EXPOSURE OVERRIDE CONTROL
United States Patent 3828226
Apparatus for use with continuous tone and halftone photographic processes which modifies the various timing cycles of the photographic lamps to compensate for variations of the chemical activity level, emulsion speed of the film utilized and temperature shifts of the chemistry. The apparatus as disclosed is logarithmically variable so that if control measurements of system density indicate a shift in operating conditions the error can be directly set on the apparatus. Manipulation of the apparatus shifts the set point of all timing cycles and thus enables the operator with one setting to proportionately vary the timing cycle for each photographic lamp used in the photographic process.
US Patent References:
Method for controlling explosure in a photographic color printer
Maddock et al. - December 1966 - 3293033
/3672767.html
Pamlenyi - June 1972 - 3672767
LOGARITHMIC CIRCUIT WITH AUTOMATIC COMPENSATION FOR VARIATIONS IN CONDITIONS OF OPERATIONS
Dreyfoos, Jr. - September 1973 - 3724954
Method for controlling explosure in a photographic color printer
Maddock et al. - December 1966 - 3293033
/3672767.html
Pamlenyi - June 1972 - 3672767
LOGARITHMIC CIRCUIT WITH AUTOMATIC COMPENSATION FOR VARIATIONS IN CONDITIONS OF OPERATIONS
Dreyfoos, Jr. - September 1973 - 3724954
Application Number:
05/316672
Publication Date:
08/06/1974
Filing Date:
12/20/1972
View Patent Images:
Export Citation:
Assignee:
Chesley F. Carlson, Co. (Minneapolis, MN)
Primary Class:
Other Classes:
355/38
International Classes:
G03B27/72; H01H47/24
Field of Search:
317/141,124 355/38,68 250/206,214P
Primary Examiner:
Miller J. D.
Assistant Examiner:
Moose Jr., Harry E.
Attorney, Agent or Firm:
Dorsey, Marquart, Windhorst, West & Halladay
Claims:
I claim as my invention
1. In a photographic process timer having initially calibrated means for controlling the illumination time of exposure lamps utilized in photographic processes, said means being calibrated according to the operating conditions at the time of calibration the improvement comprising
2. The improvement of claim 1 wherein said exposure override control apparatus comprises means for logarithmically varying the illumination time duration for each timing cycle in the same percentage.
3. The improvement of claim 2 where said means comprises a resistance network logarithmically variable whereby density measurements of finished copy can be set on said impedance network.
4. The improvement of claim 3 wherein said exposure override control apparatus is calibrated in units of density.
5. The improvement of claim 3 wherein said resistance network comprises resistors connected in series.
6. The improvement of claim 5 wherein the value of each resistor is determined according to the formula Rc = Rt (Ec /E0) wherein Rc is the control resistance to ground, Rt is the total resistance of the serially connected resistance network, E0 is the total voltage across the resistance network and Ec is the control voltage calculated from the antilogarithm of the ratio of the total voltage across the impedance network ratioed to the effective voltage to be obtained, the effective voltage being the difference between the total voltage and the control voltage.
7. In a photographic process timer having a plurality of time unit resistances and at least one comparison means, the time unit resistances being linearly variable and utilized to determine the duration of energization of exposure lamps used in photographic processes and the comparison means being utilized to determine when the time duration controlled by the time unit resistances has been achieved, the improvement comprising
8. The improvement of claim 7 wherein said exposure override control apparatus comprises means for logarithmically varying the illumination time duration for each timing cycle in the same percentage.
9. The improvement of claim 8 wherein said means comprises a resistance network logarithmically variable.
10. The improvement of claim 7 wherein said override control apparatus is selectively connected in parallel with the time unit resistances.
11. The improvement of claim 9 wherein said resistance network comprises resistors, serially connected.
1. In a photographic process timer having initially calibrated means for controlling the illumination time of exposure lamps utilized in photographic processes, said means being calibrated according to the operating conditions at the time of calibration the improvement comprising
2. The improvement of claim 1 wherein said exposure override control apparatus comprises means for logarithmically varying the illumination time duration for each timing cycle in the same percentage.
3. The improvement of claim 2 where said means comprises a resistance network logarithmically variable whereby density measurements of finished copy can be set on said impedance network.
4. The improvement of claim 3 wherein said exposure override control apparatus is calibrated in units of density.
5. The improvement of claim 3 wherein said resistance network comprises resistors connected in series.
6. The improvement of claim 5 wherein the value of each resistor is determined according to the formula Rc = Rt (Ec /E0) wherein Rc is the control resistance to ground, Rt is the total resistance of the serially connected resistance network, E0 is the total voltage across the resistance network and Ec is the control voltage calculated from the antilogarithm of the ratio of the total voltage across the impedance network ratioed to the effective voltage to be obtained, the effective voltage being the difference between the total voltage and the control voltage.
