GB713285A - Improvements in photographic printers - Google Patents

Abstract

713,285. Control of photographic printing. RADIO CORPORATION OF AMERICA. May 29, 1952 [June 7, 1951], No. 13696/52. Class 40 (3) In photographic printing apparatus of the type in which a 'scanning spot of light is focused through a transparency on to a sensitive surface the spot of light is made larger than the transparency detail at the surface of the transparency and part of the printing light is diverted to control the intensity of the scanning spot so as to alter the contrast of the printing relatively to that of the transparency except for details within the spot area which remain unaltered. The purpose of this arrangement is to simulate the printing process in which a thin defocused negative is placed in focus with a sharply focused positive transparency and the final positive printed through these. As shown a raster is produced at the screen of a cathode-ray tube 6 by "X" and "Y" deflection coils 14. The spot is then focussed on the screen of the tube by a coil 10 when it is defocussed on to the surface of the transparency 20 by a lens 18. Alternatively, a larger spot produced on the end of the tube may be focused on to the transparency. The plane of the transparency is then focused on to the printing surface 38 by a lens 37. A partially reflecting mirror 26 diverts part of the printing light to a photo-cell 28 which through a non-linear circuit 32 (v.i.) modifies the grid-cathode difference potential of the cathode-ray tube altering the relative contrast of the print and the transparency. Non-linear circuit, Fig. 2.-The output signal <SP>e</SP>c of the photo-electric cell is applied to the grid of an amplifying valve 42 which feeds the cathode-ray tube grid. A resistor 56 connected in the grid line of valve 42 has the grid end shunted by diodes 49, 50, 51, the cathodes of which are differently biased by potentiometers 45, 46, 47 so that as the input signals increase the diodes conduct one after the other causing an increasing voltage drop in resistor 56 so that a lower and lower proportion of the input signal is applied to the grid of the cathode-ray tube as the input signal rises.