US3707649A

US3707649A - Intermittent arc illumination source

Info

Publication number: US3707649A
Application number: US56342A
Authority: US
Inventors: James P Kottenstette
Original assignee: DENVER RESEARCH INST
Current assignee: DENVER RESEARCH INST
Priority date: 1970-07-20
Filing date: 1970-07-20
Publication date: 1972-12-26
Anticipated expiration: 1989-12-26

Abstract

An intermittent arc illumination source which utilizes a xenon short arc lamp operated in a pulse mode by a controlled discharge circuit operating from a low voltage alternating current source. The high current, high energy pulses are intermittently produced for conduction through the lamp at a rate sufficiently rapid to make the resulting bursts of illumination appear continuous to the human eye.

Description

United States Patent Kottenstette 1 Dec. 26, 1972 INTERMITTENT ARC ILLUMINATION SOURCE Inventor:

James P. Kottenstette, Denver,

Colo.

Assignee: Denver Research Institute, Denver City, Colo.

Filed: July 20, 1970 Appl. No.: 56,342

US. Cl ..315/239, 31 5/DIG. 5, 315/246 Int. Cl. ..I-I05b 37/00 Field of Search..315/l00 U, 239, 261, 262, 263,

References Cited UNITED STATES PATENTS 9/1968 Michelsen ..3l5/100 X 10/1966 Ahmed ....3l5/263X 12/1970 Saiger ..3l5/274X Primary Examiner-Roy Lake Assistant Examiner-Lawrence J. Dahl Attorney-Anderson, Spangler & Wymore [5 7] ABSTRACT 11 Claims, 4 Drawing l igures I 1 I 44 J 60 I j {0 6- 62: pa PATENTED DEC 26 I972 SHEET 1 BF 2 ATTO i S INVENTOR 5. KOTTENSTETTE PATENTED M02 I912 3.707.649

SHEET 2 BF 2 O l 1 l l l l l I l I O 4 8 l2 I6 20 24 FIG. 4

INVENTOR MVMTKEYS I INTERMITTENT ARC ILLUMINATION SOURCE The illumination source required for image projectionapplications is generally one of high luminance and relatively small illuminating element size. In the older state of the art projection applications, such as motion picture and slide projectors, incandescent sources provided sufficient luminance and were of a size relatively small with respect to the image being projected. However, with the advent and growth of microfilm technology, the need for a new type of illumination source became apparent. The use of incandescent sources in microfilm readers has three serious disadvantages. First, an incandescent source of the required luminance produces a substantial amount of heat which requires the design of a substantial heat dissipating system to be incorporated within the reader. This heat dissipating system usually takes the form of forced fan cooling thereby requiring additional electrical power. Also, where large numbers of such readers are used in close proximity, the heat they produce places a very substantial additional load on the air conditioning system. Secondly, the conventional incandescent source and associated cooling system requires a relatively large amount of electrical power for the production of sufficient illumination, thereby causing a significant electrical power load where a number of readers are in use. The third and primary disadvantage of using an incandescent source in a microfilm reader application is that as the film image becomes smaller, the physical size of the radiating element in the incandescent source approaches or becomes greater than the size of the film image. Specifically, at magnification ranges of approximately 60X, the physical dimensions of the conventional tungsten or tungsten halogen lamp are approximately equal to the dimensions of the film image. Accordingly, at magnifications in the order of a lSOX which are contemplated in the microfilm industry, the physical size of the incandescent illumination source becomes three to four times larger than the film image. Such relative sizes of the illumination source and the film image in the higher magnification range is optically inefficient. For the illumination radiation to be efficiently transmitted through the film image and the image resolved acceptably on the reader screen, the physical size of the radiating source should be small in comparison with the film image or at least not larger than the film image.

