US2993953A - Colour or tonal reproduction - Google Patents

Description

y 1961 G. s. J. ALLEN ETAL 2,993,953

COLOUR OR TONAL REPRODUCTION 2 Sheets-Sheet 1 I It w II l 1 mm? 9% wow mm w? %m mww @W m ow m mw wb v a g! a mo w Q Q a m m mad Filed April 22, 1957 Inventors Attorney 2 93 953 G. s. J. ALLEN ETAL COLOUR OR TONAL REPRODUCTION 2 Sheets-Sheet 2 WEE-- D mom QWN N QWN oN wm @UQNM, Syn

mom

M Attorney NN MN 7 w wt .5 wow E a a 9%; l U mwwwb m 6E nite States This invention relates to a scanning system for use in colour and tonal reproduction and has a particular application to colour correction in the production of sep aration negatives or positives for use in colour reproduction.

For colour printing, an original is generally photographed through a number of colour filters to produce colour separation negatives, and these are used to prepare printing cylinders or plates for the different colours to make multi-colour prints. However, it isknown that the separation negatives, or the positives made from them, are not usually suitable for the preparation of printing surfaces directly, owing to the difliculties of obtaining colour filters and inks of suitable complementary colour response. For example, a printing ink which is nominally cyan may also reflect some light in the magenta part of the spectrum, so that, where both inks are printed at a given element of the picture, the weight of magenta ink applied should be reduced by an amount dependent on the amount of cyan ink applied.

Accordingly, it is known to correct each element of the colour separation positive or negative of each colour in accordance with the values of each of the other colours present in that element.

In our British patent specifications Nos. 737,768 and 738,118 there are described methods of obtaining colour separation prints for use in multi-colour reproduction, in which a single light source, modulated in intensity in accordance with the output of an electrical computer, serves both to scan a colour separation print to expose the latter and to scan colour separation negatives or a colour transparency, from which colour correction information can be derived, to obtain corresponding electric signals, that part of the modulation of these signals which is due to the modulation of the intensity of. the light source being removed and the resultant signals, representing the information obtained from the colour separation negatives or the colour transparency, being applied to the computer, which is so arranged that its output varies in accordance with the required colour correction.

A number of advantages result from the use of a single light source for both analysis and reproduction, as described in these specifications. In addition to the economy of apparatus and the removal of a number of sources of failure and distortion, the problem of maintaining register between two scanning spots, in a system using a first scanning spot for analysis and a second scanning spot for reproduction, is eliminated. In sysems in which the print is exposed through a separation negative or positive, however, there remains the difliculty of providing a suitable optical system for focussing the scanning spot of light, rays from which pass through the negative or positive, on to the print to be exposed while maintaining the spot of small dimensions.

According to the present invention in a method for reproducing tones or colours in which a single light source, modulated in accordance with the output of an electrical computer, serves both to scan uncorrected transparencies to provide information from which the required correction can be computed and to expose an emulsion in accordance with the corrected information, Separate scanning beams from the light source are transmitted through a number of uncorrected transparencies and fall on a number of light-sensitive devices. The modulated electric signals from the latter are applied to the electrical computer in which the required correction is computed from the signal variations, the emulsion to be exposed being located behind the uncorrected transparency for which the collection has been computed so that the light transmitted through the latter passes through the said emulsion on its way to the corresponding lightsensitive device. Such a system has the advantage that no optical devices are required to form an image of the uncorrected separation print on the emulsion being exposed, since the two are in contact, or separated only by a screen or mask.

Although the invention is primarily concerned with colour correction in multi-colour reproduction, it can also be used for tonal correction in single colour reproduction, or in the improvement of resolution or sharpness correction in multi-colour or single colour reproduction, because the scanning spot can be made slightly larger than the resolvable elements in the picture, the operation being thus similar to photographic unsharp masking. In the application of the invention to colour correction the uncorrected transparencies will be the separation negatives or positives.

The light source is preferably pulsating in nature, in order to provide alternating electric signals for the computer, and may be amplitude-modulated or time-modulated.

