Light-exposure control unit

An improved light-exposure control unit for a photographic printer offering minimized leakage problems, elimination of special precision capacitors and a range selecting circuit which eliminates masking of the photosensitive device used to detect variations in intensity of the light source. The improved control unit including operational amplifiers connected to the photosensitive device, which is mounted after the color filter but before the negative, sending a voltage adjustable according to the desired range and proportional to the amount of light impinging thereon, to an integrating amplifier with a variable input resistance adjusted accordingly to the desired density. The integrating amplifiers output is compared to a voltage derived from a precision reference source adjustable according to the settings made to magnification and speed-factor controls. The length of the exposure period is the time period required for the integrating amplifiers output to reach equality with this voltage and is controlled automatically by the amount of light impinging upon the photosensitive device, from the source, through the color filter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved exposure control unit which is used 
advantageously in the exposing of light-sensitive materials such as 
photographic film or paper, either monochrome or color. 
The improved control unit of the present invention may be used in the 
production of all types of photographs and particularly special purpose 
photographs, such as for advertising or other commercial or governmental 
purposes, where the photographs to be produced are not of the fixed 
magnifications ordinarily used in the exposing of photographic prints for 
the public. 
2. Description of prior art 
The object of the present invention is to provide an improved design for 
the apparatus claimed within U.S. Pat. No. 3,512,952. Wherein said 
apparatus presents electrical phenomenon that complicate construction and 
requires rare and expensive components that deem the design impractical 
for mass production, specifically, there are high quality precision 
capacitors with non-standard values employed in the capacitor network used 
for density factor. These capacitors must be made to specification by a 
manufacturer at an exorbitant cost. The apparatus claimed utilizes a 
phototube to discharge the capacitor network and a triode to monitor the 
voltage on the capacitor network, this arrangement has inherent leakage 
due to the high input impedance at the grid of the triode requiring 
careful construction techniques and low leakage components such as 
TEFLON.RTM. tube sockets and wire. There are no provisions on the claimed 
apparatus for range selection, it is customary to mask the phototube with 
light-attenuating neutral density filters in order to provide longer 
exposure times. If the operator requires shorter exposure times circuit 
modifications are required. Today a wide variety of photographic materials 
are available with many rated speeds making this a serious consideration 
for improvement. 
The new design of the present invention is functionally identical from the 
operators viewpoint but constitutes an entirely different design approach. 
The new design employs an integrator with an adjustable input resistance 
adjusted accordingly to the desired density. The integrator utilizes 
precision resistors and one high quality capacitor with a moderate 
tolerance that are readily available at a reasonable cost. The new design 
of the present invention eliminates high impedance nodes with a 
photodiode/operational amplifier front end connected to the input 
resistance of the integrator. This arrangement permits voltage control of 
the charging rate of the integrators capacitor, the voltage being a 
function of light intensity thus creating the integral of intensity 
relevant to both inventions. Part of the operational amplifier front end 
contains means for adjusting the proportionality of the voltage providing 
ranges for the exposure control system. Further advantages of the present 
invention are greater accuracy, reliability and stability attributed to 
its modern solid state construction. 
BRIEF SUMMARY OF INVENTION 
This invention is directed to a device for controlling automatically the 
exposure in making of prints, either monochrome or color, regardless of 
magnification. The automatic exposure device of the present invention 
makes it possible for a person relatively un-skilled in photographic 
techniques to produce special purpose photographs which heretofore have 
required the skill and time of the professional photographic processor. 
A typical example of the use of the exposure device of the present 
invention would be as follows: A photographic print for advertising or 
other commercial purposes is to be produced. The professional processor, 
based on his or her experience, makes a first test print of small size, 
such as 4".times.5". The processor and the client examine the test print. 