One possible method of overcoming the above enumerated disadvantages of incandescent illumination sources for microfilm reader applications is the use of high pressure xenon short arc lamps as the illumination source. For example, a commercially available 75-watt xenon short arc lamp produces sufficient illumination with much less energy radiated as heat. Of greater importance for the microfilm application, however, is the fact that this same size xenon short are lamp produces a radiating arc with an area of less than 0.10 square millimeter. Therefore, the high pressure xenon short are lamp has the operating characteristic of a large amount of illumination produced in essentially a point source.

As known in the prior art, xenon short are lamps are ordinarily operated in a low voltage, continuous, direct current mode. This method of operation also requires high voltage apparatus for initially striking the arc. The electrical power supply for operation in this mode is prohibitively expensive for mass production of microfilm or image projection applications. ,Furthermore, a xenon short arc lamp operated in the continuous DC mode generates enough heat to'keep the lamp envelope temperature at about600 C. Such an operating temperature requires that the envelope be constructed of a material such as quartz. This material selection in turn requires that the lamp be handmade and therefore, quite expensive. These economic factors render the xenon short arc lamp operated in a conventional continuous DC mode infeasible for mass produced imag e projection applications. The present invention makes possible an arc illumination source which is economically feasible for mass produced image projection applications by eliminating the need for the expensive DC power supply and by operating the lamp at a lower envelope temperature which permits replacement of the quartz envelope .by high quality glass susceptible to machine fabrication.

Briefly, the new intermittent arc illumination source is a high pressure xenon short are lamp operated by a continuous series of low voltage, high current pulses of extremely short duration. The repetition rate of the pulses is sufficiently high to make the flashes of the lamp appear as continuous illumination to the eye. An inductor capacitor discharge circuit energized from a full wave rectifier bridge connected to a low voltage AC power source is used to generate the pulses. A triggering circuit operated in synchronization with the rectified alternating current, times and triggers the discharge circuit to provide the current pulses to the lamp electrodes.

Accordingly, the primary object of the present invention is to provide a means for operating xenon short are lamps with repeated high energy current pulses rather than continuous direct current.

A second objective is the provision of a very low cost power supply and control circuit operable from a low voltage AC power source to operate xenon short are lamps in a pulse mode.

Another object is to provide a means for operating xenon short are lamps more efficiently in the conversion of electrical energy to illumination, thereby lowering the generation of waste heat and lowering the operating temperature of the lamp envelope.

A fourth objective is the provision of a means for operating xenon short arc lamps at low envelope tem peratures, thereby eliminating the necessity of constructing the lamp envelope of high temperature resistant materials.

A further objective is the provision of an arc illumination source which provides a large amount of luminance at high color temperature concentrated in essentially a point source.

Another object is to provide an arc illumination source which is highly inexpensive, compact, and suitable for the illumination source in an image projection application.

Other objects of this invention will be in part apparent and in part pointed out specifically hereinafter in connection with the description of the drawings that follows and in which:

FIG. 2 is a graph showing typical relative illumination outputs of a xenon short arc lamp operated in a continuous DC mode and in a pulsed mode as a function of illumination wavelength;

FIG. 3 is a graph of the rectified input voltage, taken across the output of the full wave bridge, as a function of time, and

FIG. 4 is a graph of the voltage across the lamp discharge capacitors as a function of time.

The graphs of FIGS. 3 and 4 are drawn to the same abscissa scale with corresponding initial points to facilitate comparison of the displayed voltages at similar time points in the operation cycle.

In the power supply and pulse control circuitdiagram of FIG. 1, reference numerals l and 12 indicate the power input terminals which are to be connected to a low or medium voltage AC power source.'Reference numeral 14 indicates generally a full wave rectifying bridge comprised of

diodes

16, 18, 20, and22 and connected to power input terminals and 12 with its rectified output taken across resistor 24 at

points

26 and 28. The positive side, point 26, of the rectifier output is connected to one terminal of a charging inductor 30 with the other terminal of the charging inductor being connected to the anode 32 of a silicon controlled rectifier 34. The cathode of SCR 34 is connected to both the anode of an isolating diode 36 and one side of a discharge capacitor 38. The other side of discharge capacitor 38 is connected back to the negative side of the full wave rectifier 14, at point 28. The cathode of isolating diode 36 is connected to one side of another discharge capacitor 40 with the other side of this discharge capacitor also being connected back to the negative side of the rectifier 14, at point 28.