7. In a photographic process timer having a plurality of time unit resistances and at least one comparison means, the time unit resistances being linearly variable and utilized to determine the duration of energization of exposure lamps used in photographic processes and the comparison means being utilized to determine when the time duration controlled by the time unit resistances has been achieved, the improvement comprising
8. The improvement of claim 7 wherein said exposure override control apparatus comprises means for logarithmically varying the illumination time duration for each timing cycle in the same percentage.
9. The improvement of claim 8 wherein said means comprises a resistance network logarithmically variable.
10. The improvement of claim 7 wherein said override control apparatus is selectively connected in parallel with the time unit resistances.
11. The improvement of claim 9 wherein said resistance network comprises resistors, serially connected.
Description:
BACKGROUND OF THE INVENTION
This invention is concerned with circuitry for particularly controlling the main, highlight and flash exposure times of the lamps which are conventionally used in continuous tone and halftone photographic processes. It has become increasingly important in the photographic arts to be able to produce high quality work at a high rate or production. To meet this need the industry has been moving to electronic circuitry to provide the speed necessary for high production. Various timing circuits are being used to energize and deenergize the various lamps used in continuous tone and halftone photographic processes. Practitioners in the art using standard equipment consider both the exposure time (variable in time units of seconds) and relative illumination (variable in logarithmic units of density) when attempting to produce high quality work. In circuits currently being utilized calibration set points are established and these set points are maintained during processing of various reproductions until quality of processing is affected by the chemical activity level, the emulsion speed of the film or a temperature shift of the chemistry. Thereafter, to continue to achieve high quality work an instrument and all prior retained set points must either be recalibrated for the chemistry change or new chemicals must be utilized and an initial calibration reperformed.
SUMMARY OF THE INVENTION
The present invention eliminates the requirement of recalibrating photographic process timing circuits due to chemistry drift, variations in emulsion speeds, or the like. This is accomplished by the creation of means for varying logarithmically the set point of each timing cycle exposure time with one setting on the instrument. This setting relates directly to and compensates for the density shift due to a variation of chemical activity level, emulsion speed of the film or temperature shift of the density.
Since compensation for the variation in operating conditions is accomplished in this manner all time unit or exposure settings and illumination or density settings as measured or as retained in records or or memory banks can be utilized as they have in the past without recalibration.
Electronically this is accomplished in the preferred embodiment by providing a logarithmically variable resistance network which varies the voltage set point indicating the end of a timing cycle. The actual voltage utilized across the resistance network is equivalent to the voltage which initially determines the linearly variable exposure time. Individual resistance values are derived from voltage ratios necessary to produce a logarithmic change in all timing cycles and, thus, any variation of the control increases or decreases the exposure time in density or logarithmical steps in an amount equivalent to the density shift which occurs when the level of chemistry activity deteriorates.
DESCRIPTION OF THE DRAWING
The FIGURE is a schematic diagram showing my invention connected to the essential elements of an exemplary timing circuit for continuous tone and halftone photographic processes.
DESCRIPTION OF THE PREFERRED EMBODIMENT
My invention is designed to be used with photo process timers or light integrators which are conventionally used in continuous tone or halftone photographic processes. The basic elements of such a device are shown in the FIGURE.
The improvement which is my invention is also shown in the FIGURE comprising a logarithmically variable network designated generally by the numeral 12. It is utilized, as indicated above, to compensate for variations of chemical activity level, emulsion speed of the film and temperature shifts of the chemistry.
The circuit shown in the FIGURE can be utilized in exposing line copy, continuous tone and halftone exposures and in exposing masking films and color separation films. For exemplary purposes only, reference will be to halftone exposures. It is to be understood, however, that the control circuit 12 is effective for every process utilized.
Upon initial calibration of the overall intrument no compensation is necessary and the override control would be set at zero compensation. Normal operation of the photo process timer for halftone reproductions would be as follows.
Three basic exposure techniques are utilized in exposing photographic film to accomplish high quality halftone reproductions. These basic exposure techniques are conventionally designated main exposure which exposes the film from the illuminated copy with a halftone screen in position over the film, highlight exposure which is an exposure of the film from the illuminated copy with no halftone screen over the film and flash exposure which is an overall exposure of the film with flash lamps with a halftone screen in position over the film.
The timing device shown is designed so that for a standard density reading the circuit can be calibrated for the particular operating characteristics to be utilized such as chemical condition, film type, film speed, halftone screen, and the like. This is accomplished with time unit resistors designated 14, 15 and 16 in the FIGURE, and associated indicia where, for example, one hundred time units would represent ten seconds. Provision is made in the device with the use of switches 17, 18 and 19 to calibrate the apparatus for varying operating conditions on time unit resistor banks maintained externally from the device. These external resistor banks can be referred to as memory banks and would be calibrated for different film speeds or halftone screens or the like and thereafter when operating conditions change all that is necessary is to plug in the retained settings on the external time unit memory bands in external jacks 20, 21, 22, 23, 24, 25.
All settings will thereafter give a consistent reproduction of highlight density and shadow density when using the same film type, film speed, chemistry and halftone screen.
