US2238483A - Photomechanical color reproduction - Google Patents

Description

April 15, 1941. J, A, c, YULE' 2,238,483

PHOTOMECHANICAL COLOR REI RODUCTION Original Filed April 22, 1937 UL TRA VIOLET ORWNAL 20% 202 352 352 352 an .62 flffLfU/WT/ES EJZ L WHITE PIGMENTJ ,NFRAWED 60X 95% 95; 95% 601 25; 95; 252 RL'FlEU/WT/[J EXPOSURE THROUGH RED FILTER EXPOSURE THROUGH ULTRA VIOLET F/L TER V i'i'%;

NEGATIVE FOR BLUE GREEN PRINTER MAGENTA PR/N TER YELLOW PRINTER 3 woe/whom JohnACXule Q MM- Patented Apr. 15, 1941 PHOTOMEGHANICAL COLOR REPRODUCTION John A. C. Yule, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Original application April 22, 1937, Serial No.

138,349, now Patent No. 2,161,399, dated June 6, 1

Divided and this application November 19, was, Serial No. 241,412

Claims.

This invention relates to photographic reproduction in color and more particularly to improvements in color reproduction by a photomechanical process whereby accurate results can be obtained more conveniently than is possible with the processes at present known.

This is a division of my copending application Serial No. 138,349, filed April 22, 1937, which issued as U. S. Patent 2,161,399, on June 6, 1939.

In photomechanical color process Work, a multi-color original to be reproduced is photographed through appropriate filters upon a series of sensitive plates to provide color separation images from which the finished plates are prepared in any well-known manner. In commercial work, the multi-colored original usually consists of a sketch made by an artist, a color photograph comprising superimposed dye images or a color photograph made by a screen process.

By way of example, a color sketch made by an artist will be considered here. It is well known that the color separation images obtained from a sketch made with standard palette coloring materials will not produce in the final print correct color rendition even with the best contemporary printing inks. In practice, this deficiency which is mainly due to the inefficiencies of the inks, is generally combated by retouching the photographs and working the plates, which practice requires highly skilled artisans, and isboth costly and time consuming.

It has been proposed to modify the colors of the original sketch by applying dyes on the surface or on a superimposed film, so as to falsify their visual values and reflecting powers in such a manner as to compensate for the deficiencies in the process. However, this is merely a method of interposing hand retouching at a different point in the reproduction process. This and other related correcting methods place a great burden upon the artist and the purchaser by distorting the visual appearance of the sketch so that they must think in terms of one color value and paint or observe a different color value. One method proposed by Murray in U. S. Patent 2,008,290, suggests a process in which the original sketch is corrected without handicapping the artist or appreciably distorting the visual appearance of the sketch, by incorporating fluorescent materials in the coloring materials used in creating the original sketch and making the color separation images partly by fluorescent light.

It is an object of my invention to provide special coloring materials for creating an original to be reproduced by a photomechanical process.

Another object of my invention is to provide 'a method whereby corrected color separation a will appear from the following description and its novel features are pointed out in the appended claims.

The accompanying drawing illustrates one embodiment of the invention.

For clarity, some of the terms used throughout this specification and in the appended claims will now be defined.

A red filter is one which transmits red light and, therefore, appears red. The spectral region included as primary red cannot and need not be exactly defined but is Well known to those and minus-blue, may be used interchangeably with the terms, blue-green, magenta, and yellow, except when applied to the color of inks.

For example, the magenta printing ink may not be a pure and complete minus-green, and, hence, a certain amount of color correction may be necessary when such inks are used. It is well known that the blue color which is produced by mixing magenta and blue-green inks has a decidedly purple hue and differs thus from primary blue. It is sometimes called blue-violet.

Many of those engaged in color press-printing employ the terms, blue-printer, red-printer, yellow-printer, to mean the separation images to which the present specification refers as bluegreen printer, magenta printer, and yellow printer. This latter convention is adopted here so that no confusion will arise either between the colors applying to the negatives and the colors of the light filters used or between the terms used in this specification and those employed in various branches of color photography and ordinary color press-printing.

The term, photographic, is here used in a broad sense so that photographic layer, includes not only the silver halide type but also many similar materials such as those used as photo-sensitive resists.

