US3227923A

US3227923A - Electrodeless vapor discharge lamp with auxiliary radiation triggering means

Info

Publication number: US3227923A
Application number: US199385A
Authority: US
Inventors: Warren A Marrison
Original assignee: Thompson Ramo Wooldridge Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1962-06-01
Filing date: 1962-06-01
Publication date: 1966-01-04
Anticipated expiration: 1983-01-04

Description

Jan. 4, 1966 w. A. MARRISON 3, 7,9 3

ELECTRODELESS VAPOR DISCHARGE LAMP WITH AUXILIARY RADIATION TRIGGERING MEANS Filed June 1, 1962 2 Sheets-Sheet l PARALLEL LIGHT OUTPUT,

GHT SOURCE I i POWER I SUPPLY ELECTROLUMWESCENT 1y. 4 DE W4 RRE/V A. MARE/SON INVENTOR.

BYM Qi L VLMZJ A GENTS Jan. 4, 1966 W. A. MARRISON ELECTRODELESS VAPOR DISCHARGE LAMP WITH AUXILIARY RADIATION TRIGGERING MEANS Filed June 1, 1962 ELECTRON VOLTS 2 Sheets-Sheet 2 HSBA (APPROX) GROUND STATE 5\N\PL\F\ED ENERGY LEVEL DIAGRAM FOR 500mm 14/4 R/QzE/V A MA Q/e/so/v INVENTOR.

BY Q 59% A GENTS United States Patent 3,227,923 ELECTRODELESS VAPOR DISCHARGE LAMP WITH AUXILIARY RADIATION TRIGGER- ING MEANS Warren A. Marrison, Palos Verdes Estates, Califi, assignor, by mesne assignments, to Thompson Rama Wooldrldge Inc., Cleveland, Ohio, a corporation of Ohio Filed June 1, 1962, Ser. No. 199,385 2 Claims. (Cl. 315248) This invention relates to light sources employing vapor discharge lamps, and more particularly to improved means for triggering vapor discharge lamps of the kind that are devoid of internal electrodes and which receive energy of excitation from sources that are external of the lamp.

Electrodeless vapor discharge lamps are known in which light emission is produced through the ionizing action of electromagnetic fields on a vaporizable, light radiating substance, such as a vapor confined in an envelope, the ionizing action being affected without the aid of electrodes in the envelope. Such a lamp is comparatively simple in structure, is inexpensive to build and operate, and generally has a relatively long life because of the absence of electrodes. One application of a vapor discharge lamp using an alkali metal vapor is in frequency standard work utilizing atomic resonance phenomena. The lamp may be used to provide optical pumping for a gas cell of the same alkali metal vapor as contained in the lamp to achieve highly accurate frequency control of a radio-frequency signal. The control is attained by detecting an error signal due to variation in signal frequency, and utilizing the error signal to correct the frequency of the radio-frequency signal.

The starting of such lamps has posed several problems. One reason for this is that the power required to start the lamp is too great for continuous operation. The power required to cause the initial ionization of gases within the lamp is enough to produce much too intense illumination in continuous operation, with resulting rapid deterioration of the lamp. It also produces a range of unwanted atomic transitions resulting in inefficient use of the input power.

Another reason is that it has been found expedient to use a low power transistor exciter oscillator circuit for the continuous operation, the oscillator circuit having insuflicient power, when operated from its normal supply, to produce the initial ionization.

Prior attempts to provide a satisfactory means for starting these lamps have resulted in some instances in the use of bulky and expensive equipment. In instances where the starting equipment has been simplified, the repeated use of such equipment has caused the lamps to deteriorate so that they become progressively more diflicult to start.

Accordingly, an object of this invention is to provide improved means for triggering electrodeless vapor discharge lamps of the kind referred to Without causing deterioration of the lamps.

A further object is to provide a simple, economical, auxiliary power means which can be used in conjunction with a low power, continuous energy source to trigger a vapor discharge lamp, the auxiliary power means thereafter deenergizable without affecting the continuous operation of the lamp.

The foregoing and other objects are realized according to the invention in the provision of an auxiliary radiation means positioned to irradiate an electrodeless vapor discharge lamp so as to cause partial ionization of the lamp vapors without causing any significant emission of light therefrom. In conjunction with the irradiation from the auxiliary radiation means, an electromagnetic field is applied to the lamp vapors from a main energizing source.

