US2995978A - Photographic printer circuit - Google Patents

Description

1951 A. E. GLANDON ETAL 2,995,978

PHOTOGRAPHIC PRINTER CIRCUIT Filed Sept. 26, 1958 5 Sheets-Sheet 1 20 22) y g CONTROL U PHOTOTUBE e & L 24 ER vnunee 12: m

POWER n 11 Pl.

n C NETWORK 32 34 so 10 K K LOGARITHMIC s suMMme momma NETWORK TAMPUFIER Fj. g. 1

AdrianEGIandOn Roscoe H.6'anad INVENTORS ATTORNEYS Aug. 15, 1961 2,995,978

A. E. GLANDON ETAL PHOTOGRAPHIC PRINTER CIRCUIT Filed Sept. 26, 1958 5 Sheets-5118M. 2

AdrianllGlandon RoscoeHflanaday INVENTORS Aug. 15, 1961 A. E. GLANDON ETAL 2,995,978

PHOTOGRAPHIC PRINTER CIRCUIT Filed Sept. 26, 1958 5 Sheets-Sheet 3 fi Fig: 11

PHOTUTUBE e Z LOGARITHMIC PHOTOMETER Gf c EA l Q3 E195 E E 0: E8 :m a AdrianE.Glandon I E RosooeHL'anaday i3 E INVENTORS (00 E W M 1 i BY fiinfw.

I ATTORNEYS NEGATIVE DENSITY United States Patent The present invention relates to the automatic'control of printing exposure in the printing of color negatives, and more particularly concerns the recognition and compensation of certain errors in conjunction with such printing.

The automatic control of printing exposure in making color prints from color negatives, or transparencies, is

well known. In a typical exposure control system, a print-- ing lamp transmits light through the negative, and a predetermined fractional portion of the light passing through the negative is directed onto a printing surface. The remaining fractional portion of the light is directed onto a phototube, whose output signal is compared with a preselected reference signal. The difference between the two signals constitutes a comparison signal and represents the degree to which a standard, or normal," exposure must be changed for producing an opitimum print from the particular negative. The exposure'may be changed by varying the exposure time or' the lamp intensity or both. In the printing system embodying the present in vention, the printing time remains constant a nd the lamp intensity is adjusted to produce the optimum exposure.

Exposure control systems of the above type maybe rendered automatic by usingthe comparison signal as an error signal in a servo system, or loop, which adjusts the lamp intensity in such a direction and by such an amount as to reduce the error signal to zero. The regulation of the phototube output to produce a zero error signal is accomplished by regulating the light input to the phototube to a standard value. .Since the phototube receives a predetermined fraction of the light transmitted through the negative, and since the remainder of that light falls on the printing surface, the light intensity at the printing surface is regulated to a standard value, or aimpoin by the servo loop. It is well known that the log of the intensity of the light transmitted through a negative is proportional to the density of the negative. Therefore it is convenient to refer to the aimpoint of a printer in terms of the log intensity of the printing light.

When a printer of the type dmcribed above is employed for printing the three primary colors sequentially, or for otherwise separately regulating the printing exposure of each of these colors to a standard value, the integrated densities of the three colors in the print may be made equal to each other, in which case the print is said to be balanced to gray. Balancing the color densities in this manner has been found to produce acceptable color prints in a large percentage of a randomly selected group of negatives. However there are some negatives which, when printed by the above method, produce unacceptable prints. Various factors have been found to contribute to the occurrence of the last-named group of negatives, and include: (1) excessive contrast between the densities of various portions of the negative; (2) excessive departure from color balance in the overall photographed scene; and (3) non-uniform response in the three primary colors, with respect to various amounts of exposure of the original negative.

The first two of the above three factors depend upon the nature and lighting of the photographed subject and are independent of the negative density for any color.