Density variations of a particular piece of work to be copied are set into the apparatus to achieve precise control of the shadow and highlight densities of the finished copy. Electronically the above design is accomplished as follows.
Time unit resistances 14, 15 and 16 are provided for each illumination cycle, i.e., main, highlight and flash. As shown in the FIGURE, the time unit resistances are linearly variable resistors on the order of 100 kilohm. These resistors 14, 15 or 16 are selectively connected in parallel with a variable timing capacitor 28 or 29 and therefore, since voltage is directly proportional to resistance, any linear variation in the time unit resistors 14, 15 or 16 linearly varies the voltage impressed across and stored in the capacitor 28 or 29.
As is known in the art, density variations being related to relative illumination, are logarithmic. Therefore a 0.3 increase in density (antilog equal 1.995) will require double the time duration of illumination. This is accomplished for main and highlight illumination by the variable impedances designated 28 and 32 in the FIGURE wherein the variable resistor 32 is in series with a field effect transistor 34. This arrangement provides a logarithmically variable constant current source which will produce a current through and thus a voltage across the selected time unit resistor 14 or 15 which is variable from between zero and 0.3 density in 0.01 density steps. Since ultimately the time duration of discharge from the variable timing capacitor 28 will determine the timing cycle and since the RC time constant for discharge of a capacitor is directly related to the capacitor size, provision is also made to double capacitor 28 size every 0.3 density.
In this manner density variations may be achieved between zero and 2.0 density with the embodiment shown in the FIGURE.
As is known in the art, when flash lighting is utilized it produces different characteristic results during processing the main or highlight lamp exposure techniques. Therefore design values for density variation for the flash parameters are obtained from a different formulation and these values are incorporated into the circuit as variable resistor 36 and variable timing capacitor 29. A relay (not shown) is provided controlling interconnected switches 41, 42, 43, 46 and 47 to insert the flash time unit resistor 16, flash density impedances 29 and 36, flash lamp 51 and flash lamp phototube 52 into the timing circuit when the flash exposure is to be performed.
A more complete explanation of the design of the timing circuit described above can be found in my issued U.S. Pat. No. 3,672,767.
Electronically, calibration and operation of the timing circuit is as follows. With a standard density value, such as 1.00, set on the density dials, time unit values linearly variable in seconds of exposure are set on the time unit resistor banks 14, 15 and 16 shown in the FIGURE. Thereafter these settings will give a consislent reproduction of the highlight and shadow density when using the same film type, film speed, chemistry and halftone screen. Shadow and highlight density variations of a particular piece of work to be copied are set on the apparatus by variation in the density controls comprising the variable density resistors 32 and 36 and variable timing capacitors 28 and 29. As indicated above, the variable resistor 32 and the variable capacitor 28 compensate for density variations in the main and highlight exposures and variable resistor 36 and variable timing capacitor 29 compensate for variations in flash exposure times.
For a main exposure all switches are set to employ time unit resistor bank 14, variable density resistor 32 and variable timing capacitor 28 in operation. Variable timing capacitor 28 is charged to the proper potential from a negative source of voltage through constant current source 31, comprising variable resistor 32 and the field effect transistor 34, conductor 54, relay contact 56 in its normally closed position, conductor 58, contact 43, varaible timing capacitor 28 to ground. The voltage to which the capacitor is charged depends upon the constant current from the constant current source 31 which is controlled by variable resistor 32 as well as the linearly variable resistance value of time unit resistor 14 which is connected across the capacitor 28 through conductor 61, conductor 62, relay contact 41, switch 64, switch 17, variable resistor 14 to ground.
After the capacitor 28 has been charged to the proper value the timing cycle is initiated by depressing the start button 66 which energizes a relay 67 from a positive source of voltage through start button 66, resistor 68, relay 67 to ground. When relay 67 is energized, it is held by relay contact 69, and normally closed relay contact 56 opens to disconnect the charging circuit while normally open relay contact 70 closes to energize the lamps 72. When the lamp 72 is energized the phototube 73 conducts and the negative charge of the capacitor 28 is discharged from the capacitor 28 through contact 43, conductor 58, conductor 75, conductor 77, contact 46, phototube 73, to the positive source of voltage as indicated.
Monitoring the discharge of the capacitor 28 through phototube 73 is a differential amplifier and comparator 79 connected from point 81 through conductor 75 to the input of the differential amplifier and comparator 79. The discharge is monitored by the differential amplifier and comparator 79 to determine when the capacitor 28 has discharged to a predetermined value. When that potential has been reached the output of the differential amplifier and comparator 79 increases to a relatively large positive output within microseconds. This output is applied to the gate of a silicon controlled rectifier 84 through conductor 85, which, when it conducts, shorts the coil of relay 67 which controls the lamps 72 circuit. When the relay 67 is released its contact 70 accomplishes deenergization of the lamps 72 being utilized and thus ends the timing cycle for that exposure. Relay contact 56 returns to its normally closed position as well and the circuit may be utilized for the next timing cycle. As noted above a more complete discussion of the circuitry may be obtained from my issued U.S. Pat. No. 3,672,767.
