728 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS If the energy distribution of the illuminant is known, then the C.I.E. tristimulus specification can be derived, that is, the position of the sample can be plotted on the chromaticity diagram which gives a better indication of the colour appearance of the sample. Most commercially available spectrophotometers are fitted with an integrating sphere for making measurements of diffuse reflectance from surfaces, but it is difficult to position some portions of the body against the sample aperture of most instruments. However, extensive measurements of human skin pigmentation were carried out by Edwards and Duntley (7) using the Hardy recording spectrophotometer although this is a complex instrument and not suitable for general laboratory or field work. They obtained curves over the visible spectrum from 400-700 rim, which were sufficiently discriminating to detect the presence of reduced haemoglobin and oxyhaemoglobin as well as melanin. Similar curves were obtained in endocrinological studies by Lerner and McGuire (3) using a Cary model 14 spectrophotometer, and over a wider spectral range (from 235-700 nm) by Jacquez, Kuppenheim, Dimitroff, McKeehah and Huss (8) using a Beckman DR recording spectrophotometer. If visual properties of skin are being studied it is unnecessary to go beyond the limits of the visual spectrum (from 400-700 nm) when plotting reflection curves, but the wider range used by Jacquez yielded information about the nature of the pigments present in skin. For some investigations a full spectrophotometric curve is not required and useful information can be obtained by abridged spectrophotometry in which a small number of broad bands in the spectrum are isolated with suitable colour filters and the reflection of the sample and the standard white surface are compared at each band. Measurements of this type are useful when the general character of the spectral reflection is required, as when comparing racial differences in pigmentation, although the discrimin- ation of sharp changes in absorption is lost. The main advantages of abridged spectrophotometers are simplicity of operation and portability, and various instruments of this type have been used by a number of workers. Weiner (9), Barnicot (10), and Harrison and Owen (11) used a simple EEL reflectance spectrophotometer (Evans Electroselenium Ltd., Harlow, Essex) for anthropological studies of different races. This is a typical instru- ment of its kind and gives a satisfactory reflectance curve for skin, con- sidering that measurements are made with wide band filters at only nine points through the spectrum brightness, dominant wavelength and purity can also be obtained. The instrument consists of a reflectance head, con-

MEASUREMENT OF SKIN COLOUR IN VIVO 729 taining the optical system and photocell, and a mirror galvanometer. The head measures 11 X 9 X 8 cm and weighs 675 g, so although it can be used on humans, care has to be taken that the weight does not blanch the skin. Jarrett (12) has also used this instrument for the measurement of skin pigmentation, although it was modified by the addition of a uv light source and uv transmitting filter to enable measurements to be taken at 360 nm, where melanin absorbs heavily. Although the reflectance head of the EEL spectrophotometer is rela- tively small, it is, nevertheless, very difficult to make measurements on the face owing to difficulties in positioning the instrument correctly. It is also impossible to make measurements on small animals such as guinea-pigs or rats. The problems of correct positioning of a reflectance head on the skin of humans and small animals, without causing colour changes due to defor- mation of the skin surface, have been solved largely by the use of flexible fibre optics light guides. Fibre optics The essential principles of fibre optics are that smooth filaments of transparent materials, such as glass, conduct light with high efficiency by means of total internal reflections along the filament walls, and fibres in a bundle conduct this light independently of one another. In practice, fibres are coated with a thin layer of glass of lower refractive index than the core. Without this coating light would be lost owing to minute imperfections on the surface of the fibre and this would become serious in a fibre where each ray may undergo hundreds of thousands of reflections. Fibres can be pro- duced with a diameter as small as 2 i•m, but a diameter of 50 I•m is common in flexible light guides as this size gives a useful combination of flexibility with strength. A practical light guide consists of a bundle of several thou- sand fibres cemented together at the ends the faces of the bundle are optically polished, and the whole structure is protected by a plastic sheath or a flexible metallic spiral tube. A Hitachi Perkin-Elmer model 139 spectrophotometer was modified using a two-tailed flexible light guide (Sunbury Glass Works Ltd., Ted- dington, Middx.). An integrating sphere is usually attached to the instru- ment when making measurements of diffuse reflectance from surfaces, but the design of the sphere is such that it is possible to position only small flat samples against the sphere's aperture. The attachment was therefore removed from the instrument and replaced by the light guide. A diagram