US2754208A - Production of screened photographic images - Google Patents

Description

July 10, 1956 R. M. P. CONRAD 2,754,208

PRODUCTION OF SCREENED PHOTOGRAPHIC IMAGES Filed Dec. 8, 1952 I \.11 I I7 6 I I I Fl I I I I I I I L l[ 15 I H I 19 I I I I I I I Fig: .2.

17 m! "for 17 M 7. Com 4d fvfr M her PRGDUQTEGN F SCREENED PHOTOGRAPHIC IMAQ-ES Rudolf Michael Peter Qonrad, Bromley, England Appiication December 8, 1952, Serial No. 324,699 Claims priority, application Germany December 12, 1951 3 Claims. (Cl. 95-6) The present invention relates to the production of screened photographic images suitable for use in manufacturing photogravure printing plates.

Photogravure printing plates are usually produced in the form of a large number of small screen cells Whose lateral dimensions are substantially identical and whose depths vary in dependence upon the variations in light and shade in the image to be printed.

Such plates suiier from the disadvantage, however, that as the cells over areas corresponding to areas of light shallow, it is found that such plates are suitable for printing only a few thousand copies of the image before detail in such areas is lost. This loss of detail arises from the wear of the doctor blade and the wear between the surface of the plate and the paper on which the images are printed.

In order to overcome this disadvantage it has been pro posed to employ plates of the photogravure type commonly referred to as inverted half-tone. in producing a half-tone screen is employed whereby the are of varying lateral dimensions and whose substantially identical. Thus the density of printing plates. of the inverted half-tone plates it half-tone type Whose screen cells in accordance with the well technique.

Photogravure printing plates of the type in which the screen cells are of difierent lateral dimensions and different depths in dependence upon the variations in light and shade in the image to be printed by the plate will be referred to hereinafter as photogravure printing plates of the type specified.

In a known process for producing such plates (usually half-tone screen transparencies.

The screened positive so produced is placed in contact with the sensitised gelatin layer of a carbon tissue mounted in a printing-down frame, and printing down is effected. The screened positive is then removed and in its place is inserted the continuous tone positive, extreme care being taken to ensure that the continuous tone positive occupies a position in exact register with that previously occupied by the screened positive. Printing down is again effected whereby the unexposed dots on the carbon tissue following the first printing down operation are then exposed in dependence upon the variations in the density of the emulsion on the continuous tone positive. The treatment of the carbon tissue for producing a photo- 2,754,208 Patented July 10, 1956 ice well known in the present invention is to provide an producing a screened photographic use in manufacturing printing plates tinuous tone photographic negative, placing the transparency in a first plane, placing a photo-sensitive emulsion in a second plane, projecting an image of the transparency through a half-tone screen onto the emulsion in the second plane with the aid of a light source and a lens, removing and processing the emulsion to produce a screened the lens, removing the half-tone screen and with the aid of the light source and the lens projecting an image of the continuous tone negative onto the screened negative in exact register therewith to expose the further emulsion.

as a screen in the second exposure and the light transmitted through the dots is in accordance with the same variations in light and shade as those producing the dots on the screened negative. Thus when the final emulsion gravure printing plate of the type specified.

The invention will now be described by way of example with reference to the accompanying drawings, in which Figures 1 and 2 are diagrammatic plan views showing the manner in which the invention can be carried into effect.

In Figure l a continuous tone positive transparency 10 has its emulsion disposed in a first plane 11. An image of the picture depicted on the emulsion is projected with the aid of a light source at 12 and a lens 13 onto a photosensitive emulsion carried by a plate 14 and lying in a second plane 15. in the light path between the lens 13 and the plate 14 there is included a compensating glass 16 and a half-tone screen 17. The thickness of the compensating glass 16 is made equal to that of the plate 14. After the exposure of the emulsion on the plate 14, the plate 14 is removed and processed to produce a screened negative of the positive transparency 16.

Referring to Figure 2, the positive transparency 10 is then removed from the plane 11 and is replaced by the negative 18 from which the positive 10 was originally made by contact printing. As the positive 10 was made by contact printing, in order to keep the glass plate 18 supporting the emulsion of the continuous tone negative out of the light path between the planes 11 and 15, the plate 18 is laterally reversed so as to provide the image on the side thereof nearer the lens 13. The screen 17, as will be seen in Figure 2 is removed from the light path and as the plate 18 is laterally reversed the plate 14, after processing to produce the screened negative, is also laterally reversed and placed in the position previously occupied by the plate 14 as shown in Figure 2. It will be seen from Figure 1 that in the light path between the lens 13 and the plane 15 during the first exposure there is the compensating glass 16 and the glass supporting the screen 17. In Figure 2 the compensating glass 16 is removed and a further compensating glass 19 inserted. This further compensating glass has-a thickness equal to the thickness of the screen 17 of Figure l. A photo-sensitive emulsion carried by a further plate 19 is placed in contact with the image on the negative 14. The light 12 is again switched on and hence the the plate 19 is exposed to light directed through the negative 18 and the screened negative 14, the two images from these two negatives being in register on the emulsion carried by the plate 19.