Highlight exposure may be accomplished in the same manner by initially switching manual switch 64 to incorporate the highlight time unit resistor 15. Thereafter initiation of the timing cycle will produce a highlight exposure of a time duration dependent upon the value set on the highlight time unit resistor 15 and the density of the copy as set on the density controls 28, 32.
As indicated earlier when flash lightling is utilized it produces a different characteristic result during processing than the balance of the lighting techniques. Therefore additional circuitry is provided being essentially interconnected relay contacts 41, 42, 43, 46 and 47, variable flash density resistor 36, variable flash density timing capacitor 29, flash lamps 51 and flash lamp phototube 52 which allow presetting the desired values for flash purposes. Again a more complete discussion of the design and operation fo the flash circuitry may be obtained from my issued U.S. Pat. No. 3,672,767.
After calibration and set up of the apparatus it is not necessary to vary the time unit values which determine the exposure time as long as chemistry, film type, and the like remain constant. Successive reproductions of the same high quality can be produced by utilizing the same settings. In the past the arrangement could be utilized until the level of the chemical activity deteriorated. When this happened a density shift in the copy was noted and the apparatus would have to have been recalibrated either to the deteriorated chemistry condition or new chemicals added and the initial calibration reperformed. If the same chemistry was to be used, since effect of exposure varies logarithmically with the linear variation of illumination, it was necessary to vary each of the illumination times in the same linear percentage calculated on the basis of the original setting or to calculate density settings dependent on the actual density readings of the work to be reproduced and the density shift of the finished copy due to the changed operating conditions. Because of the difficulty of these calculations and possibility of error, the normal procedure was to recalibrate the apparatus.
The present invention provides an extremely simple and accurate method of compensating for the density shift without varying the calibration of the timing circuit or recalibrating all prior settings maintained on the memory banks of the time unit values.
This is accomplished with means for logarithmically varying the illumination time duration for each timing cycle in the same percentage deisgnated 12 in the FIGURE. By utilizing these means, all set points previously established for system control can be maintained constant.
As the preferred embodiment, this is accomplished with the resistance network 12. It should be understood, however, that with the design formulation discussed below an impedance network for control of alternating current timing circuits may be derived or more accurate and sophisticated resistance circuits can be constructed such as a parallel-series arrangement of equal value resistors to produce the same result. Resistance values have been derived based on the antilogarithm of voltage ratios yielding density variations which decrease or increase the time duration of illumination controlled by the time unit resistors 14, 15, 16 or external memory banks plugged into external jacks 20-25, if utilized.
The magnitude of the overall resistance for the control should preferably be selected within the accuracy limitation of the time unit resistances 14, 15 and 16. For example, in the embodiment shown resistance values on the order of 100 kilohm resistance are utilized for the time unit resistors 14, 15, 16. Thus an overall resistance value of three or five megohm for the control resistance 12 would draw an error current of 0.33 percent or 0.2 percent, respectively. Either of these are well within the design accuracy of the unit.
Design values for the individual control resistances 102-122 to yield a proportional logarithmic variation in illumination times are obtained as follows.
When operating under normal conditions, with no compensation from an override circuit, the selected variable timing capacitor 28 or 29 would be charged to the voltage across the selected time unit resistance bank 14, 15 or 16 (hereinafter referred to as E 0 ). When discharging, this voltage is monitored by the differential amplifier and comparator 79. The override circuit 12 is designed to vary the effective voltage across the input to the differential amplifier and comparator 79 (hereinafter referred to as E eff ) by an amount equivalent to the density shift noted on the finished copy. The value of the density shift is dialed into the conttol switch 99. As is shown in the FIGURE indicia of between +0.1 density shift to -0.1 density shift in 0.01 density steps are utilized on the control. Therefore, the ratio of the voltage across the time unit resistors (E 0 ) to the effective voltage (E eff ) must be varied logarithmically in 0.01 density steps. Ratios are accordingly obtained by deriving antilogarithms of the 0.01 units from 0.0 to 0.2. Since this ratio will hold true for any value of one of the voltages, for convenience in calculation a value of one volt can be assumed for the maximum voltage (E 0 ). From the ratios of E 0 /E eff , values can then be derived for E eff , those values being the reciprocal of E eff /E 0 . Then, since E eff is defined as the difference between E 0 and the control voltage (hereinafter referred to as E c ) E c becomes 1-E eff .
The two important parameters of the control circuit 12 are the ratio of total voltage (E 0 ) to control voltage (E c ) and the ratio of the total resistance (R t ) to control the resistance to ground (R c ). Therefore, since the load of the voltage divider is an open circuit the relationship between these ratios is E 0 /E c = R t /R c . Therefore values of R c can be obtained from the formula R c = R t (E c /E 0 ), where R t is the overall resistance value of the control circuit 12 obtained earlier of 3 to 5 megohms.
