US2313542A - Electro-optical color reproduction circuit - Google Patents

Description

March 9,1943. 1 v, Q HALL TA 2,313,542

ELECTRO-OPTICAL COLOR REPRODUCTION CIRCUIT Filed Nov. 1, 1941 4 Sheets-Sheet 1 FIG. I.

LINEAR BLUE AMPLIFIER 26 FILTER 125 Q 115 I I k LINEAR NON-LINEAR AMPLIFIER AMPLIFIER G REIE I L FLTE p, 116 I j LINEAR NoN-LINEAR LINEAR AMPLIFIER AMPLIFIER AMPLIFIER:

LINEAR AMPLIFIER R 27 EST OF GREEN MM-L y FIG. 2. I

20) l -E@"3 10 AMPLIFIER \Q\ I15 I 1615 & LINEAR NON-LINEAR LINEAR y Q L, AMPLIFIER AMPLIFIER AMPLIFIER 1 15 j I 5 BLUE I2 J FILTER 26 I A; 106 a I46 I 24 I16 G I 2 23 4: 22 34 LINEAR NON-LINEAR LINEAR I I AMPLIFIER -Q AMPLI IER GREEN I I A Z6 FILTER A LINEAR AMPLIFIER 27 y REST OF- I 6b RED CHANNEL I 21 GREEN CHANNEL VINCENT ELHALL IJEIHN l3. ETREI FFERT INVENTORS A TTORNEY March 9, 1943. v. c. HALL ETAL 2,313,542

ELECTRO-OPTICAL COLOR REPRODUCTION CIRCUIT Filed Nov. 1, 1941 4 Sheets-Sheet 2 10B 11B FIG. 3.

LINEAR AMPLIFIER I I4 G BLUE LINEAR .0 FILTER AMPLlFlER i 106 G 12G) I 26 LINEAR NON-LINEAR AMPLIFIER AMPLIFIE 2324 GREEN LINEAR REST OF F'LTER .AMPLIFIER GREEN CHANNEL Q F|G.4. I I ZOI BLUE LINEAR FILTER AMPLIFIER 15 I26) I I 145) 26 @25 LINEAR NON-LINEAR LINEAR AMPLIFIER AMPLIFIER AMPLIFIER M40 LINE R NoN-LINEAR LINEAR I AMPLIFIER AMPLIFIER AMPLIFIER GREEN LINEAR FILTE AMPLIFIER 22 J REST OF Q GREEN CHANNEL M FIG 5 I I FILTER A I 27 l IIB LINEAR 1 AMPLIFIER GREEN FILTER REST OF 1 m; 10G LINEAR AMPLIFIER 47 RED 25 \ll, t IORV Gk LINEAR 26 46 26 AMPLIFIER l REST OF VINCENT I: HALL RED I CHANNEL II:II-IN [3.5THEIFFEHT ATTORNEY March 9,

V. C. HALL ETAL Filed Nov. 1, 1941 LINEAR 4 Sheets-Sheet 5 F IG. E. BLUE AMPLIFIER 5 FILTER 2 NEAR NEAR MPLIFIER AMPLIFIER AMPLIFIER I Z4 LINEAR 23 GREEN AMPLIFIER 2 FILTER 116 :1] qt LINEAR NON-LINEAR LINEAR I06 AMPLIFIER AMPLIFIER AMPLIFIER I 1 I so I 51 I26 I36 4 RED REST OF GREEN CHANNEL] FILTER HR /53 (t LINEAR NON-LINEAR LINEAR I, j AMPLIFIER AMPLIFIER AMPLIFIER 0R Z 1 m m 14R 5 f 27 REST OF RED CHANNELj Cfi BLUE FIG. '7. FILTER 5 IIB Gt LINEAR AMPLIFIER IOB GREEN 13G LTER H MG "Y LINEAR NON'LINEAR LINEAR AMPLIFIER AMPLIFER AMPLIFIER 10G UNEAR 5 Q I AMPLIFIER GRREEENT CHITQNNEL D RED CHANNEL VINCENT E. HALL .JEIHN [3.5TREIFFERT INVENTOR A TTORNE Y March 9, 1943.

