ~hroughout the vi6ible spectrum ~nd with a great 7~3 variety of spectr~l sensitivity curve shapes are known. The choice and relat:Lve proportions of dyes depend6 upon the region of the spectrum to which sensitivity is desired and upon the shape of khe ~pectral 6ensitivity curv0 desired. Dyes with overlapping 6pectr~1 sen6itivity curves will often yield in combin~tion ~ curve in whlch the sen61tivity at each wavelength in the area o overlap i8 ~pproxi~
mately equal to the sum of the ~ensitivities of the individual dycs. Thus, it il3 possible to use combl-natione of dyes with di~ferent maxima ~o ach$eve A
~pcctral sensltivity curve with a maximum inter-mediate to the sensltizing maxima of the indivldu~l dyes.
Combinations of spectr~l 6ensitizing dyes c~n be used which result in super6ensitization--that is, spectral sensitization that ls greater in ~ome spectral reglon th n that from any concentration of one of the dyes alone or that which would re~ult from the ~dditive effect of the dyes. Supersensitization can be achieved with selected combinations of 6pec-tral sensitizing dyes and other addenda, such as stabilizers and antifoggants, development ~ccele-rators or inhibitors, coa~ing dids, brighteners snd antistatic agents. Any one of several mechanis~6 ~8 well as compounds which can be re6ponsible or super6ensitlzation are discussed by Gilman, "Review of the Mechanisms of Supersensiti~ation", Pho~o-~raphic Science and ~ , Vol. 18, 1974, pp.
418-430.
Spectrsl sensitizing dyes al60 affect the emulsions in other w~ys. Spec~r~l sensitizlng dy~
can also function as antifoggant6 or 6tabil$zers, development accelerators or inhlbitors, and halogen acceptors or electron ~cceptors, as disclosed in Brooker et al U.5. PPtent 2,131,038 ~nd Shiba et al U.S. Patent: 3,930,860.