In this manner values of overall R c to ground may be derived for each override control density step from 0.0 to 0.2. Individual values for resistors 102-122 in the control circuit 12 can be obtained by noting the difference between each successive R c .
To gain the ability to vary the control in both the positive and negative direction the 0.1 value is designated as 0.0 on the control indicia. The timing circuit is then calibrated so that at that setting the proper time duration of illumination as indicated by the time unit resistors and their associated indicia will take place. Thereafter any variation in the override control will increase or decrease illumination and thus copy density in the following manner.
Actual operation of the timing circuit with the override control is as follows. After relay contact 56 has been opened disconnecting the charging circuit and initiating discharge of the variable timing capacitor 28 or 29, constant current source 31 remains energized from the negative source of voltage through resistor 32, contact 42, field effect transistor 34, conductor 61, conductor 62, contact 41, switch 64 or conductor 65, switch 17, 18 or 19 and the selected time unit resistor 14 or 15 for main or highlight illumination, 16 for flash illumination, to ground. Current also flows through the override control circuit from the field effect transistor 34 through conductor 61, conductor 88, resistors 102-122 to ground. The set point of the time duration of the illumination is determined by that voltage which exists across the time unit resistor 14 upon initial calibration. This voltage is also impressed across the override control resistors 102-122 from point 96 through control resistors 102-122 to ground. The logarithmic proportion of this voltage E c corresponding to 0.01 density variations, taken from switch contact 101 is connected by conductor 124 to the input of the differential amplifier and comparator 79 and is utilized to increase or decrease the set point of the differential amplifier and comparator 79 so that the effective voltage E eff across the input of the differential amplifier and comparator 79 is equivalent to the original calibration voltage E 0 increased or decreased by density shift voltage E c as dialed into the override exposure control 12 through switch 99.
As is obvious from the FIGURE no matter which time unit resistance 14, 15 or 16 or an external time unit resistance memory bank plugged into external jacks 20-25, is in the circuit, since the override control 12 is selectively, by manual switches 64, 17, 18 and 19 and by relay contact 41, in parallel with each, the control voltage E c will be the result of a proportionate amount of the voltage E 0 across the particular time unit resistance 14, 15 or 16 or external time unit memory banks plugged into one of external jacks 20-25, which is currently in the circuit.
In this manner compensation for chemistry deterioration, drift, emulsion speed, or the like has been accomplished, not only for the time unit resistance values which are initial to the timing circuit but also for time unit values which are external to the apparatus which serve as memory banks and which are calibrated for other film speeds or film types.
While I have generally described my invention, it should be obvious and should be understood that it is for purposes of illustration only and that various modifications can be made within the scope of my invention. It should also be obvious that with minor modifications my invention can be applied to other forms of timing circuits and still be within the scope of the following claims.
This invention is concerned with circuitry for particularly controlling the main, highlight and flash exposure times of the lamps which are conventionally used in continuous tone and halftone photographic processes. It has become increasingly important in the photographic arts to be able to produce high quality work at a high rate or production. To meet this need the industry has been moving to electronic circuitry to provide the speed necessary for high production. Various timing circuits are being used to energize and deenergize the various lamps used in continuous tone and halftone photographic processes. Practitioners in the art using standard equipment consider both the exposure time (variable in time units of seconds) and relative illumination (variable in logarithmic units of density) when attempting to produce high quality work. In circuits currently being utilized calibration set points are established and these set points are maintained during processing of various reproductions until quality of processing is affected by the chemical activity level, the emulsion speed of the film or a temperature shift of the chemistry. Thereafter, to continue to achieve high quality work an instrument and all prior retained set points must either be recalibrated for the chemistry change or new chemicals must be utilized and an initial calibration reperformed.
SUMMARY OF THE INVENTION
The present invention eliminates the requirement of recalibrating photographic process timing circuits due to chemistry drift, variations in emulsion speeds, or the like. This is accomplished by the creation of means for varying logarithmically the set point of each timing cycle exposure time with one setting on the instrument. This setting relates directly to and compensates for the density shift due to a variation of chemical activity level, emulsion speed of the film or temperature shift of the density.
Since compensation for the variation in operating conditions is accomplished in this manner all time unit or exposure settings and illumination or density settings as measured or as retained in records or or memory banks can be utilized as they have in the past without recalibration.
Electronically this is accomplished in the preferred embodiment by providing a logarithmically variable resistance network which varies the voltage set point indicating the end of a timing cycle. The actual voltage utilized across the resistance network is equivalent to the voltage which initially determines the linearly variable exposure time. Individual resistance values are derived from voltage ratios necessary to produce a logarithmic change in all timing cycles and, thus, any variation of the control increases or decreases the exposure time in density or logarithmical steps in an amount equivalent to the density shift which occurs when the level of chemistry activity deteriorates.
DESCRIPTION OF THE DRAWING
The FIGURE is a schematic diagram showing my invention connected to the essential elements of an exemplary timing circuit for continuous tone and halftone photographic processes.
