US2008290A - Photomechanical color reproduction - Google Patents

Description

Patented July 16, I935 UNITED STATES PATENT OFFICE PHOTOMECHANICAL COLOR REPRODUCTION No Drawing. Application September 9, 1933,

Serial No. 688,828 g Claims.

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

In photomechanical process work the sketch to be reproduced is photographed through appropriate filters upon a series of sensitive plates to provide a like number of color separation images from which the printing plates are prepared in any Well known manner.

It is well known that the visual or subjective brightness of standard palette colors differs substantially from their actinic or photographic brightness and that the color separation images obtained from a sketch made with these colors will not produce in the final print correct color rendition even with the best contemporary printing inks. In practice, this deficiency is generally combatted by retouching the photographs and working the plates, which practice requires highly skilled artisans and is both 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/or reflecting powers in such a manner as to compensate for the deficiencies in the process as it has been realized in practice, which are due to the limitations inherent in the 'manner of printing, and in the available pigments that must be used for making the original and for compounding the printing inks. However this is merely a method of interposing hand retouching at a different point in the reproduction process.

Other related methods place a great burden upon the artist and the purchaser by distorting the visual appearance of the sketch so that he must think in terms of one color value and paint in a different color value. As a consequence of my invention, the original sketch is corrected without handicapping the artist or appreciably distorting the visual appearance of the sketch due to the fact that the correcting procedure .of my process is mainly technical rather than artistic.

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 color separation images may be made from originals made with the special coloring materials of my invention.

Other objects and advantages of my invention will appear from the following description and its novel features are pointed out in the appended claims.

The special coloring materials of my invention 5 are obtained by incorporating in certain of them luminous or fluorescent material in ratios such that suflicient actinic light will be available to expose the photographic material the desired amount without over-exposing the other parts of the original. The fluorescent material may be incorporated in any suitable Way as by dissolving it in any solvent which is miscible with the vehicle of the paint and then mixing it with the various colored paints by rapid stirring, or the fluorescent compound may be mixed dry with a given pigment and the mixture then ground with the vehicle, or the pigment particles may be coated with the fluorescent compound, as from a solution in a volatile solvent, or by precipitation. After removal of the solvent, there remains a dry, fluorescent, pigment powder, that can be used in paints, pastels, crayons, carbon tissues, etc. Any desired fluorescent material may be used, namely, florescein, B-naphtholdisulfonic acid, sodium salicylate, luminous alkaline-earth sulfides, anthracene, and I have found diethyl dihydrocollidine-dicarboxylate especially satisfactory for this purpose. The quantity of fluorescent material which should be used depends, among other things, upon the light source, the panchromatic emulsion, the technique and the coloring materials used. In general the quantity will be determined by the additional luminosity needed to effect color correction as above described and ordinarily the red coloring materials will require a smaller quantity than the blue and the yellow will probably require none at all. Again, some pigments have more or less fluorescence of their own. This must be taken into account by adding enough of the fluorescent compound to supplement the natural fluorescence, and raise it to the required level of intensity.

For three-color photographic prints, such as are made by modifications of the carbon and imbibition processes, three pigments or dyes are used of the following hues: yellow, bluish-red, and greenish-blue. As giving a more extended explanation of my process, however, it is more enlightening to consider the problem of painting a commercial sketch or picture in full color. For multi-color painting, a set, or palette, of pigments must be provided that will satisfactorily cover the color scale. As an example of a typical minimum palette for such purposes I might list ultramarine blue (colloidal sulfide), chromium oxide green (chromium oxide), hansa yellow (Schultz and Lehmann No. 84), cadmium orange (cadmium sulfide), permanent geranium lake (General Dyestuifs Corp), carbon black (carbon), and titanium oxide white (titanium oxide).

