US2533650A - Process of making half-tone negatives - Google Patents

Description

Dec. 12, 1950 F. E. WATTIER PROCESS OF MAKING HALF TONE NEGATIVES 2 Sheets-Sheet 1 Filed Aug. 23, 1944 /n renfor Dec. 12, 1950 wATTlER 2,533,650

PROCESS OF MAKING HALF-TONE NEGATIVES Filed Aug. 25, 1944 Z'Sheets-Sheei 2 rnev 4 Patented Dec. 1950 n PROCESS OF MAKING HALF-TONE NEGATIVES Francis E. Wattier, St. Lambert, Quebec, Canada, assignor to Powers Photo Engraving Company, Glen Cove, N. Y., a corporation of New York Application August 23, 1944, Serial No. 550,794 (01. 95-5) 2 Claims. 1

This invention relates-to a process of and to apparatus for making half-tone negatives, particularly those using photographic film as the sensitive material, and the primary object of the invention is to secure, photographically, in the negative a more faithful tone reproduction, especially in the shadows and in the high lights of the subject, than has been heretofore possible.

A further object is to produce a half-tone negative in which the light reacted portions, or dots, are so shaped and so graduated as to sharpness and opacity as to produce a superior form of dots in the positive print on. the metal plate to be etched, which superior printed dot formation facilitates such etching of the half-tone metal plate as may be necessary to obtain the required results.

A still further object is to provide a process of making half-tone negatives, which process is greatly simplified in its operation and ensures certainty of consistent results, whereby the result as to negative balance may be more nearly duplicated by one operator or the same result more nearly obtained by different operators or with different cameras.

Another object is to provide a process which does not require special expensive apparatus for relatively little light as compared to thehigh lights, so that in exposing a plate or film, the half-tone dots in the shadows are under exposed while the half-tone dots in the high lights are over exposed. Other factors, such as irradiation and lack of correct relation between the size of the stop aperture, the camera extension and the screen distance for both shadow and high light exposures, are also causes of failure to obtain correct tone values in a half-tone negative. The

aforesaid flattening and loss of tone values canits carrying out but which, on the contrary,

avoids the use of certain expensive camera apparatus now generally employed, thereby enabling less expensivecamera apparatus to be used.

Still another object is to provide apparatus for carrying out the process as aforesaid and in particular apparatus specially adapted for the making of half-tone film negatives.

Various other objects and the advantages of the invention may be asertained from the following description and the accompanying drawings.

The process now generally used in making halftone negatives has certain disadvantages and diillculties which are due to various photographic and mechanical factors. It is known that when the light rays reflected from the original copy are passing through the camera lens and through the half-tone screen, that is, when a continuous tone image is being converted into a half-tone, there is a flattening out oi detail and loss of tone values in the shadows and in the high lights of the half-tone negative, with the result the negative has a shorter range of tone values than the original copy. This flattening out of tone values at both ends of the scale is due partly to the deepest shadows in the original copy reflecting not be compensated for by any after treatment of the negative, such as the usual intensification and reduction of the wet plate process. This applies equally to making film negatives but is more detrimental in film negatives on account of the lower resolving power of film, which limits the duration of exposure and therefore limits the ability to photograph detail.

I have discovered that if the sensitive medium is exposed twice under conditions favouring shadow detail reproduction in one exposure and favouring high light detail reproduction in the other exposure, while substantially preventing overlapping of the two exposures, a new result in half-tone negative making is obtained. This new result is a great improvement over results obtained by the usual process in that the tone values of the copy are more faithfully reproduced and flattening of high light and shadow detail is avoided. The improved tone value and detail facilitate the after treatment of a wet plate negative and are of the greatest importance in the making of film negatives which do not permit of the same after treatment as a wet plate negative and in which the tone values must be obtained photographically in the first instance. The conditions favouring reproduction of shadow detail are mainly the use of a half-tone screen of special ratio in combination with a special stop in the camera lens and adequate length of exposure. The conditions favouring reproduction of high light detail are mainly the use of I the aforesaid special screen in combination with a different stop in the camera, which substantially prevents light reaching those portions of the plate or film which have been already exposed and in combination with a filter suited to the colour sensitivity of the photographic emulsion.