3,227,923 Patented Jan. 4, 1966 The strength of the electromagnetic field is insufficient by itself to trigger the lamp, but when combined with the auxiliary irradiation is sufficient to initiate light emission by ionization of the vapors. The electromagnetic field is of suflicient strength to maintain the light emission after the auxiliary radiation is removed.

Thus, the auxiliary radiation source is utilized to supply a portion of the starting power to trigger the lamp, after which the total energy can be reduced by removing the radiation and leaving only the electromagnetic field to sustain the light emission. In this way, there is achieved a reduction in the strength of the electromagnetic field that is applied continuously to the lamp.

FIG. 1 is a view partly in section and partly schematic showing a vapor discharge lamp with means for energizing the lamp according to the invention;

FIG. 2 is schematic of a circuit for heating a tungsten lamp;

FIG. 3 is a simplified energy level diagram for sodium;

FIG. 4 is a partial schematic view showing a modified form of energizing means for a vapor discharge lamp; and

FIG. 5 is a sectional view along line 55 showing in more detail the energizing means of FIG. 4.

FIG. 1 illustrates one form of the invention as embodied in a light source intended for use with frequency control apparatus utilizing atomic resonance phenomena. However, it will become apparent that the principles of the invention are applicable to light sources useful in other environments, such as, for example, in the illumination of airport runways. The light source may be one of the kind disclosed in US. Patent 2,974,243. The light source 10 includes a vapor discharge lamp 12 and a reflector 14 mounted within a housing 16. The interior of the housing 16 is provided with an annular shoulder 18 approximately at a central portion thereof for supporting a ring-like mounting bracket 20. The bracket 20 has a large central opening 22, in which the reflector 14 is mounted, and several smaller openings 24 to reduce the cross section of the bracket 20 and thus to thermally insulate the reflector 14 from the housing 16.

The reflector 14, which is made of an electrically conductive material, is generally funnel shaped, there being a neck portion 26 within which the lamp 12 is supported, and a flared portion 28 wedged within the central opening 22 of the bracket 26. The flared portion 28 of the reflector 14 has an inner surface 29 formed with the appropriate curvature and a sufiiciently bright surface texture to provide a desired reflection characteristic to the reflector 14. The inner surface 29 is given a parabolic curvature, for example, so that the light emitted by the lamp 12 from regions near the focal point F of the reflector 14 will emanate from the reflector 14 as parallel rays.

The forward or flared end of the reflector 14 is formed with an annular shoulder 30 for supporting a quartz window 31 that is transparent to the light emitted by the lamp 12. The window 31 serves in part as a dust cover for preventing foreign particles from depositing on the interior surface 29 of the reflector 14. Also, since quartz is a good reflector of long wavelength radiation, the window 31 serves to maintain a uniform temperature within the housing 16 by preventing the transmission of heat through the window 31. Both surfaces of the window 31 are preferably provided with

anti-reflection coatings

32 and 34, for example of magnesium fluoride, so as to insure maximum transmission of the desired wavelength of light emitted from the lamp 12. The window 31 and reflector 14 are fixed in position by means of an annular washer 36 of heat insulating 3 material and a retaining ring 38 which is screwed into the forward end of the housing 16.

The discharge lamp 12 has an elongated cylindrical transparent envelope 40 made of glass, for example, one end portion of which is rigidly attached to the neck portion 26 of the reflector 14 by means of an intermediate cement layer 42, such as an epoxy resin, for example. The envelope 40 protrudes from the neck portion 26 into the cavity formed by the flared portion 28 of the reflector 14.

The lamp contains an quantity of a vaporizable substance 44, preferably one of the alkali metals, such as rubidium, caesium, potassium, sodium, or lithium, which is stored on a metallic condensing member 46. The lamp 12 also contains a quantity of bulfer gas, which may be one of the noble gases such as argon, neon, helium, or krypton. The primary purpose of the buffer gas is to restrict the motion of the alkali metal ions during operation, and thereby reduce Doppler broadening.

