US3809951A

US3809951A - Electronic photoflash

Info

Publication number: US3809951A
Application number: US00287638A
Authority: US
Inventors: Z Vital; J Orban
Original assignee: Ponder & Best
Current assignee: Ponder & Best; Ponder & Best Inc us
Priority date: 1968-02-13
Filing date: 1972-09-11
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07

Abstract

There is disclosed herein an electronic photoflash circuit including means for rapidly and accurately terminating a light flash and/or blocking discharge of a photoflash energy storage capacitor. The circuit involves use of a switching device connected in series with a photoflash tube wherein the beginning of ionization of the photoflash tube affects turn on one of the series switching device. Additionally, the circuit includes means for activating a light sensing circuit, and there is disclosed a novel form of optical attenuator useful in conjuction with the light sensing circuit.

Description

' United States Patent Vital et a1.

1111" 3,809,951 [451 May 7, 1974 799,554, Feb. 13, 'continuation-in-part of Ser. Nos. 17,634, March 9,

ELECTRONIC PHOTOFLASH Inventors: Zoltan Vital, Brussels; Jean Orban,

Clabecq, both of Belgium Assignee: Ponder & Best, Inc., Los Angeles,

Calif.

Filed: Sept. 11, 1972 Appl. No.: 287,638

Related us. Application Data Continuation-impart of Ser. No. 244,279, April 14, 1972, which is a continuation-in-part of Ser. No. 1969, abandoned, ,and a 1970, and Ser. No. 174,096, Aug. 13, 1971.

US. Cl 315/149, 315/151, 315/159,

315/241 P Int. Cl. H05b Field of Search 315/149, 151, 159, 241 R,

References Cited UNITED STATES PATENTS 7/1970 Ogawa 315/151 Krusche et al 315/159 X Murata et a1. 315/151 Primary Examiner-Herman Karl Saalbach Assistant Examiner--Lawrence J. Dahl Attorney, Agent, or Firm-Lyon & Lyon [57] ABSTRACT There is disclosed herein an electronic photoflash circuit including means for rapidly and accurately terminating a light flash and/or blocking discharge of a photofiash energy storage capacitor. The circuit involves use of a switching device connected in series with a photofiash tube wherein the beginning of ionization of the photoflash tube affects turn on one of the series switching device. Additionally, the circuit includes means for activating a light sensing circuit, and there is disclosed a novel form of optical attenuator useful in conjuction with the light sensing circuit.

16 Claims, 4 Drawing Figures PATENTEB MAY 7 I974 SHEET 1 OF 3 1 ELECTRONIC PHOTOFLAS H CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of U.S. Pat. application Ser. No. 244,279 filed Apr. 14, 1972 (a continuation of U.S. Ser. No..799,554, filed Feb. 13, 1969, and now abandoned) .which is based upon Belgium Pat. No. 710,669 and Belgium Pat. of Addition Nos. 724,245, 726,076, and 727,159; of U.S. Pat. ap-. plication Ser. No. 17,634 filed Mar. 9, 1970, and based upon Belgium Pat. of Addition Nos. 729,602, 735,253 and 739,170 to said Belgium Pat. No. 710,669; and of U.S. Pat. application Ser. No. 174,096 filed Aug. 13, 1971, and based upon Belgium Pat. of Addition Nos. 755,196, 756,553, 763,925, 765,578 and 768,377 to said Belgium Pat. No. 710,669. The disclosures of said applications and patents are incorporated herein by reference. These applications and patents disclose several circuits and devices for controlling impulses generated by the discharge of a capacitor associated with a load device, such as a photoflash device.

The present invention relates to electronic photoflash devices, and particularly to devices for automatically terminating a light flash in response to the quantity of light reaching a subject being photographed, and represents improvements over the aforementioned applications and patents.

BACKGROUND In recent years, electronic photoflash devices incorporating various types of automatic flash limiting systems have been developed on both an experimental and commercial level and have enjoyed a constantly growing commercial success. These devices represent a substantial improvement over previous nonautomatic photoflashes in which no control, or only crude control, of the flash duration, or total light output, was possible. 1n recently developed systems, the light reflected from'the subject being photographed is sensed and integrated and when the total quantity of light being sensed reaches a predetermined value, a signal is produced to terminate the light flash.

lnsystems already proposed, this termination is effected by creating a short circuit across the photoflash tube. When this short circuit is created, the discharge current from an energy storage capacitor supplying the photoflash tube is shunted from the tube, so that the light flash is terminated. The energy storage capacitor then discharges completely through the short circuit and at some time after complete discharge of the capacitor, the short circuit is removed and the system is permitted to recycle back to its ready state.