3~3 Sensltizing action can be correlated ~o the position of molecular energy levels of a dye with respect to ground state and conduction b~nd energy levels o the silver halide cry~talsO These energy levels can in turn be correla~ed to polarographic oxidation and reductioD potenti~ls, a6 discussed in Photo~raphic Science and ~ neerin~, Vol. 18t 1974, pp. 49-53 (Sturmer et 81), pp- 175-178 (Leubner) ~nd pp. 475-485 ~Gilman). Oxid~tion ~nd reduction potentlals can be me~sured QS described by R. F.
L~r8e in ~t~ra~e~ Sensit_v~, Academic Press, 1973, Ch~pter 15.
The chemistry of cyanine and rel~ted dyes is illustrated.by Weissberger and Taylor, ~e~ Topics o Heterocyclic Chemi~try, John Wiley and Sons, New York, 1977, Chapter VIII; Venk~taraman, The Chemistr~
_ Synthetic es, Academic Press, New York, 1971, Chapter V; James, The Th~ of ~he Photographic Process, 4th Ed., Macmill~n, 1977, Chapter 8, and F.
M- Hamer, Cyanine Dyes and Related Compounds, John Wiley and Sons, 1964.
Among useful spectral ~ensiti~ing dyes for sensitlzing silver halide emulsions are tho~e found in U.K. Patent 742,112, Brooker U.S. P~tents 1~846,300, '301, '302, '303, '304, 2,078,233 and 2,089,729, Brooker et al U.S. Patents 2,165,338, 2,213,238, 2,23~,658, 2,493,747~ '748, 2,526,632, 2,739,964 (Reissue 24,292), 2~778,823, 2,917,516, 3,35~,857, 39411,916 and 3,431,111, Wilmanns et al ~-5- Patent 2,295,276, Sprague U~S. Patents 2,4Bl,698 and 2,503,776, C~rroll et al U.S. Patents 2,688,545 and 2,704,714, Larive et ~1 U.S. Patent 2,921,067, Jones U.S. Patent ~,945,763, Nys et al U.S. Patent 3,282,933, Schwan et al V.S. Patent 3,397,060, Riester U.S. Patent 3,660,10~, Kampfer et al U.S.
Patent 3,660~103, Taber et al U.S. Patent 6 3,335,010, 3,352,680 ~nld 3,384,486, Lincoln et ~1 U.S. P~tent 3,397,981, Fumis et al U.S. Patents 3,482,978 and 3,623,881~ Spence et al U.S. Patent 3,718,470 and Mee U.S. Patent 4,025,349. Examples of useful dye combinations, including super~ensltizing dye combina-t~ons, are found in Motter U.5. Patent 3,506,443 and Schwan et al U.S. Pa~ent 3,672,~9~. As examples of supersensitizing combinations of 6pectral sensi~lzing dyes and non-light absorbing addenda, it is ~p~cii~
cally contemplated to employ thiocyanate~ during spectral ~ensiti7.ation, B~ tRught by Leerm~ker~ U.S.
Patent 2,221,805; bis-triAzinylaminostilbenes, a~
taught by McFall et ~1 U.S. Patent 2,933,390;
sulonated aromatic compounds, aB taught by Jones et al V.S. Patent 2,937,089; mercapto-sub6tituted heterocycles, as taught by Riester U~S. Pa~ent 3,457,078; iodide, as taught by U.K. Specification 1,413,826; and ~tlll other compound6, such as those disclosed by Gilman~ "Review of the Mechani~ms of Supersensitization", cl~ed above.
Conventional amounts of dyes c~n be employed in spec~rally senEitizlng the emulsion layers containing nontabular or low aspect ratio tabular ~ilver halide grains. To realize the full advantages of this invention i~ is preferred to adsorb ~pectral sensitizing dye to the grain surfaces of the tabular grain emulslons in a 6ubstantially optimum amount--that ls, ln an amoun~ sufficient ~o realize at least 60 percent of the maximum photographic speed attaln-able from the grain~ under contempl~ted conditions of exposure. The quantity of dye employed will vary with the specific dye or dye combination chosen as well as the size and aspec~ ratio of the grains~ It 16 known in the photographic art that optimum fipec-tral 6ensitizatlon i~ obtained wlth organic dyes ~t about 25 to 100 percent or more of monolayer coverage of th~ ~otal available 6urface area of surface sensitive sllver hal~de grains, a6 dl~closed, for ~ 3 example, in West et al, "The Adsorptlon of Sen6itiz-ing Dyes in Photogrflphic Emulsions", Journal of Phys.
Chem., Vol 56, p. 10659 1952; Spence et al, "De~ nsi-tization of Sensitizing Dyes",, Journal of Physical and Colloid Ch mistry, Vol. 56, No. 6, June 1948, pp.
1090-1103; and Gilman et al U.S. Patent 3,979,213.
Optimum dye concentra~ion level6 can be cho~en by procedures taught by Mees, Th~or~ of the Photo~raPhic Process, Macmillan, lg42, pp. 1067-1069.
Although the native blue ~ensitivlty of silver bromide can be relied upon to record expo~ure to blue light, it is specifically recognized that advantage6 can be re~lized from the use of blue spectral se~sitizing dye~. Where it is intended to expose tabular grain emulsions in thelr region of natlve 6ensitivity, advantage6 in sensitivity can be gained by increasing the thlckness of the tabular grains. Specifically, in one preferred form of the invention the tabular grain emulsions ~re blue sensitized silver bromide emulsions in which the tabular grains having a thickness of leS6 than 0.5 ~m ~nd a diameter of at least 0.6 ~m have an average aspect ratio of greater than 8:1, preferably at least 12:1 and account for at least 50 percent of the total pro~ected area of the silver halide gralns present in the emulsion, preferably 70 percent and optlmally ~t least 90 percent.
Useful ~lue spectral sensitizing dyes for tabular grain emulsions can be selected from any of the dye classes known to yield ~pectral 6ensitlzers.
Polymethine dyes, 6uch as cyanines~ merocyanines, hemicyanines, hemioxonols; and merostyrylsg are preferred blue spectral sensitiæer6. ~enerally useful blue spectrsl sensitizers can be 6elected from among ~hese dye classes by their absorp~ion charac-teristics--i.e., hue. There are, however, general structural correlations that c~n ~erve as a guide in selecting useful blue sen~itizers. Gener~lly the shorter the methine chain, the shorter the wavelength of the sensitizing m~ximum. Muclei ~180 lnfluence Ahsorption. The addition o~ fused rings to nucle~
tends to favor longer wavPlength~ of ~b60rption.
Substituents can also ~lter ,~bsorptlon char~cter-i6tics. In the formulae which follow, unlesfi other-wise specified, ~lkyl groups ~nd moieties cont~in from 1 to 20 carbon atoms, prefernbly from 1 tv 8 c~rbon atom6. Aryl groups amd moietles contain rom 6 to 15 carbon ~toms and are preferably phenyl or naphthyl groups or moieties.
Preferred cyflnine blue sp~c~ral sen6itlzer~
are monomethine cy~nines; however, useful cyanine blue spectral 6ensitizers cMn be 6elected from among those of Formula 1.
I_ _zl ~ ,3 R4 Rs ~_ _z2 _ _ I
Rl-N~CH~CH~pC~C~-C-=C)m~C~CH~CH~qN~R2 (A)k (~3Q
Formul~ 1 where Zl and Z2 may be the same or different and each represent6 the elements needed to complete cyclie nucleus derived from basic he~erocyclic nitrogen compounds ~uch ~s oxazoline, oxazole, benzoxazole, the naphthoxazoles (e.g., naphth[2,1-d]~
oxazole, naphth~2,3-d30xaæole, and naphth~l,2-d]oxa-zole), thiazoline, thiazole, ~enzothiRzole, the naphthoth~azoles (e.g., naphtho~2,1-d~thiazole3, the thiazoloquinolines (e.g., th~azolo[4,5-b]quinollne) 7 selenazollne, Relen~zole, benzoselenazole, the naphtho6elenazoles (e.g., naphtho[l,2Od~elenazole~, 3Hoindole Ite.g., 3,3-dimethyl-3H-indole), the benzin-doles (e.gO, l,l-dlmethylbenz[e]indole), imidazoli~e, lmidazole, benzim$dazole, ~he n~phthimidazoles ~e.g., naphth[2,3-d]imidazole~, pyridine, and quinoline, ~ 3 which nuclei may be ~ubstituted on the rlng by one or more of a wide variety of sub~tituents au~h as hydroxy, the halogens (e.g., fluoro, chloro, bromo, and iodo), alkyl groups or substituted ~lkyl group~
(e.g~, methyl, e~hyl, propyl, isopropyl, butyl, octyl, dodecyl 9 octadecyl, 2~hydroxyethyl, 3-sulfo-propyl, carboxymethyl, 2-~yanoethyl, nnd trifluoro-methyl), aryl groups or substituted ~ryl groups ~e.g., phenyl, l-naphthyl, 2-naphthyl, 4-sulfophenyl, 3-carboxyphenyl, and 4-biphemyl), aralkyl groups (e.g., benzyl and phenethyl), alkoxy group~ ~e.g., methoxy, ethoxy, and isopropoxy), aryloxy group~
(e.g., phenoxy and l-naphthoxy), alkylthio groups (e.g., methylthio and ethylthio), arylthio groups ~e.g., phenylthio, ~-tolythio, and 2-naphthylthio3 methylenedioxy, cyano, 2~thienyl, ~tyryl, amino or substituted amino groups (e.g., anilino, dimethyl-amino, diethylamino, and morpholino~, ~cyl group~, such as carboxy te-g-, ace~yl and benzoyl) and ~ulfo;
Rl and R2 c&n be the same or differen~ and represent alkyl groups, aryl groups, alkenyl grouR6, or aralkyl groups, with or wi~hout 6ubstituent~, (e.g., carboxymethyl~ 2-hydroxyethyl9 3-fiulfopropyl9 3-sulfobutyl, 4-sulfobutyl, 4-sulfophenyl, 2-methoxy-ethyl, 2-sulfatoethyl, 3-thiosulfatopropyl, 2-phos-phonoethyl, chlorophenyl, and bromophenyl);
R3 represents hydrogen;
R4 and Rs represents hydrogen or alkyl of rom 1 to 4 carbon atoms 3 p And q are 0 or 1, excep~ that bo~h p and q preferably are not l;
m is 0 or 1 excep~ that when m is 1 both p ~nd q are ~ and at le~s~ one of Z~ and Z2 represents imidazoline 9 oxazol$n~ 9 thi~zoline, or selenazoline;
A ic an anionic group;
B is a ca~ionic group; and ^33~
k ~nd ~ may be O or 1, depending on whetherionic substituent~ ~re pre~ent. Var~nts ~re, of cour~e~ possible in which Rl and R3, R2 ~nd R5, or Rl ~nd R2 (particularly when m, p, and q are 0) together represent the ~toms nece~sarg to complet~ an alkylene brldge.
So~e representa~lve cyanine dyes u~eul a6 blue sensltizer~ ~re listed in Table 1.
Table I
101. 3,3'-Diethylthi~cyanlne bromide CH~ i J
I I Br~
15~ C2Hs C2Hs 2. 1-Ethyl-3'-methyl-4'-phenylnaphtho[1,2-d~thiazolothiazolinocyanine bromide ~-\ /s\ /s\