The condensing member 46, which may comprise a thin rod of substantially smaller diameter than that of the cylindrical envelope 40, is sealed through one end of the envelope 40 in axial alignment with the envelope 40. When made in rod form, the condensing member 46 may have a diameter that is /s to A the size of the outside diameter of the envelope 40. The condensing member 46 is formed of a material that is easily wettable by the vaporizable substance 44. In addition, the condensing member material should be an electrical conductor. When a vaporizable substance 44 such as rubidium is used, the condensing member 46 may be made of tungsten, for example.

The purpose of the condensing member is more fully described in the aforementioned US. Patent No. 2,974,243. It will suflice to say that the condensing member 46 functions to reduce noise in the lamp 12 by serving as a preferential collector of excess alkali metal vapor droplets which would normally condense on the envelope wall surfaces. A heat sink 66 attached to the condensing member 46 maintains the temperature of the condensing member 46 slightly cooler than the envelope 40 walls so that the vapor droplets condense on the member 46 rather than the envelope 40 walls.

The condensing member 46 is surrounded by a heater 48 which is mounted on the neck portion 26 of the reflector 14. The heater 48 serves to maintain the vapor pressure of the alkali metal vapor at the desired level at which light emission can occur when an energizing field is applied to the gas and vapor. The heater 48 may comprise a helical coil of insulation coated high resistance wire wound around the neck portion 26. A layer 50 of heat insulation material covers the heater coil 48. The heater 48 may be connected to a source of direct current voltage, not shown, to receive its heating current.

For supplying energizing electric fields to the lamp 12 an electromagnetic field producing element in the form of magnetic induction coil 56 is wound preferably around the end of the envelope 40 opposite the end through which the condensing element 46 is sealed. One end of the coil 56 is spaced from the end of the condensing member 46 along the length of the envelope 40 so as to leave an intermediate envelope portion 58 that is free of both internal lamp structure as well as external lamp structure. Furthermore, the lamp envelope 40 is posi tioned axially within the reflector 14 so that a substantial part of the intermediate unobstructed envelope portion 58 will be centered at the focal point F of the reflector 14. Thus, during operation of the lamp 12, a substantial portion of high intensity light emission will issue from the lamp 12 at the focal point P of the reflector 14 and emanate from the reflector 14 as parallel light rays.

One connection to the induction coil 56 is made through a terminal 60 fastened and conductively connected to the flared portion 28 of the reflector 14. The other con- 4. nection to the coil 56 is made through

apertures

62 and 64 in the reflector 14 and housing 16, respectively.

The energizing electric field for the lamp 12 may be provided by connecting the induction coil 56 in a modifled Colpitts oscillator circuit, as shown. The induction coil 56, comprising several turns of wire wound around the end of the envelope 40, is connected in parallel with two series connected

capacitors

68 and 70, the latter capacitor 70 being connected to a common ground. The coil 56 and

capacitors

68 and 70 form a tuned circuit. The junction of the

capacitors

68 and 70 is connected to the emitter 72 of a transistor 74. The collector 76 of the transistor is grounded. The high voltage end of the capacitor 68 is coupled through a capacitor 78 to the base 80 of the transistor 74.

Operating potentials are derived from a power supply 81 and voltage divider network comprising two

resistors

82 and 84 connected across the terminals 85a and 85b. The base 80 of the transistor 74 is maintained at a positive potential relative to the collector by connection through an inductor 86 to the junction of the

resistors

82 and 84. The emitter 72 is maintained at a slightly positive potential relative to the base 80 by connection through an inductor 88 and resistor 90 to the high voltage end of the resistor 84.

The oscillator circuit is designed to produce an electromagnetic field in the coil 56 that is insufficient by itself to initiate ionization of the vapor Within the lamp 12. However, once the vapor is ionized to the point where substantial light emission is produced, by means which will now be described, the electromagnetic field generated in the coil 56 is of sufficient strength to sustain the ionization and light emission.

To initiate ionization of the lamp 12 in acordance with the invention, a radiation producing means 92 is arranged to irradiate the lamp 12. The radiation producing means 92 may be in the form of an auxiliary light soure mounted in the center of the window 31 along the axis of the lamp 12. A voltage source 94 is connected through a switch 95 across the radiation producing means or auxiliary light source 92 for supplying power thereto. The auxiliary light source 92 may be a tungsten incandescent lamp or a gas discharge lamp such as one containing a filling of argon or helium. A requirement of the auxiliary light source 92 is that it must emit light of appropriate wavelength to partially ionize the vapor within the vapor lamp 12.