Each of these two factors may be compensated by increice mentally adjusting the aimpoint of the printer, i.e., by adjusting the log intensity of the light at the printing surface by an amount which is some fractional increment of the log intensity of the standard aimpoint. The compensations for these two factors are referred to, respectively, as negative classification and color correction. The third factor referred to above depends upon the integrated density of the negative. Its compensation, referred to as slope control, is achieved by varying the aimpoint of the printer as a function of the integrated negative density.

In accordance with the present invention the aimpoint of the basic servo system is shifted for negative classification, color correction and slope control during the serial printing of each of'the three primary colors. The aimpoint is shifted by adjusting the operating point of the 'phototube which constitutes an element of the servo corrective factors.

' tion of the density of the negative that is to be printed.

A further object is to modify the aimpoint of a servo system by adjusting the operating point of a phototube which constitutes an element of the system. 1

More specifically, it. is an object to modify the aim- A point of a servo system by adjusting the dynode voltage of a photomultiplier tube which constitutes an element of the system. i

Other objects of the invention will appear from the following description, reference being made to the accompany-ing drawings, wherein:

FIG. 1 is a block diagram of the basic servo system of the exposure control system, as modified by the corrective input circuits of the present invention;

FIG. 2 is a schematic diagram of the details of the control phototube of- FIG. 1;

FIG. 3 is a block diagram of the n+c network of FIG. 1;

FIG. 4 is a schematic diagram of the circuit of FIG. 1, showing in greater detail the n+0 and s networks; and

FIG. 5 is a graph showing the operation of the printer control with and without the introduction of the correction factors n+0 and s.

Basic control system No. Q,794,366. The basic control system, shown sche- 1 ma-tically in FIG. 1, includes a printing lamp 10 which illuminates a negative, or transparency 12 and projects an image thereof, by means of a lens system indicated generally at 13, through a beam splitter 14 and a printing filter 16 onto a sheet 18- of printing material. The beam splitter 14 directs a predetermined fraction of the printing light through a monitor filter 20 onto a control phototube 22. Suitable lenses (not shown) may be supplied in the light paths, in a well known manner, to cause proper focussing on the printing paper 18 and on tube 22.

The voltage output of the control phototube 22 is compared to a reference voltage E the source of which is connected to the phototube output through a load resistor R The difference between the two compared volt-- ages, designated the error signal e, constitutes the control input signal for an amplifying circuit comprising a voltage amplifier 24 in series with a power amplifier 26. The power amplifier, in turn, drives printing lamp 10. The circuit comprising lamp 10, negative 12, phototube 22 and amplifiers-24 and 26 constitutes the basic servo loop, or system, which stabilizes itself in a well known manner by varying the lamp intensity in such direction and amount as to reduce the value .of the error signale toward zero. When the error signal substantially equals zero, the voltage output of the control phototube 22 is constant and virtually equals E therefore, the log intensity of the light input to phototube 22 also must be substantially constant. Since beam splitter 14 directs a fixed fraction of the printing light onto phototube 22 and the remaining fixed traction of that light onto the printing sheet 18, the light intensity on the print sheet,

or the aimpoint of the control system, is maintained virtually constant by the basic servo system.

Expressed algebraically the operation of the basic servo system is as follows:

log H-d==log h'+log h" v 1 where log -h"=log m dh'=K 2 But the relative transmission-and reflection of the beam which holds the splitter is also constant, so that the light transmitted to the printing paperis constant:

Control phototube The control phototube 22 employed in the system is a multiplier phototube, or photomultiplier tube of the well known type shown in FIG. 2, having a photosensitive cathode 21, a series of intermediate secondarily emissive electrodes 23, called dynodes, and an anode 25. The anode 25 is connected through load resistor R to source E of anode reference potential and is connected to an output terminal 29. The successive segments 27 of a potential divider connect the successive dynodes to stepped voltage sources, illustrated in FIG. 2 as extending between a maximum dynode voltage E and a reference voltage, ground.

The relatively high sensitivity of the multiplier phototube, when used as the control phototube 22 (FIG. 1), makes possible the use of a beam splitter 14 having a high ratio of transmission to reflection. The beam splitter herefore transmits onto the printing paper most of the light transmitted by the negative 12.