This invention is primarily concerned with three color press-printing, and, hence, the terms, color separation negative, refers to the negatives used in making the blue-green printer, magenta printer, and the yellow printer. It is not used in the broad sense to include a negative from which the so called black-printer may be made. However, it is to be understood that my invention may be applied to the making of the three color printers in a four color process. In this case, the black printer may be made by any of the known methods which do not use the invisible portions of the spectrum. In fact, as will be pointed out later, certain embodiments of my invention permit the making of a black printer even by such methods.

The term, coloring material, is used herein to include a pigment, ink, dye or similar material. As is well known, the following coloring materials reflect or transmit primary red light almost completely and do not require blue-green pigments or inks for their reproduction: "magenta, red, and yellow. The following require no magenta pigments for their reproduction: yellow, green, blue-green. The following require no yellow ink for their reproduction: bluegreen, blue, and magenta. A coloring material, such as an ink, when printed in its maximum concentration in a continuous tone image or when printed as an unbroken area in a halftone image is said to be printed as a solid tone.

According to one embodiment of my invention, a color sketch or photograph is created using a set of coloring materials having predetermined refiectivities or transparencies in the ultra-violet and infrared regions of the spectrum. As will be pointed out below, this sketch or color photograph is particularly suitable to be reproduced using a different set of coloring materials such as a set of commercial inks. In the above connection, the only infrared and ultra-violet regions which are of interest, are those to which photographic materials can be made sensitive.

According to this embodiment of the invention the coloring materials which require no yellow ink for their reproduction are chosen or have other materials incorporated in them so that their ultra-violet refiectivities are high and equal. The coloring materials which require a solid tone of the yellow ink in their reproduction are chosen so that they highly and equally absorb ultraviolet. The coloring materials which require intermediate amounts of yellow have correspondingly intermediate ultra violet absorption power. When only three subtractive color coloring materials are used to create the original namely a yellow, a magenta and a blue green pigment, ob-

viously none of them require such an intermediate ultra violet absorption power. As will be pointed out later, any foreign materials which are incorporated in the coloring materials to give them the desired properties should themselves be practically invisible, i. e., should have uniform and high reflectivity throughout the visible spectrum range.

The materials which require no magenta ink in their reproduction are chosen or have materials incorporated in them so that they highly and equally reflect infrared. Those which require a solid tone of magenta-ink in their reproduction are chosen or compounded so that they absorb infrared highly and uniformly. As will be apparent from the following description, only the relative, rather than the absolute, reflectivity and absorption are of interest when opaque coloring materials are used. However, when transparent pigments or dyes are employed, it is necessary that the ultra violet transmissions be practically complete; for example those transmitting blue light have an ultra-violet transmission of over 95%. Otherwise, variations in the amounts of these pigments would appear as variations in the yellow printer. This point will be discussed in connection with the method of employing these coloring materials.

In accordance with my newly invented method, 2. colored sketch or photograph is created using these coloring materials and color separation images are made therefrom by successively exposing three photographic plates or films to the red, infrared, and ultra-violet respectively, reflected from or transmitted by this original. The color separation images obtained thereby correspond to the ones usually obtained by exposing through a red filter, a green filter and a blue filter respectively. However, the ones made according to my invention need little, if any, further correction.

Thus, according to one, embodiment of my invention, a palette of opaque artists colors is made up having predetermined refiectivities in the ultra-violet and infrared regions, so that the deficiencies, which occur when a colored sketch created from these pigments is photographed by reflected light through red, green and blue filters m a photomechanical color process, are eliminated or greatly minimized by making the corresponding color separation images through red, infrared and ultra-violet filters respectively.

As is well known, a picture having a wide and almost complete range of colors can be created from a palette of three pigments, namely: magenta, yellow, and blue-green (sometimes known as red, yellow, and blue). Some artists prefer to use a palette consisting of six or even many more pigments, especially when the medium carrying the pigments is such that it is inconvenient and difficult to blend the separate ones, but for purposes of describing my invention it will suflice to consider only the case of three pigments, and to consider that the other pigments are made up from these three. It will be apparent that the principle and method disclosed are applicable in special cases to the use of three similarly colored dyes which are suitable for employment in the making of a color photograph comprising three superimposed dye images. However, as pointed out above, errors which are absent when opaque pigments are used, are introduced unless these transparent pigments or dyes, which according to the invention reflect or transmit the ultra-violet and infrared regions, do so completely. These errors are due to the obvious fact that any transparent material whose ultra-violet absorption is neither very large nor very small, cannot exhibit a. uniform ultra-violet transmission or reflectivity when the amount employed is liable to vary over an effectively wide range. On the other hand, the opacity of opaque. materials eliminates the effects of all layers and surfaces except the outermost one.