DESCRIPTION OF THE PREFERRED EMBODIMENT
My invention is designed to be used with photo process timers or light integrators which are conventionally used in continuous tone or halftone photographic processes. The basic elements of such a device are shown in the FIGURE.
The improvement which is my invention is also shown in the FIGURE comprising a logarithmically variable network designated generally by the numeral 12. It is utilized, as indicated above, to compensate for variations of chemical activity level, emulsion speed of the film and temperature shifts of the chemistry.
The circuit shown in the FIGURE can be utilized in exposing line copy, continuous tone and halftone exposures and in exposing masking films and color separation films. For exemplary purposes only, reference will be to halftone exposures. It is to be understood, however, that the control circuit 12 is effective for every process utilized.
Upon initial calibration of the overall intrument no compensation is necessary and the override control would be set at zero compensation. Normal operation of the photo process timer for halftone reproductions would be as follows.
Three basic exposure techniques are utilized in exposing photographic film to accomplish high quality halftone reproductions. These basic exposure techniques are conventionally designated main exposure which exposes the film from the illuminated copy with a halftone screen in position over the film, highlight exposure which is an exposure of the film from the illuminated copy with no halftone screen over the film and flash exposure which is an overall exposure of the film with flash lamps with a halftone screen in position over the film.
The timing device shown is designed so that for a standard density reading the circuit can be calibrated for the particular operating characteristics to be utilized such as chemical condition, film type, film speed, halftone screen, and the like. This is accomplished with time unit resistors designated 14, 15 and 16 in the FIGURE, and associated indicia where, for example, one hundred time units would represent ten seconds. Provision is made in the device with the use of switches 17, 18 and 19 to calibrate the apparatus for varying operating conditions on time unit resistor banks maintained externally from the device. These external resistor banks can be referred to as memory banks and would be calibrated for different film speeds or halftone screens or the like and thereafter when operating conditions change all that is necessary is to plug in the retained settings on the external time unit memory bands in external jacks 20, 21, 22, 23, 24, 25.
All settings will thereafter give a consistent reproduction of highlight density and shadow density when using the same film type, film speed, chemistry and halftone screen.
Density variations of a particular piece of work to be copied are set into the apparatus to achieve precise control of the shadow and highlight densities of the finished copy. Electronically the above design is accomplished as follows.
Time unit resistances 14, 15 and 16 are provided for each illumination cycle, i.e., main, highlight and flash. As shown in the FIGURE, the time unit resistances are linearly variable resistors on the order of 100 kilohm. These resistors 14, 15 or 16 are selectively connected in parallel with a variable timing capacitor 28 or 29 and therefore, since voltage is directly proportional to resistance, any linear variation in the time unit resistors 14, 15 or 16 linearly varies the voltage impressed across and stored in the capacitor 28 or 29.
As is known in the art, density variations being related to relative illumination, are logarithmic. Therefore a 0.3 increase in density (antilog equal 1.995) will require double the time duration of illumination. This is accomplished for main and highlight illumination by the variable impedances designated 28 and 32 in the FIGURE wherein the variable resistor 32 is in series with a field effect transistor 34. This arrangement provides a logarithmically variable constant current source which will produce a current through and thus a voltage across the selected time unit resistor 14 or 15 which is variable from between zero and 0.3 density in 0.01 density steps. Since ultimately the time duration of discharge from the variable timing capacitor 28 will determine the timing cycle and since the RC time constant for discharge of a capacitor is directly related to the capacitor size, provision is also made to double capacitor 28 size every 0.3 density.
In this manner density variations may be achieved between zero and 2.0 density with the embodiment shown in the FIGURE.
As is known in the art, when flash lighting is utilized it produces different characteristic results during processing the main or highlight lamp exposure techniques. Therefore design values for density variation for the flash parameters are obtained from a different formulation and these values are incorporated into the circuit as variable resistor 36 and variable timing capacitor 29. A relay (not shown) is provided controlling interconnected switches 41, 42, 43, 46 and 47 to insert the flash time unit resistor 16, flash density impedances 29 and 36, flash lamp 51 and flash lamp phototube 52 into the timing circuit when the flash exposure is to be performed.
A more complete explanation of the design of the timing circuit described above can be found in my issued U.S. Pat. No. 3,672,767.
Electronically, calibration and operation of the timing circuit is as follows. With a standard density value, such as 1.00, set on the density dials, time unit values linearly variable in seconds of exposure are set on the time unit resistor banks 14, 15 and 16 shown in the FIGURE. Thereafter these settings will give a consislent reproduction of the highlight and shadow density when using the same film type, film speed, chemistry and halftone screen. Shadow and highlight density variations of a particular piece of work to be copied are set on the apparatus by variation in the density controls comprising the variable density resistors 32 and 36 and variable timing capacitors 28 and 29. As indicated above, the variable resistor 32 and the variable capacitor 28 compensate for density variations in the main and highlight exposures and variable resistor 36 and variable timing capacitor 29 compensate for variations in flash exposure times.
