US2885563A - Photographic exposure timer - Google Patents

Description

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Filed Oct. 5, 1954 J. R. BATINA ET AL PHOTOGRAPHIC EXPOSURE TIMER May 5, 1959 .BQZZIZQ, wife/222M256" IN V EN TOR5.

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Jam/72612 $10M Maw 2A WW United States Patent 2,885,563 PHOTOGRAE'HIC EXPOSURE TIMER Jerome R. Batina and Kenneth E. Grosse, Chicago, Ill., assignors to W. M. Welch Manufacturing Company, Chicago, IlL, a corporation of Illinois Application October 5, 1954, Serial No. 460,454 12 Claims. (Cl. 250-205) The present invention relates to an improved timing :system which is particularly devised for use in determining and controlling exposure periods in the printing of films such as a photographic negative. An improved component of the system, in the form of an adjustably biased rectifier unit, is also suited for use generally where rectification is required. This application is a continuation-in-part of our copending application, Serial No. 394,687, filed November 27, 1953, now abandoned.

The timing system operates on the general principle that a photoelectric element will originate and forward an electric signal proportional in intensity to a radiant flux to which it is subjected. If this signal is amplified and applied to an interpretative and integrative unit which will transform the signal into an output voltage expressive of a time interval, the output voltage can be used to control energization and de-energization of a light source from which the radiant flux impinging the photoelectric element was derived. In further accordance with the invention, the integrative unit produces an output voltage which rises in value at a linear rate with reference to the value of its applied voltage. Hence, since the applied voltage is directly proportional to the intensity of the original light flux at the photoelectric element, i.e., inversely proportional to the density of a film or negative exposed, it is seen that the duration of rise of the output voltage represents a time interval which is inversely proportional to the amount of light flux impinging the phototube. Such voltage is accordingly used by an appropriate control or trigger unit to de-energize the exposure light source after an exposure interval so determined by the integrator. A dense negative is thus exposed a longer time, a less dense a shorter time, and, in the main, a proper exposure will be had of negatives of average to less than average density.

However, for perfect results the linear timing response of the integrator will require some compensation or correction in the case of very dense negatives, due to failure in reciprocity of the photographic paper, and the same may be true in the case of extremely light negatives.

Other adaptations of the timing and control system Will suggest themselves to those skilled in the art, however its embodiment in an automatic photographic exposure timer is an ideally illustrative one. The requirements of such a system are exacting. It should eliminate the human factor as much as possible, and be capable of being operated with a minimum of control or adjusting manipulations. It should cope successfully with the problem of satisfactorily exposing a wide variety of photographic negatives which are the work of amateurs as well as professional photographers. It must operate satisfactorily on a full range of negatives, including those of average contrast and density, those having a predominantly light colored background and dark subject, and vice versa, and all intermediate states. The system should be versatile as regards the exposure requirements of different photographic papers.

As a factor with which the present invention does not deal in detail, whatever may be the type of opticalphotoelectric unit used in association with the system it must faithfully reproduce a projected image in reference to the intensity of the electric current which it forwards, and in this connection the system should embody an adjustment for a change in enlarger magnification. It should be at least ninety-seven percent efficient at any magnification or negative density, so that the amount of rework on the prints will not exceed three percent. It should operate so that the majority of exposures are less than one second.

Generally considered, the improved timer as embodied in a photographic exposure system having these requirements comprises a source of light for exposure of a photographic film or negative, such as a conventional enlarger; a photoelectric unit, preferably capable of forwarding an alternatingly modulated signal when exposed to an image of the negative, since it is desirable to build up the photoelectric signal in an AC. amplifier; an interpretive or translative unit in the form of an integrating amplifier to which the amplified signal is fed after being rectified and converted to steady D.C. form, and which faithfully produces a timing signal at its output which rises at a rate bearing a linear relation to the converted signal; and a control or trigger unit operated by the integrator which applies to the exposure light source a control action, translated in terms of time from the integrators applied voltage.

Reference has been made to a possible failure of material in the exposure phase, in respect to non-reciprocity of printing paper, which will have a disturbing effect on the printing result if the photoelectric signal is faithfully translated into terms of time to a unit which controls exposure. To overcome this the invention provides means operative in the rectification of the amplified photoelectric signal to optionally compensate for the non-reciprocity referred to.