They decide (1) that the print should be 11".times.14", (2) that a 
different printing material should be used, (3) that the density should be 
increased, and (4) that the color balance be changed such that the red 
tones would be more subdued and the blue tones accentuated. Having made 
these decisions, the next question is: What effect will these changes have 
on the exposure required for the printing as compared with the exposure 
used in making the test print? This question is answered automatically by 
the exposure device provided by the present invention. The processor may 
now turn the matter over to an operator not highly trained or skilled in 
photographic processing techniques. The operator, using available tables 
and charts, sets the controls on the exposure device to correspond to the 
new magnification factor (to accommodate for the desired enlargement and 
reciprocity factor), to correspond to the new speed-factor (to accommodate 
for the new type of printing material) and to correspond to the new 
density factor (to accommodate for the increase in density desired). The 
operator alters the color of the exposing light to achieve the desired 
change in color balance, then depresses the start button on the exposing 
device of the present invention and the device controls automatically the 
exposure for making the final print. 
Further objects and advantages of the invention will become apparent from 
the drawings and ensuing description thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 there is shown diagrammatically a photographic 
enlarger system comprising a light source or lamp 10, a multi-color filter 
12, a photographic negative 16, a focusing lens 17, and the printing 
material 18 contained in or supported by a housing 20, but no effort has 
been made to illustrate the structural mounting details since such means 
are well known. 
The multi-color filter is shown to include three different color filter 
elements 13, 14, and 15 which, in a typical case, will be cyan, magenta 
and yellow. Each of the color filter elements 13, 14, and 15 is also of 
varying density, increasing progressively in density from one marginal 
region to the other, and each filter element is separately adjustable, so 
by adjusting the filter elements 13, 14, and 15 a wide range of color 
filtration may be obtained. The lamp 10 and the multi-color filter 12 may, 
if desired, be a device known as a Chromega D. Lamphouse, a produce of 
Simmon Brothers, Inc. of New York State, embodying U.S. Pat. No. 
3,027,801. 
In accordance with the present invention, a photodiode 45 is disposed with 
the housing 20, in a position to receive the filtered light from the light 
source 10. Preferably, the photodiode 45 is disposed within the housing 20 
between the color filter 12 and the negative 16, but located to one side 
of the negative, in such position that the photodiode 45 is within the 
filtered light which is directed to the negative 16. Positioning the 
photodiode between the color filter and the negative, rather than between 
the negative and the printing paper, is deemed important, at least to the 
preferred embodiment of the present invention. 
A control box 21 is provided having on its front panel a focus on-off 
toggle switch 22, a power on-off toggle switch 23, a springbiased 
momentary toggle type start/stop switch 24, a material speed factor 
control switch bank 25, a magnification control switch bank 26, and a 
density factor control switch bank 27, inside the control box 21 is a 
power supply circuit and an exposure control circuit, shown schematically 
by adjoining FIGS. 2a, 2b, 2c, and 2d in sequence from left to right. 
Referring now to FIG. 2d, which shows schematically a suitable power supply 
circuit. The terminals 30 are connected to a suitable source of power, for 
example, a 115 volt alternating current line. Reference numeral 31 
identifies a fuse; 32 is a double-pole power switch controlled by toggle 
23 on the control box 21 (FIG. 1); 33 is a socket for receiving the plug 
of a cord connected to the light source or lamp 10; 34 is a step down 
transformer supplying full wave bridge rectifier 35 with current from its 
34 volt center tapped secondary winding; capacitors 37 and 38 filter the 
pulsating direct current developed by bridge rectifier 35. Power for the 
exposure control circuit provided at terminals 46 and 47 is regulated by 
integrated circuit positive voltage regulator 39 and integrated circuit 
negative voltage regulator 40 to +15 volts and -15 volts respectively. 
Transistor 41 a common emitter switch, biased by resistors 42 and 43, 
creates 120 Hz. pulses synchronous with the zero crossover region of the 
line frequency. Diode 36 isolates transistor 41 and associated components 
from the D.C. voltage imposed across filter capacitor 37. Switching for 
the light source or lamp 10 is accomplished by a solid state relay 44. 
This modern relay device is shown to contain two basic internal 
components, a light activated thyristor switch 50 and an infra-red light 
emitting diode 49. Power to the light source or lamp 10 is switched on 
when current is applied to the infra-red light emitting diode 49, through 
limiting resistor 48. This current causes infra-red light to be emitted by 
the diode 49 and this infra-red light, in turn, causes the light activated 
thyristor switch 50 to conduct maintaining a high degree of isolation 
between the control circuitry and the power lines. 