One specific palette of opaque artists colors which I have found to be useful in this connection consists of the following three mixed pigments: a

. Yellow: Hansayellow, titanium oxde, zinc oxide.

Magenta: Permanent Geranium Lake, Rhodamine Lake, cupric phosphate, white lead.

Blue-green: Peacock blue lake, white lead, zinc oxide.

Each of the three mixed pigments is compounded in such a way that they match the three printing inks to be employed in reproducing a sketch made from these pigments, when such inks are printed as a solid tone.

As pointed out above, the pigments which do not require yellow ink for their reproduction (i. e.

magenta and blue-green) are compounded in accordance with this embodiment of my invention to have equal and high ultra-violet reflectivity. The highest ultra violet reflectivity obtain- 1 reflectivity may be controlled without affecting the visual appearance by varying the relative proportion of white lead, titanium oxide and zinc oxide which respectively have very high, medium and very low ultra-violet reflectivities. These white fillers also give the pigments the desired opacity. Also the white card or canvas on which the sketch is to be painted must have an ultraviolet reflectivity approximately It will be noted that the residual 65% absorption of such a white base will be added to the absorption of the pigment, unless opaque pigments are used as in the present case. If a black pigment is used, it should absorb ultra-violet light equally to the yellow (1. e. have a reflectivity of 6%).

Also as pointed out above, the pigments which require no magenta ink in their reproduction (i. e. yellow and blue-green) must be selected or compounded to have high and equal infrared reflectivity. The above listed pigments reflect about 95% of incident infrared radiation. I have found that cupric phosphate can be used as an infrared absorbing pigment and, hence, I have added it to the magenta pigment to give an infrared reflectivity of about 25% which is relatively low (as desired). Other infrared absorbing pigments which reflect highly and uniformly throughout the visible spectrum may alternatively be employed in this connection. Actually, cupric phosphate absorbs some of the red as well as the infrared, which does not affect the operation of my invention but rather reduces slightly the range of colors which the artist is able to create with these pigments. In order to allow the artist as wide a range as possible, I refrain from adding sufllcient cupric phosphate to reduce the infrared reflectivity below 25%.

The white card or canvas on which the sketch is to be painted must reflect 95% of the infrared radiation thereon, and if a black pigment is used with this set of color pigments, it must have an infrared reflectivity of approximately 25%. Carbon black alone absorbs the infrared too strongly, but I have found that carbon black mixed with suitable portions of each of the three-color pigments has absorption characteristics in both the infrared and ultra-violet regions which make it satisfactory for use with these pigments (in which cupric phosphate is the specific material em- "ployed to increase the infrared absorption of the magenta pigment) As a white base upon which to paint the sketch, I use ordinary Bristol board coated with titanium oxide suspended in cellulose acetate, but any white card or canvas reflecting approximately 95% in the infrared and 35% in the ultra-violet will serve as well with the abovepigments.

This embodiment of my invention thus depends on four (or five) coloring materials namely; yellow, magenta, blue-green, and the white support (the fifth which may or may not be desirable, being black).

The accompanying drawing illustrates this embodiment. An original picture is made with special pigments as described above and the reflectivity at any point to any particular color is determined by the pigment used to produce that point. The letters R, Y, G, B-G, B, M stand for red, yellow, green, blue-green, blue, and magenta, respectively. Although any number of pigments having the required reflectivities to the various colors and to ultra-violet and infrared may be used, the simple case of three pigments yellow, blue-green, and magenta serves to illustrate the invention. Green is produced by mixing yellow and blue-green pigment and has ultra-violetand.

infrared reflectivities determined by the proportion of pigments used and by their ultra-violet and infra-red reflectivities. For example, a green made up of equal amounts of the above-described yellow and blue-green pigments would have an ultra-violet reflectivity of about 20% and an infrared reflectivity of about 95%.

Photographing such anoriginal through red, infrared and ultra-violet filters gives negatives to be used in the usual way for the blue green,

magenta and yellow printers respectively. In the drawing the relative densities of the negatives are indicated very roughly by the heights of the shaded areas in the blocks corresponding to the colors of the original.