Apparatus of such kind is disclosed in U.S. Pat. No. 3,033,988 and is composed of a device which controls the light of a photographic enlarger and in which a gasdischarge switching triode with an external firing electrode or a mercury tube is provided as a second switch, always with an impedance of about 0.1 ohm. Concerning the light sensor of this patent, U.S. Pat. No. 3,350,604 states, at column 3, lines 27-32; In order to achieve the high photocell dark impedance with the required high light sensitivity, the ligh sensitive element must be a photomultiplier tube. Suchtubes require a A rather complex power supply. This results in a package which is sufficiently bulky as to preclude its use in por table, camera mounted flash equipment." Another known device of the same kindis disclosed in U.S. Pat. No. 3,350,604, which is essentially based on the disclosure of U.S. Pat. No. 3,033,988, and represents a modification thereof in thatthis device makes use of a nonreactive light integrator, i.e., one without a capacitor, and the patent states that the use of this is more advantageous than that of the integrator of U.S. Pat. No. 3,033,988.

While such systems function reasonably well and offer a generally accurate control of the light produced during a single flash as a function of the nature and range of the subject being photographed, these systems do offer certain disadvantages.

Firstly, as already noted, they cause the energy storage capacitor to be completely discharged-after termination of each light flash.'This results in a great waste of energy which is not employed for producing light .flashes and increases the recycling time of the flash unit, since the capacitor must be recharged from a completely discharged state. It also reduces the total number of light flashes which can be provided by a power source having a given energy content;

Further, in the systems already disclosed, the actuation of the quenching, or flash terminating, circuit has the initial result of increasing the current through the tube so as to temporarily increase the light output from the tube and delay the light termination operation.

.In addition, despite various efforts at miniaturization of these devices, they remain relatively large, a considerable amount of space being required within the unit housing, for example, for the induction coil normally disposed in the flash tube-energy storage capacitor circuit. 1

Furthermore, those known devices which provide satisfactory operation require a separate gating. circuit to disable the flash limiting unit until a flash has been.

patents describe a number of circuits and devices which overcome many of the previously noted disadvantages of the short circuit of shunt type control system, and. basically make use of a switching element in series between a source and the load device, such as a photoflash tube. Other circuits of interest also are disclosed in U.S. Pat. No. 3,591,829 and in U.S. Pat. No. 3,612,947. Both of these patents disclose particular trigger circuit for operating a series switching element.

Accordingly, it is a principalobjectof the present invention to provide a new and improvedenergy impulse system such as for use in controlling the operation of a photoflash tube. 1

An additional object of the present invention is to provide a circuit for controllingthe operation of a photoflash tube wherein ionization of said tube affects triggering of a series switching device.

A further object of the present-invention is=tolprovide an improved form of electronic control circuit for a photoflash tube and an associated light sensing circuit responsive to operation of the control circuit.

Briefly, the objects of the present invention are achieved in a preferred manner by a circuit which includes a fast electronic interruptor in the form of a series switching device, such as a thyristor connected in series with the flash tube, and in which a gating system permitsuse of a conventional ignition circuit for the flash tube instead of requiring a special ignition circuit for igniting the thyristor, and which enables a short turn-off time for the thyristor, thereby making possible the use of a thyristor of an average turn-off time instead of a-fast turn-off time. This circuit is equipped with a compensation device for limiting on the one hand the di/dt of the discharge pulse and on the other hand the over-tension on the thyristor clamps. Furthermore, this circuit contains still another RC'device to facilitate the ignition of the flashtube while limiting the dv/dt value to the anodes of the thyristors. This circuit may also contain a device for measuring the reflected light quantity in the form ofa device equipped with an electronic gate activated by the voltage of the control electrode of the thyristor which is placed in series with the flashtube.

A further object of this invention is to provide a novel form of optical attenuator for use with a light sensing circuit or the like.

Another object is improvement of light-sensing calculating devices through provision of an optical universal attenuator for the adjustment of optical sensitivity.

These and other objects and features of the present invention will become better understood through a consideration of the following description taken in conjunction with the drawings in which:

FIG. 1 is a block diagram ofa preferred embodiment of an electronic photoflash circuit according to the concepts of the present invention;

FIG. 2 is a circuit diagram of the arrangement of FIG. 1; and

FIGS. 3 and 4 illustrate an optical attentuator according to the present concepts.