FIG. 2a shows a photodiode 45, connected in the photovoltaic mode, through 
a coaxial cable 51 (that may be extended 20 feet or so to the lamp 
housing) to an operational amplifier 52 that converts current created by 
the photodiode 45 as a result of its short circuit termination, into an 
output voltage proportional to the amount of light impinging thereon. The 
magnitude of this voltage is fixed to the desired white light level by 
resistor 53 and will decrease as the density of the filter elements 13, 
14, and 15 (FIG. 1) increases. Resistor 53, may if desired, be variable to 
allow for precise trimming of the proper exposure time. 
Operational amplifier 55 gain setting resistors 54 and 57 through 68 along 
with twelve position rotary switch 56 provide an internal calibration 
adjustment for the range of the exposing device. Gain steps were chosen to 
duplicate the attenuation values that would be achieved if the photodiode 
45 were masked with commercially available neutral density filters. These 
gain steps may also include amplification to provide shorter exposure 
times. Switch 56 is adjusted upon installation of the exposing device for 
the range needed to satisfy the time requirements of the printing material 
18. To further enhance an understanding of this range selecting circuit 
the following list of resistor values are given: 
54=100K 1% 
57=10.0K 1% 
58=12.4K 1% 
59=16.2K 1% 
60=20.0K 1% 
61=24.9K 1% 
62=31.6K 1% 
63=40.2K 1% 
64=49.9K 1% 
65=63.4K 1% 
66=80.6K 1% 
67=100K 1% 
68=121K 1% 
Referring now to FIG. 2b which shows a reference diode 71, biased by 
resistor 69 and made adjustable by trimming potentiometer 70 for a 
precision 2.50 volts at the reference diodes cathode. This precision 
voltage is amplified and inverted by the gain of operational amplifier 73 
made adjustable according to the desired magnification by a decade 
resistor network 120 in its feedback loop. An equivalent circuit and truth 
table for the decade resistor network, composed of precision resistors and 
switches, is shown in FIGS. 3 and 4 respectively. 
Referring now to FIG. 3 there is shown three digit sections in series, a 
most significant digit (MSD) 110, a next significant digit (NSD) 111, and 
a least significant digit (LSD) 112 which are identical with exception to 
the resistors values. 
Physically the switches S1 through S5 are combined into a rotary thumbwheel 
device with numerical markings on the body and the three sections, 
composed of three such devices, are joined together to form one bank of 
three decades. Switches S1 through S5 are actuated in a 1-2-2-2-2 code 
best described by the truth table of FIG. 4 and upon careful examination 
of FIGS. 3 and 4 it will become apparent that the three decade resistor 
network can produce one thousand different values of resistance ranging 
from zero ohms to 999K ohms when the value for the factor "R" in FIGS. 3 
and 4 is equal to one kilohm. 
Referring once again to FIG. 2b operational amplifier 73 exhibits a gain of 
negative unity to negative four when resistors 74 and 72 are equal to one 
third of the maximum resistance of decade resistor network 120 (333 
kilohms) having available at its output one thousand voltages within the 
range of -2.50 volts to -10.0 volts relative to the desired magnification. 
This output voltage is then divided by a factor of four by the ratio of 
resistors 75 and 76 producing an overall stage gain of negative 0.250 to 
negative unity to allow the use of standard one and two prime values of 
resistors (such as one kilohm and ten kilohms or two kilohms and twenty 
kilohms) in the decade resistor networks for operational amplifiers 73 and 
78. 
The next stage comprising operational amplifier 78, resistors 77, 79 and 
decade resistor network 121 is identical to the previous stage (consisting 
of components 72, 73, 74, and 120) and exhibits identical gain 
characteristics adjustable according to the desired speed-factor by decade 
resistor network 121, therefore, the output of operational amplifier 78 is 
an adjustable voltage derived from the precision reference source of diode 
71 adjustable according to settings made to magnification decade resistor 
network 120 and speed-factor decade resistor network 121 (controls 25 and 
26 FIG. 1) relative to the present invention wherein said adjustable 
voltage is within the range of +0.625 volts to +10.0 volts. 
