HALF TONE SCREEN EXPOSURE APPARATUS

Abstract: In the production of half-tone photographic exposures in which supplemental flashing and/or bumping steps involving exposing a photosensitive sheet to non-image light, it is desirable to eliminate operator intervention. A half-tone screen (3) of a graphic camera is uniformly exposed by a pinhole light projector (11, 13) positioned off axis (140) with respect to the main optical projection axis (9). A light diffuser (12) is positioned intermediate a light source (11) and the pinhole (13) and has a variable density light attenuator pattern (Fig. 2) consisting of a plurality of circular lines for attenuating the light passing through the diffuser and aperture in a manner to produce uniform illumination of the half-tone screen.

Description: L HALF TONE SCREEN EXPOSURE APPARATUS The present invention relates to apparatus involved in making half-tone photographic exposures which are used to reproduce continuous tone original 3 copy by printing. It is desirable to eliminate operator intervention, and thus automate, the supplemental flash¬ ing and/or bumping steps involving exposing the photo¬ sensitive sheet to non-image light. Such procedures 10 are well known to those skilled in the art and are ex¬ plained in columns 1 and 2 of U.S. Patent 4,275,961 to Fontana, incorporated by reference herein. The steps of flashing a plain non-image bearing card positioned over the original copy on the copy board, or shining a ^-- > light through the camera lens, previously performed manually, are eliminated, to more completely automate the operation of the graphic camera. In accordance with such automation, a computation device, not part of the present invention, is employed to actuate a 20 flash lamp for a precise interval as a function of various parameters which are fed to the computation device. In accordance with the preferred embodiment, a pinhole projection device having a flash lamp and a 5 single light diffuser situated between the flash lamp and the pinhole, is positioned within the bellows of the graphic camera, the pinhole facing the film platen. A variable density light attenuation pattern is formed upon the side of the light diffuser facing the flash 0 lamp, the pattern including a plurality of constant density circular lines concentric with a central bullseye point positioned at the optical axis of the projector. The spaces between the lines continuously 5 SUBSTITUTE SHEET increases along radial paths emanating from the central bullseye point in a manner to cause the film platen to be uniformly illuminated upon the actuation of the flash lamp, in spite of the fact that the pinhole pro- jector is positioned off axis with respect to the main projection axis of the graphic camera. Other objects features and advantages of the present invention will become apparent upon study of the following detailed description taken in conjunction with the drawings in which: Figure 1 illustrates a front sectional of the preferred embodiment; Figure 2 illustrates the preferred attenuation pattern; Figure 3 illustrates a second embodiment of the projector. Referring now to Figure 1, film platen 1 is illustrated with photosensitive sheet 2 and half-tone screen 3 positioned upon the film platen. A main pro- jection lens 4 is mounted upon conventional light tight bellows apparatus 6, which is employed to change the object and image distances upon changes in desired magnification. Copy board 7 is illustrated in a verti¬ cal position and bears objects which are imaged upon film platen 1 by lens 4 in the conventional manner. An auxiliary projection device 8 is situated within bellows apparatus 6 and is positioned off axis with respect to the main projection axis 9, as indicated in Figure 1. A light source such as a 3800°K quartz- halogen lamp 11 is provided, together with light dif¬ fusion means 12 and a pinhole aperture 13, as shown. The light diffusion means comprises an opal glass light scattering body, together with the above-mentioned SUBSTITUTE SHEET CMPI ^ y variable density pattern or mask schematically illus¬ trated at 10, which is affixed to body 12 at the side thereof facing light source 11. A preferred configura¬ tion of pattern 10, illustrated in Figure 2, comprises a plurality of circular constant density rings concen¬ tric with a central bullseye point, the spaces between the circular lines increasing as the distances along radial paths emanating from the central bullseye point increases. In the illustrated pattern, the line thick- nesses decrease outwardly to increase the spaces between lines, while the pitch of the line placement is constant. Alternatively, the spaces between lines could be increased by providing constant width lines having increased sep¬ arations outwardly along the radial lines. The pre- ferred pattern of Figure 2 mounted upon a single light scattering diffusion plate 12 is configured so that the light attenuation characteristic decreases with in¬ creased distances away from the central bullseye point on the axis 14, proportional to the cosine of the angle S formed by the optical projection axis 14 and straight line 16 extending between points on the radial paths and the center of pinhole 13 at optical axis 14. The use of such a pattern, positioned upon the face of a single light diffusion glass body, will result in illum- ination of the half-tone screen, upon film platen 1, of uniform light intensity in spite of the .fact that projector 8 is offset with respect to optical axis 9. If the circular line pattern mask were not employed in con¬ junction with glass diffusion plate 12, non-uniform platen illumination would result, due to the well-known 4 cosine law. Even if light source 11 had a large uniform intensity light emitting surface congruent with the sur¬ face of light diffusion plate 12, non-uniformity of the SUBSTITUTE SHEET OMPI light projected upon platen 1, through the pin-hole apera- ture 13 would still result, owing to the well-known 4 cosine law. Further non-uniformity is introduced, since it is desirable from an engineering standpoint to illum- inate the diffuser plate with a light source such as lamp 11, having a light emitting area substantially smaller than the side of the light diffusion plate fac¬ ing lamp 11. This creates a further reduction in the intensity of the light produced at the right-hand outer portions of the light diffuser 12, which reduction in¬ creases at distances further off axis from ancillary projection axis 14. Accordingly, in order to uniformly illuminate that portion of platen 1 bearing the half¬ tone screen, a line pattern having an attenuation CharaC- teristic proportional to cos Cr and a single diffuser plate is preferred. Should diffuser plate 12 be uni¬ formly illuminated by another type of light source, a cos ~ t9 attenuation pattern would be employed to uni¬ formly illuminate the half-tone screen. For example, in Figure 3, a pair of light diffusion plates 17 and 18 could be provided having cos 4 Λ circular masking pat¬ terns 19 and 21 formed upon the left-hand sides of the plates facing the smaller flash lamp light source 11'. Pattern 19 produces uniform illumination of pattern 21, since the circular cosine 9 pattern compensates for falloff in illumination intensity at further distances from axis 14'. If the second light diffusion plate 17 with pattern 21 thereon were not employed, the uniform light produced at the right-hand portion of diffuser plate 17 would produce non-uniform illumination upon the 4 platen surface, due to the cosine law. Thus, the 4 second cosine pattern 21 formed upon a second diffuser plate 18 is employed, so that light emitted at the right- hand side of diffuser 18 is less intense at central SUBSTITUTE SHEET _OMH portions and more intense at outer portions thereof to uniformly illuminate the half-tone screen. The bullseye points of the patterns are positioned on axis 14'. The Q cosine pattern shown in Figure 2 is the actual pattern employed in cameras to be manufactured by the assignee of the present invention. While the offset distance between main projection axis 9 and ancillary projector axis 14 is not critical, this distance was about 4" in the actual embodiment constructed in accordance with the invention. The pinhole size is not critical. The flash lamp was a G.E. lamp DZE/FDS which was typically illuminated for a period of 20 seconds, at 100% magni¬ fication. The Figure 2 pattern has a diameter of 2.0 inches. While the above described embodiments of the invention are preferred, other configurations will be readily apparent to those skilled in the art and thus the invention is only to be limited in scope by the language of the following claims, including reasonable equivalents thereof. For example, while an approximately 1/4" diameter pinhole is preferred to save costs, a small projection lens could form part of the optical aperture found in the right-hand portion of the projector. SUBSTITUTE SHEET

