US2183524A

US2183524A - Black printer for photomechanical processes

Info

Publication number: US2183524A
Application number: US138351A
Authority: US
Inventors: John A C Yule
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1937-04-22
Filing date: 1937-04-22
Publication date: 1939-12-19
Anticipated expiration: 1956-12-19
Also published as: DE740468C

Description

J. A. C. YULE BLACK PRINTER FOR PHOTOME'CHANICAL PROCESSES Filed April 22,

2 Sheets-Sheet 1 EFFECTIVE DOT AREA ORIGINAL FIG' .E BRIGHT LIGHT DEEP REFLECTION FACTOR BLACK WHITE YELLOW BROWN GREEN (TRANSMISSION) H n n n n I] n n H n PRIMARYCOLOR$-- 86' R 86' R BGR BGR BGR REPRODUCTION i SuBTRACTIvE BRIGHT LIGHT DEEP COLOR CONTENT BLACK WHITE YELLOW BROWN GREEN (PRINTING DENSITY) DARTS INDICATE COLOR CORRECTION P/GMENT COLORS Y M BGX V i1 55 Y I 5G Y M 86% Y M BGX i I 34 BLUE SENSITIVE j 35 GREEN SENSITIVE 7 36 RED SENSITIVE I FIG.4 34

A BLACR wI-IITE YELLOW BROWN GREEN NEGATIVE FOR I ET BLACK PRINTER b I I INVENTOR.

ATTORNEYS J. A. C. YULE Dec. 19, 1939.

.BLACK PRINTER FOR PHOTOMECHANICAL PROCESSES Filed April 22, 1957 2 Sheets-Sheet 2 FIG 5.

BEAMSPLITTER John AC. Yule INVENTOR.

ATTORNEYS Patented Dec. 19, 1939 BLACK PRINTER FOR PHOTOMECHANICAL PROCESSES John A. C. Yule, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application April 22, 1937, Serial No. 138,351

- 13 Claims.

This invention relates to photomechanical processes and more particularly to methods of making a black printer for use in four-color photomechanical processes.

It is an object of the invention to provide a direct and simple method of making a black printer from a multi-colored original, such as a color photograph, a color transparency, or even a natural scene.

It is also anv object of the invention to provide a method of making a black printer which is color corrected."

It is an object of the invention to provide a method of making a black separation image from which a black printer may be made and in one embodiment of the invention the black separation image is itself a halftone image and hence, the black printer may be made directly therefrom without the use of a halftone screen at this stage.

It is a particular object of the invention to pro vide a black printer, whose printing density at each and every point is substantially the theoretically correct value or more nearly approximating this value than any hitherto available and to provide a methodof making such a printer.

It is another particular object of the invention to provide a method of making a black printer having the above advantages which employs electrical apparatus similar to that disclosed by Murray and Morse in copending application, Serial Number 120,964, filed January 16, 1937.

It is also an object of the invention to provide such a method of making a black printer which employs the color correcting features of the apparatus discussed in that copending application.

It is also an object of the invention to provide apparatus for carrying out the above methods.

Other objects and advantages of the invention will 'be apparent from the following description when read in connection with the accompanying drawings in which:

Fig. 1 is a chart illustrating the primary color components of five typical colors appearing in 0 the original. Fig. 2 is a similar chart showingthe subtraccludes diagrammatic circles corresponding to, and illustrating the effective area of, each dot.

Fig. 5 is a perspective view of a simplified arrangement which serves to illustrate another embodiment of my invention.

According to the simplest form of the theory underlying three-color photography and process work, light reflected or transmitted by any given color may be considered (as far as the human eye is concerned) as being made up of three primary colors, red, green, and blue. The original may be reproduced additively from the three colors or subtractively from the three complementary colors, blue-green, magenta, and yellow.