A gating transformer 42 is provided in the circuit to control the conduction of SCR 34 and the conduction of a second silicon controlled rectifier 44 to be hereafter described. This gating transformer 42 is composed of a primary winding 46 and two secondary windings 48 and 50. The primary winding 46 is connected in series with a neon lamp 52 and this series combination is parallel across the conduction terminals of SCR 34. The characteristics of neon lamp 52 are such that it will only conduct current after the voltage across it reaches a particular level. The secondary winding 48 of gating transformer 42 is connected between the cathode and the gate electrode 54 of SCR 34 so that if SCR 34 is forwardly biased and neon lamp 52 begins to conduct, a positive gate pulse will be induced at gate electrode 54.

The aforementioned second SCR 44 is placed in the circuit by connecting its anode to the cathode of SCR 34 and connecting its cathode to a starting transformer 58, described below. The secondary winding 50 of gating transformer 42 is connected between the cathode and the gate electrode 56 of SCR 44 so that if SCR 44 is forwardly biased and neon lamp 52 begins to conduct, a positive gate pulse will be induced at gate electrode 56.

Starting transformer 58 is a voltage step-up transformer having a primary winding 60 and a secondary winding 62. In the particular embodiment shown the step-up ratio is approximately to l. The primary winding 60 is connected between the cathode of SCR 44 and the negative side of rectifier 14 at a point 28. The negative or low potential terminal of the secondary winding 62 is also connected to the negative side of rectifier 14.

In the embodiment of the invention shown in FIG. 1, a three electrode, xenon short are lamp 64 is the illumination generating element connected to the power supply and control circuits. The lamp 64 can be of a type well known in the prior art having a cathode 66,

starting electrode 68, and an anode closely spaced within an envelope containing xenon gas under high pressure. For operation of the lamp 64 by the power supply and control circuits of the present invention, cathode 66 is connected to the negative side of rectifier bridge 14 at point 28, starting electrode 68 is con- .nected to the positive or high potential terminal of secondary winding 62 of starting transformer 58, and anode 70 is connected through a pulse shaping inductor 72 to the junction between the cathode of diode 36 and capacitor 40. A capacitor 74 is connected between starting electrode 68 and anode 70 to sustain ignition current after the arc has been formed from the starting electrode. 7 I

A two electrode, xenon short are lamp of a type also well known in the prior art may be used with essentially the same power supply and control circuits. Such a two electrode lamp is of the same construction as a three electrode type except that the: starting electrode is omittedand the anode is used to perform the starting function. For use with a two electrode lamp the power supply and control circuits of FIG. 1 are altered by eliminating diode 36, discharge capacitor 40, and inductor 72, and increasing the capacity of capacitor 38. Starting electrode 68 then becomes both a starting electrode and an anode and anode 70 is not present in the two electrode configuration. Capacitor 74 is then paralleled across secondary winding 62 and-this parallel combination drives the lamp. I

.In the operation of the disclosed intermittent arc illumination source, input terminals 10 and 12 are connected to an alternating current power source of preferably low voltage. In the embodiment shown a l 15 volt, 6O cps source is used, but with proper choice of circuit components any ordinary low or medium voltage source may be used. In the description of operation that follows, illustrative typical values resulting from the use of a I I5 volt 6O cps source will be used but it should be understood that these values can be changed and varied proportionally with the values of the source that is used and still be fully contemplated by the invention.

