Dec- 24, 1946- s. c. CORONITI PHOTOGRAPHIC APPARATUS Filed July 28, 1943 2,413,218 2 Sheets-Sheet 2 56a (6 41>". 47' Jm ‘a 6I C m@ a m Vm BY ATTORNEY Patented Dec. 24, 1946 2,413,218 UNITED STATES PATENT OFFICE‘ 2,413,218 PHOTOGRAPHIC APPARATUS Samuel C. Coroniti, Cambridge, Mass., assignor to General Aniline & Film Corporation, New York, N. Y., a’corporation of Delaware Application July 28, 1943, Serial N0. 496,383 16 Claims. 1 This invention relates to photographic appa ratus, and more particularly ,to electronic means for controlling the exposure of a ?lm as a func tion of the optical density of the ?lm. In a continuous process for treating motion picture ?lm, the latent image, produced on a ?lm exposed in a camera, is developed into a silver image, producing a negative transparency. In a continuous process for ?nishing motion picture (01. 95-75) 2 Fig. 3 is a simpli?ed electrical diagram of the fundamental components of the control circuit. Fig. 4 is a set of curves illustrating voltage rela tions of the circuit of Fig. 3. Fig. 5 is a vector diagram of the current and voltages of the circuit of Fig. 3. Generally speaking, in the present invention a ?lm which has been exposed in a camera and then developed, and bleached, is moved longi ?lm, the latent image produced by exposure of} 10 tudinally past an element e?ective to measure » the ?lm in a camera, is ?rst subjected to devel the optical density of the ?lm. After passing opment such as would normally produce a nega this element, the ?lm is moved past a variable intensity light source. The optical density meas the ?lm is bleached, that is, is so treated as to uring element, through electronic means, con remove the metallic silver formed by the ?rst 16 trols the intensity of the variable intensityllght development. It is then reexposedthereby ren source. A constant intensity light source may dering the remaining silver bromide developable. be provided to give a predetermined basic ex That. positive latent image is developed, ?xed posure to the ?lm which is additive to the ex-‘ and washed. Thus, by so-called reversal devel posure given by the variable intensity light opment, a positive image is formed on the orig 20 source. The provision of the constant intensity tive transparency. Without ?xing that image, inal ?lm. . 'It is among the objects of this invention to provide an improved photographic apparatus for the continuous treatment of motion picture ?lm; to provide photographic apparatus for control ling a second exposure of a motion picture ?lm as a function of the optical density of the ?lm light source is not a necessary feature of the vpresent invention. Such constant intensity light may be provided by arranging the variable in tensity light source to have a minimum or basic 85 intensity at all times, or may be provided by a separate light source. Means are provided for delaying the effect of the measuring element on the variable intensity light source for a sumcient period of time to permit the ?lm to travel from 30 a point adjacent the measuring element to a after its ?rst development; to provide apparatus for controlling the secondexposure of a motion picture ?lm including an element for measuring the optical density of the ?lm and electronic point adjacent the variable intensity light source. means cperatively associated with the element The light measuring element controls the in for varying the intensity of such second exposure .tensity of the ?lm exposure through a novel elec as a function of the optical density of the ?lm; tronic control circuit. to provide photographic apparatus including an 35 Referring to Fig. 1 of the drawings, the photo element for measuring the optical density of film graphic apparatus includes a light proof housing passed thereover, electronic means controlled by Ill subdivided by partitions HI and 52 into com the element for varying the intensity of the sec partments iii, ill and it. An intermediatewall ond exposure of the ?lm and means for delaying it of the housing iii is formed with three aper the e?ect of the measuring element on the elec 40 tures l'l, it and it, each disposed centrally of tronic means for a period of time sufficient to one of the compartments it, ill and 55. Housing permit the ?lm to travel from the measuring it is formed with an extension 26 opposite the . element to the point where it receives its second compartment i3 and with an outer wall 22 spaced exposure; and to provide an improved electronic from the wall iii. Walls it and 22 form a tunnel 45 control circuit. through which passes the ?lm 2t which is to be These and other objects, advantages and fea given a second exposure. tures of the invention will be apparent from the A relatively small constant voltage lamp 25 following description and accompanying draw is mounted in housing l3. The light from lamp ‘ ings. In the drawings: 25 is directed