In Fig. 1 the heights of the vertical lines indicate the percentage of the primary colors B. G. and R. (blue, green and red) present in, i. e. re-' flected or transmitted by, the five typical colors chosen for purposes of illustrationblack, white, bright yellow, light brown, and deep green. As indicated, pure white has a reflecting factor or transmission factor F of approximately 1 for each of the primary colors and a certain light brown, for example, reflects (transmits) blue 16%,green 32%, red 63%. I use the term Fmax. to mean the reflection or transmission factor for the predominant primary color component-4n this case, red (1. e. Fmax.=.63). 1-Fmax. iS the absorption factor for this component: l/Fmax. is the opacity of this color for this component and the logarithm of l/Fmax. is the density of this original color to this, its predominant component. In reproducing the typical colorsby a threecolor process, the printing density .of the three subtractive color inks, Y. M. B.-G. (yellow, magenta, blue-green) are indicated by the heights of the vertical lines in Fig. 2. (Note: Themagenta, yellow and blue-green inks are sometimes called red, yellow, and blue, but such terminology and its accompanying confusion will beavoided in this specification.) 'Throughout'thisspecification and the appended claims, the term printing density refers to the density of the ink in the reproduction and hence to the density of the printer. It is used to include both continuous tone and halftone density where halftone" refers to any suitable process wherein relative densities are represented as relative areas of minute elements such as the well-known halftone dot or line. My invention is applicable to both continuous tone and halftone processes. In either case the term solid tone refers to the result produced when the maximum available printing density is used. The printing densities of the inks as represented by the verticals in Fig. 2 are equal to the densities of the original colors with respect to the corresponding primary color complementary to that of the ink. The darts at the side-of each vertical indicate the printing densities after color correction has been applied as will be discussed below. For clarity part of the following description will be confined to the consideration of the halftone dots, but it will be obvious that any of the equivalent systems may be used satisfactorily. The heights of the verticals in Fig. 2 are a logarithm function of the effective area of the halftone dots printingeach particular color.

Due to the limitations of three color inks and printing processes in their present state of development, a certain amount of color correction is necessary and many suitable methods of color correction are well known to those skilled in this art. Certain of these methods known as tourcolor processes, employ a black printer in addition to the magenta, yellow, and blue-green printers for the correction of the gray tones. Color correction such as hand retouching or masking is employed with either three or four color processes.

""Theore'tically, equal amounts of yellow, ma-

genta, and blue-green should appear gray; solid (tone) yellow plus solid magenta plus solid bluegreen should appear black. If a solid tone has a density of 1.0 as shown in Fig. 2, light brown will be, reproduced by say .8 solid yellow plus .5 solid magenta plus .2 solid blue-green measured as densities. If a black printer is to be used for the gray tones, this particular light brown may be considered as comprising .2 solid gray (i. e. .2 black) plue .6 solid yellow, .3 solid magenta and 0 blue-green. For correct color reproduction in a four-color process, the printed gray contents, as indicated in Fig. 2, must be equal to the lowest of the three subtractive color contents which would be used in a three-color process. In this chart, the height of the black vertical labeled X is, therefore, equal to the shortest of the yellow, magenta, and blue-green verticals, which is, in turn, proportional to the density of the original color with respect to the predominant primary color component thereof: (i. e. proportional to Expressed differently, it is proportional to the log whichever of the primary filters gives the lowest density.

Therefore, in a correct four-color process, the black printing density should be proportional to the height of the black vertical and the other three printing densities should be proportional to the difference of the heights of the corresponding vertical and the black vertical. It is common practice in certain types of work not to bother to reduce the printing densities of the color printers .when a black printer is used orto reduce them only a relatively small amount since the errors introduced by omitting this reduction appear only as increased color contrast and are often commercially acceptable.