In explanation, it has been found that most photographic printing emulsions are not strictly linear in their relation of exposure time and exposure light throughout the entire range of their response to light, especially in the low range. Here the required exposure time for a perfect result is greater than would be expected were the responsivity of the paper a linear one. This being the case, it is desirable to afford means to compensate for the non-linearity by increasing the exposure time; otherwise the insuificient photoelectric signal will be translated in linearly responsive fashion into an excessively rapidly rising timing signal at the integrator output. The means for this purpose as contemplated by the invention take the form of means for adjustably biasing a rectifier by which the amplified photoelectric signal is converted, so that the integrator will then receive a DC. signal which it can in turn modify properly for use in the exposure control operation. The adjustably biased rectifier unit is of itself an independent part of the invention, since it is well suited to other applications.

As a further refinement in the timer, selection by range switching or voltage divider is employed to lend versatility in respect to the exposure requirements of different negatives, as well as other variable factors with which those skilled in the photographic art are familiar.

As indicated above, the integrating amplifier of the system is an electronic unit whose output voltage will rise at a linear rate in relation to its applied or input vo1tage. This is due to the fact that a part of the output is reflected back to its input grid, i.e., in degenerative feed back, and the linearly rising output voltage is the product. The control or trigger unit is shown as a triode, to the grid of which this voltage is applied, and a normally closed switching relay acts when a predetermined potential level aeeaaee .the light transmitted through a negative, permits the normally closed relay of the trigger unit to keep the icircuit of the exposure light source closed until the trigger tube passes current. Thus for a higher impressed voltage, at the integrator grid, due to a light negative, its output voltage will rise more rapidly and the exposure time will be shorter. With a denser negative the trigger tube and -relay will act to hold the light source circuit closed a longer time. In either case the time interval may optionally be in a linear or non-linear relation to the density of the negative, as determined at the adjustable rectifier.

It will become more evident, as the description proceeds that, either as a component of or as an independent subcombination of the timer, the adjustably biased rectifier unit is a highly valuable improvement, for the further reason that it affords a means for controlling and compensating undesirable contact potential in a rectifier tube,

i.e., between the electrodes thereof. This permits preamplification of an A.C. signal to a considerable degree, as by thermionic circuit, transformer, or both, and full wave rectification without the effect of contact potential.

The invention provides a system which satisfies the operational requirements previously outlined, being capable of timing properly and sustainedly under a normal, single cycle operation or with automatic recycling in association with an automatic paper advance mechanism. It permits the making of more than 600 exposures an hour. Yet another advantage of the system is that it is composed of units which are individually replaceable readily when necessary, thereby insuring against any sustained interruption of operation.

The foregoing statements are indicative in a general way of the nature of the invention. Other and more specific objects will be apparent to those skilled in the art upon a full understanding of the composition and operation of the system.

A single embodiment of the invention is presented herein for purpose of illustration. It will be appreciated that the invention may be incorporated in other modified forms coming equally within the scope of the appended claims.

In the drawings:

Fig. 1 is a schematic layout illustrating optical and electrical components of the improved system and the wiring of the latter;

Fig. 2 is a block diagram showing the essential operating components in a simpler fashion; and

Fig. 3 is a schematic wiring diagram of one of the components of the system of Fig. 1, in the form of a biased rectifier arrangement for linearity compensation under certain conditions of operation, either in the installation of Fig. l or for other purposes.

Considering first Fig. 2, a simplified layout, the improved system is composed of a suitable light source 10, such as a conventional photographic enlarger or other exposure device which transmits radiant energy, under the control of other units of the system, through a photographic negative or film 11 into impingement with the cathode of a phototube 12 of a photoelectric probe unit, generally designated 13. Thus a projected image of negative 11 originates and forwards a weak electrical signal which is directly proportional in intensity to the total radiant flux impinging the phototube, i.e., inversely proportional to the density of the negative.

Since a signal which is usable, in regard to sensitivity and stability, for the present purpose can best be built up in an alternating current amplifier, the probe unit 13 is preferably a magnetically modulated one, as hereinafter described, in general accordance with the disclosure of the patent to Kalmus et al. No. 2,605,428, dated July 29, 1952.