20 ! ~ \N~ c \-~ C2~s CH3 ~ r~
3. 1',3~Diethyl-4-ph~nyloxazolo-2'-cyanine iodide /o~
CH-1;~;;,1!, ~!

~./ C2Hs C2H5 I-4. Anhydro 5-chloro-5'-methoxy-3,3'-bis-(2-~ulfoethyl)thiacyanine hydroxide, triethyl-amine 8~1 t CH--~

2 ) 2 (CH2 ) 2 (C2 Hs) 3 NH~
S3 S(~3 -3~-S. 3,3'~ (2-carboxyethyl)thiazolinoc~rbo-cyanine iodide CH-CH~CH~

(CH2)2 (CH232 COOH COOH
6. 1,1'-Diethyl-3,3'-el:hylenebenzimid~zo~o~
cyanlne iodide CzHs C2~S

nCH~

7. 1-(3-Ethyl-~-benzothiazolinylldene)-1,2,3,4-tetrahydro-2-methylpyrido-[2,1-b~ benzothiazolinium iodide // \./ ~.
~-\ /S\ / ~ + U ~l /
l \ ./ I-C2Hs 3. Anhydro-5,5'~dimethoxy-3,3'-bis(3-sulfo-propyl)~hiacyanine hydroxide, ~odium 6~1t t i~
CH3 ~ ~o/ \N/ ~ ~ OCH3 NaSO3(CH233 (C~2)3SO3 Na~
Preferred m~rocy~nine blue spectral sensi-tizers are zero meth~ne merocy~n~nes; however, useful merocy~nine blue spec~rAl ~ensitizer~ an be ~lected '7 from flmong those of Formula 2.
O
,- -Z - -, R4 11 G ' R-N~cH~cH3rce~c-cR )n \G2 Formul~ 2 where Z represents the sAme elements ~ her Z~ or Z2 o~ FormulR 1 above;
R repre~ents the same group~ ~8 either Rl or R2 of Formula 1 above;
R~ ~nd Rs represent hydrogen, an alkyl group of 1 to 4 carbon atoms, or an Aryl group (e.g., phenyl or nsphthyl);
Gl represents an alkyl group or substltuted alkyl group, an ~ryl or substituted aryl group, an aralkyl group, an ~lkoxy group, ~n aryloxy group~ a hydroxy group, an amino group, a substituted amino group wherein ~pecific groups ~re of the type~ in Formula 1, G2 can represent any ~ne of the group6 list~d for G' and in addition c~n represen~ a cyano group, ~n alkyl, or arylsulfonyl group, or ~ group represented by -C~GI, or G2 tsken together with can represent the element~ needed ~o eomplete a cyclic acidic nucleus ~uch RG tho~e derived from 2~4-oxazolidinone ~e.g., 3-e~hyl~2,4-oxazolidin-dione), 2,4~thiazolldindione (e.g., 3-~ethyl~2,4-thi-azolidindione), 2-~hio-2,4-oxazolidindione ~e.~., 3-phenyl-2-thio-2,4-oxazol~dindione), rhodanlne, ~uch a8 3-ethylrhodanine, 3-phenylrhodanine, 3-(3-di-methylaminopropyl~rhodan~ne, ~nd 3-carboxymethylrho-dan~ne, hydantoin (e.g., 1,3-diethylhyd~ntoln Rnd 3-ethyi-1-phenylhydan~oin), 2-thiohydantoin (e.g., l-ethyl-3-phenyl-2-thiohydantoin, 3-heptyl-1-phenyl-~ t~