The special screen used is of lower ratio than is ordinarily used, the ratio of aperture width to line width being approximately 35:65, instead of the usual 1:1. I have found that a screen of this ratio, in combination with a small, square stop aperture, is especially suitable for obtaining shadow detail and permits of full exposure with very little spreading or the half-tone dots in the high lights and produces small, firm dots in the deepest shadows. The square stop also promotes the highly desirable checker-board dot formation in the middle tones. A screen of this type is especially desirable for making film negatives as it compensates for the low resolving power of the film as compared with wet plate. In'addition to the foregoing, the screen Ls so constructed that it cannot be placed in the camera otherwise thanat exactly the proper distance from the focal plane and in exact parallelism therewith. The stop used for the second or high light exposure is characterized by an annular opening which guides the light rays away from those portions of the plate or film which have been exposed through the shadow stop, so that it is impossible for the high light exposure to interfere with the shadow exposure. This gives the operator complete control over contrast in the negative. The filter preferably used with the high light stop also prevents light going too far back into the shadows and affords a more nearly correct tone separation in the higher tones.

Broadly speaking, the present invention consists, as to process, in photographing copy through a screen of ratio less than 1:1, located at exactly the distance from the focal plane of the camera determined by the mesh of the screen and in making two exposures through stops having apertures of difierent sizes and shapes such that the exposure through one stop does not overlap the exposure through the other stop; and consists, as to apparatus, in stops presenting a square aperture for shadow exposure and a larger annular aperture for high light exposure, in combination with a screen closer than 1:1 and means for locating the screen at a distance from the focal plane of the camera exactly suited to the size of the screen apertures.

In greater detail, the invention consists in the features and combinations of features herein disclosed, together with all such modifications thereof and substitutionsof equivalents therefor as are within the scope of the appended claims.

In the accompanying drawings which illustrate that form of apparatus now preferred and the carrying out of the process:

Fig, 1 is a diagram illustrating the relation of the half-tone screen, the film and various camera parts including the screen supporting means and means for feeding roll film.

Fig. 2 is a plan view of a half-tone screen according to this invention.

Fig. 3 is a sectional view of the screen on the line fso'f Fig. 2.

Fig.4 is a plan view of a fragment of the halftone screen greatly enlarged.

Fig. 5 is a plan view of a stop according to this invention.

Fig. 6 is a diagram illustrating the operation of the stop and screen in the exposure of the sensitive medium to produce dots during the shadow exposure.

Fig. 7 is a diagram illustrating the operation of the stop, and screen in the exposure of the sensitive medium to produce dots during the high light exposure.

Fig. 8 is a diagram illustrating the relation of the screen to the shadow dots and high light dots.

Fig. 9 is a fragmentary cross-sectional view of a screen, showing a modified form of supporting flange.

Referring more particularly to the drawings,

41 ll designates a camera lens, I! a stop having an aperture ll, while It designates, as a whole, a half-tone screen, the rear surface i5 of which is located in the focal plane of the camera when the screen is in position therein, by reason of supporting engagement with the screen bars I. of the camera. These bars are vertically adjustable in the usual manner but are non-adjustable as regards the distance from the focal plane of the camera. A pressure board I! is located in rear of the screen and is adapted to be ur away from the screen by springs ll preferably located at the four corners of the board.

Photographic film designated ll, arranged with its emulsion side adjacent the screen, is passed for exposure between the screen and the pressure board and, during exposure, is pressed tightly against the rear surface of the screen by the pressure board when the latter is moved toward the screen by a cam Ill engaging the center of the board, or by other suitable device. The film used is preferably in the form of a. roll I! from which the strip is fed between the screen and the pressure board by feed rolls 2| located in any suitable position and passes between the shear blades 22 arranged in any suitable location, for example above a suitable receptacle 23 for severed portions of exposed film. The pressure board presses the emulsion side of the film tightly against the rear surface it of the screen so as to flatten out any undulations that may be present in the and ensure that the entire surface of the emulsion to be exposed is exactly in the focal plane of the camera. It will be understood that in practice the film and its handling arrangements are separable from the back of the camera in the same manner as the usual plate holder and are enclosed in a suitabledight-proof housing.