In the reproduction of ultramarine blue by the contemporary 4-color process, a normal color separation gives one practically as much red as blue, and a considerable amount of yellow and black, so that the actual ratios of printing inks printing together to make ultramarine blue would be of the following order: blue 90, red 80, black 50, yellow 15. As craftsmen know, this gives a purplish-black. The current practice is to adjust the various densities by laborious and diificult hand retouching in one form or another until the ratios are roughly, blue 90-100, red 15-20, black 0-5, and yellow 0. Ultramarine blue has no natural fluorescence, but by compounding fluorescent material with it, it is possible to make supplementary exposures to the fluorescent light and build up the densities of all parts containing that pigment until the required density is reached on the separation negatives. If the exposure were exactly made, and if no improvements were required over the original, all need for retouching would be removed, but, in any case, the major part is easily eliminated.

The chromium oxide green would be rendered in normal 4-color negatives as roughly, blue'50, red 30, black 40, yellow 40, which with printing inks would give a dark gray with a mere suggestion of blue-green. It is necessary to reduce the ratios to 0-5 in the case of the red, and 5-8 in the case of the black, which can be done by supplementary exposures if the green is also made fluorescent.

It is convenient to make the brightnesses, or intensities, of the fluorescences in the blue and green proportionate, so that in making the red printing negative, which requires the most correction, the plate can be exposed to the full fluorescence without having to use a filter to modify either the blue or the green.

The permanent geranium taken in this example has a natural red fluorescence. This is augmented so as to maintain the fluorescence of purples made by mixing the 'red with the blue. Permanent geranium is a bluish-red.

fIn the limited palette given in the present example there are no other colors'requiring correction. It will be seen that high fluorescence is only required in those pigments requiring process blue ink in their reproduction, especially if the separate-exposure method, described below as Procedure A," is followed.

A sketch made with coloring materials containing a fluorescent compound would not appear distorted in ordinary illumination. Such sketches should have some identifying mark or label, but even if not marked they need only be viewed in ultra-violet radiation, when the fluorescence, if present, will be plainly visible. It might be well to note that the sketch should be painted on a support having little or no fluorescence in ultraviolet light. On the copyboard with each original it will be helpful for the operator to have register marks, a scale of grays, and a fluorescent guide card carrying patches of blue-violet, green, and red, which may be used as described later.

For the first exposures, white flame arcs or any other source of white light may be used as at present. For the ultra-violet exposures, mercury vapor tubes made of ultra-violet transmitting,

white-light-absorbing glass may be mounted at either side and/ or above and below the copyboard.

The white light exposures which will be referred to as color separation exposures will be made through the proper filters and should be sufficient to record the white of the gray scale as full density. After each one, and without disturbing the plate, the white light source is extinguished and the ultra-violet is turned on. The camera, ultra-violet lamps and copyboard should be suitably enclosed to exclude extraneous light while the fluorescence exposures are made. These will be referred to as the supplementary," or booster, exposures. The following is a specific example of how a supplementary exposure may be made. The ultra-violet source is provided with filtering means to absorb the visible wave lengths between 400 m and 700 m and transmit wave lengths between 350 m and 400 m The original is then reflecting these ultra-violet rays, which are invisible, plus the fluorescence they excite, which is visible. In order that the ultra-violet radiation shall not affect the photographic plate, it is necessary to place a filter over the lens that absorbs between 350 m and 400 my, and transmits between 400 m and 700 m l such as the Corning glass filter known as Noviol-O. A plate exposed in the camera now, through the Noviol filter, will record the fluorescent regions, but will see all the other parts, including whites, yellows, etc., as black. Thus it becomes possible to build up density in the negative in certain colors without affecting the remaining colors, or

the grays or whites.

The first exposure is a perfectly normal color separation exposure, and if the negative were developed at that point it would be a normal negative, with a density in the whites of say 1.4 to 1.5 after subtracting fog.

To make the yellow printing negative, a normal first exposure is given through a blue-violet filter. The supplementary exposure is given to the fluorescent image through the Noviol filter, plus a blue-violet filter. This removes excess yellow from the colors containing blue-violet without appreciably decreasing the amount of yellow in the greens. This is possible because when the fluorescent compound is combined with the blueviolet (ultra-marine) and green paints, the resulting fluorescence tends to assume the hue of the pigments, and by using a blue-violet filter, the fluorescence from the ultramarine is transmitted, but that from the green is absorbed.