For a main exposure all switches are set to employ time unit resistor bank 14, variable density resistor 32 and variable timing capacitor 28 in operation. Variable timing capacitor 28 is charged to the proper potential from a negative source of voltage through constant current source 31, comprising variable resistor 32 and the field effect transistor 34, conductor 54, relay contact 56 in its normally closed position, conductor 58, contact 43, varaible timing capacitor 28 to ground. The voltage to which the capacitor is charged depends upon the constant current from the constant current source 31 which is controlled by variable resistor 32 as well as the linearly variable resistance value of time unit resistor 14 which is connected across the capacitor 28 through conductor 61, conductor 62, relay contact 41, switch 64, switch 17, variable resistor 14 to ground.
After the capacitor 28 has been charged to the proper value the timing cycle is initiated by depressing the start button 66 which energizes a relay 67 from a positive source of voltage through start button 66, resistor 68, relay 67 to ground. When relay 67 is energized, it is held by relay contact 69, and normally closed relay contact 56 opens to disconnect the charging circuit while normally open relay contact 70 closes to energize the lamps 72. When the lamp 72 is energized the phototube 73 conducts and the negative charge of the capacitor 28 is discharged from the capacitor 28 through contact 43, conductor 58, conductor 75, conductor 77, contact 46, phototube 73, to the positive source of voltage as indicated.
Monitoring the discharge of the capacitor 28 through phototube 73 is a differential amplifier and comparator 79 connected from point 81 through conductor 75 to the input of the differential amplifier and comparator 79. The discharge is monitored by the differential amplifier and comparator 79 to determine when the capacitor 28 has discharged to a predetermined value. When that potential has been reached the output of the differential amplifier and comparator 79 increases to a relatively large positive output within microseconds. This output is applied to the gate of a silicon controlled rectifier 84 through conductor 85, which, when it conducts, shorts the coil of relay 67 which controls the lamps 72 circuit. When the relay 67 is released its contact 70 accomplishes deenergization of the lamps 72 being utilized and thus ends the timing cycle for that exposure. Relay contact 56 returns to its normally closed position as well and the circuit may be utilized for the next timing cycle. As noted above a more complete discussion of the circuitry may be obtained from my issued U.S. Pat. No. 3,672,767.
Highlight exposure may be accomplished in the same manner by initially switching manual switch 64 to incorporate the highlight time unit resistor 15. Thereafter initiation of the timing cycle will produce a highlight exposure of a time duration dependent upon the value set on the highlight time unit resistor 15 and the density of the copy as set on the density controls 28, 32.
As indicated earlier when flash lightling is utilized it produces a different characteristic result during processing than the balance of the lighting techniques. Therefore additional circuitry is provided being essentially interconnected relay contacts 41, 42, 43, 46 and 47, variable flash density resistor 36, variable flash density timing capacitor 29, flash lamps 51 and flash lamp phototube 52 which allow presetting the desired values for flash purposes. Again a more complete discussion of the design and operation fo the flash circuitry may be obtained from my issued U.S. Pat. No. 3,672,767.
After calibration and set up of the apparatus it is not necessary to vary the time unit values which determine the exposure time as long as chemistry, film type, and the like remain constant. Successive reproductions of the same high quality can be produced by utilizing the same settings. In the past the arrangement could be utilized until the level of the chemical activity deteriorated. When this happened a density shift in the copy was noted and the apparatus would have to have been recalibrated either to the deteriorated chemistry condition or new chemicals added and the initial calibration reperformed. If the same chemistry was to be used, since effect of exposure varies logarithmically with the linear variation of illumination, it was necessary to vary each of the illumination times in the same linear percentage calculated on the basis of the original setting or to calculate density settings dependent on the actual density readings of the work to be reproduced and the density shift of the finished copy due to the changed operating conditions. Because of the difficulty of these calculations and possibility of error, the normal procedure was to recalibrate the apparatus.
The present invention provides an extremely simple and accurate method of compensating for the density shift without varying the calibration of the timing circuit or recalibrating all prior settings maintained on the memory banks of the time unit values.
This is accomplished with means for logarithmically varying the illumination time duration for each timing cycle in the same percentage deisgnated 12 in the FIGURE. By utilizing these means, all set points previously established for system control can be maintained constant.
As the preferred embodiment, this is accomplished with the resistance network 12. It should be understood, however, that with the design formulation discussed below an impedance network for control of alternating current timing circuits may be derived or more accurate and sophisticated resistance circuits can be constructed such as a parallel-series arrangement of equal value resistors to produce the same result. Resistance values have been derived based on the antilogarithm of voltage ratios yielding density variations which decrease or increase the time duration of illumination controlled by the time unit resistors 14, 15, 16 or external memory banks plugged into external jacks 20-25, if utilized.