by a mirror 23 through aperture Fig. 1 is a vertical sectional view through one 50 ii upon a photoelectric cell 30 mounted in exten form of apparatus embodying the principles of the invention. . Fig. 2 is a diagrammatic representation of the apparatus shown in Fig. 1, and a control circuit associated therewith. sion 2!. The particular photoelectric cell illus trated is responsive to farred and infrared rays, and has a substantially linear response to such . rays. A red light ?lter 213 is mounted in brackets 55 26 adjacent aperture ill and a matte base it is 2,413,218 inserted in aperture H. A variable voltage lamp I to conductors 31. A substantially constant direct 35 is mounted in compartment/l4 and is adapted current potential for the control circuit is sup- » to give a variable intensity exposure to ?lm 28 plied by any suitable source of direct current. » through aperture is. If desired a constant volt-. agelamp to may be mounted in compartment While a battery 50 has been indicated, it will be understood that the direct current energy may be obtained from alternating current source 88 through a suitable recti?er, with the use of ?lters and‘ a voltage regulator. Positive terminal ‘5i of direct current sourcetd is connected in series with a voltage divider comprising ?xed re sistances 52 and 53. From the Junction 55 oi these resistances, a voltage is applied to anode 55 of photoelectric cell 30 through a resistor 58, E5 to give a constant intensity exposure to ?lm 20 through aperture IS. The control of these lamps will be described more fully hereinafter. The ?lm 28 travels in the direction of the arrow shown .in Fig. 1. The ?lm ?rst passes over a roller 28 and then between walls It and 22. At the opposite end of the housing, ‘?lm 2B is threaded over a second roller 3|. The ?lm is maintained in constant spaced relation with re spect to light 25 and photoelectric cell 30 by means of a guide bracket 32 which presses the and a conductor 57. ' Cathode 58 of photoelectric cell 38 is connected by conductor iii to control grid 52 of a thermionic ?lm\against spaced rollers 38, 34 as the ?lm ampli?er tube 60. For a purpose to be described, passes aperture i1. a condenser 63 and a resistance 85 are connected ' ‘ The operation of the apparatus and the con to conductor Si in parallel with each other. The trol mechanism thereof, will be more apparent 20 opposite ends of condenser 63 and resistor $13 from a consideration of Fig. 2. In this ?gure, are connected by a conductor 55 to ground. A the elements shown in Fig. 1 have been given the suitable bias voltage for grid 62 of tube 58 isv same reference characters. Before passing derived from the adjustable terminal 65 of re through the apparatus of the invention, ?lm 20 sistor or potentiometer 51, connected by a con will have been processed to a permanent or to ductor 68 to positive terminal 5i of direct current a bleached image before reaching the roll 28. source 50, through conductor ‘i8. . It then passes in front of aperture H at which The anode-cathode circuit of the ampli?er tube point light rays from lamp 25 pass through ?lter 58 is connected to the grid circuit of a grid con 2d and base 21 on to photoelectric cell 38. trolled gaseous space discharge tube ‘i?, such as Through the control circuit shown in Fig. 2, the 30 a thyratron tube. In a manner to be described, operation of which will be described more fully. ampli?er tube 6!] is e?ective in varying the phase photoelectric cell 30 controls the intensity of relation of the voltages applied to the grid and illumination of variable intensity lamp 35. Thus, plate, respectively, of discharge tube 15. A con as ?lm 20 passes aperture IE, it receives an ex ductor ‘I3 connects cathode ‘H of tube 60 to a posure from lamp 35 which is a function of the junction point 74. One ‘ terminal 16 of the optical density of the ?lm as measured by photo secondary ‘winding ‘ll of transformer 86 is con electric cell 38. Generally, the intensity of the nected to junction point 14 through a ?xed con second exposure given by lamp 35 should be an denser 88. Grid electrode 8| of tube 15 is con inverse logarithmic function of the optical nected to junction point 14 through a grid cur density of the ?lm as measured by photoelectric 40 rent limiting resistor 82. The anode or plate 83 cell 30 for reasons apparent to those skilled in of thermionic ampli?er 60 is connected to the the art. The control circuit includes means for opposite terminal 84 of secondary winding Tl delaying the response of lamp 35 to the measure through a current limiting resistance 86. ments of photoelectric cell 30 for a period of time The cathode 81 of thyratron tube 15 is con su?icient for a given point on the ?lm to move nected to the mid point 88 of secondary winding from aperture ll to aperture l8. Ti. Plate 90 of tube 15 is connected by a con After passing aperture i8, ‘?lm 20 