The half-tone screen, designated as a whole by the numeral 14, is illustrated in detail in Figs. 2, 3 and 4 and comprises a frame 24, the opposite sides of which are preferably rabbeted, as at It, to receive a screen plate II and a transparent distance plate 21, preferably of glass and to hold the plates in absolute parallelism. At least two opposite edge surfaces of the screen frame are provided with ribs 28 adapted to be engaged in the grooves of the camera screen bars It. The ribs may extend entirely around the frame, as shown, to permit mounting the frame in the camera with either of two adjacent edges at the bottom. The cross-section of the ribs and of the scren bars is such that the screen cannot be inserted in the bars back to front. A cross-section of rib suitable for this purpose is illustrated and comprises front and rear parallel surfaces 29 and II, respectively, and an edge surface 3| out of normal to the front and back surfaces. The inclined edge surfaces are preferably disposed at an acute angle to the front surfaces 29 so that, when engaged in the grooves of the screen bars, which are formed to correspond with the ribs, the inclined surfaces of the ribs and bars cooperate to urge the front surfaces of the ribs against the front surfaces of the grooves and thus take up the clearance necessary for easy insertion of the ribs into the bar grooves and ensure that the rear surface of the screen is located in exactly the focal plane of the camera. The screen plate and the distance plate are held in the frame rabbets by sealing strips 32 which also exclude dust from the space between the plates and hold the plates in absolute parallelism. It

will be understood that the plates are absolutely The screen plate 26 maybe of the usual construction or may be a photographic reproduction on glass of the lines and apertures of an etched screen. The screen plate shown is of the latter variety and isset in the frame with the emulsion side'in. According to this invention, the intersecting screen lines, designated 33 (Fig. 4) are wider than the spaces or apertures between them so that the ratio of aperture width to line width isless than the usual 1:1. I now prefer a screen having a ratio of approximately 35:65, but the ratio may be somewhat higher or lower.

The thickness of the frame between the screen plate 26 and the distance plate 21 is determined by the mesh or pitch of the screen (being less for a fine mesh screen than for a coarse m sh screen) so that the distance d (Fig. 3) from the rear face l5v of the distance plate 21 to the plane of the screen lines on the plate 26 is exactly the calculated screen distance. Regardless of the thickness of the frame 24, the distance d from the rear surface of the plate 21 to the plane of the front surfaces 23 of the ribs 28 is constant. In the camera, the distance between the front faces of the screen bar grooves and the focal plane is exactly equal to the distance (D. This renders unnecessary all the usual camera mechanism for adjusting the screen with reference to the focal plane and ensures that the screen occupies exactly the proper relationto the focal plane of the camera. Thus, according to this invention, a set of half-tone screens, instead of being composed of screens which are inserted in a camera and adjusted to what is presumed to be the proper distance from the focal plane, is composed of screens which occupy a fixed position in the camera and in which the plane of the screen lines is automatically and unavoidably located at exactly the proper distance from the focal plane of the camera by reason of the distance d from the plane of the screen lines to the front surfaces 29 of the ribs 28 plus the constant distance d being exactly that required to provide screen distance proper for the mesh of the screen. Thus, consumption of time and uncertainty of proper result incident to the usual screen adjusting procedure are avoided. Moreover, the plane of the screen lines is fixed in exact parallelism with the rear surface of the distance plate 21 and as this occupies the focal plane and forms a support for the photographic film, it follows that the plane of the screen lines is unavoidably parallel to the plane of the film durin the exposure thereof.

The stops 12 according to this invention each comprise a thin plate 35 havin means 36 for transmitting a solid, square beam of light and means 31 for transmitting a hollow, approximately square beam of light. The means 31 preferably has an outside diameter approximately three times the diameter of the means 36 and has a square inner periphery of the same dimensions and disposition as the means36. The outer periphery of the means 31 is quadrilateral, each of the sides being convexly curved and preferably a circular arc, the radius of which is approximately equal to the diagonal of the means 31. In absence of any known namefor such a .figure, I shall, for convenience of reference and for 38 and 31 are centered on the major axis of the ,being integral with the plate.

plate, each toward one end thereof, with their centers the same distance from the ends of the plate, so that when either end of the plate is inserted as far as it will go in the camera lens mounting, the means 38 or 31, as the case may be, is located exactly in the axis of light projection. The plate may be formed of any suitable material which is transparent in portions so as to constitute the means "and 31 and is opaque elsewhere; or the plate may be formed of opaque material, such as metal, provided with apertures constituting the means 36 and 31. In such case the opaque material 33 within the inner periphery of the means 31 will require support and this may be afforded by means of transparent material filling the space between the inner and outer peripheries of the aperture or by means of narrow bars 39 extendin from the corners of the portion 38 to the outer corners of the aperture. These bars may be integral with the portions 33 and with the plate or may be separate and suitably attached. It is now preferred to produce the stops by stamping the same from thin sheet metal, the portion 38 and its supportingbar 39 The bars may be of ample width to have adequate strength without affecting the tubular character of thelight beam projected through the opening 31, since the bars are so much narrower than the source of light (which is the camera lens) that they do not cast shadows extending more than a few inches from the stop. The invention contemplates the provision of a series of stops as aforesaid, preferably one for each inch of camera extension, having the means 36 and 31 of each stop of a size exactly suited to the camera extension to which the stop pertains. For convenience, the means 36 and 31 will be hereinafter referred to as apertures. Each stop is provided with an identification mark 40, preferably a number corresponding to the inches of camera extension at which the stop is to be used.

