GB1509757A - Method of and apparatus for determining the exposure in the production of photographic colour prints - Google Patents

Abstract

1509757 Photographic exposure control GRETAG AG 28 Nov 1975 [13 Dec 1974] 49096/75 Heading G2A An original I is scanned point-by-point to determine the density of each point in each of three primary colours, those points whose densities fall within ranges determined from a reference original are selected and the density values of the selected points are used to determine exposure values for controlling colour balance of the three primary colours. In this way the influence of colour dominants is minimized. Preferably if the number of eligible points falls below a certain percentage of the total number of points, the number is made up using the density values of the reference original, resulting in under-correction. As shown, the original 1 is scanned using a light source 3, rotating, six-sided mirror 6, lens 7, mirror 8 and photo-cells 9. A computer 21 compares the density values of the scanned points with stored reference values, selects eligible points, and calculates colour balance. It also determines overall exposure time preferably from the five highest and five lowest density values in each colour. When the original reaches the exposure station comprising a light source 13, lens 15, filters 16, shutter 17 and sensitive material 2, the computer 21 controls the filters 16 and shutter 17 according to the calculated exposure values. Formulae are given in the specification for selecting points, for calculating colour balance from the densities of the selected points, and for calculating overall exposure time from the highest and lowest density values. Instead of using density values D, transmission values T may be used where D= -log T; the formulae being modified accordingly.