Patent GB-1496894-A (GB)
Title: Method and apparatus for determining the exposure of a printing material
IPC: G03B27/73
Filed: 1974-11-11
Abstract: 1496894 Determining printing material exposure AGFA-GEVAERT AG 11 Nov 1974 [10 Nov1973] 48769/74 Heading G2X A method of determining, for each of two colours of printing light to be used, the degree of exposure required to produce from an original transparency a mono-chromatic print having a predetermined density range on printing material of which the response to light in one of the two colours is different from the response to light in the other of the two colours, comprises evaluating the response of the printing material to exposure in each of the two colours of light by making two test exposures only, that is, a first test exposure which comprises exposing a first test portion of the printing material for a first predetermined time and through a neutral density wedge to light in one of the colours and a second test exposure, carried out after the first test exposure, which comprises exposing a second, different test portion of the printing material for a second predetermined time and through a neutral density wedge to light in the other of the colours, developing the two test portions, and ascertaining, for each test portion, the two values of wedge density which have produced, on that test portion, densities equal to the density values at either end of the predetermined print density range, respectively, and evaluating, on the basis of the four ascertained wedge density values and the density range of the original, the degrees of exposure, in both of the two colours of light, for the exposure of the printing material through the original, that must be observed in order to obtain a print having the predetermined density range. Suitable apparatus for carrying out the method comprises a neutral density wedge 4, means for supporting a strip of printing material 5, means for projecting light through the wedge on to printing material on the printing material support means, a first colour filter 6 arranged to transmit only light in one of the said two colours of printing light and which can be selectively moved by magnet 8 in response to signals from control unit 10 into and out of the path of light projected through the wedge, a second colour filter 7 arranged to transmit only light in the other of the said two colours of printing light and which also can be similarly selectively moved into and out of the path of light projected through the wedge, means for advancing printing material on the printing material support means, and exposure control means arranged to effect the movement of the first colour filter into the said light path and the exposure of the printing material through the first colour filter for a first predetermined time, thereafter the movement of the first colour filter out of the said light path and the actuation of the printing material advancing means such that printing material on the printing material support means is advanced by a distance at least equal to the length of the wedge, and the movement of the second colour filter into the said light path and the exposure of the printing material through the second colour filter for a second predetermined time. The method proceeds on the basis that from computed superposition of the constituent exposures, the intermediate values between the pure yellow and the pure blue exposure can be determined.