US2102022A - Color corrected photomechanical printing plate - Google Patents

Description

Dec. 14, 1937. A. MURRAY 2,102,022

COLOR CORRECTED PHOTOMECHANICAL PRINTING PLATE Filed July 6, 1935 INVENTOR:

Mlu'my,

ATTORNEYS.

Patented Dec. 14, 1937 i UNITED STATES PATENT OFFICE COLOR CORRECTED PHOTOMECHANICAL PRINTING PLATE Jersey Application July 6, 1935, Serial No. 30,091

6 Claims.

This invention relates to color separation negatives for producing photo-mechanical printing plates having corrected color printing values and to processes for making them.

In photo-mechanical color reproduction processes, the sketch or picture which is to be reproduced by photo-mechanical means is photographed through appropriate filters upon a series of sensitive plates to provide the required number of color separation images and the printing plates are prepared from these in any well known manner. These printing platesare then used with printing inks of an appropriate color to reproduce the sketch or picture in cblors.

Since the apparent sensory characteristics .of the colors of an original sketch or picture difier substantially from actinio or photographic effects,'the color separation images obtained from the original sketch do not represent the true visual color values of the original. The prints made from the color separation images, therefore, must be treated in some manner to correct this discrepancy between the color values required in the reproduction and those of the color separation images. The color values required in the reproduction are influenced by the fact that the color transmission of available printing inks is imperfect.

In practice, this practical deficiency in color values of the color separation negatives is overcome by hand retouching the color separation negatives or the prints made from them. This process requires highly skilled artisans and is both costly and time consuming. Numerous methods have been devised to avoid hand retouching of the negatives, such as the use of intermediate plates which correct values which are deficient in the. original negatives. The objection to these processes is that they necessitate making a number of extra plates. In my Patent No. 2,008,290, granted July 16, 1935, I have described a method for producing color. corrected plates for photo-mechanical printing by means of controlling the actinic value of the pigments used in making an original sketch. However, it is obviously not always possible to control the production of the original in this way, especially if it is not a sketch.

The principal objector the present invention is to provide amethod for correctly reproducing colored sketches or pictures by photo-mechanical means. and to avoid the necessity of retouching the plates. A furtherobjectis to produce color corrected plates for photo-mechanical printing where there is no'control over the pigmentation of the original sketch or picture. Other objects will appear more fully hereinafter.

These objects are accomplished by my invention by making use of the Herschel effect in correcting the color separation plates obtained by 5 photographing the original sketch or picture.

The Herschel effect is the well-known eifect produced when a latent image, usually of silver halide, is subjected to the action of red or infrared light and which results in a destruction of 10 the latent image. This action-is accelerated by the presence of an organic desensitizer and potassium bromide. This effect may be produced, for example, in the following'way. A gelatin emulsion containing a latent image of silver bromide is dipped in a solution of an organic desensitizer and potassium bromide. The emulsion is then exposed to red or infra-red light and it will be found that the latent image has been weakened or destroyed. I have found that this efi'ect may be made use of as described below.

y In the drawing, Fig. 1 is a diagrammatic representation of a color sketch which is to be reproduced. Figs. 2, 3 and 4 are sectional views of blue, green and red color filters respectively.

The remaining figures of the drawing are diagrammatic representations of color separation plates made by photographing the original colored sketch through the separate color filters..30 The figures in. the same horizontal row represent the various color separation plates in corresponding stages of processing. Figures 5, 6, and 7 represent respectively the blue, green and red color separation negatives made by photographing the original sketch through the blue, green and red filters. Figs. 8 and 9 represent positives made from the negatives shown in Figs. 5 and 6 respectively. Figs. 10 and 11 represent corrected positives made from the positives shown in Figs. .8. and 9, and Fig. 12 represents a positive made from the negative shown in Fig. '7. Figs. 13, 14 and 15 represent correct or corrected negatives.

In photo-mechanical color printing work, it is customary to use yellow, magenta, and bluegreen printing inks in printing the final reproduction. These are usually spoken of as yellow, red and blue inks respectively, the hues of the red and blue being practical compromises arrived at empirically The colors of theoriginabsketch are theoretically reproduced by the subtractive .method, that is,,,red in'the original sketch is reproduced by printing magenta over yellow, blue is reproduced by printing blue-green over magenta, and green is reproduced by "printing For this reason, the magenta plate made through Cal the green filter must be subjected to the greatest amount of. correction. The yellow plate made through the blue filter must be corrected to a less extent and the blue-green plate made through the red filter need not be corrected at all for color.