The magnitude of the overall resistance for the control should preferably be selected within the accuracy limitation of the time unit resistances 14, 15 and 16. For example, in the embodiment shown resistance values on the order of 100 kilohm resistance are utilized for the time unit resistors 14, 15, 16. Thus an overall resistance value of three or five megohm for the control resistance 12 would draw an error current of 0.33 percent or 0.2 percent, respectively. Either of these are well within the design accuracy of the unit.
Design values for the individual control resistances 102-122 to yield a proportional logarithmic variation in illumination times are obtained as follows.
When operating under normal conditions, with no compensation from an override circuit, the selected variable timing capacitor 28 or 29 would be charged to the voltage across the selected time unit resistance bank 14, 15 or 16 (hereinafter referred to as E 0 ). When discharging, this voltage is monitored by the differential amplifier and comparator 79. The override circuit 12 is designed to vary the effective voltage across the input to the differential amplifier and comparator 79 (hereinafter referred to as E eff ) by an amount equivalent to the density shift noted on the finished copy. The value of the density shift is dialed into the conttol switch 99. As is shown in the FIGURE indicia of between +0.1 density shift to -0.1 density shift in 0.01 density steps are utilized on the control. Therefore, the ratio of the voltage across the time unit resistors (E 0 ) to the effective voltage (E eff ) must be varied logarithmically in 0.01 density steps. Ratios are accordingly obtained by deriving antilogarithms of the 0.01 units from 0.0 to 0.2. Since this ratio will hold true for any value of one of the voltages, for convenience in calculation a value of one volt can be assumed for the maximum voltage (E 0 ). From the ratios of E 0 /E eff , values can then be derived for E eff , those values being the reciprocal of E eff /E 0 . Then, since E eff is defined as the difference between E 0 and the control voltage (hereinafter referred to as E c ) E c becomes 1-E eff .
The two important parameters of the control circuit 12 are the ratio of total voltage (E 0 ) to control voltage (E c ) and the ratio of the total resistance (R t ) to control the resistance to ground (R c ). Therefore, since the load of the voltage divider is an open circuit the relationship between these ratios is E 0 /E c = R t /R c . Therefore values of R c can be obtained from the formula R c = R t (E c /E 0 ), where R t is the overall resistance value of the control circuit 12 obtained earlier of 3 to 5 megohms.
In this manner values of overall R c to ground may be derived for each override control density step from 0.0 to 0.2. Individual values for resistors 102-122 in the control circuit 12 can be obtained by noting the difference between each successive R c .
To gain the ability to vary the control in both the positive and negative direction the 0.1 value is designated as 0.0 on the control indicia. The timing circuit is then calibrated so that at that setting the proper time duration of illumination as indicated by the time unit resistors and their associated indicia will take place. Thereafter any variation in the override control will increase or decrease illumination and thus copy density in the following manner.
Actual operation of the timing circuit with the override control is as follows. After relay contact 56 has been opened disconnecting the charging circuit and initiating discharge of the variable timing capacitor 28 or 29, constant current source 31 remains energized from the negative source of voltage through resistor 32, contact 42, field effect transistor 34, conductor 61, conductor 62, contact 41, switch 64 or conductor 65, switch 17, 18 or 19 and the selected time unit resistor 14 or 15 for main or highlight illumination, 16 for flash illumination, to ground. Current also flows through the override control circuit from the field effect transistor 34 through conductor 61, conductor 88, resistors 102-122 to ground. The set point of the time duration of the illumination is determined by that voltage which exists across the time unit resistor 14 upon initial calibration. This voltage is also impressed across the override control resistors 102-122 from point 96 through control resistors 102-122 to ground. The logarithmic proportion of this voltage E c corresponding to 0.01 density variations, taken from switch contact 101 is connected by conductor 124 to the input of the differential amplifier and comparator 79 and is utilized to increase or decrease the set point of the differential amplifier and comparator 79 so that the effective voltage E eff across the input of the differential amplifier and comparator 79 is equivalent to the original calibration voltage E 0 increased or decreased by density shift voltage E c as dialed into the override exposure control 12 through switch 99.
As is obvious from the FIGURE no matter which time unit resistance 14, 15 or 16 or an external time unit resistance memory bank plugged into external jacks 20-25, is in the circuit, since the override control 12 is selectively, by manual switches 64, 17, 18 and 19 and by relay contact 41, in parallel with each, the control voltage E c will be the result of a proportionate amount of the voltage E 0 across the particular time unit resistance 14, 15 or 16 or external time unit memory banks plugged into one of external jacks 20-25, which is currently in the circuit.
In this manner compensation for chemistry deterioration, drift, emulsion speed, or the like has been accomplished, not only for the time unit resistance values which are initial to the timing circuit but also for time unit values which are external to the apparatus which serve as memory banks and which are calibrated for other film speeds or film types.
While I have generally described my invention, it should be obvious and should be understood that it is for purposes of illustration only and that various modifications can be made within the scope of my invention. It should also be obvious that with minor modifications my invention can be applied to other forms of timing circuits and still be within the scope of the following claims.