From the foregoing, it will occur to those skilled in this art that several alternative arrangements may be employed without departing from the spirit of this invention. For example,

' the ultra-violet reflectivity may be such as to correspond to the green reflectivity instead of the blue reflectivityof the coloring materials used and, correspondingly, the infrared reflectivity would correspond to theblue reflectivity of the coloring materials used. In other case, the infrared or ultra-violet regions may be divided into two portions and the reflectivities within these portions may be controlled .to correspond to the green and blue reflectivities respectively or to the reflectivity of any two of the primary colors. This latter case would be particularly useful if, for any reason, it were desirable to also correct the blue-green printer or to utilize an infrared portion of the spectrum for the purpose of making a black printer as disclosed in copending patent application Serial Number 44,125 of Murray. On the other hand and for this same reason, the first described embodiment of the invention is perhaps more useful in connection with three color rather than four color work.

I have found that color separation negatives made up in accordance with my invention require somewhat more contrasty photographic materials and/or longer development than ordinarily used in order to obtain sufficient contrast for photo-mechanical work. Having thus described one palette of colors made up in accord,- ance with my invention and one method of employing these colors, I wish to point out that the invention is not limited to this example but is of the scope of the appended claims.

I claim:

1. A palette for preparing camera copy in a photo-mechanical process comprising a series of coloring materials in which certain of the materials have incorporated in them predetermined amounts of a material having known ultra-violet reflectivity to give them equal ultra-violet reflectivity.

2. A palette for preparing camera copy in a photo-mechanical process comprising a series of coloring materials in which certain of the materials have incorporated in them predetermined amounts of a material having known ultra-violet reflectivity to give them equal and high ultraviolet reflectivity and in which the other materials have low and equal ultra-violet reflectivity.

3. A palette for preparing camera copy in a photo-mechanical process comprising. a series of coloring materials differing in reflecting power in which certain of the materials have incorporated in them predetermined amounts of a material having a known infrared reflectivity to give them equal infrared reflectivity.

4. A palette for preparing camera copy in a photo-mechanical process comprising a series of coloring materials made up from three subtractive-color coloring materials which have incorporated in them predetermined amounts of materials having known infrared reflectivity to give one of them a relatively low infrared reflectivity and in which the other two have high and equal infrared reflectivity.

5. A set of artists colors which are reproducible in a three-color process without the use of yellow ink, having incorporated in each a prereproduction, having incorporated in each a predetermined amount of a material of known infrared reflectivity, said amount being sufiicient to impart high and equal infrared absorption to those requiring solid tone magenta for their reproduction, low and equal infra-red absorption to those requiring no magenta for their reproduction and to those requiring intermediate amounts of solid tone magenta, infra-red absorptions intermediately proportional to said intermediate amounts.

7. A set of coloring materials for use in preparing originals in a photo-mechanical printing process, certain of the materials containing predetermined amounts of a material which is substantially invisible in ordinary light and which has known reflectivity in portions of the invisible spectrum said predetermined amounts being those sufficient to make the reflectivities in one portion of the visible spectrum inversely proportional to the amounts of one of the subtractivecolor pigments required for the reproduction of the color of said coloring materials.

8. An artists color to which has been added a material having practically uniform reflectivity over the visible spectrum range and a known ultra-violet reflectivity whereby said color acquires an ultra-violet reflectivity equal to that of at least one other color with which it is to be used.

9. An artist's coloring material to which has been added a material having a practically uniform reflectivity over the visible spectrum range and a known infrared reflectivity whereby said color acquires an infrared reflectivity equal to that of at least one other color with which it is to be used.

10. A set of coloring materials for use in preparing originals in a photo-mechanical printing process, certain of the materials containing predetermined amounts of materials which are sub stantially invisible in ordinary light and which have respectively known reflectivities in different portions of the invisible spectrum, said predetermined amounts being those suflicient to make the reflectivities in one of the portions of the invisible spectrum inversely proportional to the amounts of one of the subtractive color pigments required for the reproduction of the color of the said coloring materials and to make the reflectivities in another of the portions of the invisible spectrum inversely proportional to the amounts of a different one of the subtractive color pigments required for the reproduction of the color of said coloring materials.

JOHN A. C. YULE.

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