Half-tone drop-out process

Patented Mar. 5,1946

2,395,986

UNITED STATES PATENT OFFICE

2495,986

HALF-TONE DROP-OUT PROCESS

Burtt L. Berry, San Francisco, Calif., assignor to Burtt L. Berry, Victorville, Calif., and Lyle P. Youngdahl, San Francisco, Calif.

No Drawing. Application July 2,1941, Serial No. 400,827

Claims.

This invention relates to the half tone process of making printed reproductions of drawings, photographs, and other pictures, or “copy, and the object of the invention is to provide improvements in the so-called high-light drop-out methods heretofore in use for procuring high-light areas in the reproduction which will be white and free of screen dots.

More particularly the invention comprises an improvement in such methods of producing dropout high-light half tone screen negatives in which the high-light dots are entirely or substantially closed on the negative by overexposure of the high-light areas of the copy, and the invention is based on my discovery that the high-light areas of any copy to be reproduced by the half tone screen process may be surface treated by a fluorescent substance so that by proper illumination or irradiation they will give off an actinic light to close the high-light dots on the photographic plate or film in the camera while the remainder of the copy including high-light areas not treated will be non-luminous or inactive.

The present application is closely connected to my copending application filed concurrently herewith under Serial No. 400,826, as it is drawn to cover a different phase of the invention described in said copending application.

Briefly described, the present invention comprises making the photoengraver’s copy or drawing in line, wash, or both, or crayon, pencil or other media, upon a sheet of paper, artist’s board, or other suitable material, after the surface of which sheet has been given a coating, wash, layer or substratum of a fluorescent material over its entire surface, and leaving the high-lights to be represented by the bare fluorescent layer where it is desired they be pure white or dotless in the final half tone print, and more or less covering up the active layer with the paint or other artist’s media so as to mask or block out the substratum or active layer, so that only the desired dotless high-light areas and/or desired white margins of the picture will retain the full fluorescing power of the active layer.

The fluorescent material must be held in or to the paper or surface of the artist’s board by a binder or film which will permit easily working thereover with water colors, ink, and other artists’ drawing media, and without substantially washing off or mixing with the media, and the fluorescent material should be of a type adapted to emit a photographically actinic ray upon irradiation, preferably by an invisible or substantially invisible ray such as by ultra violet light, so iu (Cl. 95—5) that after normal exposure of the plate or film in the half tone screen camera to the copy illuminated by daylight, arc light, or other electric light, the normal source of illumination is shut off and an irradiating ray is directed against the copy to cause the treated high-light areas to fluoresce and give off a strongly actinic light which will continue to impress the plate over such areas only and close up the high-light dots.

The irradiating exposure may be made before the normal exposure if desired and the result will be substantially the same, also a large closing-ln diaphragm or stop should be used on the lens, as the remaining parts of the copy not being treated are non-luminous during the irradiating exposure.

The irradiation of the copy for fluorescing the treated high-light areas is preferably done with rays totally invisible or yielding no actinic reflection from any part of an ordinary photoengraver’s copy such as an artist’s wash drawing, pen drawing, photograph, or other copy, so that the fluorescent high-lights alone will continue to impress the plate in the closing-in exposure. Such rays could be those just beyond either end of the visible solar spectrum directed against the copy with high-lights coated with a layer of any suitable fluorescent substance which will emit an actinic light under the activating action of the rays, or, in fact the irradiation may be by cathode or X rays and the fluorescent material may be barium sulphate or zinc sulphide copper activated and dampened with nickel, as used for fluoroscopes, or other substance adapted to be activated into luminosity by these rays, and in such case there would be no actinic reflection from the surrounding areas of the copy during the closing-in exposure. The same would be true if infra red rays were used.

However, as it is more convenient to use a band of rays from the ultra violet end of the spectrum for the irradiation of the high-light areas, preferably a band of ultra violet of from 4000 to 4200 Ang. (or more practically including wave lengths from about 3500 to about 4500 Ang.) there will be some actinic reflection of these rays from the general surface of the copy which was not treated with the fluorescent material, and with the use of such rays, or even slightly more visible rays, it is an advantage, and in some cases necessary to use a color filter over or behind the lens of a color to stop such reflected rays so that the rest of the photographic plate will not be affected during the closing-in exposure.

It is possible to make allowances for such addl2

3,395,886 tional effect from such violet reflection, by shortening the normal exposure of the negative to white light, but this brings in an element of uncertainty.

If violet or ultra violet rays are used for the irradiation, they may be obtained from any special lamp generating these rays and directed to the copy after the electric lamp for regular illumination has been turned off, or if an electric arc lamp or other lamp rich in ultra violet rays is used for the regular illumination of the copy, as soon as the regular exposure is finished, a suitably colored filter may be placed between the same light source and the copy to permit the continued passage of the irradiating rays to the copy while shutting off the balance of the light. Of course care is taken to have the room dark and avoid any wall reflections of any of the balance of the light of the lamp or from other sources, to any appreciable extent.

In considering fluorescent materials for the purpose of this invention, attention is called to the fact that the words fluorescent, luminescent, phosphorescent and photo-phosphorescent, are not clearly defined but appear generally to have overlapping meanings—but as used herein the word fluorescent means such materials as will emit an actinic light when irradiated with an invisible radiation or more or less visible radiation, though such materials may also be photo-phosphorescent—that is, capable of giving off light after irradiation has ceased, and frequently after exposure to ordinary or white light. Thus if a layer or coating of a chemical or material which is simply photo-phosphorescent, be used in the high-light areas of the copy and be illuminated by daylight or electric or arc light, and the light turned off, the resulting continued phosphorescent glow of these areas may alone be sufficient to close in the high-light dots, over such coated areas. Besides, to be considered is that even during the illumination of the copy by such ordinary light, it is probable that the coated highlight areas are in fact by combined reflection and photo-phosphorescent glow, acting much more strongly on the high-light dots than any untreated areas would act and are thereby hastening the closing-in action during the regular exposure.

