US3422308A - Light intensifier circuit for flash photography - Google Patents

Description

Jan. 14, 1969 B. J. SULLIVAN 3,422,308

LIGHT INTENSIFIER CIRCUIT FOR FLASH PHOTOGRAPHY Filed July 14. 1966 28 I 34 ZZKA- l 2| IOOKFD BERNARD J. SULLIVAN INVENTOR.

ATTORNEY United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE A light intensifier circuit includes a first circuit for continuously energizing a lamp from an alternating current power source for low intensity output and a second circuit for momentarily energizing the lamp for high intensity output with a rectified alternating signal for a duration of integral half cycles of the alternating current power.

The invention relates to electrical circuits in general and more particularly to an electrical circuit for controlling the intensity of a light source for producing at least two levels of intensity.

When photographs are made through the use of optical instruments such as microscopes, endoscopes, and retinal cameras, etc., it is desirable to provide a continuous source of illumination for viewing, focusing, etc., and also intense illumination for a short period of time during photographic exposure. It is difficult, if not impractical, to employ two separate light sources due to the necessity of strict optical alignment required in these instruments. Furthermore, it is impractical to substitute one type illuminant for another, particularly if the sub ject is continually moving and it is desirable to observe the subject while, or shortly before, photographing.

'In the light intensifing apparatus of the prior art, a single lamp or light bulb was generally energized at a low voltage level while a capacitor was charged to a substantially higher voltage. At a precise time, the capacitor is discharged through the lamp to provide an intense source during photographic exposure. In such cases, the intensity of the flash is a function of the amount of energy stored in the capacitorand the impedance match between the capacitor acting as a source and the lamp as a load. The duration of the flash is determined by the electrical time constant of the capacitor-filament combination and the thermal time constant of the lamp filament. The size of the capacitor employed generally changes with the characteristic of the particular lamp connected in the circuit. Accordingly, various lamps can not be readily interchanged, or several connected in parallel without compensating therefore by changes in the size of the discharge capacitor. In addition, the capacitor employed in such light intensifier circuits is in the order of hundreds or thousands of microfarads. This type of capacitor is large and expensive, particularly if the capacitor is to be charged to a relatively high voltage. Furthermore, the capacitor is generally rapidly or impulsively discharged, subjecting the capacitor to severe operating conditions that may cause an overload condition and damage after a short period of service.

It is therefore an object of this inventor to provide a new and improved light intensifier circuit.

It is also an object of this inventor to provide a new and improved light intensifier circuit for momentarily increasing the light intensity of a lamp without the use of a large storage capacitor as a source of energy for intensifying the output lamp.

-It is also an object of this invention to provide a ice new and improved light intensifier circuit adapted to provide a means for continuously energizing a lamp at a low level and momentarily energizes the lamp at a high level for a predetermined time duration.

-It is still a further object of this invention to provide a new and improved light intensifier circuit for momentarily increasing the intensity of a lamp for a time duration determined by the period of a half cycle of alternating signal applied thereto.

It is also an object of this invention to provide a new and improved light intensifier circuit that readily allows for the interchange of lamps or connection of lamp in parallel.

The intensifier circuit of the invention includes a pair of input terminals adapted to be connected to an alternating current power source and a pair of output terminals adapted to be connected to a lamp to be energized. Circuit means are coupled between the input terminals and the output terminals for applying a potential to the output terminals having a value within the rating for the continuous energization of the lamp. Switching circuit means are also provided coupled between the input terminals and the output terminals for applying a substantially higher potential to the output terminal for a time duration for at least one half of a cycle of the alternating current power source so that the lamp provides an intense source of radiation at a desired time.

The novel features which are considered to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings in which the figure illustrates an embodiment of an intensifier circuit including the invention.

Light intensifier circuit of the figure includes a filament type lamp 10 that may be suitably mounted in an optical apparatus for providing a first level of illumination for the viewing, focusing, etc., and also a second higher level of intensity for a short duration for photography. The intensifier circuit includes a pair of terminals 14 adapted to be connected to an alternating current power source (not shown such as the 60 cycle line mains. A primary winding 16 of a transformer 18 is connected to the input terminals 14. The transformer secondary winding includes a first winding portion 20 (between the terminals 21 and 27) and a second winding portion 22 (between the terminals 21 and 39). The turns ratio in the winding portions 20 and 22 is such that a substantially greater voltage is developed across the winding portion 22 than across the winding portion 20.