The anode of phototube 12 is connected to the input grid of A.C. amplifier 14, the cathode side of which is appropriately grounded, and the output of the amplifier is impressed on an improved type of adjustable full wave rectifier and voltage doubler unit 15. Unit 15 converts the amplified A.C. potential to a steady one of negative polarity at the input side of a subsequent translative unit 16, in the form of an integrating amplifier, which operates on direct current.

The rectifier output is preferably applied to integrator 16 through an appropriate voltage divider type of selector 17 (Fig. l), by which the system may be appropriately adjusted in regard to integrator input voltage, as determined, for example, by the preliminary exposure and printing of photographic test strips.

Integrator 16 acts in conjunction with photoelectric probe 13 to establish a time-integral relationship with the radiant flux of a projected image striking phototube 12, i.e., with the average density of a photographic negative. Output voltage of the integrator, linearly or nonlinearly related in its rate of rise to the flux intensity, in accordance with the setting of unit 15, is made effective by trigger or control unit 18 to determine the duration of energization of light source 10, through the agency ofa suitable holding and restorative circuit, generally designated 19 and hereinafter described in detail.

Now considering in greater detail the component units of the improved timer and the operating connections thereof, light source 10 may be a 1000 watt lamp of a conventional enlarger, energized by a volt A.C. line 20. This is of course under the control of trigger device 18 and associated holding circuit 19. Negative 11 is removably supported by the film holder of the enlarger, which projects an image of the negative onto phototube 12, either directly or by reflection.

Suitable optical composing, focusing and phototube exposing provisions may be associated with photoelectric probe unit 13, for example of the type illustrated and described in a copending application of Stanton H. Petry and Kenneth E. Grosse, Serial No. 423,960, filed April 19, 1954, now Patent 2,800,834, issued July 30, 1957, in which a projected image is reflected to the probe device from a work surface on which the photographic paper is supported. Alternatively, probe 13 may be energized directly by light transmitted from an image.

As indicated above, the probe is preferably a magnetically modulated one, phototube 12 being in the field of an electromagnet 22 whose coil 23 is supplied through leads 24 with a 6 volt, 60 cycle potential. A lead 25 connects the anode of tube 12 to the input grid of a narrow band or tuned alternating current amplifier 14, the cathode of the phototube being grounded in common with the rest of the cathode connections of the amplifier and other tubes as shown. Plus voltage of the amplifier, as of the remaining electronic tubes of the system, is through a lead 26 supplied by a B battery plus terminal.

Rectification of the output voltage of amplifier 14, and the reason therefor, have been referred to, the input terminal of adjustable rectifier unit 15 being supplied through a lead 27. The steady output of this unit is impressed on integrator 16 through lead 28 and selector voltage divider 17, by which it is adjusted as to value to compensate for any of several variable photographic factors at enlarger 10 and probe unit 13.

The input lead 27 to rectifier unit 15 is capacity coupled in an entirely conventional fashion to a standard amplifier tube 127 which has its plate voltage supplied through the primary coil of a transformer 128 connected to B voltage lead 26. This transformer further steps up the amplified A.C. output of tube 127 and applies it to the terminals of a standard bridge rectifier, generally designated 129. The result, as described above, is a high voltage D.C. output at lead 28 to voltage divider 17, the value of which voltage will reflect the intensity of radiant flux at the light source in a linear ratio.

If perfect results are to be attained in the printing of commercially available papers throughout the entire range of densties of any negative, it is highly desirable in printing dense negatives to compensate the linearity of performance of the circuit thus far described, for, as has been stated above, a greater exposure time is required in the range of low light flux impinging the printing paper than would be indicated by a truly linear reciprocity curve for the papers emulsion.

Accordingly, the invention provides a corrective D.C. biasing voltage of opposite polarity, derived from a potentiometer 130 connected between the B+ supply and ground, for application to rectifier 129. Potentiometer 130 is adjustably tapped, in the illustrated embodiment, to the cathode of the first tube of full wave rectifier 129, being so connected for the particular purpose of the tim ing system. In another application the rectifier connections may be reversed.