2-thiohydantoin, and 1,3-diphenyl-2-thiohydanto~n~, 2-pyrazolin 5-one, such as 3-methyl-:L-phenyl-2~pyr~-~olin-5-one, 3-methyl-1-(4-carboxybutyl)-2-pyr~-zolin-S-one, and 3-me~hyl-2-4-sulfophenyl3-2-pyr~-S zolin-5-one~ 2-i~oxazolin-5-one (e.g , 3-phenyl-2~
oxazolin-5-one), 3,5-pyrazolidindione ~e.g , 1,2-dl-ethyl-3,5-pyrazolidindione and 1,2-diphenyl-3~5-pyr~
zolidindione), 1,3-indandione, 1,3-dioxane-4,6-dione, 1,3-cyclohexanedione, bsrblturic ~cid (e.g., l-ethyl-b~rbituric acid and 1,3-dlethylbarbituric acid), and 2 thiobarbituric acid (e.~., 1,3-diethyl-2-th~obarbl-turic acid ~nd 1,3-bi6(2-methoxyethyl3-2-thiob~rbi-turic acld);
r and n e~ch can be O or 1 except th~t ~hen n i~
1 then generally either Z is re~tric~ed ~o imida-zoline, oxflzoline, selenaæoline, thiazoline, imidazo-line, oxazole, or benzoxazole, or G' ~nd G2 do not represen~ a cyclic system. Some repre~entative blue ~ensltlzing merocyanine dyes are li~ted below in Table II.
Table II
1. 5-(3-Ethyl-2-benzoxazolinylidene3-3-phenyl-rhodanine o I !' ~ \ /\ U-N/ ~./
il \. ,~./ \~ -S
~-/'\N/ \S/
C2 ~5 2. 5-[1-(2-Carboxyethyl3-1,4-dihydro-4-pyridlnylidene-l-ethyl-3-phenyl-2-thio-hydantoin ~ \O
O 1 l' HOOCCH2CH2-~ S

C2Hs ~ 3~3 3. 4-(3-Ethyl-2-benzothiazolinylidene~-3-methyl-1-(4-sulophenyl)-2-pyraæolin-5-one, potassium ~alt s o î fi-503 K
!~ îî N~~ N

C2Hs CH3 4. 3-Carboxymethyl-5-(5-chloro-3-ethyl-2-benzothiazolinylldene)rhodanine 11 /~ S

C2Hs

5. 1,3-Diethyl-S-[3,4,4-trimethyloxazolidin-ylldene3ethylidene]-2~thiobarbituric acid /0 \ ~ ~ C2Hs H3C-I\N/ ^~C~-CH~ S
H3C ~ ~ C2Hs Useful blue sensitizing hemicyanine dye~
includ~ those represen~ed by Formula 3.

I '' -Z - - l 2 3 4 R-N~CH-CH~pC~CL CL ~CL CL )~-~+ 4 Formula 3 (A)k wher~
Z, R, and p represent ~he same elementæ a~ in Formul~ 2; G3 and G4 may be the same or different and may represent alkyl, sub6tituted alkyl, aryl, substituted aryl, or aralkyl, æ~ illu~tra~ed for rlng sub~tituents in Formula 1 or G3 and G~ taken together complete a ring sys~em derived from a cyclic 6econdary amine, such as pyrrolidine, 3-pyrroline, 1 ~ ~'7~1~4 piperidine, piperazine te-g-, 4-mPthylpiperazine end 4-phenylpiperazine), morpholine, 1,2,3,4-tetr~hydro-quinoline, decahydroquinolinel, 3-~z~bicyclo[3,2,2]-nonane, indoline, azetidine, llnd hexahydroazepi~e;
Ll ~o L4 represent hydrogen, alkyl of 1 to 4 carbons, aryl, substituted aryl, or any two of Ll, L2, L3, L4 can represent the element6 needed to complete an ~lkylene or carbocyclic bridge;
n is 0 or 1; and 0 A and k h~ve the same def:Lnition ~ in Formula 1.
Some representative blue sensitlzing hemi-cy~nine dyes ~re listed below in Table III.
Table III
1. 5,6 Dichloro-2-[4-(diethylamino)-1,3-butadien-1-yl]-1j3-dlethylbenximidazolium iodide C~Hs Cl, ~~ ~N, C2~ls ! 11 -CH~CH-CH~CH-~

C2~s I-2. 2-{2~[2-(3-Pyrrolino)-l-cyclopenten-l-yl]ethenyl}3-ethylthiazolinium perchlor~te /S \ H ~CH2\cH /-\

~CH-C~ \O/ C10~-C2Hs 3. 2-(5,5 Dimethyl-3-piperidino-2-cyclohexen-l-yldenemethyl)-3-ethylbenzoxazolium per-chlorate ~CH3) 2 .
-c~si~
Cl04 C2Hs 1;2~.~'7~.~3t`3 Useful blue ~ensitizing hem:Loxonol dye~
lnclude those represented by Formula 4.
Gl-~ O G3 C~CLI(-CL2~CL3) N
Formula 4 where Gl and G2 repre~ent the l3ame elements ~8 in Formulfl 2;
0~3 ~ G4, L~, L2, and L3 r~apresen~ the sflme elementB as in Formul~ '3; and n i~ O or 1.
Some represent~tive blue ~ensitizing hemi-oxonol dye6 ~re listed in T~ble IV.
15T~ble IV
1. 5-(3-Anillno-2-propen-1-ylidene)-l,3-di~
ethyl-2-thiobarbiturlc acid C2Hs S~ CH-CH~CH-N~
O
C2Hs 2. 3-Ethyl-5-(3-piper~dino-2~propen-l-25ylidene)rhodanine /~3CH-CH-CH-N/\ \-~ \S/ ~--3. 3-Al1yl-5-[5,5-dimethyl~3-(3-pyrrolino)-2-cyclohexen-l-ylidene~rhod~nine 0 H3 ~ /CH3 CH2~CH-CH2\ ~
S~ \ S/ C~ \ . / -U~eful blue senslt~zing merostyryl dyes -~o -lnclude those represented by Formul~l 5.
0 / ~ G3 G2/ CH-~CH C ~ ~ 4 5Formuls 5 where G~, G2, G3, G4, ~nd n ~re ~8 def~ned in Formula 4.
Some represeDtativç! blue ~en~lt~zing mero-10 8tyryl dye~ ~re listed in T~ble V.
Table! V
1. 1-Cyano-1-(4-dimethyl~minobenzylidene)-2-pentanone 15CH3 (C~2)2-~ D~- CH3 \C3CH~