To make a corrected black printing negative it is necessary that all (subjectively) saturated colors should photograph as white. This has always seemed a photographic impossibility, but

it has now been accomplished. A normal first.

Procedure B In Procedure A, separate exposures are given to copy illuminated separately by lamps of different characteristics. In Procedure B, simultaneous exposures are given to the copy illumihated simultaneously by lamps of different characteristics. A white light exposure through a filter may take place simultaneously with an exposure to ultra-violet excited fluorescent light through the same filter.

As an example of simultaneous color-separation and color-correction by Procedure B, the copyboard is irradiated with filtered ultra-violet from mercury vapor lamps and at the same time illuminated with white light of such intensity that in the time required to make an exposure to the normal light the excited fluorescence will build up the density required icr color correction. The exposure for a corrected yellow negative would be made under the above conditions through a filter transmitting in the blue-violet only.

Procedure C In this procedure the copy is illuminated by radiation from a homogeneous or mixed source with spectral characteristics differing from those of the correcting or supplementary luminescence of the colors, and by means of filters placed between the original and the plate, say in front of the lens, an effective fraction of the luminescence is allowed to pass, while the larger part of the exciting radiation reflected from, or transmitted by, the original, is absorbed, a residual fraction of the latter being used to make the simultaneous exposure to the reflection values of the original. Thus, with a sketch painted with black, white, and fluorescent red, yellow, green, and blue-violet, pigments, illuminated with unfiltered mercury vapor light, a corrected black-printing negative is made in a single exposure with a filter transmitting red and blue-green, but absorbing the strong mercury lines in the 405 and 435 m regions and those at about 546 m and 578 m Enough of the blue and green fluorescence will come through the gap in the blue-green to make an effective exposure, and the fluorescence of the red and yellow will be visible through the red transmission band of the filter. The density of the filter is low enough that the white-to-black values of the sketch are recorded on the negative at the same time. Procedure C requires the correcting luminescence of the pigments to be diiferently distributed as compared with pigments intended for Procedures A and B.

By the blue portion of the sketch is meant all those parts requiring some process blue ink to match the hue with printing inks.

In the booster exposure on the green filter negative, enough fluorescence exposure with a Noviol or similar filter should be given to make the green patch on the guide card equal in density, the density produced by the non-fluorescent white of the surrounding card in the white light exposure. In making the blue-violet filter negative, the booster exposure should bring the density of the blue-violet patch up to that of the white card. In making the black printing negative, the supplementary exposure should make the blue-violet, green, and red, patches equal in density to white. In making a purple or violet printing negative, the booster exposure should make the density of the green patch equal that of the white card in the negative.

In making color separation negatives from this sketch, several alternative techniques or modifications of the process are possible, of which three are here outlined as follows:

Procedure A In making the red printing negative, the sketch is illuminated with carbon are light and exposed on a panchromatic plate through a green filter for a predetermined period, such as A seconds; the sketch is then illuminated with exciting light such as obtained from a mercury vapor source with an ultraviolet transmitting filter and a further exposure is made through an ultraviolet absorbing filter, such as a light yellow filter, for A times B seconds where the Factor B has been predetermined to compensate for the low reflection of commercially pure blue pigments. It will be appreciated that this compensation is effected by the increased exposure from the blue portion of the sketch due to the fluorescence of the material therein when excited by the ultraviolet illumination.

Ordinarily no correction is necessary on the blue printing negative and it is made in the usual way without any supplementary exposure to the fluorescent image as excited by ultra violet light.

The necessity of a supplemental exposure as above described is due to causes suggested above. The chief of these, and onezthat has received some quantitative study in the past, is the fact that the green, the blue-green, the blue, the violet, and the purple, of the best available commercial coloring materials, have much lower reflections in those wavelengths which they reflect most, than the yellow, the orange, and the red. Since the compounding of printing inks depends upon the known pigments just as much as the manufacture of artists colors, it is no more possible to get full saturation throughout the color scale in the one than in the other, unless, or until, there is a distinct advance in the knowledge of the manufacture of pigments. In this respect it might be pointed out that at present a blue of 60 per cent reflection in its wave-length region of maximum reflection would be subjectively accepted as a color of high purity".