It will be appreciated that special apparatus is required to ensure firstly that the continuous tone negative 18 can be placed in the plane 11 in exactly the same position as was occupied by the continuous tone positive and secondly to ensure that the screened negative 14 can be replaced with its emulsion in the plane in exactly the same position as was occupied by the emulsion during the first exposure.

Apparatus suitable for carrying out the invention and to ensure that these conditions are met is described in copending patent application Serial No. 249,056 which is a continuation in part of United States application Serial No. 191,552, nOW abandoned. The apparatus described in this co-pending application comprises a lens which communicates through two light-tight structures on either side thereof with two plate holder devices arranged in such a manner that a plate can be accurately located therein and that the plate holder itself can be accurately located in the camera. The arrangement for accurately locating the plate or plates in the plate holder as therein disclosed comprises an outer frame having a windowed panel therein against which the plates are pressed by blade springs. The plates have two fixedly positioned edges at an angle to each other. One of these edges abuts against two lays or mechanical locating elements and the other of these edges abuts against a third lay or mechanical locating element. The arrangement for locating the plate holder in the camera itself is such that either face of the plate holder can be accurately located, the means for ensuring this comprising a system of dowels projecting from both faces of the plate holder and engageable with apertures in the camera body. In the form disclosed in said copending applications the system of dowels comprises three dowels arranged like the corners of an isosceles triangle symmetrical about the vertical axis of symmetry of the plate holder. Thus with the so arranged dowels projecting from both faces of the plate holder a reversed engagement thereof with the apertures in the camera body reverses the accurately positioned plate about such axis of symmetry.

To insure that the continuous tone negative 18 can be placed in the first plane 11 in exactly the same position as was occupied by the continuous tone positive 10 therein, said positive is produced from said negative by contact printing as aforesaid with the fixedly positioned edges of the two plates aligned by such three point lays or mechanical locating means, so that the positive thus has an image completely complemental to that of the negative with reference to the corresponding registration points or elements established by the lays abutting said edges. Thus the images in these two plates can then be interchangeably located in the same position in the holder in the first conjugate plane 11 without resort to any registration by hand and eye, while the similar three lay positioning of the plate 14 in the holder in the second conjugate plane 15 enables it to be replaced therein in exactly the same photo-sensitive emulsion on,

position as it occupied during its exposure, the symmetrically arranged dowels projecting from both faces of the holders providing for the reversal of the respective plates about corresponding vertical axes of symmetry, whereby the images of the two negatives 18 and 14 are automatically placed in register on the emulsion carried by the plate 19 without resort to hand and eye adjustments.

I claim:

1. A method of producing, from a continuous tone photographic negative having two fixedly positioned edges at an angle to each other, a screened photographic image suitable for use in manufacturing a photogravure printing plate of the type specified, which method comprises the steps of (a) positioning in contact with said continuous tone negative a first sensitized transparency having two fixedly positioned edges corresponding to those of said negative and respectively aligned therewith by means of a frame having two mechanical locating elements abutting one of said edges and one such element abutting the other of said edges of said negative and transparency thus establishing corresponding registration points therefor, illuminating the so positioned negative and transparency to expose said first transparency, and processing said first transparency to produce a continuous tone positive transparency having a positive image completely complemental to that of said negative with reference to the corresponding registration points of the aforesaid edges thereof; (b) placing said continuous tone positive transparency in the first of two conjugate planes on opposite sides of a lens with its position fixed in said conjugate plane by three mechanical locating elements corresponding in position to those referred to in step (a); (c) placing in the second of said conjugate planes a second sensitized transparency having two fixedly positioned edges at an angle to each other located in predetermined positions by means of a frame having two mechanical locating elements abutting one of said edges and a third such element abutting the other of said edges; (d) then with the aid of a light source and said lens projecting an image of said continuous tone positive transparency through a half-tone screen interposed between said conjugate planes to said second transparency, and removing and processing said second transparency to produce a half-tone screened negative transparency; (e) replacing the processed halftone screened negative transparency in said second plane in the same abutting relation referred to in step (c); (f) substituting the continuous tone negative for the continuous tone positive in said first plane in the same abutting relation referred to in step ('b); (g) placing a third sensitized surface adjacent one of said negatives on the side thereof remote from said lens, removing the half-tone screen, and (h) with the aid of a light source and said lens projecting an image of the other of said negatives to said third sensitized surface through the negative adjacent thereto.