Filed Nov. l, 1941- CIRCUIT 4 Sheets-Shee'E 4 BLUE FIG. 5. I H5 355 Q26 LINEAR LINEAR 1 I I05 AMPLIFIER AMPLIFIER 36 23 GREEN I- FILTER I HG 1267 [367 I4? 2 g LINEAR N0N-LINEA LINEAR 10G I AMPLIFIER AMPLIFIER AMPLIFIER $27 icuiifiu LINEAR I AMPLIFIER REST OF BLUESILTER H5 255 GREEN CHANNEL 5/ LINEAR D" AMPLIFIERT B 5 I GREENISH FILTER 73 FIG, 5

r 7IB LINEAR AMPLIFIER 1 mm- LINEAR 77G 25G GREEN FILTER 72B AMPL'F'ER m 7 C@ I as L, "G T 75GJ I I 736 LINEAR I I AMPLIFIER W 356) NON-LINEAR HI 27B I MAGENTAISH FILTER F E f K 716 76G NEG IVE I Q LINEAR 1 AMPL IER 77R 25R NON-LINEAR 72G v RED FILTER I- AMPLIFIER $348K, IIR 75R) qt LINEAR I F AMPLIFIER 35R NON-LINEAR J 276w BLUISH FILTER I 'E MAGENTA I, 71R NEGATIVE l A PLIFIER CYAN I 27R NEGATIVE VINCENT El. HALL INVENTOR WWW A TTORNE Y Patented Mar. 9, 1943 UNITED- STATES PATENT OFFICE ELECTED-OPTICAL COLOR REPRODUCTION CIRCUIT Vincent C. Hall and John G. Streiflert, Rochester, N. Y., assignors to Eastman Kodak Company, "Rochester, N. Y., a corporation of New Jersey Application November, 1, 1941, Serial No. 417,540

12 Claims.

separation images to correspond to colors which included negative values. The realization of such a system involves the linear subtraction of,

I quantities linearly proportional to the reflectivities or transmissions of the original colored sub ject. It is an object of the invention to provide an electro-optical circuit for obtaining this type of color correction.

It is a particular object of the invention to provide for this type of color correction combined with that corresponding to the "masking method of color correction which involves the subtraction of densities. Since densities are logarithmically proportional to the reflectivities of the original, the invention must approximate the combination of the subtraction of linear and logarithmic quantities, the subtraction of logarithm quantities being equivalent to the division of linear quantities. a

In U. S. Patents 2,176,518 and 2,221,037, J. A. C. Yule has outlined an improved form of masking, which involves the addition of exposures before the subtraction of densities is applied.

Possibly this system incorporates the principles outlined by D. L. MacAdam in the Journal of the Optical Society of America, vol. 28, 11, Nov., 1938, wherein he points out that since the "additive errors outlined by Hardy and Wurzburg, combine vectorially with the errors due to the failure to simulate the phenomena of additive mixtures by the use of subtractive mixtures, properly adjusted logarithmic subtraction can correct substantially for all the errors of both types. In any case, Yules invention requires the addition of quantities linearly proportional to refiectivities and then the subtraction of quantities logarithmically proportional to reflectivities. It is an object of the present invention to provide an electro-optical circuit for accomplishing this.

All the above objects are accomplished by the fact that the circuit according to the present invention is capable of addition or subtraction of linear quantities. On the other hand, this same device is useful for obtaining the pure division of linear quantities as taught by one of us, namely Hall, in a co-pending application Ser. No. 417,541, filed concurrently herewith.

Broadly, the present invention consists of using a ribbon type light valve with substantially parallel ribbons in a magnetic field, which ribbons are independently operated and one of them is connected to the channel whose signal requires correction whereas the other is connected to the channel carrying the' correcting signal. correcting signal has relatively less response. In the preferred embodiment of the invention applied to linear corrections such as those according to the Hardy or Yule theories, each of the ribbons is operated entirely by linear amplifiers or at least by amplifiers whose output is linearly proportional to the reflectivity of the subject, 1. e. to the intensity of the light striking the photoelectric cells at the input ends of the primary color channels.

Furthermore, the Yule theory and the most useful form of the Hardy correction, require the combination of linear subtraction or addition with the subtraction or addition of logarithmic quantities. Following the teachings of U. S. 2,249,522, Hall, the logarithmic correction may be provided by exponential amplifiers and suitable electrical control circuits. Alternatively, it may .be provided by an optical tandem light valve system as disclosed in copending application, Serial No. 393,418, filed May 14, 1941, by Vincent C. Hall. Furthermore, in preferred forms of the invention the circuits may be simplified and the logarithmic and linear correction produced with scanning-beams in which the primary modulation is obtained solely by one of the two parallel ribbons in the double-ribbon light valve. Ribbon type light valves with independently controlled ribbons are known, but have not previously been applied in any way to color correction and according to the present invention gain all of the above-discussed advantages when so applied.

