GB837778A - Photographic method and apparatus - Google Patents

Abstract

837,778. Photo-electric printing control. LOGETRONICS Inc. June 19, 1957 [June 19, 1956], No. 19299/57. Class 40 (3). In a photographic method in which a variable light beam scans a subject and forms a real image on a photo-sensitive surface from which the light is directed to light-sensing means, the light beam is modified as a function of the intensity variations sensed by said means to maintain the average light intensity constant and is further modified as a function of the integrated value of light received. A light beam from CRT 10 is directed through lens 26 on to a film negative 28, which it scans under the control of horizontal and vertical sweep generators 18, 22. Light which passes through the photo-sensitive surface 30 is constrained by a light trap 34 and energizes a photo-multiplier tube 32. Electrical feedback through amplifier 42 to CRT grid 14 so modulates the beam intensity that at any instant the average light intensity over the finite area of the light spot is the same as at all other instants, i.e. the average light intensity on all portions of the photo-sensitive surface is substantially constant. Thus the contrast range of any one negative is reduced to that acceptable by the printing paper. To maintain similar levels of light intensity and therefore uniform exposure, with negatives of varying density an integral function of light intensity is used to further modify the light beam. This is derived from a separate photo-multiplier tube 60 which has an adjusting sleeve so that any particular portion of the surface 30 may be used to influence this control factor. The output of tube 60 is applied to lightintegrating switch 50, to which is also fed the signal determining the desired level of exposure from device 54. In a simpler embodiment (Fig. 1, not shown) the integral control factor is derived directly from photo-multiplier tube 32.