may be ductor 91 to one terminal of lamp 35. The op given a constant predetermined basic exposure posite terminal of lamp 35 is connected through at aperture i9 from constant intensity lamp 80, if desired. However, the lamp $0 is not necessary 50 conductor 92, ?xed resistance 93 and fuse M to terminal 85 of secondary winding ‘H. For a pur to the practice of the present invention. The pose to be described, the screen grid electrode ?lm then passes over roller 3!, after which it is 95 of ampli?er tube 60 is connected through a again developed, ?xed and washed to form a limiting resistor 96 to plate 83, and a fixed re positive image on the ?lm. Subsequent to such sistor 9'! is connected across the output of ampli treatment, the ?lm may be examined in any ?er 6G. well known manner and is then ready for use in The operation of the circuit illustrated in Fig. a projector. 2 is effective to vary the illumination of lamp 35 ~ Lamps 25, 35 and (ill are energized from a in accordance with the amount of light falling suitable source of alternating current 36 which is connected to conductors 31 through a switch (ll) on photoelectric cell 30 from lamp 25 as ?lm 20 passes~aperture I1. It will be noted that the 38 and a fuse 4|. Lamp 25 is operated at a voltage applied to grid electrode 8| of discharge lesser voltage than lamps 35 and 40. It is there tube ‘I5 is derived from junction point ‘I4. fore connected to the secondary winding 42 of a Junction point ‘H5 is the connecting point be step-down transformer 43 connected to conduc tween ampli?er tube 80 and condenser 80, which tors 37. The energization of variable intensity are thus connected in series with the secondary lamp 35 is controlled by the electronic control winding ‘ll of transformer 46. The internal re circuit as will be presently described. Constant sistance of ampli?er tube 80 is dependent upon intensity lamp an is connected across conductors the voltage applied to its grid electrode 62. Thus, 31 and has a variable resistor 44 in series there with condenser 80 being relatively ?xed, varia with, so that the predetermined basic intensity tions in the voltage applied. to grid 62 will vary of lamp 48 may be selected to give a desired the internal resistance of ampli?er tube 80. basic exposure to ?lm 20. Alternating current'energy for the control cir This, in turn, will vary the phase of the voltage applied to grid 8| with respect to the voltage cuit is derived from a transformer 46 having a primary winding 4'! connected by conductors 48 75 applied to plate 98 of thyratron 15. The plate 5 2,418,218 voltage is derived from the same alternating cur rentreference source as is the grid voltage. The voltage across secondary winding 11 is the vector sum of the voltages across ampli?er tube 60 and condenser 80. The input alternating volt age between the grid and the cathode of thyratron ‘I5 is constant in magnitude as will be made apparent hereinafter. However, the phase rela tion of the grid to cathode voltage with respect to the output voltage of tube 15 is controlled by the condenser 80 and ampli?er tube'iill. By varying the output of ampli?er 60, the phase relation of 6v . plished by a time delay circuit including con denser B3 and resistance 64 which are connected in parallel with the control grid 82 of ampli?er tube 80 and photoelectric cell 30. By proper selection of the relative sizes of condenser 63 and resistance 64, the proper time delay in the ‘ impression of the control signal from photoelec tric cell 30 on the control grid 62 of tube 60 is obtained. . Figs. 3, 4 and 5 diagrammatically illustrate the general principles underlying the electronic con trol action of the present invention. The con trol circuit in a simpli?ed form is illustrated in . Fig.3. For reference, the points of differing po is changed. The thyratron 15 is thus made to ?re . tential, such as the terminals of winding 11 of at a point during each cycle of applied plate volt ‘ transformer 46, junction point 14 and mid-point age when the instantaneous value of such input or 88 of winding 11 have been also designated with grid voltage exceeds the critical ?ring value. The the letters a, b, c and d, respectively. Variable ?ring oftube 15 is thus made dependent on the intensity lamp 35 is represented by resistance phase relation between its applied grid cathode, 20 Rp and the output eifect of ampli?er 60 by varia or input voltage, and its applied plate to cathode, ble resistance Rs. Also, the potential di?erences or output voltage. between the respective points have been desig- , The internal resistance of tube 60 is a function nated Eda, Ebd, Ede and Eac. ' of the amount of light reaching photoelectric cell In Fig. 4 are curves illustrating the operation 30. As the amount of light falling on cell 30 in 25 of the circuit through two cycles. The grid volt creases, for instance, the current ?ow through