Turning now to FIGS. 1 and 2 there is shown an exemplary emcodiment of an electronic photoflash system according to the present invention. There is shown a circuit which includes a fast electronic interruptor, constituted by a thyristor connected into series with the flashtube, and in which the gating system permits use of a conventional ignition circuit for the flashtube instead of a special ignition circuit which also must ignite the thyristor, and which enables the shortest possible turn-off time for this thyristor, thereby making possible the use ofa thyristor of an average turn-off time instead of a fast turn-off time. This circuit is equipped with a compensation device for limiting the di/dt of the discharge pulse and the overtension on the thyristor clamps. Furthermore, this circuit contains still another RC device to facilitate the ignition of the flashtube while limiting the dv/dt value to the anodes of the thyristors. This circuit may also contain a device for measuring the reflected light quantity in the form of a device equipped with an electronic gate activated by the voltage of the control electrode of the thyristor whic is placed in series with the flashtube.

The circuit of FIG. 1 includes an electric supplying device 101, a discharge capacitor 102, a flashtube 103', a conventional ignition circuit 105 for flashtubes, a

compensation circuit 104 connected into series with the discharge capacitor 102 and with the flashtube 103, a fast-working electronic interruptor 106, which is equally connected into series with the capacitor 102 and the flashtube 103, and a light quantity measuring and calculating device 107. This circuit operates as follows: The supply 101 charges the discharge capacitor 102. The voltage of this capacitor is applied via the compensation circuit 104 and via the fast working electronic interruptor 106 to the electrodes of the flashtube 103. As soon as the synchronisation contact 110 of the ignition circuit 105 is closed, the flashtube 103 ionises itself, the fast working interruptor 106 closes and the discharge current in the flashtube is established at a predetermined speed (di/dt) and at a maximum value, which are controlled by the compensation circuit 104, and this discharge current generates the luminous flash. The connection 109 takes the information of the beginning of the flash at the moment when the interruptor 106 closes and sends it to the light sensing-calculating device 107. This latter device delivers a control command of the end of the flash to the fast working interruptor 106 by intermediary of the connection 108, as soon as the sensor has received a predetermined light quantity which has already been counted since the beginning of the flash. This control signal of the end of the flash provokes the fast opening of the electronic interruptor 106 and because of this the discharge current of the capacitor 102 and the flash are interrupted.

This circuit is shown in more detail in FIG. 2 and includes, among other things, a discharge capacitor 202, the flashtube 203, an induction coil 201, a recuperation diode 204, two

thyristors

205 and 206, each with its own ignition circuit, a switching capacitor 207, and a sensor-calculator 240 (107).

This circuit operates as follows: The voltage of the capacitor 202 is applied to the electrodes of the flashtube 203 via the compensation inductance 201 and via the resistor 208, connected in a parallel way on the thyristor 205 which is normally blocked (or off). The voltage of the capacitor 202 is equally applied to the switching capacitor 207 via the

resistors

209 and 211. This voltage is equally applied to the anode of the thyristor 206, which is normally blocked, via the resistor 21 1.

As soon as the synchronisation contact 210 is closed, the flashtube 203 ionises, which has an effect the generation of a variation of the positive voltage (dv/dt) to the anode of the thyristor 205 and via the resistor 209 and the capacitor 207 (which is normally charged) equally to the anode of the thyristor 206. This dv/dt is limited by the resistor 212 and the capacitor 213 placed at the electrodes of the thyristor 205. This dv/dt is equally transmitted -tothe control electrode of the thyristor 205 via the resistor 214 and the capacitor 215 which ignites the thyristor 205. As soon as the thyristor 205 is conducting, the discharge current in the flashtube 203 is established like in a known electronic photofiash device, excepting that-the current increasing speed (di/dt) is limited by the induction coil 201.

As soon as the light sensor-calculator 240 delivers a control pulse for the end of the flash to the control electrode of the thyristor 206, this last one ignites, which has an effect to discharge the capacitor 207 into the thyristor 205 via the resistor 209. The voltage of the anode of the thyristor 205 is thus inverted during the duration of the discharge of the capacitor 207 and the thyristor 205 passes over to the blocked state. Furthermore, the ignition of the thyristor 206 generates a negative pulse to the control electrode of the thyristor 205 due to the capacitor 215 and to the resistor 214, which has as an effect to diminish the turn-off-time of the thyristor 205. Due to the fact that the thyristor 205 passes over to the blocked state, the discharge current is interrupted, the tube dis-ionises, and the flash exting'uishes. As soon as the discharge current of the capacitors 207 and 215 becomes weak enough, the thyristor 206 is blocked and the whole system goes back to the initial state. The compensation diode 204 avoids the overtension at the electrodes of the flashtube 203 and the

thyristors

205 and 206 on the occasion of the interruption of the flash by the absorption of the energy which is contained in the induction coil 201.