In carrying out the process of'the invention, the usual commercial process camera is employed and is extended and focused as usual. The proper stop l2 for the camera extension is inserted in the lens mounting with the small square aperture 36 in register with the lens. A halftone screen I 4 of pitch suitable for the work to be done is selected and inserted in the usual place in the camera and the film containing and shifting mechanism set in the position of the usual plate holder. The cam 20 is rotated to advance the pressure board l1 to clamp the film fiat against the rear surface ii of the screen, that is, against the rear surface of the distance plate It. An exposure of duration suitable to the intensity of light reflected from the copy is made. The

lens shutter is then closed and the stop reversed rotated to move the exposed film out of register with the screen and the shears 22 are operated to sever the exposed portion of film, which falls into the receptacle 23.

In the first, or shadow, exposure, which is represented by the diagram Fig. 6, the light passing; through the small stop aperture 35 and the screen apertures -is sharply focused and reacts small, relatively widely spaced areas of the filmemulassaoso sion, one such area being designated II in the diagram. The duration of the exposure is such that the relatively small amount of light reflected from the dark shadow portions of the copy has ample time to react the film emulsion sumciently to produce a sharp opaque dot on development of the negative. In the second. or high light, exposure, represented by the diagram Fig. 7, light passing through the stopaperture 31 re- .acts areas of film emulsion, designated 2 in the diagram, extending around each of the reacted portions ll resulting from the first exposure. The opaque portion 38 within the aperture 31 prevents any substantial amount of light reaching the film areas ll during the second exposure, such light as does reach these areas ll being mostly the result of irradiation. In the second exposure, the shorter duration thereof combined with the interruption of light by the filter substantiall prevents the small amount of light reflected from the dark shadow portions of the copy affecting the film emulsion. It will thus be seen that in the high light portions of the film, the dots are caused to grow in size but "the opacity of their central portions is not increased, whereas in the deep shadow portions the dots neither grow nor are increased in opaqueness. Thus, the photographer can, by judicious timing of the first and second exposures, control the contrast in the negative. The aforesaid effects are due not alone to the stop apertures used but to the combination of these with a screen of ratio less than 1:1. If a. screen of the usual 1:1 ratio was used with the stops of this invention, the size and spacing of the film areas reacted during the first exposure would be such that not enough space would be left between them with the result that the areas reacted during the second exposure would overlap to some extent on the areas reacted during the first exposure. This overlapping of exposures would prevent full realization of the selective effect of the two exposures. With the normal, diagonal disposition of screen lines and the width of the lines being approximately twice the width of the screen apertures; and with the stop apertures arranged with their diameters horizontal and vertical, the 1:3 ratio of the diameters of the stop apertures 36 and 31 produces high light-dots and middle tone dots which are firmly connected at their corners, as illustrated in Fig. 8, and thus produce the desirable checker-board pattern. The. dot formation resulting from the combined effects of the special screen and of the square stop apertures is superior to dot formation resulting if circular stop apertures are used either with the special screen or with an ordinary screen.

I A film negative produced by double exposure through the stops and screen as hereinbefore described exhibits a superior gradation of tones and superior dot formation and-does not require any after treatment but is ready for use in making positive prints on metal. As the screen distance and sizes of stop apertures are correctly fixed by the apparatus used, the photographer can produce the optimum result with certainty and without the customary expenditure of time in screen adjustment and stop selection. The superior tone gradation and superior dot formation in the negative are translated in the positive print on metal and greatly facilitate the etching of the half-tone plate, as will be readily understood by those skilled in the art, and results in the production of a superior plate.