The full wave rectifier bridge 14 produces a I20 cps, direct, varying voltage across its output, points 26 and 28, as shown in FIG. 3. A varying voltage of the illustrated waveform is therefore also the one impressed across the lamp control circuit. Since SCR 34 is nonconducting until a positive gate pulse is received at its gate electrode and neon lamp 52 is also nonconducting until its breakdown voltage level is reached, no current will be conducted by the circuit at the beginning of a cycle. When the voltage across neon lamp 52 rises from its zero level at time t 0, FIG. 3, to the breakdown or conduction level, the neon lamp begins to conduct and current flows through primary winding 46. In the embodiment shown this breakdown level is approximately 70 volts. As current flows through the primarywinding, current is induced in secondary windings 48 and 50 producing a positive gate pulse at gate electrode 54 and a negative pulse at gate electrode 56. SCR 34 is then switched into conduction by the positive pulse and SCR 44 remains nonconducting since the negative pulse received at its gate does not operate to effect switching. With SCR 34 conducting, capacitors 38 and 40 begin charging and a magnetic field is created in charging inductor 30. As the varying voltage cycle continues and the voltage begins to decrease from its peak, the magnetic field in inductor 30 starts to collapse and in doing so induces a voltage in the inductor which adds to the charge voltage on capacitors 38 and 40. With complete field collapse capacitors 38 and 40 will have been charged to approximately 350 volts by virtue-of the charging inductor. Also with complete field collapse capacitors 38 and 40 will have been charged to their peak voltage and SCR 34 immediately becomes reverse biased and nonconducting.

With the reverse biasing of SCR 34, the polarity across the neon lamp 52 and primary winding 46 combination will also be reversed and when the breakdown voltage of the neon lamp is again reached it will begin to conduct causing current to flow through the primary winding from capacitor 38 to inductor 30. This current flow induces current in both secondary windings 48 and 50 and produces a positive pulse at gate 56 of SCR 44 and a negative pulse at gate 54 of SCR 34. The negative pulse to gate 54 does not affect the now nonconducting characteristics of SCR 34. The positive pulse to gate 56, however. switches SCR 44 to a conducting mode and capacitor 38 begins to discharge through SCR 44 and primary winding 60 of starting transformer 58; Capacitor 40 is, of course, prevented from discharging through this path by isolating diode 36. Discharge current passing through primary winding 60 induces current in the secondary winding 62 and since starting transformer 58 is a voltage step-up transformer, a high voltage positive pulse is produced at the starting electrode 68 of lamp 64. This high voltage pulse at the starting electrode results in a large potential difference across the starting electrode and cathode of lamp 64 thereby causing a starting arc and triggering lamp 64 into conduction and illumination. Once the conduction arc is established from the starting electrode, capacitor 40 will begin to discharge through inductor 72 and lamp anode 70 to cathode 66 thus sustaining the conduction arc and lamp illumination. The discharge of capacitors 38 and 40 is of very short duration and upon completion the lamp will cease conduction and illumination. The described operational cycle is then continuously repeated for steady state operation. I

The graphs of FIGS. 3 and 4 compared with each other illustrate the time relations of the capacitor charge and discharge portions of each cycle. The waveform of FIG. 3 is that of the output of the rectifier bridge 14 taken at point 26 and the waveform of FIG. 4 is that across capacitor 38 taken at point 76. Starting at time t O the bridge output voltage begins to rise but since the breakdown voltage of neon lamp 52 has not yet been reached, it is nonconducting and the voltage across capacitor 38 (and capacitor 40 for that matter) is zero as shown by FIG. 4 at point A. When the bridge output voltage reaches approximately 70 volts, neon lamp 52 begins to conduct switching SCR 34 into conduction and capacitors 38 and 40 begin to charge as shown by the rising waveform at point B of F IG. 4. The capacitors continue to charge until the magnetic field of inductor 30 is fully collapsed and then because SCR 44 is switched into conduction as described above, capacitor 38 rapidly discharges as illustrated at point C. It is during this period of discharge that lamp 64 is triggered into conduction and produces illumination. After capacitor discharge the voltage across capacitors 38 and 40 is 'zero, point D of FIG. 4, as is the voltage across the short are lamp electrodes and will remain at a zero level until the next cycle when the bridge output again reaches volts and SCR 34 is switched into conduction.

