GB1458358A - Method and apparatus for colour inspection - Google Patents

Abstract

1458358 Measuring colour photo-electrically SIRA INSTITUTE Ltd 26 Feb 1974 [5 Feb 1973] 5655/73 Heading G1A A test pattern is inspected by comparing, for a selected area, a signal representing the achromatic radiation from that area with a first reference signal and, with the use of further reference signals representing the desired colour of that area, determining the acceptability of the colour of the test pattern. A signal S i representing the general achromatic radiation received from a given area "i" of a test pattern is compared with a first reference signal W i representing achromatic radiation received from a corresponding area of a master pattern, an error signal (S i -W i ) indicating that at least one primary colour component of that area of the test pattern is incorrect, e.g. red, green or blue ink applied in a colour printing process. It is stated that mutually cancelling errors in two or more colour components of a given area are unlikely to occur, and that the probability that such complementary errors would escape detection by the method can be made negligible by inspecting a suitable number of areas, particularly if these are chosen so that different colours predominate in respective areas. The achromatic error signal (S i -W i ), weighted by reference chromaticity co-ordinates r i , g i , b i derived for a corresponding area of the master pattern, provides signals r i (S i -W i ), g i (S i -W i ), b i (S i -W i ) from which the error in one or more colour components can be derived by computation. Optical apparatus, also described in Specification 1326881, comprises a source 1, Fig. 1, illuminating a strip-like area of an object O bearing a test pattern, a portion of the reflected image being selected by a slit S in a plate 3 for presentation to a photomultiplier tube 8 which thus provides a signal representing the average light, irrespective of colour, reflected from a narrow strip of the test pattern. A radial grating 5, further described in British Specification 970369, is arranged as a chopper to modulate the beam received by the tube 8 for convenience of signal amplification. As the object O is moved perpendicularly to the length of the slit S the tube 8 provides a variable signal which, after analogue processing at 11, 12, 16, Fig. 2, is sampled in digital form to provide signals Si. The master pattern is similarly treated to obtain signals W i but is additionally scanned with colour filters 9 to give signals representing the chromaticity co-ordinates r i , g i , b i . Each area may be scanned individually achromatically and by colours in turn; or the whole master pattern may be scanned completely four times. Signals derived from the measurements may be passed to a pen recorder 13, or to a data processor 14 controlling the printing process.