The sensor-calculator 240 operates as follows: In the initial state the

capacitors

241 and 242 are charged at voltages which are determined respectively by the resistor 243 and the Zener diode 244, and all the transistors are in a blocked state. As soon as the thyristor 205 conducts, the positive voltage which appears on its control electrode makes the transistor 245 conducting which permits the integration of the photocurrent of the sensor 246 in the capacitor 242. As soon as the base-emitter voltage of the transistor 247 becomes negative, this last one conducts and applies the positive voltage of the capacitor 241 to the control electrode of the thyristor 206.

Turning now to FIG. 3, the same represents an example of realization of a universal attenuator. A light sensor 301 is placed in a non-transparent tube 302. In this tube 302 is provided a transversal split 303 in which the universal attenuator 304 can glide. The attenuator 304 is constituted by a ruler composed of a succession of windows having increasing attenuation coefficient from 305 to 313. For example, the window 305 has an attenuation coefficient equal to zero and will be used in the case when the optical sensibility of the light sensor will be 50 percent of the nominal value, the window 310 having an attentuation of one aperture will be used in the case when the optical sensibility of the light sensor is equal to the nominal value, and the window 313 has an attenuation coefficient of less than 2 apertures and will be used for a light sensor of which the sensibility will be 200 percent ofthe nominal value. The attenuation difference between two successive windows is of one quarter of an aperture, which permits in all cases an adjustment of the sensibility with a precision of one quarter of an aperture. FIG. 4 represents a device working with the same principle, but with a universal attenuator constituted by a disc 404 instead of a ruler 304.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. in an electronic flash device including an energy source, a flash tube and a first electronic switch connected in series, a turn-off circuit connected with said first electronic switch and including commutating means and a second electronic switch connected with said first electronic switch for controlling the turn-off of said first electronic switch in response to a terminating signal from a light sensing circuit, the improvement.

said light sensing circuit including a light sensor and gating means coupled with said light sensor for enabling the operation of said light sensor, means connecting said. light sensor with said second electronic switch of said turn-off circuit for supplying said terminating signal to said second electronic switch upon the generation of a predetermined quantity of light from saidflash tube, and

means connecting said impedancemeans to said gating means to cause said gating means to turn on when said firstelectronic switch is turned on to thereby enable said light sensor.

2. A device as in claim 1 wherein said impedance means comprises a capacitance and a resistance connected from a junction between a main electrode of said second electronic switch and said commutating means to said control electrode of said first electronic switch.

3. A device as in claim 2 wherein said impedance means further includes a resistance coupled between said control electrode and a main electrode of said first electronic switch means, and

said gating means is connected to the junction of said resistances and the control electrode of said first electronic switch, and

said commutating'means includes a capacitance and a resistance connected in series from the junction of said flash tube and said first electronic switch means to a main electrode of said second electronic switch means' 7 4. A device as in claim 1 including transient voltage limiting means connected in parallel with said main electrodes of said first electronic switch, said transient voltage limiting means com prising capacitance and resistance connected in parallel with said main electrodes of said first electronic switch means.

5. A device as in claim 1 including inductive means and unilateral current conductive means coupled in parallel with said inductive means, the parallel combination being connected in series with said source, flash tube and first electronic switch, said inductive means functioning to limit transient current flow in said flash tube upon firing thereof, and aid unilateral current conductive means functioning to absorb energy of said inductive means upon turn-off of said flash tube.

6. A device as in claim 1 wherein said means connecting said light sensor with said second electronic switch comprises integrating circuit means including an integrating capacitor coupled with said light sensor for integrating a signal from said light sensoras a function .second electronic switch.