Operation of the intermittent arc illumination source utilizing a two electrode short are lamp in place of the three electrode type is essentially identical to that described above except that all of the stored energy is discharged into the lamp through starting transformer 58. That transformer performs both functions of initially striking the are to start conduction and carrying the dischargecurrent to maintain conduction and illumination.

By operating a xenon short are lamp with the above disclosed circuit in the described manner, the lamp only conducts current and therefore emits illumination for a very short period of time during each cycle. In the embodiment described each cycle has a duration of approximately 854; milliseconds while the discharge period and therefore the lamp conduction pulse is approximately 10 microseconds. Of course, the conduction pulses and resulting illumination bursts occur at such a rapid rate that they appear as continuous illumination to the human eye. As a practical matter it has been found that the pulses should occur at a rate of from approximately 30 to 800 per second and that their duration should be less than 0.01 of the length of the cycle or interval between pulses. Because the discharge capacitors release their energy through the short are lamp in a period of time which is very much shorter than the required charge time, the conduction current through the short are lamp is of a high value for a short time duration. For beneficial operation of the invention, the circuit parameters should be chosen to give conduction currents of greater than approximately 50 amperes. It is this intermittent operation by short duration, high current pulses which results in new and desirable operating characteristics for the are illumination source of the present invention.

Since the electrical energy pulses flowing across the short are lamp electrodes and forming the illumination are are of high current, the magnetic field induced by such current flow is of substantial strength. This strong magnetic field encircling the arc tends to confine it within a limited region of the lamp enclosure. Such are confinement results in three major effects: (1 the pressure within the actual arc is increased thereby increasing the plasma temperature and the effective illumination color temperature; (2) are mixing with the surrounding gas is minimized so that the temperature of the lamp envelope walls is decreased; and (3) the emitting volume of the arc is decreased thereby closer approximating a point source of illumination.

The effect enumerated as (1) above is illustrated by FIG. 2. In that graph the relative outputs of a xenon IObOlZ 047 I Ml short are lamp operated in a Continuous D.C. mode, curve E, and the same lamp operated in an intermittent pulsed mode in accordance with the present invention,

curve 'F, are plotted as functions of illumination wavelengths. Also shown in this graph is a typical human eye response curve, curve G, of relative sensitivity versus illumination wavelength. It is clearly seen 7 from FIG. 2 that the xenon short are lamp operated in a pulsed mode produces a significantly greater amount of illumination in the lower or ultraviolet end of the spectrum than does the same lamp continuously D.C. operated. It is also apparent that for the pulsed operated lamp a substantially larger amount of flow wavelength or blue illumination is produced within the human eye response curve. It has been found that the effective illumination color temperature of the lamp operated in the conventional D.C. mode is approximately 6000 K while the effective temperature for pulsed mode operation is approximately 9000 K. Such an increase in color temperature produces a significantly whiter" appearing illumination source.

Of particular importance to the present invention is the effect enumerated in (2). When xenon short are lamps are operated in the conventional D.C. mode the current through the lamp is relatively low, in the2 to 10 amperes range, and an arc is continuously burning between the electrodes. Under such circumstances the magnetic field tending to confine the arc is very weak and theplasma in the arc mixes very readily with the gas in the-lamp envelope. The ease of arc and gas mixture coupled with the continuous burning of the arc during the entire operating cycle heats the lamp envelope walls to a nominal temperature of approximately 600 C. for D.C. mode operation. Such an envelope wall temperature requires that the envelope be constructed of quartz or other similar high temperature resistant, transparent materials. As a practical matter quartz is the only material found suitable for xenon short are lamp envelopes and because of its peculiar qualities the lamps must be hand-blown and handmade, therefore, making them relatively expensive. However, in the operation of a xenon short are lamp in a pulsed mode as disclosed by the present invention, a substantial magnetic field. is present tending to confine the arc and the arc is only burning a very short period of time during each cycle; i.e., the are only burns approximately 1/800 of the time. These two factors operate to drastically reduce the envelope wall temperature from 600 C. to approximately 150 C. A high temperature resistant envelope material is now not required for operation at this lowered temperature. Therefore, the envelopeof a short arc lamp operated in the presently disclosed pulsed mode may be constructed of a high quality, heat resistant glass rather than quartz. The use of such glass in place of quartz renders the lamp susceptible to mass production methods rather than handmade techniques thereby lowering lamp construction costs by many factors.