Referring to FIG. 2c there is shown a focus switch 80, controlled by toggle 
22 on control box 21 (FIG. 1) for energizing solid state relay 44 (FIG. 
2d) therefore the lamp 10. A spring-biased momentary start/stop switch 83, 
controlled by toggle 24 (FIG. 1) for actuating D type flip flop 84, which 
synchronizes the control circuit to the 120 Hz. zero crossover pulses 
provided by transistor 41 (FIG. 2d) eliminating spurious power line noise. 
Flip flop 84 is controlled by the output of flip flop 85 which serves as a 
latch for the current status of the control system provided by comparator 
91 and momentary start/stop switch 83. Operational amplifier 101 functions 
as an integrator with capacitor 86 as a feedback element and this 
integrator is controlled by D type flip flop 84 with CMOS switches 87 and 
88 as an interface. Decade resistor network 122 is the input resistance 
for the integrating amplifier 101 and is variable according to the desired 
density-factor. This decade resistor network 122 is identical to those 
shown in FIG. 2b (120, 121) with exception to resistance. In order to keep 
integrating capacitor 86 small in value and size (1.5 mfd) a variable 
resistance of zero to 9.99 megohms is required ("R"= 10K ohms for FIGS. 3 
and 4). Resistor 89 protects CMOS switch 87 from large values of in rush 
current developed while discharging capacitor 86. Diode 82 isolates 
parasitic components of CMOS FET source follower 81, which functions as a 
buffer amplifier, from the shorting action of switch 80. Diode 90 isolates 
the output of comparator 91 from the shorting action of start/stop switch 
83. Comparator 91 compares the output of integrating amplifier 101 to the 
voltage developed by the circuit of FIG. 2b, relative to the desired 
magnification and speed-factor, to determine the end of a timing cycle, 
resistors 92 and 93 provide hysteresis to prevent oscillations from within 
comparator 91. Resistor 95 minimizes error, due to input bias current, 
developed across resistor 92. Diode 94 isolates the non-inverting input of 
comparator 91 from the voltage imposed at its output by pull-up resistor 
96 and is a germanium type for a low forward voltage. Resistors 97, 98 and 
99 are needed to pull up the inputs of flip flops 84 and 85. Capacitor 100 
insures that flip flop 85 will be cleared when the control circuit is 
energized. 
The manner in which the exposure control device of the present invention 
operates will now be described by considering its use in photographic 
printing. 
Assume that a test print of small size, such as 4".times.5", has been made 
and that the processor and the client are examining the print. They decide 
that the print should be 11".times.14", that it should be on different 
material, and that the red tones should be subdued and the blue tones 
accentuated, having made these decisions, the processor or his operator 
adjusts the enlarging device to provide the increased magnification, he 
adjusts the lens opening, and he adjusts the color filters 12. These 
adjustments are made in accordance with calibration tables and charts 
which are available to the operator. 
Having done these things the operator next flips the power switch 23 to the 
on position. This closes the double-pole switch 32, in FIG. 2d, and turns 
on the power to the system. Next the operator may flip the focus switch 22 
to the on position to close switch 80, of FIG. 2c, energizing solid state 
relay 44. This connects power to the enlarger lamp 10 and allows the 
operator to adjust the focus and compose the picture. The focus switch 22 
is then returned to the off position, extinguishing the enlarger lamp 10. 
The operator then adjusts thumbwheel switch bank 26 to read the new 
magnification factor, and in doing so he adjusts the feedback resistance 
of operational amplifier 73, of FIG. 2b, to provide a different voltage at 
its output. The operator then adjusts thumbwheel switch bank 25 to read a 
speed factor which corresponds to the rated speed of the printing material 
being used. The adjustment of switch bank 25 changes the feedback 
resistance of operational amplifier 78 (FIG. 2b) and the voltage at the 
output of operational amplifier 78 is adjusted accordingly thereby 
changing the voltage at the non-inverting input of comparator 91. The 
operator next adjusts thumbwheel switch bank 27 to achieve the desired 
density correction and in doing so removes connections for the particular 
resistor, or the particular combination of resistors, necessary to set the 
input resistance of the integrating amplifier 101 according to the desired 
density. 