The auxiliary light source 92 is mounted at a suitable distance from the lamp 12 so that the light rays 96 given off by the auxiliary light source 92 and deflected by the reflector 14 converge in a region R adjacent to or coincident with the focal point P of the reflector 14. The position of the auxiliary light source 92 along the axis of the lamp 12 is such that the source 92 does not obstruct any of the light rays emanating from the lamp 12 and made parallel by the reflector 14. With this arrangement, the parabolic reflector 14 serves the double purpose of focusing an intense beam of light from the auxiliary light source 92 in the region R and of rendering parallel the light rays produced from the vapor at the region F.

In operation, as noted previously, the field produced in the coil 56 is not sufficient in the absence of irradiation from the auxiliary light source 92 to trigger the lamp 12 into full ionization. However, upon irradiation from the auxiliary light source 92, the vapor within the lamp 12 becomes partially ionized. The ions formed thereby are then acted upon by the fields produced in the coil 56 as follows:

When the oscillator circuit is energized the alternating magnetic field produced axially of the induction coil 56 induces a circumferential electrical field at right angles to the magnetic field. The circumferential electric field is impressed upon the vapor ions that have been produced by irradiation from the auxiliary light source 92, where upon more ions are produced through multiple collisions with'the vapor and gas molecules. In addition to the circumferential electric field, there exists an alternating electric field between the high voltage end of the induction coil 56 (opposite the grounded end) and the reflector 14 (which is grounded). The reflector 14 thus constitutes a second electromagnetic field producing element, the first element being constituted. by the induction coil 56. Since the composite of the two electricfields is concentrated in the central region 58 of the lamp 12, betweenthe conidensing member 46 and the. induction coil 56, there will be a concentration of ionization produced in this region. Accordingly, the lamp12 will be triggered into light emis-v sion, with light of relatively high intensity beingemit'ted from the focal point regions of the reflector 14.

Once the lamp 12 has been ionized sufliciently to pro duce the required amount of light,-the auxiliary light source 92 can be deenergized, since the energy required to sustain light emission is substantially less than that neces sary to initiate emission. The electric field produced in the coil 56 will be adequate to sustain the light emission. It has been found that the magnitude of the electric field needed to start the lamp 12 with the aid of an auxiliary light source 92 is at least twenty-five percent less than the magnitude of field required when the light source 92 is not present. Since this reduced field is present during continuous operation of the lamp 12, the life of the lamp is extended considerably. In order to maintain an electrodeless discharge in the lamp 12 by means of high frequency currents in the coil 56 surrounding the lamp 12 it is necessary that there be some degree of ionization of the medium within the envelope 40. The discharge will be maintained continuously provided that the rate of ionization by collision induced by the high frequency electromagnetic field is at least equal to the recombination rate within the medium. It is the recombination of electrons with positive ions and the reversion to lower energy states that is responsible for the spectral emission of a vapor lamp 12. v

, In general, once ionization has been initiated, the state of ionization may be sustained by a relatively small amount of applied high frequency powerin the coil 56. This is because ions moving with long free paths under the influence of the high frequency field can readily .accumulate .the energy required to create new ions through collision processes.

However, the amount of high frequency field which is capable of sustaining ionization at a low level is not sufficient to initiate a discharge without auxiliary means. It is such auxiliary means with which this invention is concerned.

There is some evidence that throughout matter, and in particular in the medium within the lamp envelope 40, there exist spontaneously a small number of unbound electric charges. Those may be the result of cosmic radiation, or of ionization by collision through thermal motion or of the presence of very small radioactive impurities in some way associated with the'equipment.

A very small number of such charges, if acted upon by a high frequency field, will result in a general avalanche breakdown ofthe medium, if the intensity of the field is sufficiently increased to the point where ionization by collision exceeds the normal rate of recombination. However,.the amount of high frequency field that is required to initiate an electrical breakdown in this way causes a runawaycondition in the discharge that results in an over-abundance of high energy transitions and the production of many unwanted spectral components in the emitted light as well as overheating and rapid deterioration of the lamp.