That is, the present invention involves a ribbon type light valve with independently controlled parallel ribbons one of which is operated by a color signal and the other by a correcting color signal. The invention will be more fully understood from the following description thereof when read in connection with the examples disclosed in the accompanying drawings, in which: Fig. 1 illustrates a circuit for subtraction or addition of linear quantities according to the r invention combined with an electrical method Of course, the ribbon connected to the for the subtraction of quantities logarithmically proportional to refiectivities.

Fig. 2 illustrates a similar circuit in which the logarithmic co.rection is provided by optical tandem light valves.

Fig. 3 shows a condensed form of the circuit shown in Fig. 2.

Fig. 4 is-similar to Fig. 2 and employs crossed ribbons in a single valve for the optical tandem feature.

Fig. 5 illustrates an arrangement for giving a special type of color correction involving the subtraction and multiplication of quantities linearly proportional to refiectivities.

Fig. 6 illustrates an optical tandem system for logarithmic correction by red and green of the blue signal together with linear correction of the blue signal, according to the invention.

Fig. 7 shows a condensed form of the circuit shown in Fig. 4.

Fig. 8 shows the condensed form of the circuit shown in Fig. 1.

Fig. 9 illustrates a complete three-color system, in which the corrected blue and green negatives correspond to a modification of Fig. 2 and the red negative involves only the Hardy type of correction.

Since the various forms and types of color correction involve elaborate explanations, but are well understood by those skilled in the art, and since it can be obtained from the references mentioned above, no detailed description thereof will be given in this specification. However, attention is drawn to some of the pecularities of the language used in this art so that there will be no misunderstanding. An original painting or scene is said to have certain red, green and blue refiectivities, which can be separately measured and which are used to set up the primary color signals in an electro-optical color reproduction system. These refiectivities at each point of the original correspond to the amount of red, green and blue light refiected from that point. In the electro-optical system, the channels, the signals in the channels, the scanning beams, the valves, etc., are all referred to as red, green, and blue, but, of course, they are not actually colored these colors. That is, the red signal is the signal corresponding to the red reflectivities, and the red scanning beam is not red light, but is usually white light scanning a sensitive material which eventually will be a record of the red reflectivities of the original. Since these final records are used to make printers which print respectively with cyan, magneta, and yellow inks, these latter terms are sometimes applied to the various elements, but such nomenclature will not here be used.

Ribbon type light valves are well known and are illustrated in the conventional manner in the accompanying drawings, the magnets producing the required magnetic field not being shown. In Hall's patent and his copending application mentioned above, the fact that the intensity of one color signal multiplied by a constant minus the intensity of the correcting color signal can be made to simulate the division of the two signals is described and will not separately be described here. That is, the actual effect theoretically given by the circuits shown will be specified as 2 well as the relationships they have been found to simulate in practice, but no explanation of this will be attempted since such explanation can at best only be qualitative and very involved and is unnecessary.

In Fig. 1 the light from the original being scanned through a suitable beam splitter passes through a blue filter MB and a green filter IOG to impinge upon photoelectric cells H3 and 0 whereby blue and green" signals are established in corresponding circuits. Attention is drawn to the similarity of this figure with Fig. 1 of U. 8. 2,249,522, Hall. The current passes through suitable linear amplifiers B to I20, non-linear amplifiers HE and IIG whose nonlinearity is in the form of exponential functions so that the modification in the circuits corresponds to the division of logarithmic quantities at different gammas as described in Hall's patent. Through suitable control circuits II, II, II, and I9, between the linear amplifiers I23 and I26, and between other linear amplifiers II B and MG, the exact degree of correction, which actually is where Rb is the blue reflectivity and R; is the green reflectivity and the C's are constant, can be controlled. This corrected signal is fed into and operates a ribbon type light valve whose ribbon IBB and aperture IBB are shown. Thus by means of a lamp 2! and lenses 28 there is produced a scanning beam for scanning a sensitive film on a cylinder 21, which scanning beam is modulated by the valve I53 in accordance with the blue signal, corrected by the green signal.

