US1407029A - Photoprocess screen and method of preparing same - Google Patents

Description

J. A. H. HATT.

PHOTOPROCESS SCREEN AND METHOD OF PREPARING SAME. APPLICATION FILED OCT.I4, 1920.

1,407,029, I Patented Feb. 21, 1922.

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PEO'IOPROCESS SCREEN AND METHOD OF PREPARING SAME.

Specification of Letters Patent.

Patented Feb. 21, 1922.

Application filed October 14, 1920. Serial No. 417,011.

Screen and Method of PreparingSame, of

which the following is a specificatlon.

The invention relates to the art of printing and more particularly to the preparation of. half-tone printing plates for either black or colored printing. The obj cot of the invention is to improve the tonal quality of half-tone reproductions without the aid of hand work, requiring no special manipulations of the diaphragm of the lens during exposure, when making a negative, or any moving of the source of light when the screen is used for making half-tones by the printing-through-the-screen method.

- The invention is illustrated in the accompanying drawings, in. which Fig. 1 illustrates one part of the screen and Fig. 2 the other element of the screen of Fig. 3, while Fig. 3 represents the complete screen; Fig. 4 is a section on lines 4-4 of Fig. 3. All views are obviously exaggerated and greatly enlarged.

In order to prepare a screen of the present invention, I first provide a number of glass plates suitably coated with a sensitized film, preferably a bic-hromated colloid. The coating is preferably of the type described in my Patent No. 1,317,493, of Sept. 30, 1919 the characteristic of which is that the sensitized film upon the glass, in so far as it is not acted upon by rays of light, shall be capable of ready removal therefrom. I next provide a regular cross-line screen of suitable mesh, i. e., a standard screen of the art or a special screen, such, for example, as the one invented by me and described in my Patent No. 1,175,445 of Mar. 14, 1916. This screen is placed in front of one of the glass plates sensitized as described. A plain sheet of glass will usually be required between the screen and the sensitized glass plate in order to secure the proper screen distance, which may be defined as the distance which produces a round dot on the sensitized plate through the rectangular, clear glass 0pen ings of the screen. This well known practice is shown, for example, in the illustrations accompanying my Patent No. 1,134,381 Of APIIl 6, 1915. After being placed in a suitable printing frame the plate is then exposed to an are light. By varying the time of exposure this round dot may be made larger or smaller at will. The plate is now developed, for instance, by being immersed in water (of. my Patent No. 1,317,493), and the round dot image which remains is dyed until it is quite black or opaque. The dyelng can be satisfactorily accomplished by well known appropriate aniline dyes. The result of this treatment is illustrated in Fig. 1, in which the glass plate 1 is coated with a series of spaced opaque dots 2. The space between the dots is not covered by a film but is clear glass.

This plate of Fig. 1 is then used for the purpose of making the other half of my new screen. The plate of Fig. 1 is brought into contact with a second sensitized plate similar to the one from which Fig. 1 was prepared and a proper exposure made. The second plate is then developed in the same manner as the first plate was developed and it is also dyed but to a different degree. The

same dye that was used in making the first plate can be used in making the second plate but the second plate must be left in the bath a shorter period of time or the dye bath must be diluted or a specially prepared dye bath may be used, the result accomplished being in either case, to produce a translucent field 3 inteispaced with clear glass openings 4, the latter corresponding in this particular method of producing the second plate to the position of the dots 2 of Fig. 1. The complete plates, of which Figs. 1 and-2 are very greatly enlarged portions, will, of course, if.