2. 5-(4-Dimethylaminobenzylidene-2,3-diphenyl~
thiazolidin-4-one-1-oxide 2~~-\ 0 Il ~ \./ ~ .// ~0_.~ ~\CH
i~ ,1! o 3. 2-(4-Dimethylaminocinnamylidene)thiazolo-[3,2~a]benzimidazol-3-one ." O
~ CH-C~-CH_-/ \- ~ CH3 Spectral 6ensitization can be undertaken at any stage of emulsion preparation heretofore known to be useful. Most commonly spectral sensitiz~tlon is undertaken in the ~rt ~ub6Pquent ~o the completion of chemlcal sensitization. Xowever J lt is specificslly recognized that ~pectral sensitiz~tion can be under-taken alternatively concurrently with chemicalsensitization, can entirely precede chemical sen iti-zation~ and c~n even commence prior to the ~ompletion of sil~er halide grAin precipitAtlon~ A~ tau~ht by Philippaerts et al U.S. Patent 3,628,960, ~nd Locker et al U.S. Patent 4,225,666. As taugh~ by Locker et al, it is specific~lly contemplated to dis~ribute introduction of the spec~ral aen~itlzing dye into the emulsion ~o that a portion of the spectral ~en~itiz-ing dye is present prior to chemic~l 8en61tiz~tionand a remaining portlon i8 introduced after chemical 6ensitizstion. Unlike Locker et al, it ls ~pecifi-cally contemplated that the ~pectral ~ensitizlng dye can be added to the emul~ion ~fter 80 percent o the silver halide has been precipitated. Sensitizat~on can be enh~nced by pAg adju~tment, including varla-tion in pAg which completes one or more cycles 7 during chemical ~nd/or spec~ral Ben~itization. A
~pecific example of pAg ad~u~tment i8 provided by Research Disclosure, Yol. 181, May 1979, Item 18155.
As taught by Kofron et al U.S. Patent 4,439,520, high aspect ra~io tabular ~rain silver halide ~mulsions can exhibit better ~peed-granularity relationships when chemically and ~pectr~lly ~en6i-~ized than have heretofore been achieved u~ingconventional sil~er halide emulsions of like halide content~
In one prefPrred form, 6pectral sen~itizer~
can be incorporated in ~he tabular grain emulsions prior to ch4mic~1 sensiti7ation. Similar result~
have also been achieved in some in6tances by lntro-ducing other adsorbable materials, such ~8 f~nish modifier6, i.nto the emulsion6 prior to chemical R enB itization.
Independent of the prior incorporation vf ~d~orbable materials, ~t is preferred to employ thiocyanates durlng chemical sen~tization in concen-trntlonOE of from about 2 X 10- 3 to 2 mole percent, based on ~ilver, a8 taught by Dam&chroder U.S. Patent 2,642,361, cited above. Other rlpening agent~ cnn be used during chemical sensiti2ation.
In ætill a third approach, which can be practiced in combination with one or both of ~he above Appro~che6 or separate'Ly thereof, it 1~ prefer-red to ad~ust the concentr~tLon of ~ er and/or halide ~alts present immedlnl:ely prior to or during chemical sensitlz~tlon. Soluble ~ilver salt~, ~uch as silver ~cetate, silver triLfluoroacetate, and sil~er nitrate, cnn be introduced aæ well as ~ilYer s~lts c~pable of precipltatiilg onto the grain surfaces, 6uch as ~llver thiocyanate, silver phos-ph~te, silver c~rbona~e, and the llke. Fine sil~er halide (i.e., silver bromide ~nd/or ehloride) grains cspable of Ostw~ld ripening onto the tabular grsin surfaces can be introduced. For example, a Lippmann emulsion can be introduced during chemlcal ~en~itiza-tion. M~skasky U.S. Patent 4,435,501, discloses thechemical sensitlzation of ~pectrally sensitized high aspect ratlo tabular grain emulsions at one or more ordered d~screte site~ of the tabular grain6. It is believed that the preferenti~l adsorption of spectr~l ~ensitizing dye on the cry6tallographic surfaces formlng the ma~or f~ce~ of the tabular grain6 allows chemical sensitlzation to occur selectively at unlike erystallographic surfaces of the t~bular grains.
The preferred chemical sens~tizers for the highest attained speed-granularity relationship~ are gold and sulfur ~ensitlzer6, gold and ~elenium ~enæitizers, ~nd gold, ~ulfur~ and selenium ~ensitiz-ers. Thus, in a preferred form, the high a~pect r~tio t~bular grain silver bromlde e~ulsions cont~in a middle chalcogen, such as 6ulfur and/or selenium, whieh'may not be detectable, ~nd gold, whIch i6 detectable. The emul~ionæ al~o uæually contain ~ 3 detectable levels of thlocyanate, although the concentration of the th~ocyan~te in the 1nal ~mul-