Where a chemical or material is used for the sub-stratum or active layer which is both photophosphorescent as well as fluorescent to invisible radiation or substantially invisible radiation, or to deep violet radiation, the subsequent irradiation of the treated copy after the normal exposure will intensify the light emitted from the high-light areas or maintain it at a high state of activity for any desired length of time, but where the treating chemical or material is only fluorescent to invisible radiation it may nevertheless be equally effective to close in the high-light dots, as this depends on the efficiency of the material, the thickness of the coating, and the power or strength of the irradiating rays. Therefore in my appended claims, unless otherwise qualified the language is intended to cover activating material having any or all of the above explained properties.

In making the closing-in exposure, in addition to the use of a large stop in the lens, the half tone screen may be moved further from the plate, or swung away or removed entirely from in front of the plate if the construction of the camera permits, to thereby hasten or more completely effect the closing-in exposure.

CO

Besides the fluorescent materials mentioned may be the luminescent or fluorescent grades of sulphides of calcium, strontium, barium, and zinc generally combined with a minute quantity of bismuth or thorium oxide. The quality of such materials generally known in trade as “fluorescent grade” has been found to give good results. Other substances such as anthracene, eosine, also thioflavine are useful as they are fluorescent under ultra violet rays of from 3500 to 4500 Ang. without being photo-phosphorescent. Mixtures of the above materials may be used.

In usuing the fluorescent materials for the purpose of my invention they are very finely ground up and mixed with an adhesive liquid to form a paste or paint which is applied to the surface of the drawing paper or artist’s board or canvas with a brush, or sprayed on or with a roller if the areas are large, or in any other manner to secure an even layer, or incorporated throughout the substance of the paper at the time of manufacture. In making the copy the outlines or edges of the bare high-light areas may be left sharp or abrupt or they may be blended gradually into the adjacent areas as may be desired. A satisfactory vehicle or binder for use in forming the active layer is equal quantities of a 5 or 6% aqueous solution of gelatine and a 20% aqueous solution of gum arabic, to which mixture is added about 50% by weight of the fluorescent material, thoroughly mixed, say anthracene, as it gives but a pale yellow tint when applied to the artist’s board, and the fluorescent light is a greenish white. A weak aqueous solution of white shellac cut with ammonia also forms a satisfactory binder for the active layer.

When ultra violet rays are used in irradiating the fluorescent areas, the use of a color filter before the lens to absorb any ultra violet which may be reflected from the copy, may be dispensed with by mixing a small quantity of a violet ray absorbent with the water color paint with which the drawing is made, or with the black paint only with which the artist makes up his washes. A suitable material for this purpose is the aniline dye known as water soluble Brilliant Green, or Saffronine Red or mixture, about 10% thoroughly incorporated with the paint will be found effective for the purpose.

Of course, If an irradiating ray is used which has no actinic reflecting power on the high-light surrounding areas of the wash drawing, such as the X ray or cathode ray or invisible ultra violet or infra red ray, no absorbent in the water color wash, nor ray filter over the lens, need be used, as the irradiation by these rayr on a suitable fluorescent material left bare in the high-lights of the picture or copy will cause these areas only to be luminescent in the closing in exposure.

In the application of. the invention to plain photographic copy, the photograph may of course be made over a fluorescent layer as described, but I have also discovered that if a plain photograph, preferably on mat surface paper, be merely soaked in a thin liquid solution or suspension of the fluorescent material so as to produce an even coating in the whole surface and dried, such a treated photograph while manifestly having the fluorescent material over its whole surface, still will be much more luminous in the high-lights when irradiated, for the reason that much of the fluorescent quality penetrates in and is lost in the darker portions of the photograph, and therefore the high-lights will be illuminated so much more strongly as to affect to a great extent

3,395,986 the closing in of the high-light dots under the general process described above. A suitable fluorescent wash for the purpose would be a thin solution of white shellac in alcohol and carrying about 5% of anthracene.

It is of course evident that where in the preparation of the copy some high-light areas have inadvertently been covered up by the artist, he may “paint” them in again by applying an overlying layer of the fluorescent coating with a brush.

From a consideration of the preceding description of the invention, it will be seen that its purpose is to produce a half tone screen negative of any desired copy and in which negative the dot structure in any desired high-light areas may be substantially or completely closed to produce a dotless high-light. Such negatives of course may be used for the various half tone methods of printing, such as the making of ordinary half tone cuts therefrom or the use of the negative in making photolithographs or photolithographic transfers in the well known way. However, when the negatives are used for making etched cuts on sheet metal, it is evident that in the shallow half tone etching on copper such areas would require routing out, though with the four way dusting method as applied to zinc etching and particularly with coarse screen effects as used in newspaper cuts, sufficient depth of etching may be obtained so that routing will be dispensed with in all except the larger areas.

From what has been described of the process and the variations in carrying it out, it will be understood that if the “copy” or picture to be half toned is translucent and made on a transparent or translucent base sheet, the photographing of it to make the screen negative may be made while illuminating it from the rear, and also irradiating it from the rear. In fact the irradiating of the copy from the rear would in case of fairly densely shaded copy, be sufficient to close the high-light dots on account of the great actinic value of the emitted light from the bare high-light areas, while the lower value of the emitted light under the shaded areas would produce the dot structure required for the rest of the copy. Such a variation of the process is intended to come within my appended claims as the equivalent of the surface method of illumination and irradiation.