:In one form of the invention a signal from a photomultiplier representing a correcting colour is subtracted directly from a signal from a photomultiplier corresponding to the colour to be corrected. This subtraction removes to some extent the modulation in the signal which is due to the modulation of the light source. This arrangement has been found to give good colour correction. An alternative method is to pass the signals from the photomultipliers through logarithmic conversion circuits, the output signals from which represent density values. If necessary, the modulation due to the light source can be removed by exposing a further light-sensitive device to the face of the cathode ray tube and passing the output through a logarithmic circuit, the signal from which is then subtracted from the signal derived from the logarithmic circuits associated with the colour chan nels. In a further embodiment the demodulating signal is obtained from the computer which is used to modulate the light output of the cathode ray tube.

The invention may also be applied to the production of screened positives or negatives, for example by inserting a contact screen between the emulsion to be exposed and the corresponding transparency.

The source for the scanning spot of light may be obtained in various ways, for example from a mechanical system involving a rotating mirror drum. -In such a system, a small lamp of high luminous intensity is placed at the focus of a lens which forms a beam of parallel rays. This beam falls on a rotating mirror drum and is reflected through a second lens, to bring it to a focus to form a spot image. In this way the spot is caused to scan repeatedly along a straight line. If the lamp is now moved slowly in a direction parallel with the axis of the drum, spot image is caused to produce a complete raster scanning a rectangular area.

However, the most convenient source for the scanning spot is the screen of a flat-faced cathode ray tube, on which a complete raster is produced by means of the usual electrostatic or electromagnetic deflection arrangements. This has the advantage of virtual lack of inertia.

In order that the invention may be better understood, several embodiments thereof will now be described, by

3 way of example, with reference to the accompanying drawings, in which:

FIGURE 1 shows diagrammatically an embodiment of the invention in which the signals in the colour chan nels are converted to represent density values before being applied to the computer;

FIGURE 2 shows diagrammatically an embodiment in which the colour channel signals are applied to the computer in a form in which they represent transmission values directly; and

FIGURE 3 shows the use of a contact screen to produce a screened positive or negative.

In FIGURE 1 there is shown a cathode ray tube 6, constituting the light source, provided with deflection coils 8 connected to time base circuits 10, which produce a rectangular raster on the tube face, and with a focussing coil 12 connected to a focussing circuit 14. The intensity of the light spot on the face of the cathode ray tube is governed by the signal which is applied by way of conductor 16 to the grid of the tube. This signal passes through a gate circuit 18 which is controlled by a square-wave generator 20 oscillating at a convenient high frequency, and as a result the light spot on the face of the tube is pulsating in character. The signals derived from light-sensitive devices exposed to light from the face of the cathode ray tube are therefore alternating signals, and the design of the subsequent circuits is considerably simplified.

Light spot from the scanning spot on the face of the tube 6 passes through a lens 22 and a part of this light is deflected by partially silvered mirrors 23 and "24 on to the red and green separation negatives 25R and 25G, the remainder of the light from the lens 22 passing straight through to the separation negative 258. The emulsion 26 to be exposed is placed behind and in contact with the corresponding separation negative, in this case the negative 25B. The emulsion 26 is backed by a filter 28 such that it passes only light to which the emulsion is insensitive, that is to say, it prevents the transmission of light to which the emulsion is sensitive. This prevents unwanted exposure of the emulsion due to light reaching the latter through the back surface of the photographic plate. Assuming the emulsion to be sensitive only to the blue end of the spectrum, the backing 28 absorbs blue light but transmits light in the remainder of the spectrum.

Light passing through the separation negative 25B and the photographic plate 26-28, is collected by a lightintegrating unit 32B and directed to a photomultiplier 34B, the output of which passes through a cathode follower circuit 36B.

The light passing through the separation negative 253 is diffused by the emulsion of the photographic plate 2628, and diffusing plates 30R and 306 are placed behind the separation negatives 25R and 25G to diffuse the transmitted light in a similar manner. Light-integrating units 32R and 32G are placed between the diffusing plates and photomultipliers 34R and 346, the signals from which are applied to cathode followers 36R and 36G. The cathode follower circuits 36R, 36B and 36G are included in order to provide a low output impedance.