age Ede is represented by the broken line curve grid resistor 56 increases. This increases the G. The output voltage of tube 15 is represented voltage drop across resistor 56, and thus varies by the solid line curve P. In the instant illustra the voltage applied to grid electrode 62 of am tion, the grid voltage G lags the plate or output pli?er tube 60. The internal resistance of tube voltage P by a electrical degrees. The grid voltage 60 is thus made dependent on the amount of G m'ust obtain a certain critical value k‘before tube light reaching photoelectric cell 30. As men ‘l5 can become conductive or ?re. As shown in tioned above, the output of tube 60, or corre Fig. 4, the plate voltage obtained the positive po spondingly its effective resistance as applied to tential If at the time the grid attains its critical po the input circuit of tube 15, varies the relative tential k and tube 15 starts to ?re. Tube 75 con phase relation between the input and output volt tinues to ?re for the rest of the positive half cycle, ages of thyratron tube 75. This in turn deter— to point m. Tube 15 remains non-conducting until mines during what portion of each cycle of ap its plate voltage P reaches the value Z’f’, when plied plate voltage tube 15 will be conductive. the grid voltage G again reaches the magnitudek. As lamp 35 is in series with the output of tube 15, 40 The tube 75 will again become conductive and re its average intensity of illumination is accord main conductive for the remaining half cycle to ingly controlled as a function of the eiiective point m’. resistance of ampli?er tube 60. This action is The conducting time Zn of, tube 15 may be made . described in detail hereinafter with reference to ‘smaller or larger by varying the electrical phase Figs. 3, 4 and 5. displacement ¢ of the grid voltage G with respect By selection of an ampli?er tube 60 with suit to the plate voltage P. In Fig. 4, this would be ’ able characteristics, and by design of the param represented by shifting curve G to the right or eters of the circuits of tube 60, the response of the left with respect to curve P. The maximum lamp 35 to the control exerted by photoelectric time for conduction of tube 15 is practically for cell 30 may be made to bear any desired relation. 50 a complete half cycle or about 180". Thus, by As stated above, in the present instance, the in varying the phasedi?‘erence between the plate tensity of illumination of lamp 35 is designed to ’ and grid voltages of tube 15, the amount of time vary inversely, and as a logarithmic function of during which tube 15 is conductive is correspond the optical density of ?lm 20. ingly varied. ' The variation of the parameters of the circuits Fig. 5 is a vector diagram of the relation of of ampli?er 60 may be accomplished in any de phase angle (1) and the grid and plate voltages sired manner. In one practical embodiment of Ede and Eda. The current I ?owing through the the invention, a pentode of the type “6F6” has resistance R; and condenser 86 here leads the been used satisfactorily as ampli?er tube Kill. The voltage Eba across transformer winding ‘H by an design of the parametersof the circuits of tube 50 60 angle a. The voltage drop Eat, across resistance is improved by the use of the screen grid resistor RE is in phase with current I. The voltage Ecb 96 and shunt resistor 9? connected across the out across condenser Bil lags 90° with respect to cur put of tube Eli. In the described embodiment, rent I. The vector sum of the voltages Esc'and with the use of a red light ?lter 24, a red light Ecb is equal to the total voltage Eba of the sec responsive photoelectric, cell 30 having a linear 65 ondary ll of transformer Mi. This relation holds response, and a pentode for ampli?er til, the cir for all conditions. The vectors Bob and E210 make cuit parameters necessary to produce proper cOn a right angle with each other for all values of trol of lamp 35 are e?ectively obtained in practice. the angle oz. Hence the locus of point e for all One other criterion is taken into consideration values of on from zero to 90° will be a semi-circle in the operation of the illustrated apparatus. It with ?xed vector Eba as diameter. takes a small interval of time for a given point The voltage between grid 8i and the cathode on ?lm 2m to move from aperture ll to adjacent ill of tube 15 is represented by the vector Ede aperture E8. The imposition of the control sig which is a radius of such circle. Since the volt nal from photoelectric cell 30 on the control cir ages Ebd and Eda are one-half of Eba,‘ the magni cuit is delayed for this interval. This is accom 75 tude of vector Ede is equal to that of Eta and the input or grid voltage of discharge tube 15 with respect to the output or plate voltage thereof 2,413,218 8 cell; means for directing an image bearing photo Eda. Vector Ede is thus constant in magnitude