According to the invention this is combined with a second ribbon type valve having substantially parallel ribbons 22 and 23 and an aperture 24 mounted in the scanning beam. The ribbon 22 is operated linearly through a linear amplifier 20 in accordance with the signal input in the blue channel, 1. e., in accordance with the intensity of the light striking the phototlectric cell lIB. Simiiarly, the ribbon 23 is operated by amplifiers 2| linearly in accordance with the green signal. By arranging the ribbons 22 and 23 to move in opposite directions, the effect can be made additive, whereas the modulation of the blue signal by the green signal electrically is effectively the subtraction of logarithmic quantities, so that the whole combination corresponds to Yules photographic system referred to above. Alternatively, if the ribbons 22 and 23 move in the same direction with increasing signal respectively, this involves the subtraction of quantities linearly proportional to the input signals and hence, the whole combination gives the correction of additive errors combined with correction by masking, i. e. by subtraction of logarithmic quantities.

The same eifect is obtained with the arrangement shown in Fig. 2, but in this case the subtraction of logarithmic quantities is obtained by having a glow lamp 30in optical tandem with the light valve IBB so that as before the correction is which by proper selection of constants can be made to simulate the division of refiectivities over all the'range required in practice.

This arrangement shown in Fig. 2 can be further simplified as shown in Fig. 3, if the nonlinear amplifier I3G has an exponential function such that the need for another non-linear amplifier in the blue circuit is eliminated. That is. the ribbon 36 is operated entirely by a linear amplifier 35B so that there is no chance to change the exponential or gamma in the blue signal. For example, if in Fig. 3, the non-linear amplifier "G has a .5 exponent, the eifect of the ribbon is masked by a .5 gamma mask, 1. e., by the output of the glow lamp 86. Since the ribbon at has a linear response, it is also aifected linearly (either addition or subtraction) by the ribbon 25 operated by a linear amplifier 2i as before. Going bacl; to Fig. 2, it is pointed out that a .5 mask could in obtained in this more complicated arrangement by having ISG operate at .25 eitponential and i813 operate at .5 exponential, the ratio being the effective mashing factor.

Fig. 4 also shows an optical tandem system, in which the optical tandem valves are successive ribbons c9 and ii crossed in a single ribbon type light valve having an aperture 632. As indicated by the use of similar numerals, the other elements of this Fig. 4 correspond exactly to Fig. 2.

n the other hand, Fig. corresponds to an entirely difierent type of color correction, at least as far as the theoretical intensity of the scanning beam is concerned. In this figure, the red channel is also shown and each of the channels has solely linear amplifiers 53b3, 856, and 35R respectively. In this arrangement, the ribbon 3S operated by the blue signal has its response linearly reduced or increased by a parallel ribbon 13 in the same light valve having an aperture 69. Thus this valve gives either (Rb+Rg) or (RbRg). To simulate the eilect of masking except for the logarithmic feature, both by green and red, an additional valve also having independently operated parallel ribbons is placed in optical tandem with the valve containing the ribbon 3%. One of the parallel ribbons 35 is operated by the green signal in series with the ribbon and the other ribbon lid is operated linearh' by the red signal. The aperture of the second valve is shown at 5?. The net output of this arrangement is given by the following formula (Rt i C5Rg) (Cu C'IR;,' (18120 which in practice simulates R iCS Q B IU r more or less.

In Fig. 6 the blue valve 55B is in optical tandem or optical series with valves 58 and 52 having apertures 5i and 53 respectively and operated by proper exponential functions of the green and red channels for masking. As in Figs. 1 and 2 the linear correction is provided by a valve having independently operated ribbons 22 and 23. The valve 5d replaces the glow lamp 30 of Fig. 2 and additional correction by the red signal through valve 52 is provided in this figure.

The similarity of Figs. '7 and 4 becomes apparent when one notices that the ribbon 36 in Fig. '7 is the sole modulator for giving the primary modulation to the scanning beam. Its effect is reduced or increased linearly by the ribbon 23 and is reduced logarithmically by the ribbon 55 mounted at right angles to the other two ribbons in the same light valve and in optical tandem with these other two ribbons. As in the case described in connection with Fig. 3, the control ribbon 35 is operated entirely linearly and hence the non-linearity of the amplifier I3G is exactly the masking factor.

Similarly Fig. 8 is a. simplified form of Fig. 1, the non-linear amplifier "B being eliminated and the exponent of the non-linear amplifier 13G being adjusted so that the right degree of correction is provided in a linear amplifier or modifier 60 controlled by suitable control ll.