The signals from the cathode followers are applied to logarithmic circuits 38R, 38B and 38G, respectively, which provide output signals representing the logarithms of the input signals. As an example, each logarithmic circuit could consist of a high resistance in series with a germanium crystal rectifier. With this arrangement the current through these two components is substantially proportional to the input voltage, and the output voltage appearing across the rectifier is proportional to the logarithm of the current flowing through it, and hence to the logarithm of the input voltage. The photomultipliers provide signals which are proportional to the product of the brightness of the spot on the face of the tube and the transmission factor of the element of the corresponding uncorrected negative which is being scanned at the instant in question. The output of the logarithmic circuits therefore represents the sum of the logarithms of the spot brightness and the transmission factor of the scanned element of the corresponding separation negative. The logarithm of the transmission factor is proportional to the inverse density of the negative.

To enable the modulation which is due to the intensity variation of the light spot to be removed from the colour channel signals a photoelectric cell 40 is exposed directly to light from the face of the cathode ray tube and its output signal is applied through a cathode follower 42 to a logarithmic circuit 44 similar to the circuits 38R, 38B and 38G. The output of the logarithmic circuit 44 is applied with the output of circuit 38G to a subtracting circuit 466, which therefore provides an output signal representing the logarithm of the transmission factor of the scanned element of the separation negative, that is to say, the inverse density of the scanned element. This output signal is therefore suitable for the preparation of a magenta printer (uncorrected) for the subtractive printing process. Similarly the output of logarithmic circuit 44 is subtracted from the signals derived from circuits 38B and 38R in subtracting circuits 46B and 46R to provide signals suitable for the preparation of yellow and cyan printers. The circuits 46G, 46B and 46R may each consist of a T-resistance network, the two signals being applied to the two arms of the network, the signal to be subtracted being applied in a negative sense. The output signal, representing the difference, is then taken across the common resistance of the network.

The output signals from the subtracting circuits are applied directly to masking circuits 506, 5013 and 5011 and also to inverting and attenuating circuits 48G, 48B and 48R. The inverting and attenuating circuit in each channel applies an output signal, suitably attenuated, to each of the two masking circuits in the other channels. Thus, considering the blue filter signal (yellow printer) the signal from circuit 46B, which represents the inverse density of the uncorrected negative or the density of the required printer (neglecting correction), is applied to the masking circuit 50B in which it is combined with inverted and attenuated signals from the green and red filter channels.

The corrected signals from the masking circuits are applied through circuits 51R, 51B and 516, for deriving the anti-logarithms of the signals, to limiting circuits 52R, 52B and 526. These limiting circuits prevent the brightness of the spot on the cathode ray tube from increasing beyond a certain level and may comprise two cathode followers to the grid of one of which the signal pulses are applied, the grid of the other receiving pulses of constant height, this height representing the limiting amplitude. The cathodes of the two valves are connected by a diode and a resistance in series, the diode being so directed that when the signal pulses are less than the simultaneous standard pulses the diode conducts and the signal pulses are applied by way of a lead connected to the junction of the diode and the resistor to the grid of the cathode ray tube. When the signal pulses are of greater amplitude than the standard pulses the diode is non-conducting and the standard pulses are applied through the resistor to the output of the circuit. The signals from the limiting circuits are applied to spot brightness compensation circuits 53R, 53B and 53G which compensate for the non-linearity of the relationship between grid potential of the cathode ray tube and the intensity of the spot on the screen. As this non-linearity is different for each of the colours, separate compensation circuits are required for the three channels.

A switch S4 enables the output from one of the spot brightness compensation circuits to be applied through J the gate circuit 18 to the control grid of the cathode ray tube.

In an alternative embodiment the light-sensitive cell 40, cathode follower 42 and logarithmic circuit 44 are omitted and the demodulating signal to be applied to the circuits 466, 468 and 46R is obtained from the control grid circuit of the cathode ray tube.

FIGURE 2 shows a simpler embodiment of the invention, but shows the correcting circuits for only one channel. In this embodiment the cathode ray tubeand associated circuits, the lens systems and the arrangement of the separation transparencies, photographic plate, diffusing plates and photomultipliers are the same as in FIGURE 1.