for all such values of 0:. graphic ?lm, which has been exposed, partially Movement of vector Ede in either a clockwise or a counterclockwise direction will vary the relative magnitudes of the vectors Ebc and Eca, and likewise, the phase angle ¢ between the plate voltage Eda and grid voltage Ede. It will be apparent that either the voltage . developed, and bleached, successively past said photoelectric cell and said variable intensity light source; a light system for directing a beam of light through'said ?lm upon said photoelectric across the condenser 80 or that across the re cell; a grid-‘controlled discharge device control ling the energization of said variable intensity sistance R; can be varied from Zero to the re light source; a reference source of alternating spective full v2 ues' of the transformer secondary 10 current; circuit means for applying a voltage from said source to the plate of said‘ device; voltage Em; and the phase angle ¢ thus varied means, including a- capacitance and grid-con from 0° to 180°. ‘ trolled electronic means in parallel circuit rela Referring again to Fig. 3, either resistance B; tion with said capacitance, for applying a volt may be held constant and condenser 80 varied, or vice versa, to vary the, phase relation es of the 15 age from said source to the grid of said device; circuit means connecting the control grid of said grid or ‘input voltage of tube 15 with respect to electronic means to said photoelectric cell; and the plate or output voltage thereof. As pointed means for delaying the action of said photo out in connection with Fig. 4, this correspond electric cell on said electronic means in-accord ingly varies the time interval during which tube 15 is conductive. In practice, ‘either resistance 20 ance with the spacing between said photoelectric cell and said light source. 3. Apparatus'for use in a photographic process comprising a photoelectric cell; a variable inten sity light source spaced from said photoelectric 80 is held constant, and resistance Hg is elec tronically varied by ampli?er tube 60. In turn, .25 cell; means for directing an image bearing photo Hg or condenser 80, or both, may be varied man ually, mechanically, electronically or automati cally. In the illustrated embodiment condenser tube 603 is herein automatically controlled by photoelectric cell 38. Variations in the conductivity or internal re sistance of ampli?er tube 60 as controlled by the graphic ?lm, which has been exposed, partially developed, and bleached, successively past said photoelectric cell and said variable intensity light source; a light system for directing a beam of amount of light reaching photoelectric cell 30, 30 light through said ?lm upon said photoelectric cell; a grid-controlled discharge device control will vary the ‘phase relationship a of the voltage ling the energization of said variable intensity applied to grid ill of tube 15 with respect to the light source; a reference source of alternating voltage applied to plate 99 thereof.v current; circuit means for applying a voltage The control circuit including the time delay from said source to the plate of said device; means, including a capacitance and grid-con trolled electronic means in parallel circuit rela_ an, and the load comprising lamp 35, is of gen tion with said capacitance, for applying a voltage eral application. That is, it may be used other from said source to the grid of said device; cir wise than in the speci?c case illustrated in the drawings. Likewise, the control of photoelectric 40 cuit means connecting the control grid of said electronic means to said photoelectric cell; and cell 30 on variable intensity lamp 35 can be exer means, including a resistance and a capacitance cised through a control circuit different from that connected in parallel circuit relation with each illustrated, through the use of suitable equivalent circuit comprising condenser 63 and resistance . B4, ampli?er tube 63, discharge tube 15, condenser electronic means. . other and with said photoelectric cell and the grid It should therefore be understood that while a 4. of said electronic means, for delaying the action of said photoelectric cell on said electronic means speci?c embodiment of the invention has been for the time interval necessary for a reference shown and described, to illustrate how the princi ples of the invention may be applied, the inven tion is not limited thereto, but may be otherwise embodied without departing from the principles thereof. I claim: ' 7 1. Apparatus for use in a photographic process comprising a photoelectric cell; a variable in tensity light source spaced from said photoelec tric cell; means for directing an image bearing photographic ?lm, which has been exposed, par tially developed, and bleached, successively past said photoelectric cell'and said variable intensity point on said ?lm to travel from