One method of making a black printer which is well known to the trade, is from a separation negative made on a panchromatic plate through a yellow filter. By such a procedure, there is recorded not only the densities of neutral areas of the original causing equal absorption of the three primary colors but also the eifective density of pur colors such as magenta which absorbs the primary green portion of the incident light. Obviously, in reproducing a pure magenta, magenta ink only should be used and a black printer should have a zero printing density at this point. Hence, as is well known, considerable costly and time-consuming hand-retouching is necessary on a black printer made by this method.

Another method of making a black printer as disclosed by Murray in copending application Serial Number 44,125, comprises making a black separation negative through an infrared filter. It the multi-colored original comprises only blacks which absorb infrared and magentas, yellows, and blue-greens, which transmit or reflect infrared (i. e. no blacks or grays made up of equal amounts of these yellows, magentas, and blue-greens) a black printer obtained by this method will be correct according to the above discussed theory and will require little, if any, further retouching.

According to the present invention a method of making a black printer is provided which directly employs the above theory and which exposes a photosensitive surface such as a sensitive photographic layer or a photoelectric cell to each or the three colors reflected or transmitted by the original and employs the intensity of the primary color which is most predominantly reflected or transmitted, to control the printing density of the black printer. Thus, according to my invention, there is provided for the first time a method of making a black printer which is free from the necessity of using hand retouching and which is applicable to any original irrespective of the absorption-properties of the coloring materials of that original in the invisible portions of the spectrum. As mentioned previously, the best available colored inks have limitations, but, my invention provides a black printer capable of realizing the full abilities of these inks without recourse to hand retouching.

My invention also provides a method of making a color corrected black printer which comprises illuminating the original with white light (this term is used in its usual sense and all the variations in white which are well known in color photography need not be discussed here) which is, of course, made up of the three primary colors. The light reflected or transmitted by each point of the original is, therefore, made up of three light energies whose intensities are respectively proportional to the reflection or transmission factor of the original for the three primary colors. I use the term point of the original to mean a small elemental area and since the invention is primarily concerned with half tone reproduction, the term, "point of the printer may be considered as the area containing only one half-tone dot. In one embodiment of my invention (discussed below) these light energies are translated into electrical energies. Color correction corresponding to the masking method or to any similar method of color correction, may be introduced by modifying the energies (luminous or electrical) corresponding to at least one of the primary colors in accordance with the energies corresponding to at least one other of the primary colors.

As described in the Murray and Morse application mentioned previously, this modification may be introduced after part or all of the energies have been translated into their electrical form. On the other hand according to the simple masking method, the modification is applied directly to the light energies.

In the masking method of color correction, a

negative image of the original is made through a suitable'filter and processed usually to low relative density and lowcontrast. This image'or a positive made therefrom is used'to mask the light coming from the original." If the original is a transparency, the mask may be placed in contact with and on either side of the original so that it intercepts the light, before or after it passes through the original. In fact, various suitable arrangements may be provided wherein the mask is in register with the original or a real image of it. If the original is opaque the mask may have one-half the required masking density at each point and may be placed in contact with the original so that the light reflected thereby goes through the mask twice, once before striking the original and once after it has been reflected thereby. It is obvious that any of these masking methods may be employed in the present invention.

According to the embodiment of my invention illustrated in Fig.3, a negative from which a correct black printer may be made, is produced directly by forming a halftone image of a multicolored original on a three layer subtractive color film, preferably a monopack. White light represented by arrows 30 passes through or is reflected from an original multi-color transparency 3| and through a halftone screen 32 to expose a monopack film 33' comprising three

photosensitive layers

34, 35, and 36. It is obvious that the "white light 30'should be preferably that for which thesensitivities of the three layers '34, 35 and 36 are balanced. The original 3|, the screen 32, and the film 33 are shown as if mounted in a process camera having a lens 31. Any well known and equivalent set-up for making a halftone neggood definition inand registration of the three layers is obtainable thereby. In this specification and the accompanying claims the term photographic is employed in a broad sense so that photographic layer includes not only'the silver halide type, but also those employed as photo sensitive resists.