Assuming that the blue filter channel (yellow printer) is to be corrected by a signal derived from the green filter channel, the output of the photomultiplier 34B is applied through the cathode follower 368 to a subtracting circuit 56. The output from the photomultiplier 34G is also applied through the cathode follower 36G to the subtracting circuit 56, but passes through an inverting circuit 57. The subtracting circuit may be a T-resistance network having a common resistance in which the currents from the blue filter and green filter channels oppose one another. This subtraction process cancels to some extent the modulation present in both signals which is due to the modulation of the light spot on the face of the cathode ray tube. The resultant signal rom the subtracting circuit is applied to a non-linear circuit 58 which limits the amplitude of the signals, thereby preventing the brightness of the spot on the cathode ray tube from increasing beyond a certain level. This limiting circuit may be of the same kind as that described in connection with FIGURE 1. The output of the circuit 58 is applied through a spot brightness compensation circuit 53B and through the gate 18 to the grid of the cathode ray tube. As a result, there is formed on the face of the latter a light mask each element of which has the intensity required to correct the yellow printer separation negative 25B and to form a corrected positive on the photographic plate 2628.

The method according to the invention can be used to carry out the electronic equivalent of a photographic technique which may be called successive two-stage masking. Considering the masking of the yellow printer by the magenta printer, in the photographic process a first corrected yellow positive is made by combining the uncorrected yellow negative with a mask which has previously been made by combining the uncorrected yellow negative and the uncorrected magenta negative. The first corrected yellow positive is registered with the uncorrected magenta negative to produce a further mask, which is registered with the uncorrected yellow negative to make a second corrected yellow positive. This process can go on indefinitely, the amount of the correction carried out becoming progressively less at each stage. In the application of the method according to the present invention to this process, the signals from the yellow and magenta channel photomultipliers are passed through logarithmic circuits and the signal in the magenta channel is demodulated to remove the effect of variations in spot brightness. The resulting signal in the magenta channel, representing density values of the uncorrected magenta negative, and the signal in the yellow channel, representing density values of the corrected yellow negative, are fed in phase opposition to a summing amplifier, the out put of which is fed through an antilogarithmic circuit and a limiting circuit to the grid of the cathode ray tube.

The method according to the invention can be applied to the production of screened positives and negatives. FIGURE 3 shows an arrangement employing a contact screen, which is a thin film having a cyclically varying density pattern in two mutually perpendicular directions, to produce corrected screened positives or negatives. The contact screen 62 is placed between and in contact with the separation negative 25B and the photographic plate G 2628. Dots are formed on the resulting photograph, the area of the dots depending on the total" quantity of light passing through the original. With this arrangement the square-wave generator 20 and the gate circuit 18 become unnecessary.

Alternatively, a ruled screen can be placed between the uncorrected transparency and the lens system which focusses light from the light source on to the uncorrected transparency, so that the combination of a stop in the lens system and the ruled screen provide the required variation of light distribution across the uncorrected transparency and the emulsion to result in an image in dot form.

In some circumstances it may be desirable to insert a mask between the uncorrected transparency and the emulsion to be exposed, for example to modify a picture or compensate for a variation in density in the picture, or to remove part of a picture.

Although the invention has been described in terms of amplitude modulation, the light source can equally well be time-modulated. The time-modulated pulses for application to the grid of the cathode ray tube can be obtained by using an oscillator to initiate the reversal of a trigger circuit, the return of which is delayed to an extent dependent on the amplitude of the amplitudemodulated signals obtained from the computing circuits, these signals representing the correction to be applied In this case the signals from the photomultiplier will be amplitude modulated in accordance with the transmission factor of the corresponding separation negative but time modulated in accordance with the variation of the light spot intensity. The light spot modulation can be removed from the signals by demodulator circuits controlled by pulses of constant duration, which can be obtained from the oscillator used in the production of the input signal for the cathode ray tube.