'Phototube 22 is required to control the light intensity at which the servo system stabilizes for a given negative density. Therefore, some means must be provided to change the operation point of that phototube, preferably without affecting the gain of the basic servo system, while as a matter of design the systemgain must be consistent with requirementsforstability, accuracy of regulation,

and degree of rejection of extraneous influence on the circuit. The transfer characteristic of the multiplier phototube is given by where z :anode current, in amperes, v

=cathode sensitivity, in amperes per lumen, =current amplification from cathode'to anode, in amperes per ampere, h=light intensity at cathode, in foot candlw, A=cathode area, in square feet.

The current gain, 7, is dependent on dynode voltage, the relationship being exponential in character:

'Y= d where:

a=a constant; 1 E =dynode voltage; and m=a constant.

Combining Equations 6 and 7,

i =C'aE h"A (8) When the phototube is operated in a system which regulates to 'keep its anode current nearly constant,

where E =referenoe voltageapplied toanode load resistor; R =value of anode load resistor; and e=error voltage output.

The error voltage e approaches zero as the closed-loop servo system reaches balance. At balance,'-for practical purposes,

E =i R ==R CaE Ah" 10) For a given circuit and phototube type, R C, a and A are constants, and Equation 10 may be written E =kE h" 11 where k=R CocA Thus, a change in dynode voltage causes a change in the light intensity at balance. 7

The transfer gain of the phototube, as employed in the self-regulating system, may be defined as the rate of change of error output voltage with respect to log intensity light input:

phototube transfer gain (12) .i log h Since the output voltage (i R is a linear function of the input light intensity, a given percentage in light input will cause an equal percentage change in output voltage.

where:

I! %X =percentage change in light input; and

-X10O=pereentage change in error output voltage,

ER based on output voltage, i,R which equals E at balance.

g 2.3Ea v (17) showing that phototube transfer gain is a function only of the reference voltage employed, andindependent of dynode voltage and of anode load resistance. f

I The dynode voltage controls the current amplification of the phototube, and therefore can be to control the light level at which the system stabilizes, according to the relations shown in Equations 6 and 11 above, without affecting system gain. The dynode voltage determines only the cathode illumination, h", .required to produce a given anode current, i

The value of dynode voltage applied to tube 22 (FIGS.

1 and 2) determines the light level at which the servo 15 system stabilizes for a given negative density. This voltage is adjusted, by means of the present invention, to accomplish three control functions, viz: '(a) setting the aimpoint of the servo system, that is, setting the exposure to the desired value for a normal, standard or average negative; (b) shifting the aimpoint to introduce the factors previously identified as negative classification and color correction; and (c) shifting the relationship of negative density to print exposure for introducing the factor identified as slope control.

Negative classification and color correction- The shifting of theaimpoint of ,the system to introducenegative classification and c'olor correction represented as follows, based upon Equation 5:

log H=d+(K'+n+c) where:

n=negative classification increment'in log units; and

c=color correction increment in logjnnits v Equation 18 shows that if negative classification and color correction are introduced, their values are included in the log intensity of the light ative 12 (FIG. 1).

Circuitwise, the corrections "n transmitted through negand c' are applied to the control phototube as incrementalchanges of its dynode voltage. It is desired to obtain a given change in intensity for a given corrective increment:

i dl If: E (19) Thus, for a desired ratio change in log light intensity at system balance, a certain ratio change in dynode voltage is required. Expressed logarithmically, Equation 19 becomes I! 10% (th 10g (hi) Where two independently applied ratio changes in log light intensity are required for the two separate corrective effects (It and c),

da da la i where n and c are the corrective changes expressed in 6 log units. Substituting Equations 23 and 24 in Equation 21 and combining,

: lo /mdm/m FIG. 3 shows the basic circuit employing a summing amplifier 30 to achieve the relationship of Equation 28. At the summing point at the amplifier input, the currents i and i must add to zero:

If the reference voltage, -E is made equal to the value of uncorrected dynode voltage, E and the factors f=10 /m 32 g: IO /m (33) then Equation 31 is the equivalent of Equation 28.