Loss of saturation appears to be inherent in the contemporary processes of printing, themselves. It apparently results from absorption of the ink vehicle by the paper, and color changes in some vehicles during drying. In the case of half-tone screen printing from relief and planographic forms, which at the present time embraces the bulk of all color printing, the color mixture is partly additive, partly subtractive, and since the theoretical basis of the process is subtractive color mixture, the extent to which additive color mixture enters in represents loss of saturation in some mixtures, increase in others, and hue changes in others.

Another complicating factor is that process inks vary in transparency. Yellows are opaque with high reflection, blacks have low reflection but high hiding-power, while blues and reds can be obtained with high transparency.

In the present state of the art, experience and reference to actual, printed, halftone color scales, are the only guides to the results of color mixture in these processes.

No attempt is here made to give examples of exact exposures and the like for to do so would make it necessary to specify in detail the properties of the coloring materials used in making the sketch, the characteristics of the photographic emulsion and the spectrophotometric absorption curves of the filters used. Such detailed specifications would not aid a worker in this art to practice the invention since each worker would have to dei ermine by experiment the corrections necessary to obtain the best results with the materials he has available.

An important feature of this process is its flexibility and adaptability. In the event of future improvements in the spectral quality of process inks it will only be necessary to decrease proportionately the color-correction exposures. Similarly. one is not confined to any specific inks at any time, but the proper color correcting exposures must be established for the inks used in a given case.

It is to be understood that phosphorescent material may be used for the luminous substance instead of fluorescent material in which case the supplementary exposure would be made in the dark and a relative long exposure would be necessary.

While my invention has been described as applied to photomechanical process work it is obvious that it may be used wherever color separation images are to be made from subjects which are originally prepared for this purpose, or which are of a nature such that they may easily be altered as required by my invention. Such alteration may readily take the form of retouching the subject with a transparent or translucent fluorescent solution.

In the claims, the term color saturation is to be read as meaning the percent reflection or transmission of the characteristic hue. Thus low saturation colors denotes colors having relatively low percents of reflection or transmission of their characteristic hues.

What I claim as new, and desire to secure by Letters Patent of the United States is:-

1. The method of producing color separation images for use in a photomechanical color printing process, which comprises creating the original sketch or color photograph with coloring materials differing in color saturation and having incorporated in certain of the colors a quantity of fluorescent material, successively exposing a series of sensitized layers to light reflected by said original with a suitable filter interposed and giving a supplemental exposure to certain oi said sensitized layers while exposing the original to radiation of a wave-length which excites the fluorescent material and positioning in front of the sensitized layer a filter which absorbs light of the wave length of the exciting light.

2. In a photomechanical process the method of making a color separation image corrected for diflerences in the efliciencies of the several colors appearing in the original sketch or color photograph which comprises incorporating in the low saturation colors a quantity of fluorescent material and, in addition to the ordinary exposure, making a supplementary exposure to light emitted by the fluorescent material when excited.

3. The method of producing a corrected color separation negative of an original color sketch which comprises exposing a photographic layer to the natural reflection of the coloring materials in the original and to separable radiation of luminescent material associated with certain of the coloring materials.

4. The method of making a correctly separated record of the red from a multi-colored original in which the colors requiring process blue ink in their reproduction contain a quantity of fluorescent material which comprises exposing a sensitive layer first to visible light reflected by the original and then to light emitted by the fluorescent material upon being irradiated with exciting radiation.

5. In the art of producing polytone prints, the method of predetermining the tones of the printing plates to correspond to the process inks to be used which comprises creating the camera copy therefor with paints containing fluorescent material in predetermined amounts which are based in part upon the reflecting powers of the several paints, making ordinary color-separation exposures from said copy and in addition supplementing at least one of the color-separation exposures with an exposure through a proper filter to light emitted by the fluorescent material.

ALEXANDER MURRAY.