Instead of the beam-splitting part-silvered mirror system shown in FIGURES 1 and 2, the light spot on the face of the cathode ray tube can be focussed on the three separation negatives by three separate lens systems, arranged side by side in front of the cathode ray tube.

The production of the corrected positive in contact with the uncorrected separation negative, or separated therefrom only by a screen or mask, eliminates the need for a large condenser lens within the imagefor1ning part of the optical system. In addition it facilitates the use of a single light source for both analysis and reproduction, with the advantages of economy and removal of distortion and registration problems.

The system as described so far is applicable only to three-colour reproduction. A considerable amount of colour printing is carried out by using four colours, a black printer being used in addition to the three colour printers mentioned above. The invention is quite suitable for producing a black printer transparency by one of a number of different methods. In the conventional photographic method of producing separation transparencies, a black printer positive separation is frequently made directly from one of the colour separation negatives without further modification. This method can be used in the scanner described above, but it is possible to improve on it by adding or subtracting values according to the densities of the other two colour separation transparencies.

In the case where a black printer is to be used the colour correction signals will normally be compressed to some extent by a function of the black printer signal to allow for the neutral component which will be provided by the black printer.

The method according to the invention is particularly valuable for producing the black printer since it is more important that the resolution of this printer should be very high. By virtue of its contact printing nature, the method described above is capable of transferring all the resolution information from one of the colour separation transparencies to the emulsion which will form the black printer,

We claim:

1. Apparatus for reproducing visual images, comprising a light source, a number of uncorrected transparencies, means co-operating with said light source for scanning said uncorrected transparencies simultaneously and in synchronism, element by element, light-sensitive means behind said transparencies which convert the light passing through said transparencies into electric signals of corresponding magnitude, an electrical computer means arranged to receive said electric signals and to generate an electrical correction signal representing the correction required for the scanned element of one of said uncorrected transparencies, means for applying said correction signal to said light source to modulate the latter in accordance with said correction, and an emulsion to be exposed placed behind and substantially in contact with said uncorrected transparency for which the correction has been computed, so that light passing through said uncorrected transparency to the light-sensitive means associated therewith also passes through said emulsion and exposes it in accordance with corrected light values.

2. Apparatus according to claim 1, including a further light-sensitive device arranged to receive light directly from said light source and to provide a corresponding electric signal, said electrical computer including logarithm-deriving circuits receiving as input signals said signals from said light-sensitive devices behind said transparencies and from said further light-sensitive device and providing electric signals representing the logarithms of said input signals, means for subtracting said logarithmic signal derived from said further light-sensitive device from each of said logarithmic signals derived from the light-sensitive devices behind the transparencies and for generating output signals representing the resultants of said subtractions, said electrical computer further comprising correction circuits to which said output signals are applied.

3. Apparatus according to claim 1 further comprising a backing filter arranged behind the emulsion and substantially preventing the transmission to the emulsion of light to which it is sensitive.

4. Colour correction apparatus comprising a light source, a number of uncorrected colour separation transparencies, means co-operating with said light source for scanning said uncorrected colour separation transparencies simultaneously and in synchronism, element by element, light-sensitive means behind said separation transparencies which convert the light passing through said separation transparencies into electric signals of corresponding magnitude, an electrical computer means arranged to receive said electric signals and to generate an electrical colour correction signal representing the correction required for the scanned element of one of said uncorrected separation transparencies, means for applying said correction signal to said light source to modulate the latter in accordance with said correction, and an emulsion to be exposed placed behind and substantially in contact with said uncorrected separation transparency for which the colour correction has been computed, so that light passing through said uncorrected separation transparency to the light-sensitive means associated therewith also passes through said emulsion and exposes it in accordance with corrected light values, said exposed emulsion serving, after development, for the production of a printer for the corresponding colour.

5. Apparatus according to claim 4, in which said electrical computer includes means for dividing said signals from said light-sensitive devices one by another to produce colour correction.

6. Apparatus according to claim 5, in which said dividing means includes logarithm-deriving circuits for producing electric signals representing the logarithms of the signals from said light-sensitive devices, and means for subtracting one said logarithmic signal from another.