The filament of the lamp 10 is connected across a pair of output terminals 11 and 12. A low level energizing potential of a magnitude within the voltage rating for the continuously energizing lamp 10 is developed by the series circuit including the winding portion 20' and the diode 24 connected across the terminals 11 and 12. The lamp 10 is continuously energized at this low level to provide the low intensity light source required for viewing, focusing etc.

A switching circuit is inclined in the intensifier circuit to provide a high intensity energizing potential across the terminals 11 and 12 of a controlled time duration energizing the lamp 10* for producing a high intensity light flash. The switching circuit includes a controlled rectifier 26 having its anode and cathode electrodes connected in a series circuit between the terminals 27 of the first winding portions 22 and the output terminal 12. The controlled" rectifier is adapted to be rendered conductive when its anode is positive with respect to the cathode and is extinguished when the polarity of the voltage reverses. By controlling the duration of the firing pulses applied to the controlled rectifier gate electrode the number of half cycles of pulses of conduction is controlled.

A firing circuit for the controlled rectifier 26 includes series. circuit including a diode 28, a resistor 30, and a capacitor 32 connected across the first winding portion 22. The diode 28 functions to charge the capacitor 32 to a direct current potential. The junction of the resistor 30 and the capacitor 32 is connected through a switch 34 and diode 35 to the controlled rectifier gate electrode and also to a discharge resistor 36 connected in a series with the diode 38 to a terminal 39 of the second Winding portion 20. A suitable filter capacitor 37 is connected between the secondary terminal 21 and the junction of the diode 38 and the resistor 36 to provide a substantially constant voltage for maintaining the diode 35 nonconductive (keeping the controlled rectifier cut off) until the switch 34 is closed. The switch 34, may for example, be mechanically coupled to a shutter of camera, remotely located, or be a semi-conductor circuit for periodically firing the controlled rectifier 26 at a desired pre determined rate.

When the switch 34 is closed, the capacitor 32 discharges through the resistor 36, the diode 38 and the capacitor 37 to generate a voltage pulse at the gate electrode of the controlled rectifier 26- of sufiicient amplitude to render the controlled rectifier conductive. If it is desired to limit the conduction of the controlled rectifier 26 to one half cycle of the applied alternating current, the resistor 36 and the capacitor 32 R-C time constant is set to be in the order of the period of one cycle of the applied alternating current or less. If it is desired to fire the controlled rectifier 26 for several consecutive half cycles, the R-C time constant is increased accordingly, The values illustrated in the figure allow a single half cycle of current flow through lamp when the switch 34 is momentarily closed. The ratio of the resistor 36 over the sum of the resistors 30 and 36 is selected so that the capacitor 32 charges at a rate so that the switch 34 may be continuously depressed and the lamp 10 is not energized for a greater number of consecutive half cycles of current pulses than desired. For example, with the values of the components in the figure, the lamp 10 is energized for only one half cycle when the switch 34 is continuously closed.

As previously mentioned, the controlled rectifier 26 is rendered conductive :for at least a half cycle or a plurality of half cycles (as determined by the R-C time constant of the capacitor 32 and the resistor 36) to apply a high potential of a controlled duration across the lamp 10, causing the lamp to produce a light of high intensity (flash). It should be noted that the duration of the high potential applied to the lamp 10 is not determined by a time constant including the filament of the lamp (such as the capacitor and filament combination of the lamp as in the prior art) but rather on the frequency of the applied energizing alternating current potential and a number of integral half cycles of conduction. Furthermore, any number of lamps can .be connected in parallel with the lamp 10 (within the capacity of a transformer 18 and the controlling rectifier 26) Without substantially changing the time duration of the flash or the amplitude of the high potential applied to the lamps.

In addition to the foregoing, the value of the capacitor 32, as indicated, is 100 microfarads which is small compared to that of the storage capacitor of the prior art generally requiring substantially greater values.