6~0ns csn be grea~ly reduccd by known emulslon washing techniques. In various of the preferred forms indicated above ~he tabular silver bromid~
grainæ can have ~nother silver salt at their ~urf~ce, 6uch as silver thiocyanate or nnother ~ilver rhlor-ide, altho~gh the other silver ~lt may be pre3ent below detectable levels.
Although not required to realize all of their sdv~ntages, the image recording emulsions are preferably, ~n accordance with prevailing ~nu~actur-ing practice6, æubstantially optimaIly chemically ~nd spectr~lly~sensitized. That i6, they prefer~bly achleve ~peeds of at least 60 percent of the maximum log speed attainable from the grains in the spectral region of 6ensitiza~ion under the contemplated conditions of U6e and proces~ing. Log speed iB
herein defined ~s 100 ~l-log E~, where E iB measured in meter~candle-seconds at a density of 0.1 ~bove fog. Once the ~llver halide grains of an emulsion layer have been characterized, it is possible to es~imate from fur~her product analysis and perfor~w ance evaluation whether an emulslon layer of a product appear6 to be substan~ially optimally chemi-cally and spectrally sensltlzed in rel~tlon to comparable commercial offerings of other manufacturers.
In addition to the silver halide grain6, 30 6pectraI and chemlc~l sensltlzer~, vehicles, and hardeners described abo~e, the photographic element6 c~n con~ain in the emul6ion or other l~yers thereof brighteners, antlfoggants, stabilizer6, scattering or &bsorbing materials, coating aids, plasticizers~
lubri~ants, and mQt~ing 8gent~ ~ ~S described in Research Disclosure, Item 17643~ ci~ed above, Sections V, VI, YII9 XI, XII, and XVI. Methods of t~