a point opposite said photoelectric cell to a point opposite said variable intensity light source. '4. In a device for photographic printing;.means for scanning a ?lm including a light source and a photo-responsive element, a variable source of _ radiation for effecting an exposure and means for progressing ?lm past the said scanning means and source of radiation, and means'for varying the intensity of the source of radiation as a function of the amount of light-falling on the photo-responsive element which includes in cir light source; a light system for directing a beam 60 cuit with said element, a discharge tube the out put of which is in series with said variable source of light through said ?lm upon said photoelectric of radiation, a current supply for said tube, a cell; a grid-controlled discharge device control ling the energization of said variable intensity light source; a reference source of alternating current; circuit means for applying a voltage from said source to the plate of said device; means, including a capacitance and grid-con capacitance in series with the voltage input of said discharge tube, and an electronically variable resistor effective for changing the phase angle between the input and output voltages of said discharge tube. ' 5. In a device for photographic printing, means for scanning a ?lm including a light source and tion with said capacitance, for applying a voltage from said source to the grid of said device; and 70 a photo-responsive element, a variable source of radiation for effecting an exposure and means circuit means connecting the control grid of said for progressing ?lm past the said ‘scanning means electronic means to said photoelectric cell. and source of radiation, and means for varying 2. Apparatus for use in a photographic process the intensity of the source of radiation as a func comprising a photoelectric cell; a variable inten sity light source spaced from said photoelectric 75 tion of the amount of light falling on the photo , trolled electronic means in parallel circuit rela 2,418,218 9 responsive element which includes in circuitwith said element, a discharge tube the output of ' which is in series with said variable source of radiation, a current supply for said tube, a capaci tance in series with the voltage input of said discharge tube, and a resistor comprising a therm- ' ionic ampli?er e?ective for changing the phase angle between the input and output voltages of ' said discharge tube. - 10 ' tion of the amount oi light falling on the photo responsive element which includes in circuit with said element, a discharge tube, a current supply for said tube, means for varying the output of said tube including a capacitance and a thermi ' onic ampli?er, and other means forming a part of said circuit and including a capacitance and a resistance in parallel with said circuit by which > the effect of changes in output current from said 6. In a device for photographic printing, means '10 photo-responsive element upon said thermionic for scanning a ?lm including a light source and. a photo-responsive element, a variable source of ampli?er and on the output of said discharge tube is delayed so that said changes shall become ef fective only after a reference point on the ?lm has'moved from the scanning point to the vari radiation for e?ectlng an exposure and means for progressing ?lm past the said scanning means and source of radiation, and meansfor varying 15 able source of radiation. the intensity of the source of radiation as a func tion 01 the amount of light falling on the photo . 10. In a ‘ device for photographic printing, means for scanning a ?lm including a light source and a photo-responsive element, a variable source of radiation for e?'ecting an exposure and means responsive element which includes'in circuit with said element, a discharge tube the output of which is in series with said variable source of 20 for progressing ?lm past the said scanning means radiation, a current supply for said tube, a ca and source of radiation, and means for varying pacitance in series with the voltage input of said the intensity of the source of radiation as a 'func_ discharge tube, and a resistor comprising a ther-, tion of the amount of light falling on the photo mionic ampli?er effective for changing the phase responsive' element which includes in circuit with angle between the input and output voltages of 25 said element, a discharge tube having a control said discharge tube, said thermionic ampli?er grid, a current supply for said tube, electronical having a control grid, said grid having its poten ly functioning means by which the output of said tial varied in accordance with changes in current tube is varied, saidmeans including a capacitance ?owing from the cathode of said photo-respon ' and a thermionic ampli?er tube having a control sive element. _ 30 grid and plate, the