In a 'copending concurrently filed application by the same inventor as the present application, entitled Electrodeless Vapor Discharge Lamp With Auxiliary Voltage Triggering Means (STL 464), controlled means are 6 disclosed for making use of a transient high energy, high frequency excitation to initiate an avalanche breakdown without being harmful to the lamp.

Inthe present application, other means are utilized which have a somewhat similar end result through increasing the number of free charges existing in the active medium of the lamp which are acted upon by the applied high frequency electromagnetic field. 1 The following'discussion is based on the use of alkali metals as the active medium, but the same principles apply, with suitable modification, to other materials.

It is well-known that the ionization potentials for the alkali metals range from 5.39 electron volts to 3.89 electron volts and that these energies correspond to the quantum energies of light radiation from 2300 angstroms to 3187 angstroms according to the following table:

Table I Ionization Threshold Alkali Metal Potential (E), Wavelength (A),

electron volts angstroms Lithium 5. 39 2, 300 5.14 2, 412 4. 34 2, 857 4.18 2, 966 Oaesium 3. 89 3, 187

The above table is taken from Kaye and Laby Table of Physical and Chemical Constants, Longmans, Green and Co., eleventhedition, page 184, with the threshold wavelength A being computed from the expression E \=12397.8 10 e.v.-cm., also taken from the above reference, on page 192.

As a consequence of this phenomenon, if, in the embodiment shown in FIG. 1, the illumination provided by the light source 92 contains light of wavelength equal to or less than the threshold values shown in Table I, the corresponding alkali metals will be ionized from the ground state by the interaction of the light with the vapor. In the particular case of rubidium, for example, some rubidium atoms will be ionized if the light reaching the interior of the lamp envelope is of wavelength 2966 angstroms or less. The actual amount of such ionization depends upon the cross section of interaction of photons with rubidium atoms, the density (and hence the temperature) of the'rubidium vapor, and upon the intensity of illumination of the stated wavelength. It will be evident that, by controlling the aforementioned factors, a vast increase in the number of free charges existing in the active medium of the lamp can be obtained which is accompanied by a significant decrease in the high frequency excitation required to cause avalanche breakdown of the medium and the emission of light of wavelenth characteristic of the medium.

Analogous results are obtained by the use of other alkali metals, taking account of the characteristic ionization potentials and the corresponding threshold wavelengths required for ionization.

Analogous results are obtainable also by the use of material other than the alkali metals such as, for example, calcium, tellurium, and argon, having ionization potentials of respectively, 6.09 e.v., 6.07 e.v., and 15.68 e.v.

In order to make full use of the effect just described it is necessary to provide envelopes for the auxiliary light source 92, and for the spectral lamp 12, which are transparent to the threshold radiations involved. Standard glasses, such as fuzed quartz are available from which such envelopes can be made suitable for use with alkali metal vapor lamps.

If the auxiliary light source consists of a tungsten filament lamp, such as a flashlight lamp, only a small amount of the total light is of short enough wavelength to produce ionization even with the alkali metals which have the lowest ionization potentials among the elements. However, if the tungsten filament is operated in the range from 3000" K. to 3400 K. there will be adequate short wave radiation to cause initial ionization of caesium or rubidium which presently is of greatest concern. In order to ionize lithium by the direct irradiation effect now considered, a tungsten filament should be operated at a temperature above 3600 K. Because of the short lengths of time involved in each starting cycle for a spectral lamp, such temperatures are entirely feasible.

Curves showing the spectral distribution of the radiation from a block body for various temperatures are given on page 12 in Measurement of Radiant Energy, edited by W. E. Forsythe, McGraw-Hill Book Co., 1937. These curves closely approximate the spectral distribution from hot tungsten.

A simple means for heating a lamp filament to high temperature for short duration is illustrated in FIG. 2.

A capacitor 97 is charged to relatively high voltage from a DC power source 98 through a resistor 99. When a switch 100 is closed, a surge of abnormally high current flows through the light source 92 causing it to light to high intensity momentarily. The normal voltage of the light source 92 may be in the range, for example, of /s to of the supply voltage. The resistance of the resistor 99 is high enough to prevent the steady flow of enough current to light the light source 92, but will charge the capacitor 97 on the order of seconds when the switch 100 is open. A transistor switch such as shown in the aforementioned copending application could be used with some advantage.