7. A device as in claim 1 wherein, said gating means comprises a semi-conductor switch having first, second and third electrodes, the first of said electrodes being coupled with the control electrode of said first electronic switch, and the second and third electrodes thereof being connected in series with said light sensor, and said means connecting said light sensor with said second electronic switch comprises an integrating capacitor coupled with said light sensor for integrating a signal from said light sensor as a function of light received thereby, and includes a semiconductor switch coupled with said integrating capacitor and responsive to a predetermined magnitude signal integrated by said integrating capacitor to apply said terminating signal to a control electrode of said second electronic switch. 8. A device as in claim 7 wherein said impedance means comprises a capacitance and resistance connected from a junction between a main electrode of said second electronic switch and said commutating means to said control electrode of said first electronic switch. 9. A device as in claim 8 including transient voltage limiting means connected in parallel with said main electrodes of said first electronic switch, said transient voltage limiting means comprising capacitance and resistance connected in parallel with said main electrodes of said first electronic switch means. 10. A device as in claim 8 wherein said impedance means further includes a resistance coupled between said control electrode and a main electrode of said first electronic switch means. 11. An electronic flash device including an energy source, a flash tube and a first electronic switch connected in series, a turn-off circuit connected with said first electronic switch and including connecting means and a second electronic switch connected with said first electronic switch for controlling turn-off of said first electronic switch in response to a terminating signal from a light sensing circuit, comprising a pair of supply conductors, means connecting said energy source with said supply conductors, and means connecting said flash tube and said first electronic switch to said supply conductors, means connecting said turn-off circuit to said supply conductors, said commutating means of said turnoff circuit being coupled from the junction of said flash tube and said first electronic switch to said second electronic switch, impedance means coupled between said turn-off circuit and said control electrode of-said first electronic switch for facilitating turn-on of said first electronic switch upon ioniiation of said flash tube and for facilitating turn-off of said first electronic switch when said turn-off circuit receives said terminating signal from said light sensing circuit, said light sensing circuit including a light sensor and gating means coupled in series with the light sensor for enabling the operation of said light sensor, means coupling the series combination of said light sensor and gating means between said supply conductors',

means connecting said impedance means to said gating means to cause said gating means to turn on when said first electronic switch is turned on to thereby enable said light sensor, and

means connecting said light sensor with said second electronic switch of said turn-off circuit for supplying said terminating signal to said second electronic switch upon the generation of a predetermined quantity of light from said flash tube.

12. A device as in claim 11 wherein said means connecting said light sensor with said second electronic switch is connected with a control electrode of said second electronic switch and comprises an integrating capacitor coupled with said light sensor for integrating a signal from said light sensor as a function of light received thereby, and includes semi-conductor switch means responsive to a signal of predetermined magnitude integrated by said integrating capacitor to apply said terminating signal to said control electrode of said second electronic switch.

13. A device as in claim 11 wherein said impedance means comprises a capacitance and resistance circuit connected from the junction of said commutating means and said second electronic switch to one of said supply conductors.

14. A device as in claim 13 wherein said capacitive and resistance circuit includes a capacitor and two resistors connected in series, said control electrode of said first electronic switch being connected to the junction of said two resistors and the junction of said two resistors being connected to said gating means.

15. A device as in claim 1 including optical attenuator means coupled with said lightsensor, said optical attenuator means includes a nontransparent housing having an opening through which light from said flash tube can reach said light sensor, and said attenuator means includes an attenuation member having areas of increasing attenuation coefficient movably mounted with respect to said housing for enabling selective attenuation of the light from said flash tube impinging upon said light sensor.

16. A device as in claim 11 including optical attenuator means coupled with said light sensor, said optical attenuator means includes a nontransparent housing having an opening through which light from said flash tube can reach said light sensor, and said attenuator means includes an attenuation member having areas of increasing attenuation coefficient movably mounted with respect to said housing for enablingselective attenuation of the light from said flash tube impinging upon said light sensor.

Claims

1. In an electronic flash device including an energy source, a flash tube and a first electronic switch connected in series, a turn-off circuit connected with said first electronic switch and including commutating means and a second electronic switch connected with said first electronic switch for controlling the turn-off of said first electronic switch in response to a terminating signal from a light sensing circuit, the improvement comprising said first electronic switch having main electrodes connected in series with said tube and said source, and having a control electrode, impedence means coupled between said turn-off circuit and said control electrode of said first electronic switch for facilitating turn-on of said first electronic switch upon ionization of said flash tube, said light sensing circuit including a light sensor and gating means coupled with said light sensor for enabling the operation of said light sensor, means connecting said light sensor with said second electronic switch of said turn-off circuit for supplying said terminating signal to said second electronic switch upon the generation of a predetermined quantity of light from said flash tube, and means connecting said impedance means to said gating means to cause said gating means to turn on when said first electronic switch is turned on to thereby enable said light sensor.

