US1995958A - Universal screen for preparing half-tones - Google Patents

Description

March 26, 1935. c. w. BENNETT 5 UNIVERSAL SCREEN FOR PREPARING HALF-TONES Filed March 3, 1935 SMALLEST .1 ,Cl9'. 1

/ DIAPHRAGM FOCAL DISTANCE scnesu SEPARATION DISTANCE V CAMERA EXTENSION RGEST QIAPHRAGM 42 C. WBenneii Inventor J4 Horn 63 Patented Mar. 26, 1935 UNITED, STATES PATENT OFFICE UNIVERSAL SCREEN FOB PBEPARiNG HALF-TONES Charles w. 1mm, Douglaston, N. Y., minor,

New York N. Y., a corporation of Application March 3, 1933, Serial No. 859,487

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This invention relates to screens for making half-tones photographically and has for its object to provide a screen for universal application as well as a method of producing the same which 5 will be cheaper in manufacturing costs and more efiicient to use than those heretofore proposed.

With these and other objects in view the invention resides in the novel steps and combinations of steps constituting the method of production, as well as in the novel article produced thereby, all as will be disclosed more fully hereinafter and particularly pointed out in the claim.

Referring tothe accompanying drawing formmg a part of this specification in which like numerals designate like parts in all the views,-

Fig. l is a diagram illustrating the method of determining the screen separation distance;

Fig. 2 is a diagram illustrating the method of determining the coverage with a light intercepting screen of given opening when using the largest diaphragm opening; and

Fig. 3 is a representatiomgreatly magnified, of a portion'of the contact screen produced by my method. a

In order that the precise invention may be more clearly understood it is pointed out that contact half-tone screens heretofore produced are of a character which do not properly render the values of the picture-to be reproduced. It is obvious that if extreme contrast must be registered, the screen must be capable of delineating a range of contrast from point black to point white. This necessitates a cut-out to the minimum light on one end of the scale and a complete cut-out of the maximum light on the other end of the scale. It may be seen therefore that what is requiredis a density wedge with maximum density at a point and sumcient to interceptat that point all of the maximum light. The density should decrease from the maximum point to a minimum point at which minimum point all of the minimum light reflected. from a picture to be reproduced, should be cut' out. vIt may readily be seen that this would enable the production of a tone scale ranging from black to white. The variation in between the two extreme pointsmay be gradual or in steps.

In the production of such a density wedge for the purpose above meant, it is obvious that the extreme dense portions up to the maximum point must be light enough so that a print through may be obtained by the maximum light. At the same time, on the other extreme, the density must be lightenough so thatjeeble lights above the minimum are registered. This means that the total area of a usable screen ,must be covered but the extremes are limited as mentioned above. Contact screens have been made heretofore by interposing a cross line screen between a diaphragm and the screen supporting medium. This cross line screen has been placed in what is known as half-tone relation with respect to the screen supporting medium. It is pointed out that with the screen in half-tone relation to the screen supporting medium, a complete coverage of the areas extending between maximum and minimum density required, as pointed out above, could not be had without obtaining a density too great at the dense portion.

I have discovered that the densest ,portion of the screen canbe prepared with a gradual shading to the minimum portion in the following manner. A camera set up is obtained with a lens diaphragm system and a contact screen supporting medium, with a cross line or other light intercepting screen interposed between the two, and although in the following disclosure reference is made by way of example to a cross line screen, it is to be understood that my invention contemplates screens having lines otherwise formed as well as lines, dots, or figures creating geometric or other designs. A light throughthe cross line screen openings, will focus at the emulsified surface of the contact screen supporting medium. For example with a camera extension of 29% inches. with an 18 inch focus lens, a diaphragm opening of 0.075 inch, and a 'cross line screen opening of inch the following ratio where x equals the screen separation and 11% equals the difference between the camera extension and the focal length of the lens, will result in a value of 1 inches which is the amount of screen separation which I have found most de-' pose of illustration) and passing through an opening (also enlarged) of the light intercepting screen 5, focusing upon 'the light sensitive surface of the contact screen 6. The dot and dash lines indicate a foreshortening of the camera set-up in order to accommodate the diagram to the sheet of drawings, and the various dimensions hereinbefore mentioned are appropriately identified by the printed notations.

Having fixed the screen separation as above, a maximum diaphragm opening is calculated (see Fig. 2) so that the secondary. beams of light, indicated at 8 and 9, forming a penumbra around the central dot as obtained in half tone making with a cross line screen, just cover all intervening spaces on the screen supporting medium 6. Thus according to this invention, the minimum diaphragm opening is the controlling factor in the disposal of the cross line screen. With these limits of diaphragm openings, the exposure may .be made with a continuously opening or closing diaphragm, and the rate of opening or closing of the diaphragm will determine the rate of slope of the density wedge. In actual practice it is unnecessary to make a continuous exposure with such a constantly varying diaphragm. I have discovered for instance that if, say, six exposures are made with the two extreme diaphragm openings as mentioned above and four intervening ones, a screen is obtained with practical working quality. That is to'say, I have discovered that with a 2%, diameter, 18 inch focus process lens and a camera extension of 29% inches, with a 60 lines per inch cross line screen of the 1:1 variety separated from the contact screen supporting medium by 1 inches, with the light reflected from a pair of arc lights playing upon a sheet of blotting paper at the proper copy distance, and the commer-' cial process film as the contact screen material, (said film held under fvacuum contact against glass in the plate holder and in substantial parallelism to the light intercepting screen), a l4 second exposure successively through each of the following diaphragm openings .075 inch diameter .107 inch diameter .151 inch diameter .215 inch diameter .303 inch diameter .430 inch diameter say five minutes at to F.