The foregoing discussion relates to the case in which the auxiliary radiation is of suflicient energy, i.e. sufliciently low wavelength, to actually ionize some of the active medium in a spectral lamp in a single step from the ground state. In this case, the operation is clearly understood as described.

It has been found by experiment that it is possible to obtain a considerable reduction in starting power by the use of radiation of lower energy, that is, of longer wavelength than is required to ionize directly from the ground state. It has been found further that light tending toward short wavelength, that is toward the blue end of the spectrum is more effective in this than is red light. Up to a certain point the reduction in starting power increases with the intensity of illumination.

These observations lead to the conclusion that, before the application of high frequency exciting field, the number of free charges in the active medium of the spectral lamp is increased in some degree by the presence of light of wavelength longer than that normally associated with the ionization potential.

An explanation of this phenomenon is as follows, referring to the energy level diagram of a typical metal, sodium, shown in FIG. 3. The energy level diagram of FIG. 3 is taken from the textbook entitled Resonance Radiation and Excited Atoms, by Allan O. G. Mitchell and Mark W. Zemansky, University Press, Cambridge, 1934, page 13, Fig 2.

It is well known that an atom can be raised from the ground state to any of several excited states by exposure to light of specified wavelengths. Atoms so excited remain in the higher state for a short time and then revert to the ground state with the emission of a quantum of radiation of the same frequency. This phenomenon is known as resonance radiation.

In particular, referring to the energy level diagram for sodium, it can be seen that atoms can be raised from the ground state to the first excited 3 P and 3 P states by exposure to light of wavelengths 5890 A. and 5896 A., respectively. From these levels, it requires only 3.0 electron volts, corresponding to radiation of wavelength of 4133 A. or shorter to cause ionization. Due to the short lifetime in the first excited state, only a small proportion of the atoms can be ionized by this process. However, the number greatly exceeds the number of ions resulting from residual processes in a dark environment and is sufficient to cause a substantial reduction in the electromagnetic energy required to excite the electrodeless discharge.

The invention was operated successfully with the fol- Coil 56 consisted of 23 turns of No. 25 enameled copper wire wound with an inside diameter of approximately 7 millimeters. The auxiliary light source 92 consisted of a 2.8 volt flashlight bulb.

According to another embodiment, an auxiliary radiation source can be provided by incorporating a suitable radioactive material within the lamp 12. The radioactive material can be provided in addition to or in place of the light source 92. For example, krypton is preferably incorporated in the argon or other gas used as a buffer gas in the lamp 12. The krypton 85 emits beta particles which interact with the alkali vapor molecules as well as the noble buffer gas to partially ionize the same in a manner similar to that attributed to the auxiliary light source. Since krypton 85 has a half-life of 10.6 years, its useful life as an auxiliary radiation source should be more than adequate. Further advantages of krypton 85 are that it is relatively safe and convenient to use, is chemically inert, and does not react with the rubidium vapor.

According to another embodiment shown in FIG. 4, an auxiliary radiation source is provided in the form of an electroluminescent device 101 mounted on the end of the lamp 12. The electroluminescent device 101 may comprise a strip of translucent or transparent plastic, such as polyethylene terephthalate, formed into a sleeve 102. The plastic sleeve 102 supports, in successive layers, a transparent conductive coating 104, a layer 106 of plastic embedded electroluminescent phosphor, and an opaque, light reflective, conductive coating 108. The two

conductive coatings

104 and 108 serve as electrodes which are connected through a switch 110 across a source 112 of alternating voltage. A suitable phosphor for the layer 106 is copper activated zinc sulfide embedded in ethyl cellulose or polystyrene. Such a phosphor will emit blue light when subjected to an alternating electric field. However, other well known electroluminescent phosphors which emit light of short wavelength (suitable for assisting in the initial ionization of the spectral lamp) may be used.

When the switch 110 is closed, the source voltage 112 will impress an alternating electric field across the electroluminescent device 101 to cause the latter to emit electroluminescent light. The electroluminescent light will partially ionize the vapor molecules in the lamp 12, whereupon the electric field produced in the coil 56 will act on the partially ionized vapor molecules to produce the desired light emission from the lamp 12.