of the same size, have the dots 2 of one of the plates located at the same relative position as are the clear glass openings 4 of the other plate and there will be the same number of dots 2 on one plate as there will be openings 4 in-the other. Inasmuch as in putting the two plates together there is a relative displacement of the plates to bring about an alternate and intermediate relation between the dots 2 and the apertures 4 (as shown in Fig. 3), Fig. 2 is drawn so as to represent that portion of one of the two plates which corresponds to a correspondmg portion of the other plate (i. e. the plate is the plates of Fig. 1 and of Fig. 2, are

then placed together in such a manner that the clear lass 4 of Fig. 2 and the opaque dots 2 of l ig. 1 do not overlap. This may be conveniently. accomplished by shifting the relative position of the two plates with respect to each other. The displacement of one of the two plates will amount in actual practice to a very minute distance equivalent to the width of a line or about 1/100th of an inch for a coarse screen. After the two plates are cemented or sealed together, the slight projection of the edges of the glass plates due to the shifting of one of the plates on the other, may be corrected by grinding the edge until the same is even.

The result is shown illustra-tively in Fig. 3,

in which is illustrated a complete screen which consists of three portions, first a general ground of translucent film 3 and upon that ground interspaced throughout the entire surface of the screen a series of clear glass openings 4 and a series of opaque dots 2.

From the foregoing description it will be readily observed that instead of proceeding as described, master negatives for the production of these screens may be made by almost any photographic process. It must be observed, however, as already pointed out, that in order to make a satisfactory working screen of proper optical qualities, it is essential that no film be allowed to remain over the clear glass portions of the screen and that when a photographic process is used, it must be one which will leave no such film'over the clear glass portions. The film, even if transparent, seems to diffuse the light and prevents it focusing properly and, consequently, acts to prevent the formation of the hard dot which is required. In placing the two plates 1 and 2 in proper juxtaposition to form the complete screen of Fig. 3, the two plates may be either placed directly together, as shown in Fig. 4, or they may be slightly spaced apart to form what is known as an air space screen, the advanta es of which are set forth in my Patent o. 1,066,611 of July 8,1913.

- In order to appreciate the advantages of the present invention, referencewill be had to the existing state of the art. The earliest commercial screen and the one most widely used at the present time consists of two single-line-ruled-glass plates cemented together so that the lines cross each other at right angles. These lines are ordinarily of equal width with the clear glass space between the lines so that the entire screen will glass openings occupying about one-quarter of the area of the screen, the remainin three-quarters.of the area being occupied by the opaque lines. For exact reproduction of tonal values such a screen has many defects. With considerable care such a screen can be made to reproduce the tonal values of a picture with reasonable correctness between the blacks and the middle grays, when the screen is used for making a screen negative. On the other hand, it will produce with a fair degree of accuracy the tones of a icture from the middle grays up to the hig -lights when used for making a printing through the screen half-tone. (A printing-through-a-screen half-tone is made by placing a continuous tone negative in contact with the metal or stone plate or surface previously sensitized and then placing the screen at the proper distance above the negative and exposing the combination of screen, negative and sensitized plate to the direct rays of an arc lamp or the sun). When a negative is made through such a screen it is customary to manipulate the exposures by using alternately various sizes of diaphragms with the object of improving the tonal values and the resulting plate is finally re-etched by hand and frequently in addition to these special manipulations, various engraving tools are employed to cut away the excess in the lighter tones caused by the defective operation of the screen on the negative.

In order to improve the quality of the tonal reproduction in the case of the printing-through-the-screen method it has been" found necessary, when the ordinary screen is used, to move the source of light or to move'the printing frame. These operations .have a series of small rectangular clear require great care and special skill. I have Y myself invented two types of screens which are described in patents already referred to in this specification, and while each of these screens has been a distinct improvement over the opaque cemented cross-line screen, they have nevertheless failed to enable ine toproduce that perfection in the reproduction of tonal values which I have now been able to achieve by the use of the present invention. One characteristic of the present invention which enables me to obtain these important results is that instead of employing rectangular clear glass openings round holes are used. For the mere difference between the rectangular and the round shape of the apertures, I do not, however, lay