The biasing connection described above permits a setting of unit 15, at its potentiometer 130, in which its output voltage may be in linear or non-linear ratio to its input, in the latter case compensating for the variation introduced by paper failure and the like. Moreover, this is possible whether the disturbing phenomenon exists in the low or high range of light flux impinging the paper, or both. A non-linear relation of the rectifier input in respect to time, as determined by integrator 16, necessarily follows.

A further advantage of the adjustable biasing arrangement described, equally benefitting the illustrated adaptation as well as an adaptation in a different setting, resides in the fact that it aifords means to control and offset contact potential in the rectifier. This is particularly desirable in an instance in which the A.C. voltage to be rectified has been considerably amplified as it is applied to the input of the unit.

The amplified direct current voltage, inverted to negative polarity, is supplied to the input grid of the first tube 29 of integrator 16. This is a pentode whose operating voltages are derived from a voltage divider 30, and the anode of tube 29 is connected to a triode 31, with a degenerative feed back hookup from the cathode of the latter through a capacitor 32 to the input grid of tube 29. Thus the integrator circuit provides a rising voltage output, of time rate of rise linearly or non-linearly proportional to input signal in accordance with the setting of linearity control potentiometer 130, by reflecting a part of the former back to its input side.

There is also a grounded restorative or automatic recycling circuit associated with the grid of integrator tube 29 through leads 34, 35 to a part of holding unit 19, for the purpose of discharging capacitor 32 and returning the integrator to normal condition at the end of each timing cycle, particularly when automatic paper advance is employed. This involves a double pole, double throw relay in unit 19, schematically shown and generally designated 36. Relay 36 comprises a normally open contact arm 37 and a normally closed contact arm 38, to the terminals of which leads 34-, 35 are connected, both arms being under the control of relay coil 39.

Contact arm 37 and coil 39 of relay 36 are in series and are also wired, respectively, to one terminal of the 110 volt line 20 and to a terminal contactor 40 of a normally closed relay, generally designated 41, whose coil is series connected in the output circuit of a triode 42 of the control or trigger unit 18. The other contactor terminal is connected by lead 43 with the second terminal of line 20. The input grid of tube 42 is supplied through a connection to the cathode of second integrator tube 31, and plate voltage for the system is supplied from a B battery of tube 42.

Trigger unit 18 of the system may include a multiple button range switch panel 44 in the output circuit of tube 42. By this means cathode voltage may be adjusted as desired to accommodate the system once and for all upon initial installation to the timing of film exposure, for example as may be dictated by different vogues or regional tastes in difierent parts of the country.

The present system may be used optionally for printing alone, either repeated cycling or single exposure, or for preliminary composing without exposing the photographic paper, as in association with a probe device according to the application of Petry and Grosse, identified above. Accordingly, the circuit employs a normally open relay 45 whose coil may be connected between lead 43 and the coil 39 of relay 36 by means of a manually adjustable selector arm 46 of a print-compose switch. 47. The contactor terminal of relay 45 is series connected with the filament of enlarger lamp 1% in a linesupplied circuit which parallels that of holding unit 19. The contacts of switch 47 are connected to an instantaneous switch 48, both in shunt relation to the contactor arm 37 of relay 36, and switch 48 is the master actuating switch for the system for the timing cycle, as distinguished from the composing cycle.

In the operation of the improved timer system, let it first be assumed that it is desired to compose prior to printing, with the assistance of the device of the Petry et al. application. In such case the phototube 12 of probe unit 13 is shielded oil and the timer circuit is ineffective. Arm 46 of print-compose switch 47 is positioned in the dotted line position of Fig. l, and a circuit exists through relay 45 to close its contactor and complete a circuit through illuminate lamp 10 for composing.

When it is desired to time an exposure and printing cycle the selector arm of switch 47 is shifted to the solid line position, and phototube 12 is exposed to view an image projected by the enlarger lamp when the latpositive connection to the plate ter is lighted. Upon closing of instantaneous switch 48,

a circuit is completed from one side of line 20 through the closed contactor 40 of relay 4-1, through coil 39 of relay 36, and through switch 43 to the other side of the line. The coil of relay 45 is also energized to complete the lamp circuit, and energization of coil 39 does two things: it completes a holding circuit through its contact arm 37 and it opens its contact arm 38, thus opening the grounding circuit for the grid of integrator tube 29 through leads 34, 35.

