f. Soc. Cosmetic Chemists 1.8 3-17 (1967) ¸ 1967 Society of Cosmetic Chemists of Great Britain The rapid assessment of. colour for routine storage testing j. D. CHESHIRE and T. C. CORBY* Presented at the Symposium on "Colour", organised by the Society of Cosmetic Chemists of Great Britain in Eastbourne, Sussex, on 27th April 1966. Synopsis--The limitations of conventional colour measurement systems are discussed from the point of view of applicability to routine storage testing. The requirements for routine testing are simplicity, speed and ease of interpretation, and these are fulfilled by a technique described for transparent and opaque coloured materials. Visual assessments of hue changes are made by reference to standard colour scales while changes in saturation and tone are combined into one instrumental measurement of colour "depth." INTRODUCTION In a cosmetic laboratory developing a wide range of products, storage testing is concerned with many aspects of product stability, especially the maintenance of consumer appeal which commands a high proportion of development time. Consumer appeal covers those sensory characters of the product that are apparent before purchase and during use. If the pack and container of the product have been expertly chosen, colour is usually the first quality manifested by the product itself. Per- fume, consistency and performance assessments follow later. For this reason, colour-stability assessment is of prime importance to the cosmetic chemist. This paper describes a method of measuring changes in coloured pro- ducts after storage. In our laboratory, it has been found that storage testing of new product developments is most efficiently carried out by a separate team with full responsibility for organizing both the storage and inspection of products according to a schedule laid down by the formulation chemist. Many routine tests, including colour assessment, are carried out *Beecham Toiletry Division, Brentford, Middex. 3

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS under the schedule and the results are reported to the chemist concerned. The several advantages of this system are outside the scope of this paper, but obviously any technique developed must be compatible with the storage testing system. The emphasis, in our method for colour assessment, is therefore upon simplicity of operation, speed and ease of interpretation. Simplicity is desirable so that relatively unskilled operators can obtain reliable results with a minimum of supervision. Speed is important to satisfy the overall throughput of stored samples and to avoid a bottleneck at the colour- assessment stage. Ease of interpretation is necessary to provide the chemist with immediate knowledge of the storage characteristics of his formulation without recourse to calculations, graphs or specifications. We did not find that any of the standard methods outlined below satisfied these requirements for the reasons indicated. ESTABLISHED TECHNIQUES OF COLORIMETRY Tristimulus systems Established methods of colorimetry are discussed by Judd and Wyszecki (1), and by Judd and Nimeroff (2). Details of the ( .I.E. system of tristimulus colorimetry are described and practical instruments for specifying colours on this three-colour system are enumerated. They fall into three groups :-- (a) Spectrophotometric colorimetry Using a suitable spectrophotometer, it is possible to obtain a spectrum of any coloured material. The spectrum obtained is a complete measure of the colour, but it does not convey a clear visual idea of the colour, nor does it indicate how close the colour may be to any standard. By com- puting the spectral reflectance or transmittance of the coloured matehal with each tristimulus function of the "standard observer's" eye, it is possible to convert the spectral data to C.I.E. chromaticity coefficients. However, for the cosmetic chemist interested primarily in rapid testing, the skill and time required to use a spectrophotometer followed by a calculating machine render this method impracticable. (b) Photoelectric tristimulus colorimeters Instruments designed to measure tristimulus values directly depend on filters corresponding to each of the blue, green and red functions of the standard observer. Using a standard illuminant C (equivalent to average daylight) the colour is measured photoelectrically through each