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?

280 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS THE LECTURER: Variations between instruments of the same type will be reason- ably small but between instruments of different types the variation between the readings is fairly high. This is because instruments differ in measuring techniques, i.e., they view the patterns in different ways. Some look at the pattern in a given direction, others view the pattern over all directions as in the integrating sphere type of instrument. Differences between the absolute readings on different instruments are not important when colour differences are being measured on a particular instrument. Provided the reproducibility of measurement is good colour difference measurements will be reliable. I do not have any information in this respect for the Spectronic 20. It may also be used as a spectrophotometer but has the disadvantage that it measures with a fairly broad waveband, i.e. 20 nm, which may give rise to errors when measuring reflectance curves with sharp peaks. DR. A. W. MIDDLETON: Could you give us the reference to the Simon and Goodwin paper, and could you tell me how you sort out metameric colours since I assume that one of these instruments measures all its colours in one reference light? THE LECTURER: Generally speaking all the instruments I have referred to will measure colour under standard illuminant 'C' which is designed to correspond closely with daylight. Some ooe the instruments are also capable of measurements under other illuminants, i.e. tungsten lighting. With these instruments it is possible to assess colour differences that would be apparent under both daylight conditions and tungsten, and hence the degree of metamerism. The accuracy of assessment of metameric patterns is somewhat limited by colorimetric means. It is also a difficult problem with visual judgments because when metameric pairs of patterns are compared, the differences between observers' visual characteristics begin to show up very markedly, and widely differing opinions about a colour match may be obtained particularly when there are large age differences between observers. I certainly do not think instrumentation is the answer to the problem because instruments are only designed to have a response approximately equal to the visual response of the average observer. It is a reliable proposition only when comparing patterns which are non-metameric, for example where the standard is dyed, or coloured, with the same colorants as the sample. In practice a standard is set up from production at some stage, and compared instrumentally with subsequent production material. The reference you require is: The rapid graphical computation of small color differences by F. T. Simon and W. J. Goodwin, published by the Union Carbide Corporation, U.S.A. MR. J. M. TRmCLE: I have carried out an investigation of several differential colorimeters with a view to using theIn for quality control in industry, and there is a large spectrum of colours in eye cosmetics and face powders which are very low in green reflectance and the G value is well down. Even the most sophisticated colorimeters that I have looked at, do not seem to be able to perceive the differ- ences which must constitute say, two or three MacAdam units, where there is a G value lower than say 20%. Is this a fact of all differential colorimeters? Are there more sophisticated ones than, say, the Colormaster which I have looked at and which failed miserably in this area?