I will now describe my process in detail with reference to the accompanying drawing.

In the colored sketch to be reproduced, which I have illustrated diagrammatically in Fig. 1, the portion l a may be considered as representing a blue sky, the portion 4a as representing a red sun and the portion 1a as representing a green foreground. The border Illa represents the white portions of the sketch.

The three color separation plates made from this sketch are photographed through three color separation filters A, B and C shown respectively in Figs. 2, 3 and 4. The filter A is a Wratten No. 47 blue filter, which has a light transmission from about 360 to about 520 millimicrons. The filter B is a Wratten No. 58 green filter, which has a light transmission from about 480 to about 600 millimicrons. The filter C is a Wratten No. 25 red filter, which has a transmission above about 600 millimicrons.

These three filters are placed successively in the camera in front of sensitive plates which may be silver halide sensitized plates, and three separate exposures made of the original sketch. These exposures result in the yellow separation negative shown in Fig. 5, the magenta separation negative shown in Fig. 6 and the blue-green separation negative shown in Fig. 7. As shown in Fig. 7, the red portion 4a of the original sketch records to maximum density as shown at 6b. The blue portion la of the original sketch does not record through the red filter and the portion 3b, therefore, remains unexposed. The portion 1a of the original sketch records to a slight extent through the red filter and, as shown at 9b, the negative is slightly exposed at this portion.

As shown in Fig. 6, the green portion la of. the original sketch does not record to maximum density through the green filter and, for this reason, the portion 8b does not have sufiicient density. The portion la representing the blue of the original sketch records through the green filter to some extent resulting in an exposure of the portion 2b shown in Fig. 6. The red portion 4a of the original sketch does not record through the green filter and the portion 5b shown in Fig. 6 remains unexposed.

As shown in Fig. 5, the blue portion la of the original sketch does not record to maximum density through the blue filter. Consequently, the portion lb of Fig. 5 is not fully exposed. The green portion 1a of the original sketch records to a slight extent as shown at lb and the red portion 4a does not pass through the filter, leaving the unexposed portion to. It will be seen that in each of the three negatives, the white portion Illa of the original sketch has recorded to maximum density as shown at lflb, llb and I2b.

In order to correct for the deficiency in the blue and green portions of the negatives, shown in Figs. 5 and 6, positives are made from them as represented in Figs. 8 and 9. In these positives the light and dark portions are reversed in the usual manner,

Considering first the positive shown in Fig. 9, instead of developing it, the plate is'first desensitized by dipping it into a desensitizing solution. The following solution is suitable for this purpose:

Phenosafranin (1-1000 solution) 20cc. Potassium bromide (10% solution) 5cc. Water 75 cc.

The plate is bathed for about 2 minutes in this solution and the developed color separation plate made through the red filter, that is the plate represented by Fig. 7, is then placed over it and an exposure made under the red filter C by red light. This exposure is made for about 6 seconds and the light used is a .white flame arc of about 35 amperes. Mechanical means of registering the two plates must be provided, since there is no visible image on the green filter positive.

Of the two plates used in this exposure the plate shown'in Fig. 9 has not been developed,

while the plate shown in Fig. 7 has been developed and fixed. The plate shown in Fig. 9, therefore, carries only a latent image, while the plate shown in Fig. 7 contains a developed and fixed image. The bleaching effect, known as the Herschel effect, therefore, takes place and bleaches the latent image contained on the plate of Fig. 9. Since the red light is kept from acting on the portions of the plate covered by the darkened portions of the image on the plate of Fig. 7, these portions are not bleached. The bleaching, therefore, takes place only in the portions 20 and 80. Since the light can act with greatest intensity on the clear portion 3b of the plate shown in Fig. 7, the portion 2c is bleached to the greatest extent and results in the lightened portion 2d shown in Fig. 11. The light also acts on the portion 80, but since the image represented at this portion is much lighter than that at 20, the image at this portion is totally destroyed as shown at 8d. The portion 50, since it lies under the fully exposed portion 6b is not afiected and is represented in Fig. 11 at 5d. The plate shown in Fig. 11 can then be developed and fixed.