The feedback network of FIG. 3 is shown in block form as the n+0 network 28 in FIG. 1, where the output of network 28 is applied to the dynode voltage input of the control phototube 22 through summing amplifier 30.

In the full circuit diagram of FIG. 4, several modifications of the basic circuit of FIG. 3 are made. Since the printer performs red, green and blue exposures of the paper sequentially, individual potentiometer networks are provided to allow setting the factors 1'- and g independently for each color. The switching from the red to the green to the blue network for each factor f and g may be accomplished by means of a gang switch 33, which may simultaneously control the substitution of each filter 16 and 20 for another filter of a difierent color.

Additional amplifiers 36 and 38 are used to provide power for the networks in which the factors f and g are developed. Amplifiers 30, 36 and 38, as well as an amplifier 40, which is employed as described hereinafter, are preferably of the type known as operational amplifiers and described in Korn and Korn, Electronic Analog Computers, Chapter 5, McGraw-Hill, 1952.

Since the range of voltage output of the circuit to the phototube dynodes is not large compared to the normal unmodified dynode voltage, a fixed voltage supply 31 is connected in the output of the summing amplifier 30, so that the latter amplifier is required to furnish only a voltage corresponding to the correction factors applied.

Slope control The third exposure modification, identified as slope control, depends upon the integrated density of the negative. Its relationship to negative classification and color correction is best shown in FIG. 5, in which log intensity of the printing light is plotted against negative density. The line a represents the operation of the basic servo system, wherein the intensity of the printing light, or aimpoint, is maintained constant, regardless of the density ofzthe 'negative. Theflinesa rreprcsentsihe aimpoint as it is incrementally shifted when negative classification and color correction have been considered. A particular value of negative density d isselectedas that for a normal negative, and the slope control factor is applied to the system as an over-correction or under-correction for negatives departing from d Line. a, shows the system response for a particular value of over-correction, and line a; corresponds to some amount of under-correction.

The slope control factor is applied to the basic servo loop of the exposure control system'by deriving a signal that is dependent upon negative density and applying an appropriate function of that signal to the dynode-voltage input of the control phototube1-22 (FIGS. 1 and 4). The required signal, dependent upon negative density, may be taken from the printing lamp, whose log intensity is seen from Equation 5, above, to be proportional to negativedensity. This signal is derived by means of a second multiplier phototube 32, which is arranged as a logarithmic photometer, as disclosed in connection with FIG. 2 of US. Patent No. 2,413,706, granted January 7, 1947, to N. R. Gunderson. The salient characteristic of photometer 32 is that: its output dynode voltage is related to the intensity of the light source substantially as follows:

n a 5 m log m) r e and e represent dynode output voltages corresponding to lamp intensities H and H, respectively; and m is a constant. 7

where Alinear approximation of Equation 34, sufiiciently accurate over the range of operation of photometer 32, is

C C 11 log where:

C is a constant;

e =output of photometer 32 for a normal negative;

.and

H =intensity of printing lamp for a normal negative.

The output of photometer 32 is applied to a computing circuit comprising an s network 34 (FIG. 1), wherein a multiplying operation occurs. One multiplying factor is the density of the negative to be printed, expressed as a deviation from the density of a normal-negative. The second multiplying factor is the selected slope control valve and determines the degree of increase or decrease of the aimpoint as a function of the deviation from normal density. Otherwise stated, the first multiplying factor determines the horizontal position of 11,; in FIG. 5, whereas the second factor determines the slope of the line a.