The effect mentioned in (3) above is particularly advantageous where the illumination source is used in image projection applications. In such applications, the smaller the illumination emitting volume is in relation to the image to be projected, the less extensive, and therefore, less expensive, the collimating and focusing optics need to be. Practically, an image of the same size or smaller than the illumination sources volume is impossible to enlarge and project efficiently, therefore the advantage 'of a very small emitting volume in the present illumination source allows the projection of very small images. I I The use of the present inventions power supply and control circuits also allow control of the xenon short arc lamp's spectral characteristics. This spectralcontrol is accomplished by varying the time duration of the current pulsewhile holding the total energy dissipated in each pulse constant. Thus if the duration of the pulse is increased the current level will be decreased and the operating spectral characteristics of the lamp will be shifted toward the characteristics of the lamp in D.C. operation. For example, in FIG. 2 curve F would be shifted toward curve'E as the pulse duration increased. This spectral shift is the result of the longer pulses lower current level, its correspondingly weaker magnetic confining field, and the resultingly lower arc temperature. The pulse time duration is increased by increasing the inductance of inductor 72 and is likewise shortened by decreasing that inductance.

While the illumination source of the present invention has been discussed in context with the problems associated with sources for image projection applications, it should be well understood that the source as v disclosed and described has applications in illumination fields as wide as the need for illumination itself. Several very probable applications would be as a vehicle running lights, optical ranging systems, microscope illuminators, or even specialized wide area lighting systems.

What is claimed is:

1. An intermittent arc illumination source devoid of a separate DC power source comprising: a xenon lamp having at least cathode and anode electrodes spaced apart to define a spark gap therebetween; pulse generating means including charge storage capacitors connected across said cathode and anode of the lamp and said means being connected to a source of relatively low power AC electrical energy operating in the frequency range of from 30 to 800 pulses per second, said generating means being operative in response to a' triggering signal generated as a function of the frequency of said AC source to produce intermittent DC electrical pulses between the anode and cathode which establish arcs intermittently therebetween and provides a peak current flow thereacross of not less than approximately 50 amperes; and, triggering means including a full wave rectifier means connected to said generating means operative to initiate the pulses produced by the latter in timed relation to the frequency of said AC source to I produce pulses at a predetermined rate between approximately 30 and 800 pulses per second and of a duration of not longer than approximately 0.01 of the time interval between each pulse.

2. The intermittent arc illumination source as set out in claim 1 in which:

a starting transforming means is interposed between the pulse-generating means and the xenon lamp to raise the voltage level of the pulses toa value sufficient to initially strike an arc between the said lamp anode and cathode.

3. The intermittent arc illumination source as set out in claim 1 in which: the pulse generating means consist of a capacitive energy storage means which is alternately charged from the relatively low power AC electrical energy source and discharged through the anode and cathode electrodes of the lamp to produce DC pulses thereacross said alternate charge and discharge cycles being controlled by the triggering means.

4. The intermittent arc illumination source as set out in claim 1 in which:

the triggering means consists of a full wave rectifying bridge whose output is selectively switched into the pulse generating means by a first electrically controlled switch, and a second electrically controlled switch which selectively connects the output of the pulse generating means to the lamp.

5. The intermittent arc illumination source as set out in claim 2 in which:

the xenon lamp has three electrodes spaced apart to define a spark gap therebetween, said electrodes functioning as a cathode, starting electrode, and anode respectively; and,

a portion of the pulse generating means output is passed through the starting electrode and the remainder of the said output is directed to the anode without passing through said starting transforming means.

6. The intermittent arc illumination source as set out in claim 3 in which:

an inductor is connected to the energy storage means cooperating therewith to induce a greater charge into said energy storage means.