3. A device as in claim 2 wherein said impedance means further includes a resistance coupled between said control electrode and a main electRode of said first electronic switch means, and said gating means is connected to the junction of said resistances and the control electrode of said first electronic switch, and said commutating means includes a capacitance and a resistance connected in series from the junction of said flash tube and said first electronic switch means to a main electrode of said second electronic switch means.

4. A device as in claim 1 including transient voltage limiting means connected in parallel with said main electrodes of said first electronic switch, said transient voltage limiting means comprising capacitance and resistance connected in parallel with said main electrodes of said first electronic switch means.

5. A device as in claim 1 including inductive means and unilateral current conductive means coupled in parallel with said inductive means, the parallel combination being connected in series with said source, flash tube and first electronic switch, said inductive means functioning to limit transient current flow in said flash tube upon firing thereof, and said unilateral current conductive means functioning to absorb energy of said inductive means upon turn-off of said flash tube.

6. A device as in claim 1 wherein said means connecting said light sensor with said second electronic switch comprises integrating circuit means including an integrating capacitor coupled with said light sensor for integrating a signal from said light sensor as a function of light received thereby, and including semiconductor switch means responsive to a predetermined magnitude signal integrated by said integrating capacitor to apply said terminating signal to a control electrode of said second electronic switch.

7. A device as in claim 1 wherein said gating means comprises a semi-conductor switch having first, second and third electrodes, the first of said electrodes being coupled with the control electrode of said first electronic switch, and the second and third electrodes thereof being connected in series with said light sensor, and said means connecting said light sensor with said second electronic switch comprises an integrating capacitor coupled with said light sensor for integrating a signal from said light sensor as a function of light received thereby, and includes a semi-conductor switch coupled with said integrating capacitor and responsive to a predetermined magnitude signal integrated by said integrating capacitor to apply said terminating signal to a control electrode of said second electronic switch.

8. A device as in claim 7 wherein said impedance means comprises a capacitance and resistance connected from a junction between a main electrode of said second electronic switch and said commutating means to said control electrode of said first electronic switch.

9. A device as in claim 8 including transient voltage limiting means connected in parallel with said main electrodes of said first electronic switch, said transient voltage limiting means comprising capacitance and resistance connected in parallel with said main electrodes of said first electronic switch means.

10. A device as in claim 8 wherein said impedance means further includes a resistance coupled between said control electrode and a main electrode of said first electronic switch means.

11. An electronic flash device including an energy source, a flash tube and a first electronic switch connected in series, a turn-off circuit connected with said first electronic switch and including connecting means and a second electronic switch connected with said first electronic switch for controlling turn-off of said first electronic switch in response to a terminating signal from a light sensing circuit, comprising a pair of supply conductors, means connecting said energy source with said supply conductors, and means connecting said flash tube and said first electronic switch to said supply conductors, means connecting said turn-off circuit to said supply condUctors, said commutating means of said turn-off circuit being coupled from the junction of said flash tube and said first electronic switch to said second electronic switch, impedance means coupled between said turn-off circuit and said control electrode of said first electronic switch for facilitating turn-on of said first electronic switch upon ionization of said flash tube and for facilitating turn-off of said first electronic switch when said turn-off circuit receives said terminating signal from said light sensing circuit, said light sensing circuit including a light sensor and gating means coupled in series with the light sensor for enabling the operation of said light sensor, means coupling the series combination of said light sensor and gating means between said supply conductors, means connecting said impedance means to said gating means to cause said gating means to turn on when said first electronic switch is turned on to thereby enable said light sensor, and means connecting said light sensor with said second electronic switch of said turn-off circuit for supplying said terminating signal to said second electronic switch upon the generation of a predetermined quantity of light from said flash tube.

15. A device as in claim 1 including optical attenuator means coupled with said light sensor, said optical attenuator means includes a nontransparent housing having an opening through which light from said flash tube can reach said light sensor, and said attenuator means includes an attenuation member having areas of increasing attenuation coefficient movably mounted with respect to said housing for enabling selective attenuation of the light from said flash tube impinging upon said light sensor.

16. A device as in claim 11 including optical attenuator means coupled with said light sensor, said optical attenuator means includes a nontransparent housing having an opening through which light from said flash tube can reach said light sensor, and said attenuator means includes an attenuation member having areas of increasing attenuation coefficient movably mounted with respect to said housing for enabling selective attenuation of the light from said flash tube impinging upon said light sensor.