In the system of the preferred embodiment, as soon as the power switch 32 
was closed, and before the timer start switch 24 is depressed, capacitor 
100 held the clear input of flip flop 85 low therefore its Q output is 
low, consequentially holding flip flop 84 cleared. Integrating amplifier 
101 is in the idle state through the action of CMOS switch 87, which is 
on, and CMOS switch 88, which is off, isolating the integrator from 
photodiode amplifiers 52 and 55. The enlarger lamp is not yet on so 
photodiode 45 is not yet illuminated. There is no voltage present at the 
outputs of operational amplifiers 55 and 101. 
The operator then places printing material of the desired type in position 
and is now ready to make the exposure. He depresses the spring-biased 
momentary start/stop switch 24, to start the exposure device, momentarily 
grounding the preset input of flip flop 85 through the contacts of switch 
83. The Q output of flip flop 85 immediately goes high releasing flip flop 
84 from its cleared state. Upon the positive edge of the next zero 
crossover pulse, in a train of pulses present at the clock input of flip 
flop 84, output Q of flip flop 84 will go high. This will cause CMOS 
switch 87 to turn off and CMOS FET 81 to conduct applying voltage to solid 
state relay 44. The enlarger lamp 10 is now on. Simultaneously, the Q0 
output of flip flop 84 goes low causing CMOS switch 88 to conduct. The 
integrating amplifier 101 is now active, photodiode 45 is illuminated, a 
voltage directly proportional to the intensity of the light in the housing 
20, appears at the output of operational amplifier 55 therefore a current 
automatically variable by light intensity and preset according to density 
factor flows out of the virtual ground at the inverting input of 
operational amplifier 101. Operational amplifier 101 will consequentially 
source current through capacitor 86, into said virtual ground, causing a 
positive going voltage ramp with a slope automatically variable according 
to light intensity and manually variable according to density factor at 
its output. This ramp appears at the inverting input of comparator 91. It 
will reach equality with the voltage from FIG. 2b (relative to 
magnification and speed factor) present at its non-inverting input and the 
output of comparator 91 will switch to ground. Diode 94 will conduct 
creating hysteresis for the comparator 91 by the voltage divider action of 
resistors 92 and 93. Diode 90 will conduct clearing flip flops 84 and 85. 
The Q output of flip flop 84 will then go low extinguishing the enlarger 
lamp 10 and discharging capacitor 86 through resistor 89 via CMOS switch 
87. Simultaneously the Q0 output will go high turning CMOS switch 88 off 
placing the circuit in its original condition ready for the next exposure. 
At any time during the timing cycle switch 83 may be actuated to clear flip 
flop 85 and reset the circuit to its ready state. 
It will be seen that the circuitry of FIGS. 2a, 2b, 2c, and 2d provide a 
control system for controlling the exposure of the sensitive material 
according to settings made to a plurality of adjustments relating to (1) 
magnification, (2) reactance, or photosensitivity, of the photosensitive 
material, (3) density factor, (4) light intensity, (5) range. The device 
of the present invention is designed to manually accept information on 
four of these factors. The system automatically compensates for changes in 
light intensity and changes in intensity resulting from adjustments to 
filter elements 13, 14, and 15. 
Summarizing, the operator who has previously set the range of the exposure 
device adjusts control switches on the control unit to accommodate for the 
reactance of the sensitive material and for the magnification desired. 
These two adjustments control the voltage applied to a comparator that 
determines the exposure time from the voltage output of an integrator. The 
slope of the integrators voltage output, hence time, is manually 
adjustable by the settings made to control switches for density factor and 
automatically controlled by the intensity of the light received by the 
photodiode disposed within the lamp housing. The intensity of the light 
received by the photodiode is a function of the brightness of the lamp as 
determined by its ratings and the voltage applied thereto, and of the 
adjustments made to the color filters. Exposure time is therefore variable 
by all five of the previously mentioned factors. 
While the preferred embodiment of this invention has been described in 
great detail, it will be obvious to one skilled in the art that various 
modifications may be made without departing from the scope of the 
invention. Therefore, the scope of the invention should not be determined 
by the embodiment illustrated, but by the appended claims and their legal 
equivalents.