Timing ensues for a period which ends when the input voltage at tube 42 has been brought by integrator 16 to a critical value sufiicient to cause that tube to pass current, whereupon relay 41 is energized to open the holding circuit at contactor 40. De-energization of coil 39 of relay 36 opens its contact arm 37 and closes its contact arm 38, thus grounding capacitor 32 and restoring integrator tube 29 to normal condition, so that there will be no recommencement of timing operation in the event an unintended signal enters the system. De-energization of relay 45 of course breaks the lamp circuit.

We claim:

1. A timing system comprising electrical probe and.

signal forwarding means having a photosensitive unit to originate and forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected, an integrator unit electrically connected to and supplied by said means, said integrator unit having a degenerative feed-back action to convert said signal to an output voltage of the integrator unit which rises at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, and a further unit to which said rising output voltage is applied, said further unit being actuated by said output voltage to initate a control signal after a predetermined time interval determined by the rate of rise of said voltage.

2. A timing system comprising a photosensitive unit to originate and forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is sub jected and provided with modulating means to impart an alternating current characteristic to said signal, an alternating current amplifier receiving the modulated signal and forwarding an amplified signal still proportionate in value to said radiant flux at said photosensitive unit, a rectifier converting said amplified signal to a steady one, an integrator unit supplied by said rectifier, said integrator unit having a degenerative feed-back action to convert said signal to an output voltage which rises at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, and a further unit to which said rising output voltage is applied, said further unit being actuated by said output voltage to initiate a control signal after a predetermined time interval determined by the rate of rise of said output voltage.

' 3. A timing system comprising a photosensitive unit to originate and forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected and provided with modulating means to impart an alternating current characteristic to said signal, an alternating current amplifier receiving the modulated signal and forwarding an amplified signal still proportionate in value to said radiant flux at said photosensitive unit, a rectifier converting said amplified signal to a steady one, an adjustable supply of voltage applied to said rectifier to optionally set its output in a linear or non-linear ratio to its input, an integrator unit supplied by said rectifier, said integrator unit having a degenerative feedback action to convert said signal to an output voltage which rises at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, and a further unit to which said rising output voltage is appiied, said further unit being actuated by said output voltage to initiate a control signal after a predetermined time interval determined by the rate of rise of said output voltage.

4. A timing system comprising an electrical circuit to energize a light source, a photosensitive unit to forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source, an integrator unit electrically connected to and supplied by said photosensitive unit, said integrator unit having a degenerative feed-back action to convert said signal to a voltage rising at a rate in direct linear ratio to the value of said signal as impressed on the integrator unit, a control unit to which said rising voltage is applied, said control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said applied voltage, and a control circuit operatively connecting said control unit with said first named electrical circuit to cause said last named signal to alter the circuit upon expiration of said time interval.

5. A timing system comprising an electrical circuit to energize a light source, a photosensitive unit to forward an alternating current electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source, a rectifier unit converting said alternating current signal to a steady voltage output said rectifier having means adjustably biasing the same to control the output thereof optionally as to its linearity or non-linearity in ratio to the rectifier input, an integrator unit electrically connected to and supplied by said rectifier unit, said integrator unit having a degenerative feedback action to convert said rectifier output voltage to a voltage rising at a rate in direct linear ratio to the value of said output voltage as impressed on the integrator unit, a control unit to which said rising voltage is applied, said control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said applied rectifier output voltage, and a control circuit operatively connecting 8 said control unit with said first named electrical circuit to cause said last named signal to alter the circuit upon expiration of said time interval.

6. A timing system comprising an electrical circuit to energize a light source, electrical probe and signal forwarding means having a photosensitive unit to forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source, said signal forwarding means including magnetic modulating means to impart an alternating current characteristic to said signal, an alternating current amplifier receiving the modulated signal and forwarding an amplified signal still proportionate in value to said radiant flux at said photosensitive unit, a rectifier converting said amplified signal to a steady one, an integrator unit electrically connected to and supplied by said rectifier, said integrator unit having a degenerative feed-back action to convert said signal to a voltage rising at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, a control unit to which said rising voltage is applied, said control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said voltage, and a control circuit operatively connecting said control unit with said first named electrical circuit to cause said last named signal to alter the circuit upon expiration of said time interval.