For the purpose of this embodiment-of my invention, the image in this photographic pack is preferably processed in monotone, (for example, to .form the ordinary silver image). This image may be developed in any suitable manner, dyed,

reduced and/or intensified, provided that sufiie I cient contrast is obtained thereby. If, for any reason, sufficient contrast is not available by any of these processes, the final negative image may be copied twice on high contrast materials by making a positive and then another negative H from which the black printer may be finally made.

When this film is developed the individual halftone dots in

thelayers

34, 35, and 36 appear as shown in the highly magnified cross section -repre-' sented in Fig. 4. The particular color examples chosen correspond to those discussed in connection with Figs'.'1 and 2. As shown, each of the halftone dots in this black separation negative may be considered as comprising three super-imposed dots, one corresponding to the blue'light fromthe original, one corresp'ondingto the green light from the original, and one corresponding to the red light from the original. It -will be seen that the effective printing area of each dot, as shown by the accompanying circle, is propor tional to the area of the'largest of the three component dots. (Note: These halftone dots do not actually appear circular in form especially at the higher densities where they'merge and overlap and finally fill in solidly.) ,In accordancewith my inventiomtheeffective printing area of each dot (the so-called dot formation) is thus controlled by the'int'ensity of the predominant primarycolor of the light producing that particular dot.

A black printer made from this black separation negative will comprise a'dot formation having a printing density proportional to the density of the original to the predominant primarycolor component thereof which is equal to the printer density of the least predominant subtractive color printer under a three color process as required by the theory illustrated in Fig. 2.

To allow for further color correction by means of masking, I have found that a negative mask s milar to that usually employed for correcting the yellow (or magenta) printer gives useful and desirable results when employed in the making of this black' separation negative.

Fig. 5 shows broadly the principle employed ii an electro-optical system for making color separation images from a multi-colored transparency This and many other similar arrangements are discussed in the Murray and Morse application previously mentioned. A white light source It illuminates a multi-colored original II which is mounted on a transparent cylinder which is re tated on a screw, not shown, whereby a scanning beam, successively coming from difierent elemental areas' on the transparency II is focused by an optical system I2 through a suitable beam splitter l3 and three primary color filters R, G, and MB onto three photoelectric cells I5R,

These photoelectric cells |5R, |5G and |5B, together with their accompanying electrical circuits |6R, I6G, and IE3, control three light valves HR, HG, and NB res'pectively,each of which controls the illumination from a light source I8 and optical system l9 falling on aphotosensitive surface 20. This portion of the. drawing is shown in broken lines since it is not pertinent to the present invention. The photosensitive layers 20R,

MG, and 20B, are mounted on a cylinder 2| to'be i scanned by'the light bea'ms controlled by the valves HR, MG and I 1B. The previously mentioned Murray andMorse application describes methods whereby the output of any one of the three photoelectric cells may be modified in accordance with the output of at least one other of the cells so'that the' light valves Ii are operated to give the separation images which are corrected without further masking. Various types of suitable light valves arewell known and need not be described in detail here.

In accordance with the present invention three additional light valvsTI'R, H'G, and llB corresponding respectively to |'|R.,' HG, and NE,

successively intercept the light coming from a source I8 through an optical system 59' and scanning a sensitivephotographic layer 23 sim'i larly mounted on'a cylinder; 2 I. Since the valves,

(for example HR) are usually operated so that the separation images obtained therefrom are negative (i. e. the valve opening at HR is greater for greater output from the photoelectric cell ISR), the auxiliary valves II'R, ,etc., are-connected to operate in the opposite way. The amount of light striking the photographic layer 23 isdetermined by the valve which has an area (H'R, I'IG, or I'I'B) which is effectively less than that of the other two valves and is independent of the area of the other two. Preferably the opticalsystem I9 is arranged so that ,the light beam is intercepted by the three light valves I1 is collimated, but if a divergent or convergent beam is employed (e. g. to add weight to the rela tive eifect of one of the valves with respect to.

another or to compensate for errors in valve construction) the invention operates similarly since it concerns only the valve having the least effective" area, (which may differ from actual area.)