It is now apparent that the auxiliary radiation means of the invention provides a simplified and economical means for starting an electrodeless vapor discharge lamp, while permitting a reduction in the continuous power requirements of the lamp.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A vapor discharge light source, comprising:

a light reflector having a predetermined focal point;

a vapor discharge lamp including a cylindrical envelope axially aligned with said reflector, with a central portion of said envelope symmetrically positioned at said focal point;

a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field;

radiant energy producing means disposed adjacent to said discharge lamp envelope along the axis thereof and arranged to subject said vapor discharge lamp to radiant energy of appropriate wavelength and sufficient strength to cause a partial ionization of said vapors Without causing any significant amount of light emission therefrom; and

electromagnetic field producing means arranged to subject said vapors to a concentration of electromagnetic field in said central envelope portion,

the magnitude of said concentrated electromagnetic field being insuflicient, in the absence of said radiant energy, to ionize said vapors sufiiciently to produce light emission therefrom;

the magnitude of said electromagnetic field being suflicient, in the presence of said radiant energy, to initiate light emission by ionization of said vapors and to maintain said light emission after said radiant energy is removed.

2. A vapor discharge light source, comprising:

a light reflector having a predetermined focal point;

a vaporizable substance within said envelope the vapors of which are adapted to emit light when subjected to an electromagnetic field,

an auxiliary light source disposed adjacent to said discharge lamp envelope along the axis thereof and arranged to subject said vapor discharge lamp to light energy of appropriate wavelength and suflicient strength to cause a partial ionization of said vapors without causing any significant amount of light emission therefrom; and

the magnitude of said concentrated electromagnetic field being insufiicient, in the absence of said light energy, to ionize said vapors sufiiciently to produce light emission therefrom;

the magnitude of said electromagnetic field being sulficient, in the presence of said light energy, to initiate light emission by ionization of said vapors and to maintain said light emission after said radiant energy is removed.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Singer, Advances in Quantum Electronics, 1961, Columbia University Press, pages 96 and 97 relied upon.

GEORGE N. WESTBY, Primary Examiner.

D. E. SRAGOW, Assistant Examiner.

Claims

1. A VAPOR DISCHARGE LIGHT SOURCE, COMPRISING: A LIGHT REFLECTOR HAVING A PREDETERMINED FOCAL POINT; A VAPOR DISCHARGE LAMP INCLUDING A CYLINDRICAL ENVELOPE AXIALLY ALIGNED WITH SAID REFLECTOR, WITH A CENTRAL PORTION OF SAID ENVELOPE SYMMETRICALLY POSITIONED AT SAID FOCAL POINT; A VAPORIZABLE SUBSTANCE WITHIN SAID ENVELOPE THE VAPORS OF WHICH ARE ADAPTED TO EMIT LIGHT WHEN SUBJECTED TO AN ELECTROMAGNETIC FIELD; RADIANT ENERGY PRODUCING MEANS DISPOSED ADJACENT TO SAID DISCHARGE LAMP ENVELOPE ALONG THE AXIS THEREOF AND ARRANGED TO SUBJECT SAID VAPOR DISCHARGE LAMP TO RADIANT ENERGY TO APPROPRIATE WAVELENGTH AND SUFFICIENT STRENGTH TO CAUSE A PARTIAL IONIZATION OF SAID VAPORS WITHOUT CAUSING ANY SIGNIFICANT AMOUNT OF LIGHT EMISSION THEREFROM; AND ELECTROMAGNETIC FIELD PRODUCING MEANS ARRANGED TO SUBJECT SAID VAPORS TO A CONCENTRATION OF ELECTROMAGNETIC FIELD IN SAID CENTRAL ENVELOPE PORTION, THE MAGNITUDE OF SAID CONCENTRATED ELECTROMAGNETIC FIELD BEING INSUFFICIENT, IN THE ABSENCE OF SAID RADIANT ENERGY, TO IONIZE SAID VAPORS SUFFICIENTLY TO PRODUCE LIGHT EMISSION THEREFROM; THE MAGNITUDE OF SAID ELECTROMAGNETIC FIELD BEING SUFFICIENT, IN THE PRESENCE OF SAID RADIANT ENERGY, TO INITIATE LIGHT EMISSION BY IONIZATION OF SAID VAPORS AND TO MAINTAIN SAID LIGHT EMISSION AFTER SAID RADIANT ENERGY IS REMOVED.