7. A timing system comprising an electrical circuit to energize a light source, electrical probe and signal forwarding means having a photosensitive unit to forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source, said signal forwarding means including magnetic modulating means to impart an alternating current characteristic to said signal, an alternating current amplifier receiving the modulated signal and forwarding an amplified signal still proportionate in value to said radiant flux at said photosensitive unit, a rectifier converting said amplified signal to a steady one, said rectifier having means adjustably biasing the same to control the output thereof optionally as to its linearity or non-linearity in ratio to the rectifier input, an integrator unit electrically connected to and supplied by said rectifier, said integrator unit having a degenerative feed-back action to convert said signal to a voltage rising at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, a control unit to which said rising voltage is applied, said control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said voltage. and a control circuit operatively connecting said control unit with said first named electrical circuit to cause said last named signal to alter the circuit upon expiration of said time interval.

8. A timing system comprising an electrical circuit to energize a light source, electrical probe and signal forwarding means having a photosensitive unit to forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source and provided with modulating means to impart an alternating current characteristic to said signal, an alternating current amplifier receiving the modulated signal and forwarding an amplified signal still proportionate in value to said radiant flux at said photosensitive unit, a rectifier converting said amplified signal to a steady one, an integrator unit supplied by said rectifier, said integrator unit having elements connected in a degenerative feed-back hookup and acting to connect said signal to a voltage rising at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, a control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said voltage, and a control circuit operatively connecting said control unit with said first named electrical circuit 9 to cause said last named signal to alter the circuit upon expiration of said time interval.

9. A timing system comprising an electrical circuit to energize a light source, a photosensitive unit to forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source, an integrator unit supplied by said means, said integrator unit having elements connected in a degenerative feedback hookup and acting to convert said signal to a voltage rising at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, a. control unit to which said rising voltage is applied, said control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said voltage, and a control circuit operatively connecting said control unit with said first named electrical circuit to alter the same upon expiration of said time interval.

10. A timing system comprising electrical probe and signal forwarding means having a photosensitive unit to originate and forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected, an integrator unit supplied by said means, said integrator unit having elements connected in a degenerative feed-back hookup and acting to convert said signal to a voltage which rises at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, and a control unit to which said rising voltage is applied, said control unit being actuated by said rising voltage to furnish a signal after a predetermined time interval determined by the rate of rise of said voltage.

11. A timing system comprising an electrical circuit to energize a light source, electrical probe and signal forwarding means having a photosensitive unit to forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source, an integrator unit having its input side supplied by said means and acting to convert said signal to an output voltage rising at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, a control unit to which said rising output voltage is applied, said control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said voltage, and a control and holding circuit operatively connecting said control unit with said first named electrical circuit to cause said last named signal to alter the circuit after expiration of said time interval, said control and holding circuit including means periodically operated in response to operation of said control unit to change the potential at the input side of said integrator unit.

12. A timing system comprising an electrical circuit to energize a light source, electrical probe and signal forwarding means having a photosensitive unit to forward an electrical signal proportionate in intensity to that of a radiant flux to which the unit is subjected by said source, an integrator unit having its input side supplied by said means having thermionic tubes connected by a capacitor connected between an input electrode of one tube and an output electrode of a succeeding tube and acting to convert said signal to an output voltage rising at a rate in direct linear ratio to the value of the signal as impressed on the integrator unit, a control unit to which said rising output voltage is applied, said control unit being actuated by said rising voltage to transmit a signal after a predetermined time interval determined by the rate of rise of said applied voltage, and a control and holding circuit operatively connecting said control unit with said first named electrical circuit to cause said last named signal to alter the circuit after expiration of said time interval, said control and holding circuit including means periodically operated in response to operation of said control unit to change the potential at the input electrode of said first integrator tube by discharge of said capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,484,299 Labrum Oct. 11, 1949 2,539,100 Rado Jan. 23, 1951 2,612,630 Greenleaf Sept. 30, 1952 2,654,858 Feller Oct. 6, 1953

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