Claims: 1 WHAT WE CLAIM IS : 1. In a graphic camera having a copyboard, a film platen for positioning a photosensitive sheet and a half-tone screen thereover, together with a main ■ projection lens positioned upon a projection optical axis for focusing images of objects upon said copyboard at said film platen, characterized in that it includes an auxiliary projection device (8) for exposing said photo¬ sensitive sheet (2) to light, said projection device in- 10 eluding a light source (11, 11') positioned at one por¬ tion of said auxiliary projection device; a light trans¬ mitting aperture (13) positioned at another portion of said auxiliary projection device, and light diffusion means (12) having a variable density pattern associated 15 therewith for causing said film platen (1) to be uni¬ formly illuminated upon the actuation of said light source by light passing through said aperture. 2. A device as claimed in claim 1, character¬ ized by said light diffusion means (12) having one por- 20 tion thereof facing said light source (11, 11') and another portion opposite thereof facing said light transmitting aperture (13) said variable density pattern being on said one side of said light diffusion means facing said light source, said pattern including a plur- 25 ality of light attenuating arcuate lines, the spaces between said arcuate lines continuously increasing in a manner to cause said film platen (1) to be uniformly illuminated by light passing through said aperture upon the actuation of said light source. 30 3. A device as claimed in claim 1 or 2, characterized by said auxiliary projection device (8) being positioned off axis with respect to said main pro¬ jection optical axis (9) and upon a second optical axis 35 SUBSTITUTE SHEET (14, 14') separate from said first optical axis, said light transmitting aperture (13) being positioned upon said second optical axis. 4. A device as claimed in claim 3, character- ized in that said light transmitting aperture (13) con¬ sists of a pinhole positioned along said second optical axis (14, 14') . 5. A device as claimed in claim 3 or 4, characterized in that said second optical axis (14, 14') is parallel to said first optical axis (9) . 6. A device as claimed in any one of the pre¬ ceding claims, characterized in that said light diffus¬ ion means (12) is located optically intermediate said light source (11, 11') and said aperture (13) . 7. A device as claimed in any one of the preceding claims, characterized in that the variable density pattern of said light diffusion means (12) in¬ cludes a plurality of light attenuating circular lines concentric with a bullseye point, the spaces between said circular lines continuously increasing along radial paths emanating from said bullseye point in a manner to cause said film platen (1) to be uniformly illuminated upon the actuation of said light source (11, 11') • 8. A device as claimed in claim 5, character¬ ized in that said pattern has an attenuation character¬ istic which decreases as the distance along radial paths emanating from said bullseye point increases, proportion- g al to the cosine of the angle between said second op- tical axis (14, 14') and a straight line between points along said radial paths and the center of said aperture (13) . SUBSTITUTE SHEET 9. A device as claimed in any one of the pre¬ ceding claims, characterized in that said light diffusion means (12) consists of a single light diffusing plate having said pattern formed upon one side thereof. 10. A device as claimed in any one of claims 7 to 9, characterized in that said light attenuating lines have a constant density. 11. A device as claimed in claim 7 and includ¬ ing any one of the preceding claims, characterized in that said bullseye point coincides with said second op¬ tical axis (14, 14*) . 12. A device as claimed in any one of the pre¬ ceding claims, characterized by said light diffusion means (12) having one side thereof facing said light source (11, 11') and the other side thereof facing said light transmitting aperture (13) , said sides of said light diffusion means having an area substantially larg¬ er than said light transmitting aperture and said light source. SUBSTITUTE SHEET OMPI

Inventors: NELSON ERIK KARL, NELSON, ERIK, KARL
Applicants: ITEK CORP, ITEK CORPORATION
Application year: 1983
Main classification: G03B27
IPC codes: G03B27, G03F5