INSTRUMENTAL COLOUR MEASUREMENT AND CONTROL 275 Table IV Overall colour difference from control (traces) [ Instrumencolouroverall Mean overall Sample [ visual colour Observer Observer Observer Observer difference difference 1 2 3 4 (traces) (MacAdam Units) Red 1 12.0 9.4 4.6 6.0 7.6 $.$ Red 2 4.5 2.8 2.4 2.2 2.9 3.0 Red 3 4.1 2.2 2.4 2.2 2.6 3.2 Red 4 2.8 1.4 1.4 1.0 1.6 1.3 Red 5 2.0 1.0 1.4 1.0 1.1 1.2 Grey 1 2.0 1.0 2.4 2.8 1.6 2.0 Grey 2 2.8 -- 1.7 1.4 1.0 1.0 Grey 3 2.0 1.4 2.1 2.8 1.9 1.8 Grey 4 1.0 0.5 1.7 1.5 1.1 1.6 Grey 5 1.0 0.5 1.7 2.0 1.2 0.9 Grey 6 1.0 0.5 1.7 2.8 1.4 1.3 Blue 1 9.8 4.5 4.9 6.0 6.1 8.0 Blue 2 6.9 4.9 6.7 6.0 5.9 8.6 Blue 3 8.2 4.9 6.7 6.0 6.3 7.9 Blue 4 9.4 5.4 6.7 6.0 6.6 8.4 Brown 1 60.5 25.6 16.3 $.9 27.0 24.5 Brown 2 25.3 $.2 4.2 2.8 9.7 5.7 Brown 3 8.0 4.0 4.2 2.0 5.0 3.1 Brown 4 4.0 2.8 2.7 2.8 2.8 4.1 Brown 5 2.8 2.2 2.2 2.5 2.3 3.5 Brown 6 2.0 [ 2.0 2.4 2.0 2.0 0.9 ON-LINE INSTRUMENTAL COLOUR CONTROL When on-line colour control of continuous material is being considered the instrument must be suitable for the machine environment as Well as being stable, reliable and capable of producing a quantitative description of colour difference, which agrees closely with human perception. The Colorere measuring system, developed by Instrument Development Laboratories Inc., U.S.A.*, is an automatic on-line colour monitor which satisfies all these demands. It comprises three units: the optical measuring head, the illuminator, and control console with recorder. The Colored• is a three filter photometer with the transmission characteristics of the filters so designed that when they are combined with the energy output of the light source and the spectral response of the three photomultipliers, the output from the instrument corresponds approximately to the C.I.E. tristimulus values X, , *Now part of Kollmorgen, Inc.

276 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Y and Z. It employs the flicker photometer measurement principle, which involves a rapid continuous comparison between sample and standard, giving a direct measurement of the percentage difference in reflected energy. The optical head is linked electrically to the physically separate control console to provide remote control and display. The Colorede has been used on-line in a paper machine (8) and the arrangement of the instrument is shown in Fig. 3. Use of the Colorede measuring system The reflectance values of the standard pattern are obtained by direct reading relative to a white tile these readings are held electronically on the control console and serve as the future standard. The colour difference, •X 1, bY 1 ZkZ 1 • ' ' between the standard values X1 y1 Z1 and the moving substrate, at the end of the production line, is regarded as three simultaneous continuous signals for display on (i) three colour difference meters, (ii) a three pen chart recorder, (iii) the IDL colour analysis display computer. Accuracy of the Colorede measuring system The accuracy of the Colorede as a colour measuring instrument for X, Y, Z values of the ( .I.E. system has been obtained by comparison with an instrument widely used for colour measurement, the Coloreye Signature Large Sphere Model Colorimeter, on a range of shades with reflectance readings from 10-90%. (Table V). Value X Y Z Table V Average difference between Coloreye- Colored e --0.222 0.535 --1.237 95 % Confidence interval --0.682 to 0.238 0.162 to 0.908 --1.530 to 0.944 There is a small, but statistically significant difference between the Cotorede and the standard instrument, but the Colorede can be described as