Since the opening of the valves I1 is inversely proportional to the output of the photoelectric cells IS, the black separation image formed on the photographic layer 23 is a positive The density at any point on this positive is logarithmically proportional to the area of the least open valve and, hence, logarithmically propor-' tional to the output of the photoelectric cell receiving the predominant color from that portion of the original. Wherefore, when the set-up is arranged to give approximately linear response, the density at each point on this positive is proportional to the density of the corresponding point of the original to the primary color which is the predominant component of this corresponding point of the original.

The original I I may be illuminated and scanned.

by reflected or transmitted light in any suitable manner. If it is a multi-color negative of a multi-color original, the connections to' the valves I'I' must be reversed so that the image 23 is always a positive of the original scene. On the other hand, the three color separation images 20R, 20G and 208 may be recorded as either positives or negatives as desired.

The photographic layer 23 may be a photosensitive resist mounted on a cylinder so that a rotary black printer may be made directly, provided a -halftone pattern is introduced at some point in the system, or preferably it may be a film on which a positive is recorded from which a negative is made which in turn is used in printing the black printer. Color correction may be introduced by placing a mask over the original II or by any of the methods proposed in the Murray and Morse application mentioned previously, without departing from the spirit of the present invention. Using a modification of my invention similar to that shown in Fig. 5, it may be possible to operate one light valve in accordance with the intensity of .the maximum of three electrical currents in the circuits IBR, lliG, and IE3. However, such an arrangement will not be described in detail since it is simpler to .use three separately operated light valves II'R, II'G; and I 'l'B to control one light beam as shown and described.

On the other hand, one systemfor choosing the maximum of three electric currents is the invention of Hall and Morse and is described "in their application Serial No. 215,822, filed June 25, 1938.

In general, when color correction is employed, the printing density of the black printer should equal the least predominant subtractive color content (expressed as density) after the correction has been applied. In Fig. 2 this corrected black printer" density is indicated by a dart on the side of each black vertical (in. For example, when color correction is introduced in a process for reproducing deep green, the printing density of the black printer is reduced from .4 to .2 to correspond to an equal reduction in the magenta color content as shown. Since color correction diflerentiallymodifies the subtractive color contents, there are. some cases where the blackprinter density is controlled by .say the blue green content beforecorrection and by the magenta content after correction has been introduced. In such cases, which are relatively rare, the black printer density is not controlled by the actual predominant primary color of the original.

In any case, the black printer density is prob ably not equal to the density of the original to its predominant color component, but to a value 'less than this by an amount due to the color correction and also by an amount (uniform over the whole printer) sufficient to give zero printing densities in the highlights (1. e. the lightest portions of the original). In the original, the highlights may have a fairly high density (e. g. a color screen plate may have a density of 1.0 in the highlights).

Therefore, when no color correction is employed, the black printer density (at any point) is equal to the density of (the corresponding point of the original to the predominant primary color component of that point less the density of white (a highlight) in the original to the same primary color. There may be no actual highlight or white in the original, but a white" should be understood as being associated therewith whether actually present or not. In other words, the black printer density is equal to the logarithm of the ratio of two opacities-the opacity of the original to its predominant primary color divided by the opacity of, white" to the same color.