. In order to counterbalance the bleaching action on the density representing the white border it is desirable to give a flash exposure to white paper in addition to the color separation exposure and before desensitizing. The developable density produced by this flash exposure should be at least equal to the decrease in density of the white border produced by the. red light bleaching exposure.

A similar process is carried out in bleaching the plate shown in Fig. 8. However, the exposure of this plate need be suflicient only to bleach the portion I0. The plate is desensitized as before and subjected to an exposure under the plate of Fig. 7 and the red filter C,but the exposure is made for only about 4 seconds. This results in total destruction of the image at lc as shown at Id. The image as shown at 1c is not affected. This plate is then developed and fixed.

The blue-green separation plate of Fig. 7 is then printed as a positive shown in Fig. 12, It need not be corrected, but it is printed tothe same gamma or degree of contrast as the positives shown in Figs. 10 and 11.

The corrected positives representing the three color components are then printed through a half tone screen to obtain corrected half tone negatives. These are shown in Figs. l3, l4 and 15. Half tone printing plates may be made from these in any well known manner.

As an alternative method, I may make' corrected yellow and magenta negatives directly in the following manner. Original negatives are made as before by exposure in a camera through the blue and green filters. These negatives are then removed from the camera and immersed in a desensitizing bath. They are then replaced separately in'the camera and the secondary exposures made through the red filter C. This causes a bleaching to take place in these negatives. If a half tone screen is placed in front of the camera, the half tone screen negatives may be made directly. This method, however, has a disadvantage that an exceedingly powerful light source is required in order to secure suflicient bleaching during the second exposure.

As another alternative, corrected color separation negatives may be made from the image carried on lenticular film. Separation negatives are made from the image on the lenticular film by projecting it through masks which select the desired color method. .For example, the magenta negative is made by projecting through a green filter mask opening with white light plus a sup plementary exposure through a red filter mask opening with a red filter. In a similar manner the yellow color separation negativeis made by projecting through a blue filter mask opening with white light plus a supplementary exposure through the red filter mask opening with a red filter. "The blue color separation negative is made by projecting through the red filter mask opening only, exposing and developing so that the gamma of the blue negative corresponds to the gamma of the magenta and yellow negatives by projecting with a graded element half tone screen in contact with the negative emulsion. -Fu1ly corrected screen negatives may be made in one step.

It is to be understood that I am not limited to the examples given above, but that I comprehend all modifications and equivalents coming within the terms of the appended claims.

I claim:

1. The method of making a color-corrected plate for photo-mechanical printing, which comprises forming a green filter latent image of a colored object in a light-sensitive silver halide emulsion, desensitizing the light-sensitive material, and exposing the latent image under a red filter negative of the object to a red light to destroy portions of the latent image having a denrected plates for photomechanical sity greater than that corresponding to the correct color of the image.

2. The method ofmaking a color-corrected plate for photo-mechanical printing, which comprises forming a blue filter latent image of a colored object, in a light-sensitive silver halide emulsion, desensitizing the light-sensitive material, and exposing the latent image under a red filter negative of the object to a red light to destroy portions of the latent image having a density greater than that corresponding to the correct color of the image.

3. The method of making a color-corrected plate for photo-mechanical printing, which comprises exposing a light-sensitivesilverhalide emulsion under a green filter to a colored object to form a negative image of the green portions of the object, printing a positive latent image therefrom, desensitizing the light-sensitive material containing the positive latent image and exposing said latent image under a red filter negative of the object to a-red light to destroy portions of the latent image having a density greater than that corresponding to the correct color of the image.

4. The method of making a color-corrected plate for photo-mechanical printing, which comprises exposing a light-sensitive silver halide emulsion under a blue filter to a colored object to form a negative image of the blue portions of the object, printing a positive latent image therefrom, desensitizing the light-sensitive material containing the positive latent image and exposing said latent image under a red negative of the object to a red light to destroy portions of the latent image having a density greater than that corresponding to the correct color of the image.

5. The method of making a set of color-corp n which comprises forming blue, green, and red color separation negatives of an original in silver halide emulsions, printing positives of the color separation negatives representing colors requiring correction in sensitive silver halide emulsions, and before developing, desensitizing said positives and then exposing them separately to red light under a color filter negative representing another color, then developing and fixing the corrected positive directly from the red filter negative.

ALEXANDER MURRAY.

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