I claim:

1. The electrical circuit comprising:

a pair of output terminals adapted to 'be connected to a lamp for energization thereof;

a pair of input terminals adapted to be connected to an alternating current source of power;

first circuit means coupling said input terminals to said output terminals to provide an energizing potential for continuously energizing said lamp,

control means including first and second terminals exhibiting a controllable unidirectional current path therebetween and a control terminal for rendering said paths conductive;

second circuit means connecting said first and second terminals of said control means between said input terminals and said output terminals to provide a substantially higher potential to said output terminals in the form of a rectified alternating current signal when said path is rendered conductive, and

third circuit means, including a switch coupled to said control terminal for rendering said path conductive for at least a portion of one half cycle of said alternating current power when said switch is activated.

2. The electrical circuit as defined in claim 1 wherein:

said first means includes a transformer including a first winding coupled to provide an energizing potential that is less than the potential of said alternating current source.

3. The electrical circuit as defined in claim 2 wherein:

said second circuit means includes a second winding on said transformer providing a substantially higher alternating current potential than said first winding.

4. The electrical circuit as defined in claim 1 wherein:

said control means comprises a cont-rolled rectifier;

said second circuit means connects said controlled rectifier between said input terminals and said output terminals, and said third circuit means includes a firing circuit for said controlled rectifier circuit, and means for coupling said firing circuit to said controlled rectifier to render said controlled rectifier conductive for at least one half of a cycle of said alternating current source.

5. The electrical circuit as defined in claim 4 wherein:

said means for coupling said controlled rectifier to said input terminals includes a transformer, and

said controlled rectifier being connected in a series circuit between said transformer and said output terminals.

6. The electrical circuit as defined in claim 4 wherein:

said firing circuit comprises a capacitor and a rectifying circuit coupling said capacitor to said input terminals for developing a charging potential for said capacitor, and

said means for coupling said firing circuit to said controlled rectifier comprises a switch coupled to said controlled rectifier for applying said charging potential thereto for rendering said controlled rectifier conductive.

7. A light intensifier circuit comprising:

a lamp;

a transformer adapted to be connected to a sixty cycle power source; circuit means coupling said transformer to said lamp for applying a potential thereto having a value within a rating for the continuous energization of said lamp;

a switching circuit coupled between said transformer and said lamp adapted to be actuated for a period of integral half cycles of said sixty cycle power source for applying a substantially higher potential to said lamp in the form of a rectified alternating current signal.

8. A light intensifier circuit as defined in claim 7 wherein said switching circuit includes a controlled rectifier connected in a series circuit between said transformer and said lamp.

9. A light intensifier circiut as defined in claim 8 wherein said switching circuit includes a charging circuit and means for connecting said charging circuit to said control rectifier for rendering said controlled rectifier conductive for a duration of at least one half cycle of said sixty cycle source.

10. A light intensifier circuit as defined in claim 9 wherein said charging circuit includes a capacitor and rectifying circuit connected in series with said transformer for charging said capacitor to a predetermined voltage,

and means for connecting said charging circuit to saidcontrolled rectifier includes a switch for applying said voltage between the gate and cathode electrodes for said controlled rectifier for rendering said controlled rectifier conductive.

11. A light intensifier circuit as defined in claim wherein said charging circuit comprises a series circuit including a diode, a capacitor and a resistor connected across a secondary winding of said transformer for developing a charge across said capacitor, and said means for connecting said charging circuit to said controlled rectifier comprises a switch and a resistor connected in a series circuit between said capacitor and a secondary winding of said transformer so that when said switch is closed, said charge is applied to gate electrode of said controlled rectifier to render said controlled rectifier conductive.

12. A light intensifier circuit as defined in claim 11 wherein said capacitor is discharged through said series circuit including said switch and said resistor sufiiciently within the period of one cycle of said applied alternating current power source whereby said controlled rectifier conducts for a time duration in the order of the period of one half cycle of said applied alternating current.

References Cited OTHER REFERENCES A Shutter-Synchronized Circuit For Transiently Brightening an Incandescent Lamp Applicable To Color Photomicrography: by Edwin Gordy and GeorgefS. Siever, Published in Photographic Science and Engineering, vol. 8, No. 5, September-October 1964 (copy available in Scientific Library).

US. Cl. X.R.

JOHN W. HUCKERT, Primary Examiner.

R. F. POLISSACK, Assistant Examiner.

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