Furthermore, when ordinary masking color correction is employed, the black printer density (at any point) is equal to the density of the (corresponding point of the) original to one particular primary color increased by a correcting density and reduced by the density of "white" in the original to the same primary color. In this case the primary color is the one which is most predominant of the three after the correcting densities have been applied thereto and the correcting densities are proportional to densities of the original to at least one other of the primary col- 'ors (ordinary masking proportions as determined by the contrast to which the mask is developed or the electrical constants in the corresponding electric hook-up) For example in the case of "deep green which reflects 10% blue, 40% green and 10% red, the corresponding uncorrected printer densities are yellow 1.0, magenta .4 and blue-green 1.0. (These densities are in practice reduced by an amount which makes the "white" density for each printer equal to zero). Due to limitations of the inks, it is necessary to reduce the magenta printer density by one-fifth of the blue-green printer density (i. e. by .2). Therefore, the black printer density must equal the reduced magenta printer density (i. e. .4-.2=.2) (again corrected to make the highlights print white) Thus according to this embodiment of my invention (whether the color correction is introduced electrically or by some form of ordinary ing the original with white light made up of three energies corresponding to the three primary colors. the light reflected or transmitted thereby comprises reduced energies corresponding to the primary color components 01' each point of .the original. In the electrical embodiment of the invention, these reduced energies are recorded momentarily as changes in photoelectric cell characteristics and thus converted into electrical energies which subsequently control the light energies exposing the corresponding points of the printer. The analogy in the'embodiment of my invention employing a sensitive photograph pack is as follows: the reduced energies are recorded as relative densities in a photographic negative which subsequently control the light energies exposing the corresponding points of the printer. In both cases, the color correction is introduced by modifying the energies representing one of the primary color components in accordance with the corresponding energy representing one or both of the other primary color components.

It will be noted that in the embodiment shown in Fig. 4 the efiect (the size of the halftone dot) of the most predominant primary color overshadows and blocks out the efiect of theother two colors; in the embodiment shown in Fig. 5 the elfect (the area of the smallest valve) of the most predominant primary color again 'overshadows and blocks out the efiect of the other two colors. In the embodiment specifically invented by Hall and Morse, the efl'ect of the two lesser currents is blocked out by the greatest current so that only the greatest current controls the result. In the accompanying claims, this action is referred to as blocking--the meaning being obvious. In each case the greatest response blocks the other two and then solely controls the blackprinter.

Having thus described my invention and a number of arrangements with which it'may be practiced, I wish to point out that it is not limited to these specific arrangements but is of the scope of the appended claims.

What I claim and wish to protect by Letters Patent of the United States is: 4

1. In a photomechanical process, and method of making a black separation image of a multicolored original which comprises exposing a multi-layer photographic pack with layers sensitive to difierent regions of the visible spectrum in halftone printing relation through a halftone screen to the original and developing the pack.

2. In a photomechanical process, the method of making a black separation image of a multicolored original which comprises making a masking image from and for said original, illuminating the original with white light exposing a multilayer photographic pack with layers sensitive to difierent regions of the visible spectrum through said masking image and a halftone screen in halftone printing relation to light from the original and processing the pack in monotone to a high contrast.

3. In a four-color photomechanical process, the method of making a black printer from a multicolored original which comprises illuminating the original with white light exposing a multi-layer photographic pack whose layers are differentially sensitized to the three primary colors, through a halftone screen in halftone printing relation to light from the original, processing the pack to give a negative halftone image wherein the effective area of each element of the halftone pattern Due to absorption by the original,

is controlled by the intensity of the predominant color constituent of the light producing said elements and making a positive of said negative halftone image.

4. In a photomechanical process, the method of making a black separation image of a multicolored original which comprises scanning the original with the three primary colors, establishing electrical energy corresponding to each of the primary color components of each point of the original, controlling the-efiective area of three light valves 'each inversely andrespectively in accordance with one of the three energies so established, scanning a sensitive photographic layer with light transmitted successively by the three valves and developing the layer whereby the density, of the layer is controlled by the valve having the effective area which is least.

5. In a photomechanical process, the method of making a black printer from a multi-colored original which comprises scanning the original with the three primary colors, controlling three light valves each in accordance with the absorption by the original of one of the primary colors, scanning a sensitive photographic layer with light transmitted successively by the three valves whereby the intensity of the light is controlled by the valve whose area is efiectively least and developing the layer.