7. Apparatus according to claim 4, in which difiusing plates are placed behind said uncorrected separation transparencies from which said correction signals are derived, said plates diffusing light transmitted through said transparencies to the same extent as the light transmitted through the transparency corresponding to the colour to be corrected is diffused by said emulsion placed in contact therewith.

8. Apparatus according to claim 4, in which said light source is a cathode ray tube and the correcting signal applied thereto is a pulsating time-modulated signal.

9. Apparatus according to claim 4, in which said emulsion which is exposed is a black printer.

10. Apparatus according to claim 4, in which a contact screen is placed between said uncorrected transparency and the emulsion to be exposed.

11. Colour correction apparatus comprising a light source, an electrical computer means generating an electric correction signal for modulating said light source, a number of uncorrected transparencies, means co-operating with said light source for scanning said uncorrected transparencies simultaneously and in synchronism, element by element, light-sensitive means behind said transparencies which convert the light passing through said transparencies into electric signals of corresponding magnitude, a further light-sensitive means arranged to receive light directly from said light source and to provide a corresponding electric signal, said electrical computer means including logarithm deriving circuits receiving as input signals said signals from said light-sensitive means behind said transparencies and from said further lightsensitive means and producing electric signals representing the logarithms of said input signals, means or subtracting said logarithmic signal derived from said further light-sensitive means from each of said logarithmic signals derived from the light-sensitive means behind the transparencies and for generating output signals representing the resultants of said subtractions, said electrical computer means further comprising correction circuits to which said output signals are applied and which generate said electric correction signal for one of said uncorrected transparencies, and an emulsion to be exposed placed behind and substantially in contact with said uncorrected transparency for which the correction has been computed, so that light passing through said uncorrected transparency to the light-sensitive means associated therewith also passes through said emulsion and exposes it in accordance with corrected light values.

12. Colour correction apparatus comprising a light source, an electrical computer means generating an electric correction signal for modulating said light source, a number of uncorrected transparencies, means co-operating with said light source whereby it scans said uncorrected transparencies simultaneously and in synchronism, element, by element, light-sensitive means behind said transparencies which convert the light passing through said transparencies into electric signals of corresponding magnitude, said electrical computer means including subtracting means for subtracting the electric signal representing the correcting colour directly from the signal representing the colour to be corrected to provide said corsection signal for one of said uncorrected transparencies, and an emulsion to be exposed placed behind and substantially in contact with said uncorrected transparency for which the correction has been computed, so that light passing through said uncorrected transparency to the light-sensitive means associated therewith also passes through said emulsion and exposes it in accordance with corrected light values.

13. Apparatus according to claim 12, in which said electrical computer includes an amplitude-limiting circuit to which the signal from said subtraction circuit is applied.

14. Colour correction apparatus comprising a light source, an electrical computer means generating an electrio correction signal for modulating said light source, a number of uncorrected transparencies, means co-operating with said light source for scanning said uncorrected transparencies simultaneously and in synchronism, element by element, light-sensitive means behind said transparencies which convert the light passing through said transparencies into electric signals of corresponding magnitude, in which said electrical computer means includes demodulating means for removing from the signal representing the correcting colour the modulation due to the variation of the brightness of the light source, and a subtraction circuit for subtracting said signal from which the said modulation has been removed from the signal representing the colour to be corrected, in which signal the modulation due to the variation of the brightness of said light source is still present, the output signal from said subtraction circuit representing the said correction signal which modulates the brightness of the light source, and an emulsion to be exposed placed behind and sub- 10 stantially in contact with said uncorrected transparency for which the correction has been computed, so that light passing through said uncorrected transparency to the light-sensitive means associated therewith also passes through said emulsion and exposes it in accordance with corrected light values.

References Cited in the file of this patent UNITED STATES PATENTS 2,710,889 Tobias June 14, 1955 2,740,828 Haynes Apr. 3, 1956 2,757,571 Loughren Aug. 7 1956 2,790,844 Neugebauer Apr. 30, 1957 2,799,722 Neugebauer July 16, 1957 2,842,610 Crosfield et a1. July 8, 1958 FOREIGN PATENTS 753,340 Great Britain July 25, 1956

Images