Process and apparatus for printing transparencies

Abstract

A method for controlling exposure in the direct printing of color transparencies, the method comprising comparing the neutral density of an original transparency required to be printed with a reference transparency to obtain a measure of the difference in neutral density, this difference then being used to control the exposure time. Control is effected by increasing the exposure time for that portion of the density difference which exceeds a first positive limit by a predetermined amount which is less than the increase in exposure time required by the positive density difference below the first positive limit and decreasing the exposure time for that portion of the density difference which is less than a second negative limit by a predetermined amount which is less than the decrease in exposure time required by the negative density difference above the second negative limit. Apparatus for carrying out the method comprises a printing light source for illuminating the original and the reference transparencies and circuitry for obtaining the neutral density difference. Filters for interposing between the light source and the original transparency are controlled by a non-linear circuit which determines the amount by which the exposure is to be increased or decreased by the above predetermined amounts.