- with the other features of the present inven tion. In my Patent No. 1,175,445 I endeavored to accomplish approximately the same object which my new screen produces but the screen of that patent, consisting as it did of series of ruled lines with clear glass spaces between each series, required the rulings to be sufiiciently dense or opaque to form where they crossed an altogether opaque tone. Where the ruled lines did'not cross, therefore, the tonal value which resulted was not regulata'b-le but was governed by the necessity of the situation as called for at the places where the lines crossed. If the translucent lines of the screen, i. e. the rulings where they did not cross each other, were less dense or opaque for the reproduction of the various tonal values, a result akin to that following the use of the new screen would have resulted but it was impossible to make a satisfactory screen of that character because of the necessity of having an opaque spot where the lines crossed. The translucent lines could not be made any less opaque than they, had to be in order to produce the opaque spot when superposed and crossed.

I have, therefore, in making the present invention, proceeded upon the foundation of a totally difi'erent manner of making a screen than that which was adopted in con nection with the patents referred to. I now use two plates, one of which (Fig. 2) consists of a transparent film with small round clear fglass openings interspaced therethrough, the

1m being dyed but nevertheless much lighter in color than the translucent portions of the screen of my prior patent above referred to. The clear glass 0 enings of this plate occupy approximatel t e same positlons as the rectangular c ear glass openings in the regular or standard screens. The second plate which I now use (Fig. l) which is placed either in contact with the first named plate or is placed in front of it with an air space between, consists of small, round, opaque dots of about the same size and number as the clear glass openings of the first named plate, the remainder of said second plate bemg clear glass. These two plates are placed together in such a manner that the opaque dot will be located in the field of the translucent portion of the first plate (see Figs. 3 and 4). The result is a screen consisting of clear glass round apertures surrounded by a field of very translucent color and within that translucent field there will appear round, opaque dots. The resulting halftones produced by the use of this new screen have the sametexture as those made from the regular cross-line screens, the only dif-' ference being the perfect rendering of the tonal values. The use of my new screen involves no expert manipulatlons or special treatments but the tonal" values will almost automatically flow from the fact that the clear glass spaces, being of the proper size and number, will permit the direct passage of light in so far as the same is desired while the opaque dots prevent the passage of light to the extent deslred, whereas the intermediate translucent part ofthe screen will, according to the intensity of the light or the length of exposure, cause just the right light values to be imparted to the sensitized surface in connection with which the screen is used.

What I claim is:

1. A screen comprising a transparent foundation, a coating of translucent material partially but not wholly covering the transparent foundation so as to leave clear transparent spots through said coating and I a series of opaque spots located in non-interfering relation with respect to the clear transparent spots.

screen comprising a transparent foundation and opaque, spaced spots thereon, a second transparent foundation coated with translucent material, the translucency being interrupted by spaced, clear glass spots, the two plates being superposed and united in such a way that the opaque spots and the clear glass spots do not overlap.

3. A screen comprising two glass plates united together, one of said plates carrying upon the surface thereof a series of opaque dots, the remaining surface of said plate being uncoated, the second plate being coated with a film of translucent material interrupted by a series of clear glass dots, the union between the plates being such that the opaque dots of one plate alternate with the clear glass dots of the other.

4. A screen comprising two plates of transparent material suitably united together, and located in the space between the two plates, a layer of periodically perforated translucent material and a serles of minute pieces of opaque material, the minute pieces of opaque material being positioned in juxtaposition to the translucent material.

5. The process of producing screens which comprises preparing a transparent plate carrying upon the surface thereof a series of dots of non-transparent or opaque material, preparing a second transparent plate coated with a lightly translucent material periodically perforated to exhibit clear glass spots, and then placing the two plates in juxtaposition so that the opaque spots of one plate alternate with the clear glass spots of the other.

6. The process of making screens which comprises. producing a series of dots upon a transparent plate coated with a sensitized film consisting of material capable of ready removal except where acted upon b light, removing the film except where the ots applate and dyeing the remaining portion of the film to a light trenslucency, and then placing the two plates together so that the 10 opaque spots of one will alternate with th clear dots of the other In testimony whereof I have hereunto set my hand.

JOSEPH ARTHUR HENRY HATT.

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