2. A method according to claim 1, in which' the negatives positioned in said abutting relations in steps (e) and (f) for the practice of step (h) are laterally reversed about corresponding axes of symmetry from the positions occupied by the positive transparency and the second sensitized transparency in steps (b) and (c).

3. A method according to claim 1 in which the third sensitized surface is placed adjacent the half-tone screened negative in step '(g) References Cited in the file of this patent UNITED STATES PATENTS 2,096,794 Dultgcn Oct. 2 6, 1937

Claims (1)

1. A METHOD OF PRODUCING, FROM A CONTINUOUS TONE PHOTOGRAPHIC NEGATIVE HAVING TWO FIXEDLY POSITIONED EDGES AT AN ANGLE TO EACH OTHER, A SCREENED PHOTOGRAPHIC IMAGE SUITABLE FOR USE IN MANUFACTURING A PHOTOGRAVURE PRINTING PLATE OF THE TYPE SPECIFIED, WHICH METHOD COMPRISES THE STEPS OF (A) POSITIONING IN CONTACT WITH SAID CONTINUOUS TONE NEGATIVE A FIRST SENSITIZED TRANSPARENCY HAVING TWO FIXEDLY POSITIONED EDGES CORRESPONDING TO THOSE OF SAID NEGATIVE AND RESPECTIVELY ALIGNED THEREWITH BY MEANS OF A FRAME HAVING TWO MECHANICAL LOCATING ELEMENTS ABUTTING ONE OF SAID EDGES AND ONE SUCH ELEMENT ABUTTING THE OTHER OF SAID EDGES OF SAID NEGATIVE AND TRANSPARENCY THUS ESTABLISHING CORRESPONDING REGISTRATION POINTS THEREFOR, ILLUMINATING THE SO POSITIONED NAGATIVE AND TRANSPARENCY TO EXPOSE SAID FIRST TRANSPARENCY, AND PROCESSING SAID FIRST TRANSPARENCY TO PRODUCE A CONTINUOUS TONE POSITIVE TRANSPARENCY HAVING A POSITIVE IMAGE COMPLETELY COMPLEMENTAL TO THAT OF SAID NEGATIVE WITH REFERENCE TO THE CORRESPONDING REGISTRATION POINTS OF THE AFORESAID EDGES THEREOF; (B) PLACING SAID CONTINUOUS TONE POSITIVE TRANSPARENCY IN THE FIRST OF TWO CONJUGATE PLANES ON OPPOSITE SIDES OF A LENS WITH ITS POSITION FIXED IN SAID CONJUGATE PLANE BY THREE MECHANICAL LOCATING ELEMENTS CORRESPONDING IN POSITION TO THOSE REFERRED TO IN STEP (A); (C) PLACING IN THE SECOND OF SAID CONJUGATE PLANES A SECOND SENSITIZED TRANSPARENCY HAVING TWO FIXEDLY POSITIONED EDGES AT AN ANGLE TO EACH OTHER LOCATED IN PREDETERMINED POSITIONS BY MEANS OF A FRAME HAVING TWO MECHANICAL LOCATING ELEMENTS ABUTTING ONE OF SAID EDGES AND A THIRD SUCH ELEMENT ABUTTING THE OTHER SAID EDGES; (D) THEN WITH THE AID OF A LIGHT SOURCE AND SAID LENS PROJECTING AND IMAGE OF SAID CONTINUOUS TONE POSITIVE TRANSPARENCY THROUGH A HALF-TONE SCREEN INTERPOSED BETWEEN SAID CONJUGATE PLANES TO SAID SECOND TRANSPARENCY, AND REMOVING AND PROCESSING SAID SECOND TRANSPARENCY TO PRODUCE A HALF-TONE SCREENED NEGATIVE TRANSPARENCY; (E) REPLACING THE PROCESSED HALFTONE SCREENED NEGATIVE TRANSPARENCY IN SAID SECOND PLANE IN THE SAME ABUTTING RELATION REFERRED TO IN STEP (C); (F) SUBSTITUTING THE CONTINUOUS TONE NEGATIVE FOR THE CONTINUOUS TONE POSITIVE IN SAID FIRST PLANE IN THE SAME ABUTTING RELATION REFERRED TO IN STEP (B); (G) PLACING A THIRD SENSITIZED SURFACE ADJACENT ONE OF SAID NEGATIVES ON THE SIDE THEREOF REMOTE FROM SAID LENS, REMOVING THE HALF-TONE SCREEN, AND (H) WITH THE AID OF A LIGHT SOURCE AND SAID LENS PROJECTING AN IMAGE OF THE OTHER OF SAID NEGATIVE TO SAID THIRD SENSITIZED SURFACE THROUGH THE NEGATIVE ADJACENT THERETO.

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