While the invention relates primarily to the making of half-tone film negatives, the process may be applied to the making of half-tone wet plate negatives. It will be understood, of course, that in such case, contact of the wet plate and the half-tone screen is not permissible. wherefore the back plate of the screen will be located forwardly of the focal plane of the camera but in other respects the screen construction will be unchanged. In particular. the space relation 01' the screen ribs 2a to the plane of the screen lines will be determined by the pitch of the screen. A wet plate negative made according to this invention will have the same superior characteristics of tone gradation as a film negative, with the result that the treatment of the wet plate negaat the focal plane of the camera and of ensuring that it is in exact parallelism with the plane of the screen lines.

In the foregoing description of the process of my invention, no mention has been made of the.

customary practice of "flashing," that is, of exposing the sensitive medium to light reflected from an all white surface and passing through a very small stop aperture, the p pose of the flashing being to increase the opacity of the centers of the half-tone dots. According to my inven-.

tion, by reason of making two exposures, the first of which is through a small stop and may be of longer duration than is usual, the shadow dots.

are normally adequately exposed, so that the said first exposure in many cases serves the purpose of the usual flashing and renders the same need.-

lms. In cases where it is desired to harden the shadow dots, flashing may be resorted to but the amount thereof required to produce the desired effect wil be less than would be required if the negative had been made by the usual process.

While the screen illustrated in Fig. 3 is so constructed that the screen distance d is greater than the constant distance d it will be understood the distance d is arbitrarily selected and 1 may be greater than the screen distance d, as is illustrated in Fig. 9, in which the fiange 28 is offset. In this construction the distance d equals d -d'. This construction is very convenient in the case of fine pitch screens, for which the screen distance d is only a fraction of an inch.

Except as to the shape of the stop and screen apertures, this invention applies equally to making half-tone positives for production of rotagravure plates.

Having thus described my invention, I claim:

1. A process of making half-tone negatives, which process comprises photographing copy by passing light from the copy, through an approximately square stop aperture and through a halftone screen of a ratio of aperture width to screen line width substantially less than 1:1; and subsequently photographing the copy on the same sensitive medium by passing light from the copy through a larger approximately square, annular stop aperture the opaque center of which is similar in size and shape to the square stop aperture, and through the same screen, the two exposures being in register-, being unequal and aoaaoao made with the diagonals or the stop apertures parallel to the lines of the screen. i

2. A process of making half tone negatives, which process comprises exposing spaced areas of a light sensitive medium to light passing from copy'through a small substantially square stop aperture and exposing areas of said medium surrounding those first exposed and at a dinerent time to light passing from the copy through a larger stop aperture of quadrl-convexo-lateral outer perimeter surrounding an opaque area of approximately the same size and shape as the square aperture, the light passing during each of said exposures also through a cross lined halftone screen having an aperture width less than the line width, the diagonals of both said stop apertures being parallel with screen lines, said expcsures being unequal and in register.

FRANCIS E. WA'I'IIER.

REFERENCES CITED The following references are of record in the the of this patent:

UNITED STATES" PATENTS Number Name Date 532,326 Levy Jan. 8, 1895 802,598 Richter Oct. 24, 1905 1,167,505 Larsen Jan. 11, 1916 1,289,129 Douthitt Dec. 31, 1918 1,575,408 Bassani Mar. 2, 1926 5 ously Number Name Date 1,732,051 Johnson 1 Oct. 15, 1929 1,832,656 Powers Nov. 17, 1931 2,044,184 Powers June 16, 1936 2,060,640 Shaw Nov. 10, 1936 2,062,401 Disque et a1 Dec. 1, 1936 2,095,909 Bennet Oct. 12, 1937 2,100,346 Misuraca Nov. 30, 1937 2,115,233 Powers Apr. 26, 1938 2,123,830 Groesbeck, Jr July 12, 1938 2,126,479 Landrock et a1 Aug. 9, 1938 2,145,427. Morris, Jr. Jan. 31, 1939 2,306,936 Chambers Dec. 29, 1942 FOREIGN PATENTS Number Country Date 30,272 Sweden Feb. 11, 1911 Germany May.27, 1919 OTHER REFERENCES Clerc, llford Manual of Process Work, published by Ilford Ltd., London (3rd edition 1941),

pages 163 and 164 cited.

Clerc, Ilford Manual of Process Work, previcited; pages 168-171 additionally cited. Fruwirth Diffraction Theory of Half-Tone," part 14, Odd Screen Rulings, The American Photo-Engraver, 1935, pages 109 to 115 cited.

Halftone Stops, by J. S. Mertle, in the Ameri- 30 can Photo-Engraver, vol. 21, No. 10, Sept. 1929,

pages 937 to 951 cited.

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