7. The intermittent arc illumination source as set out in claim 3 in which:

the capacitive energy storage means consists of first and second capacitors in parallel with an output being taken across each capacitor.

8. The intermittent arc illumination source as set out in claim 4 in which:

the first electrically controlled switch is cyclically activated by a predetermined voltage level of the rectifying bridge output; and

the second electrically controlled switch is cyclically activated by a predetermined voltage level of the pulse generating means output.

9. The intermittent arc illumination source as set out in claim 5 in which:

that portion of the pulse generator output directed to the anode is passed through an inductor whose size is chosen to effect critical damping of the pulse.

10. The intermittent arc illumination source as set out in claim 5 in which:

a capacitor is connected between the lamp starting electrode and anode which is adapted to sustain an are between the electrodes for the duration of the pulse 11. The intermittent arc illumination source as set out in claim 7 in which:

an isolating diode is connected between the two capacitors and adapted to allow both of said capacitors to be charged from a common path but not to allow the second capacitor to be discharged into the first capacitor.

Claims

1. An intermittent arc illumination source devoid of a separate DC power source comprising: a xenon lamp having at least cathode and anode electrodes spaced apart to define a spark gap therebetween; pulse generating means including charge storage capacitors connected across said cathode and anode of the lamp and said means being connected to a source of relatively low power AC electrical energy operating in the frequency range of from 30 to 800 pulses per second, said generating means being operative in response to a triggering signal generated as a function of the frequency of said AC source to produce intermittent DC electrical pulses between the anode and cathode which establish arcs intermittently therebetween and provides a peak current flow thereacross of not less than approximately 50 amperes; and, triggering means including a full wave rectifier means connected to said generating means operative to initiate the pulses produced by the latter in timed relation to the frequency of said AC source to produce pulses at a predetermined rate between approximately 30 and 800 pulses per second and of a duration of not longer than approximately 0.01 of the time interval between each pulse.

2. The intermittent arc illumination source as set out in claim 1 in which: a starting transforming means is interposed betweEn the pulse generating means and the xenon lamp to raise the voltage level of the pulses to a value sufficient to initially strike an arc between the said lamp anode and cathode.

4. The intermittent arc illumination source as set out in claim 1 in which: the triggering means consists of a full wave rectifying bridge whose output is selectively switched into the pulse generating means by a first electrically controlled switch, and a second electrically controlled switch which selectively connects the output of the pulse generating means to the lamp.

5. The intermittent arc illumination source as set out in claim 2 in which: the xenon lamp has three electrodes spaced apart to define a spark gap therebetween, said electrodes functioning as a cathode, starting electrode, and anode respectively; and, a portion of the pulse generating means output is passed through the starting electrode and the remainder of the said output is directed to the anode without passing through said starting transforming means.

6. The intermittent arc illumination source as set out in claim 3 in which: an inductor is connected to the energy storage means cooperating therewith to induce a greater charge into said energy storage means.

7. The intermittent arc illumination source as set out in claim 3 in which: the capacitive energy storage means consists of first and second capacitors in parallel with an output being taken across each capacitor.

8. The intermittent arc illumination source as set out in claim 4 in which: the first electrically controlled switch is cyclically activated by a predetermined voltage level of the rectifying bridge output; and the second electrically controlled switch is cyclically activated by a predetermined voltage level of the pulse generating means output.

9. The intermittent arc illumination source as set out in claim 5 in which: that portion of the pulse generator output directed to the anode is passed through an inductor whose size is chosen to effect critical damping of the pulse.

10. The intermittent arc illumination source as set out in claim 5 in which: a capacitor is connected between the lamp starting electrode and anode which is adapted to sustain an arc between the electrodes for the duration of the pulse

11. The intermittent arc illumination source as set out in claim 7 in which: an isolating diode is connected between the two capacitors and adapted to allow both of said capacitors to be charged from a common path but not to allow the second capacitor to be discharged into the first capacitor.