Each of the above discussed figures show the correction of the blue scanning beam. Fig. 9 shows all three beams. blue, green and red for printing the yellow, magenta, and cyan printer negatives respectively. According to the theoretically correct spectral sensitivities for a typical additive process, the primary blue should include a negative greenish section, primary green should include a negative blue and a negative 1 red section, and primary red should include a negative section in the blue-green region. In practice the correcting factor corresponds respectively to green, red plus blue, and green re spectively. However, in Fig. 9 the correcting 15 colors areshown theoretically different and are labeled respectively greenish, magentish, and bluish. For simplicity the lenses of the optical systems in the scanning beams are omitted from this figure. The blue scanning beam is produced by a light source 263 modulated primarily 5 cell MB which operates a linear amplifier MB and a ribbon 133 which is parallel to the ribbon 86B. expression (Rb-Rn). However, this beam requires color correction of the masking type, i. e.

the subtraction of a quantity logarithmically pro-- portional to green or in some cases to yellow. In this figure the non-linear amplifier 15G operates a ribbon 'ilG in optical tandem with the valve 383 to give the color correction of the required type. Similar correction is obtained in the green scanning beam from the lamp 25G, similar numerals being used to illustrate similar features and elements, G and R being substituted for B and G respectively. However, in both of these 40 cases, the masking signal coming in through HG should also be corrected linearly before it is used to give the masking effect. Theoretically, this requirement is not compatible with the requirement that the ribbon i'iG must be operated by a non-linear amplifier. However, for practical purposes, the same effect can be obtained by having a non-linear amplifier 18G carrying the magenta signal into a ribbon MG and parallel with the ribbon HQ in the same valve. Theoretically this correction is :R:.) but isefiectively (Rg-Rm) As before, similar numerals are used for similar elements in the three channels and B, G, and R refer to the blue,

green and red channels. In practice, the elements NB, HB, and 123 can be eliminated, the ribbon 133 being operated directly from the am plifier 35G. Similarly, elements 10G, HG and "G can be replaced by a linear combination of the output of the amplifiers 35B and 35R. And again, the elements 10R, HR, and 12R can be omitted and replaced by connections into the amplifier 35G.

In all of these arrangements shown, the indementioned above, parallel ribbons mounted for independent operation by two different colors for color correction can be utilized by Hall's invention to give pure division of reflectivities.

Having thus described various embodiments of 76 the invention, we wish to point out that it is not Thus the main modulation is given by the,

limited to these structures but is of the scope of the appended claims.

What we claim and desire to secure by Letters Patent of the United States is:

1. In an electro-optical color reproduction circuit with at least three channels respectively for carrying the primary color signals, color correcting means consisting of a ribbon type light valve with substantially parallel ribbons, an amplifying circuit connecting one of the channels to one of the ribbons for deflecting the ribbon in accordance with and relatively greatly with respect to the signal incident into said one of the channels, and a second amplifying circuit connecting at least one other of the channels to the other ribbon for deflecting it in accordance with and relatively less greatly with respect to the signal in-' cident into said other of these channels.

2. Color correcting means according to claim 1 in which each of the amplifying circuits deflect the corresponding ribbon linearly relative to the respective incident signal.

3. Color correcting means according to claim 1 in which each of the amplifying circuits is purely linearly and is connected to the output of a purely linear portion of the corresponding channel.

4. An electro-optical color reproduction circuit comprising at least three channels respectively for carrying the primary color signals, means for modifying the signal in one of the channels exponentially in accordance with the signal in at least one other of the other channels and linearly in accordance with the signal in at least one of said other channels, said modifying means including for its linear part a ribbon type light valve with substantially parallel ribbons and two linear amplifying circuits respectively connecting the ribbons to said one of the channels and to at least one of said other channels.

5. An electro-optical color reproduction circuit comprising at least three photoelectric cells for respectively receiving the primary color components of a colored subject, means for producing a scanning beam modulated primarily by the output of one of said cells, means including an exponential amplifier connected to the output of another of the cells for exponentially modifying the modulation of said beam, a ribbon type light valve with substantially parallel ribbons in the path of said beam, linear amplifying means operating one of the ribbons linearly in accordance with the output of said one of the cells and linear amplifying means operating the other ribbon linearly in accordance with the output of another of the cells.