addition and coating and drying proce~dures can be employed, ~s described in Section XIY and XV.
Conventional photographic supports c~ln be employed J
as described in Section XYII~ These photographic elements are capable of producing st~ble, view~ble sllver images on development in aqueous alkal~ne processing solutions and fixing out.
In a preferred form the ~ilver im~ge produc-ing photographic elements of this inven~lon Are radiogrRphic elements~ In ~dldition to the ~e~tures specific~lly desc.ribed above the radiographic elements of this invention c~ln lnclude ~dditlonal featurcs conventional in r~diographic applications.
Exemplary features of this ~ype are di~closed, ~or example, in Research Disclosure, Vol. 184, Augu6t 1979, Item 18431; For example, the emulsions can contain antikink agents, as set for~h $n Paragraph II. The radiographic element ean contain antistatic agents and/or layers, as 6et forth ln Paragraph III.
The radiogrephic elements c~n contain overcoat layers, as 6et out in Paragraph IV.
Preferred radiographic elements are of the type disclosed by Abbott et al U.S. Patents 4,425,425 and 4,425,426, cited above. That is, at least one 2S tabular grain emulsion layer is incorporated in each of two imaglng units located on opposite ma~or surf~ce6 of a support capable of permitting ~ub6tan-tially specular transmission of imag~n8 radiatlon.
Such radiographic supports are most preferably 3~ polyester film suports. Poly(ethylene terephthalate) film ~upports are 6pecific~11y preferred. Such supports as well as their preparation ~re ~isclo~d ln Scarlett U.S. Pa~ent 2,823,421, Alles U.S. Patent 2,779,6~4, and Arvid~on and Stottle~yer U.S. Pst~nt 3,939,000. Medical radiographic elements are u6ually blue ~cinted. Generally the tinting dyes are added directly to thP molten polye6ter prior to extrusion ~ 3 And therefore must be thermally stable. Pre~rred tinting dyes ~re anthr~quinone dye , such a~ those dlsclosed by Hunter U.S. Pat~nt 3,438,195, H$bino et ~1 U.S. P~tent 3,849,139, Ara~ et ~1 U.S~ Pat~nts 3,918,976 and 3,933,502, Okuyama e~ ~1 U.S. Pat~nt 3,948,664, and U.K. Pa~ent6 l,250,983 ~nd 1,372~668 The crossover ~dvantages re~ult~ ng from cmploying tabular gr~$n emulsion~ a8 t~RUght by Abbott et ~1 c~n be further improved by employlng conventional cros~-10 over exposure control approaches" ~as di~clc:sed inItem 18431 > P~r~gr~ph V.
The preferred spectr~l æen~itlzing dyes for these r~diogr~phic elementfi ~re chosen to exhibit an absorption.peak ~hift in thelr adsorbed state, 15 u~u~lly ln the H or J b~nd, to ~ region of the spectrum corresponding to the wavelen~th of electro-magnetic radlation to wh~ch the element i~ intended to be imagewlse expoæed. The electromagnetic r~diA-tion producing imagewise exposure i6 typically emitted from pho6phors o~ intenslfying screenæ. A
separate intensifying screen expo~e6 each of the two imaging units loc~ted on opposlte ~ides of the support. The intenslfying screen6 can emit llght in thP ultr~violet, blue, green, or red portlons o the spec~rum, dependlng upon the ~pecific phosphor6 chosen for incorporation therein. In a ~pecifically pr~ferred form of the invent~ on the spectr~l senPl-tiz~ng dye is a ~arbocyanine dye exhibiting a J band absorption when nd60rbed to the tabular gr~ins in ~
æpec~r~l region corresponding to peak emission by the intensifying æcreen, usually the green region of the ~pectrum, The ~ntensifying ~creens can themselve6 form ~ part of lthe r~diographic elements, bu~ usually they are separate element6 which are reused to provide exposure6 of 6uccessive radiogr~phic elements.
Intensifying 6creenæ are well known in the radiv-graphlc nrt. Conventional intensifylng screen6 and their components are di6clo6ed by _~enrch Di6clo-6ure, Vol. 18431, cited above, Paragraph IX, ~nd by Rosecrants U.S. Patent 3,737,313.
S To obtain a vlewable sllver image the photographic or, in preferrecl applic~tions, r~dio-graphic elemen~s are developed in an ~queous alkaline processing solution, ~uch a5 an aqueous alkaline developer ~olution or, where the developing agent i~
incorpora~ed in the photogr~E~hic element, in sn aqueouæ ~lkaline activ~tor 601ution. To enh~nce silver covering power development can be undert~k~n as t~ught by Dickerson V.S. }'atent 4,414,304. In the practice of this lnvention dlrect or chemical development is favored over physical development.
Following development the residual 6ilver halide i6 removed from the pho~ographic elemen~s of thi6 invention by fixing out. This avoid6 an incr~nse ln minimum density attributable ~o delayed conver6ion of silver halide to sllver. In other words, i~ render6 the silver image produced by development s~able.
Development and fixing out together with other optional, but common attendant 6teps, ~uch &S 8top-ping development, washing, toning, and drying, can be undertaken following practices well known in the art, such as the materlals ~nd procedure6 useful for ~ilver im~ging iden~ifled in Re6earch Disclo6ure, Item 17643, cited above, Sec~ions XIX, ~X, and XXI.
Examples The lnven~ion can be better appreciated by reference to the following specific examples:
Examples_l thou~h 5 The~e ~xample6 illu6trate a reduction of dye Rtain in ~n X-ray f~lm having a negative work~ng latent ~ma~e forming tabular grain silver bromide emul6ion layer and a gelatin ovPrcoat. Silver lodide is present in either ~he emul610n layer or overcoat ln the example X-ray films and absent from the X-r~y films identified as controls.
To prPpare ~he X-ray ilms a high aspeet ratio tabular grain silver bromide emul6ion was employed wherein grea~er thEIrl 50 percent o~ the total grain pro~ected area was accounted for ~y t~bular grains havlng an average diameter of about 1.6 ~m, a thickness of about 0.11 ~m, and ~n aver~ge ~pect rato of about 14:1~ The tabular gra1n emul~ion was optimally spectrally sensitized with anhydro-5,5'-di-chloro-9-ethyl~3,3'-di(3-sulfopropyl)oxacarbocyanine hydroxide (hereinafter reerred to ~s Dye I). For super sensitlzation about 2 4 X 10~l percent by weight, b~sed on total halide, iodide in the form of potassium iodide was added to the emulsion after addition of the dye. The emul6ion was coated on a polyester film support at 1.98 g/m2 silver and 2.92 g/m2 gel~tin. The gelatin overcoat was applied at Oo91 g/m2 gelatin. The coating WAS hardened with bis(vlnylsulfonylmethyl~ ether ~t 2.5~ of the total gelatin.
In the example X-ray films a 0.08 ~m sil~er iodide emulsion W8S added either to the tabular grain silver bromide emulsion forming the emulslon layer or to the gelatin formlng the overcoat at the levels of silver indicated in Table YI. All emulsion melts were held at 40C for about 8 hours.
Samples of the X-ray films were exposed through a graduated den~ity step tablet to a MacBeth~ sensitometer for 1/50th second to a 500 watt General Electr~c ~MX- pro~ector lamp cali-brated to 2650K filtered wlth a Corning C4010-filter to simulate a green emitting X-ray screen exposure. The X-ray fllm samples ~ re then proeessed through an Eastman Kodak RP X-Omat~Y roller trans-port ~rocessor, Model MB. Proce sing wa~by develop-ment in Kvdak RP X-Omat Developer MX-116~ or 21 -~8-seconds at 35.5C followed by flxing iD Kodak RP
X-Omat Fixer MX-1088~ for 16.5 seconds at 35~C. To complete flxing out the X-ray film samples were washed in deionized water for 12 second6 At 8.5C~
S The sen6itometric result6 are tabulated ln Tsble VI. Maximum and minimum densities were measured wi~h neutral white light extending over the entire visible spectrum. Re~idusl dye ~tain wa8 measur~d as the differenee between den~ity ~t 505 nm, which corresponds to the dye absorptlon pe~k, And the density at 400 nm. Dye stairl Wfl~ measured in minimum density areas of the X-ray film sa~ples a8 well ~ ~t density levels of 0.25, 0.50, and 0.75~
As shown in Table VI, dye ~tain in the control coating was at its maximum in min$mum density sreas and decreaséd slightly in 0.25, 0.50, and 0.75 density areas. Addition of the ~ilver iodide emul-sion to the tabul~r graln silver bromide emulsion caused A Blight increase in dye stain ~n minlmum density areas, but low~red dye density in 0.50 and 0.75 density areas with the net efect b~ing a pronounced lowering of dye 6~ain. When silver iodide was added to the overcoat layer, dye stsin w~s lowered in minimum density as well as 0~25, O.S0, ~nd 0.75 density areas. Although the 8 hour melt holding of the sllver iodide ~n the ~abular 8rain silver bromide emulsion prior to coating resulted in a loss of sensitivity~ no Rensitivity 106s was experienced when the silver iodide was added to the o~ercoat.
The unusually long melt hold was intcnded ~o exagger-ate the effect of the silver iodide in the t~bular grain sil~er bromide emulsion and could ea~ily have been minimized to reduce lo~s of ~en~itivity.
To demonstrate the re~tricted scope of the dye st'ain problem a control X-ray film was prepared and processed as described above, differing only by -49~
the eature~ specifically identlfied below. An approxim~tely spherical 8rain silver bromoiodide emulsion containing 3.4 mole percent iodide, b~sed on tot~l halide, and having ~ mean graln dlameter of S 0.75 ~m was optim~lly spectrally sensltized with Dye I and anhydro-5-chloro-9-ethyl-5'~phenyl-3'-~3-sulfobutyl)3-(3-sulpropyl)ox~lc~rbocy~nlne hydroxide, ~odium salt- The emulsion was coated at 2.47 ~/m2 silver and 2.85 g/m2 gelatin. Slnce no silver iodide w~s added, the 8 hour melt hold was omitted.
The results, reported in T~ble VI, ~how A
comparable green speed, but wi.th gre~tly reduced dye stain, This $11ustrates that dye stain is not normally a matter o concern for nont~bular sllver bromoiodide emulsions containing substantially optimum amounts of spectral sensitizing dye.

