GB1155482A - Improvements in or relating to Photographic Printing Apparatus - Google Patents

Abstract

1,155,482. Controlling contrast. PENTACON DRESDEN KAMERA-UND KINOWERKE, VEB. June 14, 1966, No.26382/66. Headings G2A and G2C. In a process for printing from a negative on to a photo-sensitive sheet, the contrast is assessed by scanning the illuminated negative by a photoelectric device which automatically determines and controls the main exposure and, if necessary, an auxiliary overall exposure. If the range of values of the contrast of the negative (log K N ) is less than the copying range of the photo-sensitive paper (KU p ), then no auxiliary exposure is necessary [Range I]. If log K N is greater than the copying range KU p , then a maximum auxiliary exposure is required [Range III]. If the two are the same [Range II] the amount of the auxiliary exposure is determined automatically. The theoretical considerations involved are discussed, and Fig. 4 shows an arrangement in which they are applied to the contrast assessment of a film frame as illuminated by a lamp 2, the light transmitted being received by a photo-cell 4 via a scanning device 3, shown in more detail in Fig. 5. Between the illuminated negative frame 1 and the photo-cell 4 is an apertured partition 12, a pair of optical wedges 10, 10' having the same wedge angle and so disposed initially as to form a parallel plate, and a cylindrical lens 11. The wedges are rotated in the same direction at constant but different angular speeds. Thus the illuminated spot on lens 11 which is received by cell 4 is scanned spirally round a circular area. This is distorted to an elliptical area by the lens 11, so that the illuminated spot on the negative film frame 1 which is received by cell 4 is scanned spirally round an elliptical area. The output from the cell is fed to a series of computing circuits R 1 , R 2 , R 3 which carry out the necessary calculations and feed the resultant information in the form of controlling electrical pulses to the light sources 5 and 6 when the film negative 1 has moved on to the printing station. Lamp 5 effects the main exposure, projecting the image through an objective lens 7 on to the photosensitive paper 8, and the auxiliary overall exposure is effected by lamp 6, the light from which is reflected by a semi-transparent mirror 9 into the main light path.