278 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS calculator programmed with shading factors, which in turn was linked to a continuous colour addition system for dye solutions, on-line colour control to 1.5 MacAdam units tolerance was regularly attained for a wide range of shades. These shading factors were obtained from the ICI colour match prediction system called 'IMP', and enabled the calculation of dye additions to correct a dyeing on to shade. Factors with an influence on the accuracy of the colour control were the opacity of the substrate and variations in the substrate pass-line at the plane of measurement of the Colorede. Results, similar in accuracy to those reported above, for uniformly diffusing paper substrates, were obtained on textile fabrics and polythene coated pigmented board. It is expected that coloured film and plastic substrates could be accurately monitored and controlled by similar techniques. ADVANTAGES OF ON-LINE QUALITY CONTROL The advantages resulting from on-line quality control of coloured substrates by a continuous colour monitor and recorder are numerous, the more important being listed below: 1. Speed of detection of colour drift. With the Colorede measuring system, quality control is part of the manu- factuddng process. The standard is held electronically and deviations outside the agreed tolerance are automatically recorded. 2. Accuracy, inherent in a system which considers the total colour error (shade, strength and lightness) and records the difference without regard to ambient conditions and human foibles. :3. Availability of continuous information on which sound action can be initiated. CONCLUSIONS Colour control by instrumental measurement, both for batchwise and continuous colouring processes, is now a practical proposition, particularly for mateddais with surfaces having uniform structures. A number of in- struments have the necessary reliability, and colour difference calculating techniques, together with equipment for their convenient application, are available. Official recommendations regarding instruments and colour difference calculations have yet to come but many have been used with satisfactory

INSTRUMENTAL COLOUR MEASUREMENT AND CONTROL 279 results. A trial exercise is the best approach to the establishment of the reliability of a particular technique. Instrumental colour control is capable of improved quality of pro- duction and has economic advantages in reducing the quantity of reject material, and the volume of re-processing work in colour correction. Capital outlay in the installation of equipment for this purpose must, however, be carefully weighed against the expected economic saving. (Received: 22nd February 1968) REFERENCES (1) MacAdam, D. L. J. opt. Soc. America 89, 247-274 (1942). (2) Simon, F. T. and Goodwin, W. J. (3) Billmeyer, F. W. and Saltzman, M. Principles of Colour Technology (1967) Wiley & Sons, New York. (4) Proc. 15th Session C.I.E. Vol. A, p.113. (5) Glasser, L. G., McKinney, A. H., Reilly, C. D. and Schnelle, P. D. J. Opt. Soc. America 48 736 (1958). (6) Godlove, I. H. J. opt. Soc. America, 41 760 (1951). (7) Chickering, K. D. J. opt. Soc. America, õ? 537 (1967). (8) Paper Maker p.50 (December 1967). DISCUSSION MR. J. C. THORNTON: Could you please tell us over what range of materials such equipment could be used? THE LECTURER: There is no restriction on the materials that can be measured with this type of equipment. It is true that the accuracy achieved will vary according to the type of material examined. Measurements on very uniform surfaces, such as a piece of plastic, a ceramic tile, or a paint film--are very reproducible. Measurements of small colour difference between two similar patterns will, therefore, be reliable. With other types of material the reproducibility can decrease considerably. This will also depend on the equipment used. Instruments referred to in the paper are among the best, and give least problems with non-uniform surfaces. Decreases in instrument accuracy are, however, compensated to some extent by the fact that in these cases the eye is less sensitive to colour differences. In general, provided the instrument is good and allows for the problem of non-uniformity of material, and the instruments I have referred to today all do, there should not be any significant prob- lem. It is certainly advisable to carry out some tests before installing any piece of equipment to ensure that the reproducibility of readings on your patterns is sufficient- ly good to be well inside the size of tolerance that you are intending to set. MR. K. M. GODFREY: Can we have your opinion on instruments costing less than f500 as work I have done indicates that results with the Spectronic 20 are as repro- ducible as the Colormaster, although the figures obtained by the two instruments are different?