output of said ampli?er tube 7. In a device for photographic printing, means at said plate being connected with both the cur for scanning a ?lm including a light source and rent supply and the variable source of radiation, a photo-responsive element, a variable source of the control grid of said tube being connected to radiation for effecting an exposure and means the cathode of the photo-responsive element, the for progressing ?lm past the said scanning means 35 circuit through said variable source of radiation and source of radiation, and means for varying " being completed by a connection to the anode of the intensity of the source of radiation as a func the discharge tube. tion of the amount of light falling on the photo 11. In a device for photographic printing, responsive element which includes in circuit with means for scanning a ?lm including a light source said element, a discharge tube the output of 40 and a photo-responsive element, a variable source which is in series with said variable source of radiation, a current supply for said tube, a con of radiation for effecting an exposure and means ‘for progressing ?lm past the said scanning means and source of radiation, and means for varying trol grid therefor, and a thermionic ampli?er connected to the cathode of the photo-respon the intensity of the source of radiation as a func sive element and by which the phase relationship 45 tion of the amount oi‘ light falling on the photo between the output Voltage of said discharge tube responsive element which includes in circuit with and the grid voltage supplied thereto may be said element, a discharge tube having a control varied. grid and an anode, a current supply for said tube, 8. In a device for photographic printing, means an electronically functioning means by which the for scanning a ?lm including a light source and 50 output of said tube is varied and by which the a photo-responsive element, a variable source of effect of changes initiated at the photo-respon radiation for effecting an exposure and means for sive element is delayed, including a capacitance progressing ?lm past the said scanning means between the current supply and the control grid ' and source of radiation, and means for varying of the discharge tube, a thermionic ampli?er the intensity of the source of radiation as a func 65 tube having a control grid and a plate, means tion of the amount of light falling on the photo connecting said control grid to the cathode of responsive element which includes in circuit with said photo-responsive element, means connect said element, a discharge tube, a current supply ing the plate of said tube to the variable source for said tube, and a thermionic ampli?er con of radiation and the current supply, said con nected to the cathode of the photo-responsive 60 nection to the source of radiation having its cir element and by which the output of said tube is cuit completed through to the anode of the dis varied, and other means forming a part of said charge tube, and a capacitance and resistance circuit for delaying the e?ect of changes in out connected in parallel to the circuit between the put‘ current fromthe photo-responsive element cathode of the photo-responsive element and the upon the output of the discharge tube so that 65 control grid of the thermionic tube. said changes shall become eiiective only after a 12. Mechanism as de?ned in claim 11 further reference point on the ?lm has moved from the characterized by an additional and normally in scanning point to the variable source of radiation. variable source of radiation. _ 9. In a device for photographic printing, means 13, Mechanism as de?ned in claim 11 further for scanning a ?lm including a light source and a 70 characterized by an addition Within the circuit photo-responsive element, a variable source of ra-' - which includes a battery, a connection there diation for effecting an exposure and means for from through a resistance to the'anode of said progressing ?lm past the said scanning means photo-responsive element, a voltage divider con and source of radiation, and means for varying nected to a ground line from that part of the the intensity of the source of radiation as afunc 75 circuit formed by the capacitance and resistance 1 2,413,218 ii in parallel and by which the effect of changes initiated at the photo-responsive element are de layed. 1 l4. Mechanism as de?ned in claim 11 in which the thermionic ampli?er tube has a screen grid and a connection leading therefrom through a resistance to the output of the plate of said tube. 15. Mechanism as de?ned in claim 11_ in which 12 current supplied to the control grid of the dis charge tube is limited by a resistor. 16. Mechanism as de?ned in claim 11 in which a biasing voltage for the control grid of the thermionic ampli?er tube is supplied through a variable resistor. SéMUEL C. CORONITI.