Referring to FIG. 4, the s network is seen to corn prise an analog multiplier of the type disclosed and claimed in thecopending application Serial No. 756,585, filed August 22, 1958, by A. E. Glandon. Briefly, the multiplier comprises an input lead 35, which connects the output of photometer 32 directly to the input of summing amplifier 30 through a resistor-R Lead 35 also is connected tothe input of amplifier 30 through a series network comprising a rcsistor'R an amplifier '40 in parallel with a resistor R a potentiometerR, and a resistor R The summing amplifier 30 may be considered as an output stage of the analog multiplier, with the n+0 network constituting a feedback path parallel to amplifier 30. With this configurationin mind, the output'E of the multiplier is related to its input as follows:

where:

R is the equivalent resistance of the n+0 network for any settings of factors and g; and p is the ratio of output to input of potentiometer R The setting of potentiometer R to adjust the value p determines the slope of the line a in FIG. 5.

The other multiplying factor, determining the horizontal positionof d in FIG. 5, is introduced as a reference potential e in the circuit shown in FIGS. 1 and 4. 1 Referring to FIG. 4, a source of potential e is connected to the inputs of amplifiers 40 and 30 through a pair of resistors R and'R respectively. The value of e determines the value of e that produces a zero output from the multiplier, i.e., it determines the horizontal position of d in FIG. 5. The equation for the multiplier circuit, including the input of e,, is as follows:

nc 3 Bn 6 no 3 ue (R.R. R. la-.121 R.)

The output of the "s" network is applied through the summing amplifier 30 to the dynode input of the control tion as described hereinabove and as defined in the appended claims.

We claim:

1. In an exposure control system for a photographic printer, the combination comprising: means for projecting the image of a transparency onto a printing plane in which a sheet of light-sensitive printing material is to be located; a variable-intensity printing lamp for illuminating said transparency to project an image thereof onto said printing plane; means for regulating the intensity of said lamp until the intensity of the projected image reaches a selected value and including a photomultiplier tube having a plurality of control dynodes and disposed for illumination by said lamp through said transparency, said photomultiplier tube being coupled to said lamp for controlling the intensity of said lamp; means for supplying control voltage to said dynodes; means for generating a first signal which is a function of the density of said transparency; means for generating a second signal representing the density of a standard transparency; means connected to both of said generating means for comparing said first and second signals to generate a difference signal; means connected to said comparing means for multiplying said difference signal by a multiplying factor to generate a correction signal; and means interconnecting said last-named generating means and said control-voltage-supplying means for changing said dynode voltage as a function of said correction signal, thereby changing said value of image intensity as a function of said correction signal.

2. The exposure control system defined in claim 1, wherein said means for generating the first signal comprises a logarithmic photometer adapted to receive light directly from said printing lamp.

3. The exposure control means defined in claim 2, with manually operable means connwted to said multiplying circuit for selectively adjusting the latter, to thereby selectively adjust said multiplying factor.

4. In an exposure control system for a photographic printer, the combination comprising: means for projecting the image of a transparency onto aprinting plane in which a sheet of light-sensitive printing material is to be located; a variable-intensity printing lamp for illuminating said transparency to project an image thereof onto said printing plane; means for regulating the intensity of said lamp until the intensity of the projected image reachesaselected value and includinga photomultiplier tube having a plurality of control dynodes and disposed for illumination by said lamp through said transparency, said photomultiplier tube being coupled to said lamp for controlling the intensity of said lamp; with means for supplying control voltage to said dynodes; means for deriving a correction signal which is a function of the density of said transparency; a summing amplifier having an input and an output; means for applying said correction signal to the input of said summing amplifier; means for applying the output of the summing amplifier to the dynodes of said photomultiplier tube; and a multiplying feedback circuit inter-connecting the output of the summing amplifier to the input of said summing amplifier, for multiplying said dynode voltage by a selected factor and thereby changing the intensity of said projected image as a function of said factor.

5. The exposure control system defined in claim 4, wherein said feedback circuit includes two series-connected sets of multipliers, each set comprising three parallel-connected, manually settable potentiometers, and respective selector switches for selecting a potentiometer in each set, with means for operating said selector switches in gang.

References Cited in the file of this patent UNITED STATES PATENTS 2,561,243 Sweet July 17, 1951 2,757,571 Loughren Aug. 7, 1956 2,794,366 Canaday June 4, 1957

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