6. "A device for making ablack printer to be used in the photomechanical reproduction of a. multi-colored original which comprises photoelectric means for scanning the original with the primary colors, a plurality .of light valves, means including the photoelectric means for controlling each of said light valves in accordance with one of the primary colors from the original, a light source positioned to project a beam. successively through the light valves whereby its intensity is varied in accordance with the aperture of the ,valve whose area is efiectively less than the others, and a scanning arrangement including said beamwhereby a sensitive photographic layer suitably mounted on said device will be scanned by the beam.

7. In a photomechanical process, the method of making a black separation image of a multicolored original which comprises scanning' the original with the three primary colors, establishing electrical energy corresponding to each of the primary color components of each point of the original, modifying the energies corresponding to atleast one of the primary colors in accordance with the energies corresponding to at least one other of the primary colors, controlling the effective areas of three 'light valves each corresponding to one of the primary colors in accordance with the respective energies so established and modified, scanning a sensitive photographic layer with a light beam which has passed successively through the three valves and developingthe layer wherein the density is controlled by the valve having an elfective area less than either of the others.

8. A device for making a black printer to be used in the photomechanical reproduction of a multi-colored original which comprises photoelectric means for scanning the original with the three primary colors whereby an electrical energy is established Whose intensity is proportional to each of the primary color components, means for modifying one of said energies in accordance with at least one other of said energies, ,a plurality of light valves, means including the photoelectric means and modifyingmeans for controlling each of said valves in accordance with a different one of the energies as modified. a light source positioned to project a beam successively through the light valves whereby its intensity is varied in accordance with the aperture of the valve whose area is effectively least and a scanning arrangement including said beam whereby a sensitive photographic layer suitably mounted on said device will be scanned by the beam.

9. In a photomechanical process for reproducing a multicolored original, the method of making a black printer which comprises illuminating the original with white light comprising the three primary colors, exposing respectively to the three primary colors from the original, three photosensitive surfaces whose responses are directly in accordance with the intensities of the respective primary colors incident thereon, blocking the two lesser responses by the greatest of the three responses and forming at each point on the printer in accordance with the greatest response, a printing density proportional to the density of the corresponding point of the original with respect to the predominant primary color of that point of the original.

10. In a photomechanical process for reproducing a multicolored original, the method of making a black separation negative which comprises illuminating the original with white light comprising the three primary colors, exposing respectively to the three primary colors from the original, three photo-sensitive surfaces whose responses are directly in accordance with the intensities of the respective primary colors incident thereon, blocking the two lesser responses by the greatest of the three responses and form- 7 ing at each point on the negative in accordance with the greatest response, a density in accordance with the predominant primary color component of the corresponding point of the original.

11. The method of making a black printer according to claim 9 and including the additional steps of making a color correcting mask from and for the original and registering the mask with the original before exposing said three, photo-sensitive surfaces.

12. In a photomechanical process for reproducing a multicolored original having a white area associated therewith, the method of making a black printer which comprises illuminating the original with white light consisting of energies corresponding to thethree primary colors, absorbing a portion of each of said energies at each point of the original whereby there comes from each point of the original a reduced energy corresponding to each primary color component of that point of the original, exposing three photo-sensitive surfaces whose responses are proportional to the energies incident thereon respectively to the three reduced color component energies, blocking the two lesser responses by the greatest of the three responses and forming in accordance with the greatest of the three responses at each point on the printer a printing density which is proportional to the logarithm of the ratio obtained by dividing the respective reduced color component energy for said white area by said greatest of the three reduced energies for the point of the original corresponding to said point of the printer.

13. The method of making a black printer according to claim 12 and including the additional step of modifying the energies corresponding to one of the primary colors in accordance with the respective energies corresponding to at least one of the other primary colors before the step of blocking the two lesser responses by the greatest of the three responses.

JOHN A. C. YULE.