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'rhe invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that vari~tions and modifications can be effected within the spirit 5 and scope of the invention.

Claims (16)

WHAT IS CLAIMED IS:

1. In a photographic element capable of producing a stable, viewable silver image on develop-ment in an aqueous alkaline processing solution and fixing out comprising a support and one or more image recording silver halide emul-sion layers each comprised of a dispersing medium and latent image forming silver halide grains, said halide consisting essentially of chloride, bromide, or mixtures thereof, at least one of said image recording silver halide emulsion layers being comprised of spectral sensitizing dye adsorbed to the surface of tabular latent image forming silver halide grains having a thickness of less than 0.5 µm and an average aspect ratio of at least 5:1 accounting for at least 35 percent of the total projected area of said latent image forming silver halide grains present in said silver halide emulsion layer, the improvement comprising high iodide silver halide grains of less than 0.25 µm in mean diameter located in proximity to said tabular silver halide grains and limited to a concentration capable of being dissolved on fixing out.

2. A photographic element according to claim 1 in which the iodide present in said high iodide silver halide grains is less than 5 mole percent of the total halide present in said photo-graphic element.

3. A photographic element according to claim 2 in which the iodide present in said high iodide silver halide grains is less than 3 mole percent of the total halide present in said photo-graphic element.

4. A photographic element according to claim 1 in which said high iodide silver halide grains have a mean diameter of less than 0.1 µm.

5. A photographic element according to claim 1 in which said high iodide silver halide grains are present in said image recording silver halide emulsion layer containing said tabular latent image forming silver halide grains.

6. A photographic element according to claim 1 in which said high iodide silver halide grains are present in a hydrophilic colloid layer adjacent to said image recording silver halide emulsion layer containing said tabular latent image forming silver halide grains.

7. A photographic element according to claim 6 in which said hydrophilic colloid layer containing said high iodide silver halide grains overlies said image recording emulsion layer.

8. A photographic element according to claim 1 in which said spectral sensitizing dye is present in an amount sufficient to form a monolayer coverage of from 25 to 100 percent of the total available surface area of said tabular silver halide grains.

9. A photographic element according to claim 1 in which said high iodide silver halide grains are comprised of at least 90 mole percent iodide, based on total halide.

10. A photographic element according to claim 9 in which the halide content of said high iodide silver halide grains consists essentially of iodide.

11. A photographic element according to claim 1 in which said tabular grain containing image recording silver halide emulsion layer is a high aspect ratio tabular grain emulsion layer wherein the silver halide grains having a thickness of less than 0.3 µm and a diameter of at least 0.6 µm have an average aspect ratio of greater than 8:1 and account for at least 50 percent of the total projected area of the silver halide grains present in said emulsion layer.

12. A photographic element according to claim 1 in which said tabular grain containing image recording emulsion layer is a thin, intermediate aspect ratio tabular grain emulsion layer wherein the tabular silver halide grains having a thickness of less than 0.2 µm and average aspect ratio of from 5:1 to 8:1 account for fit least 50 percent of the total projected area of the silver halide grains present in said emulsion layer.

13. In a radiographic element capable of producing a stable, viewable silver image on develop-ment in an aqueous alkaline processing solution and fixing out comprised of first and second image recording silver halide emulsion layers each comprised of a dispersing medium and latent image forming silver bromide grains, a film support interposed between said emulsion layers capable of transmitting radiation to which said image recording emulsion layers are responsive, at least said first image recording silver halide emulsion layer being a high aspect ratio tabular grain emulsion layer wherein the latent image forming silver bromide grains having a thickness of less than 0.3 µm and a diameter of at least 0.6 µm have an average aspect ratio of greater than 8:1 and account for at least 50 percent of the total projected area of the latent image forming silver bromide grains present in said first emulsion layer, said high aspect ratio tabular emulsion layer containing spectral sensitizing dye in an amount sufficient to form a monolayer coverage of from 25 to 100 percent of the total available surface area of said tabular silver bromide grains, the improvement comprising high iodide silver halide grains of from 0.25 µm to 0.01 µm in mean diameter located in proximity to said tabular silver bromide grains of said first emulsion layer in a concentration of less than 3 mole percent, based on total halide present in said first emulsion layer.

14. A radiographic element according to claim 13 in which said high iodide silver halide grains consist essentially of silver iodide and have a mean diameter of less than 0.10 µm.

15. A radiographic element according to claim 14 in which said silver iodide grains are present in a hydrophilic colloid layer adjacent said first silver bromide emulsion layer.

16. A radiographic element according to claim 14 in which said first and second silver bromide emulsion layers are each high aspect ratio tabular grain emulsion layers and said high iodide silver halide grains are located in proximity to each of said high aspect ratio tabular emulsion layers.