6. An electro-optical color reproduction circuit in accordance with claim 5 in which the modulated beam producing means includes a light valve and an amplifier for operating the valve by the output of said one of the cells and a control circuit is connected between the exponential amplifier'and the operating amplifier for exponentially modifying the signal in the operating amplifier.

'7. An electro-optical color reproduction circuit according to claim 5 in which the modulated beam producing means includes a light valve and an amplifier for operating the valve by the output of said one of the cells and another light valve operated by the output of the exponential amplifier is in optical tandem with the lastmentioned valve.

8. An electro-optical color reproduction circuit according to claim 5 in which said one of the ribbons constitutes exculsively the means for modulating the scanning beam by the output of said one of the cells and acontrol circuit is connected between the exponential amplifler and the amplifying means operating said one of the ribbons for exponentially modifying the signal in the latter amplifying means.

9. An electro-optical color reproduction circuit according to claim 5 in which said one of the ribbons constitutes exclusively the means for modulating the scanning beam by the output of said one of the cells and another light valve operated by the output of the exponential amplifler is in optical tandem with the ribbon type valve.

10. An electro-optical color reproduction circuit comprising at least three photoelectric cells for respectively receiving the primary red, green, and blue components of a color subject, means for producing at least three scanning beams modulated respectively primarily by the outputs of the three cells, blue modifying means for increasing linearly and decreasing exponentially the signal in the beam corresponding to blue in accordance with the signal for at least one of the other cells and green modifying means for increasing linearly and decreasing exponentially the signal in the beam corresponding to green in accordance with the signal from the red cell, the blue modifying means including an exponential amplifierconnected to the output of at least one of the green and red .cells, means operated by said exponential amplifier for decreasing the signal in the blue beam, a ribbon type valve with substantially parallel ribbons in the blue beam, linear amplifying means for operating one of the ribbons linearly in accordance with the output of the blue cell and linear amplifying means for operating the other ribbons in the opposite direction linearly in accordance with the output of said at least one of the red and green cells, the green modifying means including an exponential amplifier connected to the output of the red cell, means operated by the latter exponential amplifier for decreasing the signal in the green beam, a ribbon type valve with substantially parallel ribbons in the'green beam, linear amplifying means operating one of the latter ribbons linearly in accordance with the output of the green cell and linear amplifying means operating the other of the latter ribicons in the opposite direction linearly in accordance with the output of the red cell.

11. An electro-optical color reproduction circuit comprising at least three photoelectric cells for respectively receiving the primary red, green, and blue components of a color subject, means for producing at least three scanning beams modulated respectively primarily by the output of the three cells, blue modifying means for increasing linearly and decreasing exponentially the signal in the beam corresponding to blue in accordance with the signal from the green cell and green modifying means for increasing linearly and decreasing exponentially the signal in the beam corresponding to green in accordance with the signal from the red cell, the modifying means including exponential amplifiers connected respectively to the output of the green and red cells, means operated by the exponential ampliflers respectively for decreasing the signal from the blue and green beams by factors inversely proportional to between the .4 and .8 power of the green and red signals, a ribbon type valve with two substantially parallel ribbons in each of the blue and green beams, a linear amplifier for operating one of the ribbons of the valve in the blue beam linearly in accordance with the output of the blue cell, a linear amplifier for operating the other ribbon of the latter valve linearly in the opposite direction with about a sixth to a third the response in accordance with the output of the green cell, a linear-amplifier for operating one ribbon of the valve in the green beam linearly in accordance with the output of the green cell and a linear amplifier for operating the other ribbon in the latter valve linearly in the opposite direction with about a sixth to a third the response in accordance with the output of the red cell.

12. An electro-optical color reproduction cir-' cuit comprising a plurality of channels, three of which carry respectively at their inputs, signals corresponding to the positive portions of theoretically perfect primary colors, and at least one of which carries a signal corresponding to the negative portion of one of said colors, means for producing three scanning beams modulated primarily respectively in accordance with said positive portion signals, a ribbon type light valve with two substantially parallel ribbons in the beam corresponding to said one of said colors one ribbon being connected to and operated by the channel corresponding to said one of said colors, and the other being connected to and operated in the same direction by the negative portion signal, and means for modifying at least one ofthe beams exponentially in accordance with a positive portion signal different from the one modulating said one of the beams.

vmcnn'r c. HALL. JOHN G. STREIFFERT.

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