512 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS presence of the mercury line radiation and the low efficiency of the red- producing phosphors inevitably spoil to some extent, even seriously, the imitation of daylight. This is apparent as soon as coloured objects are looked at, not only whites and greys under the highly efficient "daylight" fluorescent lamps, which look white but have a serious deficiency in the red part of the spectrum, tea and coffee look yellowish and unpleasant, red carpets look brown, butter acquires an unnatural jaundiced appearance, complexions look dirty, etc. These phenomena are grouped under the term "colour rendering of light sources". To a certain degree, one can be trained or brain-washed to disregard abnormalities in colour rendering, but not entirely. No one with normal colour vision would ever accept mercury or sodium street lamps as giving satisfactory colour rendering. On the other hand, many of the better fluorescent lamps are acceptable, at least for many purposes, as imitations of daylight, or just as pleasant sources of light. There is obviously, somewhere, a kind of threshold of acceptability, an engineering tolerance within which the spectral composition of light sources must lie in order to render colours satisfactorily. At first sight it might seem that the determination of a threshold of acceptability is in- superably complex, involving as it does variations in spectral composition all through the spectrum, each part of the spectrum contributing its quota to the general appearance of all colours in a realistic situation. No doubt this apparent complexity inhibited experiment for many years, so that the only methods of controlling colour rendering were common sense, guess- work and the misguided application of existing data from other fields of colour vision. In fact, the problem is not so recalcitrant as it seemed. For example, a series of carefully designed psycho-physical experiments carried out under my direction at the National Physical Laboratory showed that a comparatively simple system of colour rendering assessment could be used with great practical success. The first step in the colour rendering assessment of a source of light is to have an analysis of its spectral quality. The question is, how detailed need the analysis be? As a basis for colorimetric calculations, an analysis of 40, or 80, or even 400 bits of information is used, each "bit" being a relative energy measurement in a spectral band. Since the calculation ends up with only three bits of information, viz. the two chromaticity co-ordinates and the luminance factor, it would seem that a lot of information has been thrown away. It is interesting to follow up this analysis of visual per- ceptions into bits of information. One bit can be regarded as the total in- tensity of the source, a measurement including the whole spectrum. Two bits

EYESWEI•-T AND COLOUR SCIENCE IN COSMETICS 513 can give the equivalent colour temperature of the source by comparing the intensities in the two halves of the spectrum. Three bits give the colour, either of a source or of an object illuminated by it. One-bit information is provided by a photometer, two-bit information by a colour temperature meter, three-bit information by a colorimeter. Six bits of information were found sufficient for adequate assessment of colour rendering in all practical cases so far treated: the six bits are the relative luminosities in six spectral bands which together cover the whole visible spectrum. The use of more than six (or possibly seven) bits would be impossible experimentally since, on the average, an observer does not notice anything wrong with colour rendering until about half of any one of these six spectral bands is missing. COLOUR RENDERING ASSESSMENT Experiments on the trichromacy of colour vision stretch back in time over more than 100 years, those on colour rendering over a mere 10 or 15, so it may be useful to give a very brief general outline of the latter, based on the NPL work which may be taken as representative. Light from a source accurately equivalent to daylight was dispersed into a spectrum and then recombined. The apparatus was essentially a very large double monochromator so designed that the whole of the central spectrum was recombined without loss. At the location of the central spectrum, screens couldbe inserted so as to produce any desired modification, sudden or progressive, in the composition of the finally emergent light. If the latter was used to illuminate a picture, an object such as tea, coffee, butter, meat, etc., or a face, the colour rendering could then be judged while changes were made in the composition of the illuminant. By signalling the first onset of perceptible change, the observer made a determination of threshold, or tolerance, in colour rendering. There are, of course, many more experimental details and conditions involved, which can be studied in the original communications (9), but the final results which emerged were unexpectedly simple. Colour rendering is always assessed by conscious or subconscious comparison with something, there is no absolute measure of it. The most logical standard with which to compare is a phase of daylight, since day- light is the natural and most common illuminant, and it is in relation to daylight that our colour perception system has evolved. It is further pro- posed that the colour rendering properties of a source should normally be assessed in relation to a phase of daylight of the same equivalent (correlated)