It is pointed out that the intercepting cross line screen 5 is not in half-tone relation according to the ordinaryconception of the term, for it is generally conceded that half-tone relation with the screen used is approximately ninety timesthe diameter of the screen opening, which would result in a separation ranging from to of an inch, while I use 1% inches separation. In using a cross line screen with separation. in half-tone making, it is customary to assume for an 18 inch focus lens that the camera extension is to the screen separation as the diaphragm opening is to the screen opening. For the purpose of preparing my.half-tone contact screen,

however, I have found that this ratio does not give. the best results, but rather the camera ex tension distance minus the focal length of the lens is to the-screen separation as the minimum tion, I find that instead of using the camera extension distance alone, as is the custom in present half-tone calculation, I must use the dimension expressed by the difierence between the camera extension and the focal length of the lens, wherefore it maybe seen that the screen separation used in preparing my half-tone contact screen, as calculated from the minimum diaphragm opening used, is intermediate between that required by calculation as used today and a screen separation as would be used in ordinary practice.

It is behaved that novelty in the preparation of my contact half-tone screen resides in the selection of proper screen separation for the minimum diaphragm used. It is pointed out with particular reference to Fig. 3 that the, maximum density in the area covered by the light passing through the'openings of the intercepting cross line screen, is at the central point as indicated by the numeral 11 and that there is a decrease in the density in all directions from this central point. Conversely, the points farthest removed from the above mentioned central dense spot and indicated at 12, are the lightest or least dense portions and these lightest points underly the intersections of the lines on the cross line screen. Therefore, considering the space on the halftone contact screen medium lying between the lines A-A, BB, C-C, 13-1) and underlying opening of the diaphragm when the diaphragm is opened continuously, or the number of separate exposuresmade at dlfiere'nt diaphragm openings, as well as the time of the individual exposures.

Fig. 3 is a representation magnified to substantially sixty times the dimension of the actual screen produced in order to better illustrate the variable density from point maximum to point minimum. I 7

Thus it will be seen that by this invention there is produced a contact half-tone screen which when highly magnified will show a plurality ofgeometric areas each of, which has'a variation of tone. That is to say, it will be seen that the center of each area appears as a point which issharply focused, and that the remaining points making up the balance of the area appear radiating from the center point in progressive degrees of focus as well as progressive change of quantity, equivalent to that passed by an intercepting cross line screen when a separation is maintained between the cross line screen and the sensitized medium. It is further pointed out that for a universal rendition of copy from the most contrasty to the softest, a mask of the above nature is necessary. Consider for a moment the reproduction of the most extreme contrast, the minimum density, at the mnimum point on the screen, is required to mask out reflected light from the extreme black portions of the copy. On the other hand, the densest portion, being a point, allows the printing with the maximum light to the finest point. On the black copy at the maximum point the printing can be carried to the desired point, even to the extent of lightening the whites of the copy, while the blacks'will register their proper values through the points of minimum density and areas surrounding them. For use in the camera, in contact with the sensitive medium, thisscreen enables a universal rendition of all types of copy.

Where an adjustment ofcontrast is desired, as is customary in the making of half-tones with the intercepting screen, adjustment of contrast is obtained by what is known as flash on white copy, followed by the regular exposure on the copy to be reproduced. It may be seen that this shifts the shadow rendition toward the high light without markedly affecting the extreme high light. This same flash is desirable with the contact half-tone screen in some cases. In fact the making of half tones with the contact half tone screen, thereby eliminating the element of screen separation, enables the making of half-tones by the unskilled photographer, since there must be dealt with only the question of flash and total exposure. The relation of diaphragm openings to screen separation, and camera extension or change of size of copy, is entirely dispensed with.

Therefore, it will be understood that this invention is not limited to the specific examples given in the preceding paragraphs in order to make the disclosure more clear, since the smaller diphragm openings may vary in size, though they will be relatively minute, there may be a wide there may have to be made a slight adjustment to insure coverage. This adjustment will depend upon the sensitive emulsion used, and with the developer in which the contact screen emulsion is developed. The adjustment is less the more sensitive the recording medium is to the register of tones from light exposures.

It is obvious that those skilled in the art may vary the details of construction as well as arrangements of parts without departing from the spirit of the invention, and therefore it is not desired to be limited to the foregoing except as may be required by the claim.

What is claimed is:-

In the production of a contact screen for universal application in photographically making half-tones, and wherein there is employed a screen in a camera for intercepting the light rays reaching the photographically sensitized surface or the contact screen medium, the method of determining the separation distance of the light intercepting screen from the contact' screen medium according to the formula A:x=B:C in which a: is the separation distance, A is the difference between the camera extension and the focal length of the camera lens, B is the diameter of the smallest lens diaphragm opening used, and C is the dimension of one of the openings of the light intercepting screen.

CHARLES W. BENNETT.

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