450 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS production batch (1). The basic philosophy of the instrumental approach is to establish techniques, associated with physical instruments, that will enable the color perception of the normal individual to be expressed quan- titatively and to provide a universally acceptable reference system for us• in scientific and commercial application of color. As a result of our experimental investigation, we have established that it is desirable to employ the simplest practical application of instrumental reflectance colorimetry. This is meant in the sense that a minimum back- ground of work is needed before the colorimeter is put into routine service. It is first necessary to establish that the chosen instrument has sufficient reproducibility of measurement on the colored system of interest and then to set tolerance limits in terms of instrumental readings by correlation with the visual assessment method previously employed. This can normally be done in the course of production by routinely operating both the instru- mental and visual methods side by side for a period of time long enough to determine the suitability of the instrument for this purpose. We have used the scheme of color classification of the Commission Internationa/e de l'Eclairage (C.I.E.), a trichromatic system of colorimetry which is de- scribed in detail in a number of publications (2-4). This system enables a color to be specified by three individual wavelengths of the visual spectrum matched by normal observers, using adjustable amounts of three physically defined light sources or primaries, red, green and blue. This description of the color perception of the normal eye in terms of three defined standards was adopted by the C.I.E. from the data provided by Wright and Guild of the International Commission on Illumination (I.C.I.) in 1931 (5). The National Bureau of Standards unit of color differences has been derived in the manner of the C.I.E. system, and its use in individual applications is widespread in the United States (6). The instrument which we have used is the Color-Eye, Model D, manu- factured by the Instrument Development Laboratories, Incorporated, Attleboro, Mass. This is a photoelectric tristimulus colorimetric instru- ment. It transcribes the tristimulus specifications of a surface color, according to the I.C.I. Standard, to arbitrary values which may be recorded and graphed for the purpose of matching. A short discourse on reflectance colorimetry is in order (7). A light beam, C.I.E. Illuminant A, is reflected from a sample to a phototube. The Color Eye utilizes the flicker photometer principle. In the flicker system, the flicker motor drives an optical mirror so that light from the sample and standard are alternately focused on a photomultiplier tube. If light from the sample and standard are identical, the photocell output will be unchanged by the flicker. If the two differ, an alternating current will be generated in the photocell circuit. Regardless of the brightness level of the sample or standard, or the light

EVALUATION OF HAIR DYES 451 transmission, or the intensity of illumination, a given per cent difference in color will give a finite a.c. voltage to the meter. In effect, the sensitivity of the photocell is varied inversely with brightness of light reaching it, just as the iris of the human eye and accommodation of the brain act to decrease the eye sensitivity when a brighter object is viewed. The range of automatic accommodation of the instrument is comparable to that of the eye, but it is faster and has no fatigue factor with time. To obtain the tristimulus values, all one has to do is to insert into the light beam, prior to its reaching the photocell, the desired primary filters. The electronic system automatically transmits the a.c. output through a component necessary to express the difference between sample and standard in terms of reflected light. The per cent values, or readings, of the instrument are designated by the letters X, Y and Z, which are known as the tristimulus values, each letter representing the amount of one of the primary stimuli. The results obtained from the instrument indicate that, when two sam- ples have the same values of X, Y, Z, the quality of the reflected light from the samples for the specified illuminant is approximately the same. There- fore, dye classification by the equivalence of the tristimulus values appears to give sufficiently good approximations for matching. It is not our desire to go into the physical or optical principles necessary to describe reflectance or color absorbancy instruments. Nor do we intend to delve into the mathematical procedures required for the definition of tri-color stimuli used in the colorimeter. This may be found, for those who may be interested, in the many excellent publications on the subject (2-4, 6). The dyed human hair swatches, which are customarily used to evaluate color pick-up visually, proved to give poorly reproducible results when they were read on the colorimeter. This was probably due to the non- uniform affinity of the hair for the dye materials. It was decided that a suitable substitute be made for human hair. By experimentation, it was found that the monochromatic absorption of a dye on certain textile fabrics, as on the hair, is proportional to the concentration of the dye present. Textile fibers which were found to be unsuitable included synthetics, such as cellulose acetates and nylon, silk, cotton and linen (8, 9). Wool, be- cause of its keratin protein composition, was found to be satisfactory (10). Many of the woolen materials that were tried also had undesirable qualities. Cashmere, flannel and some worsteds gave a surface with too great a nap. A tightly woven wool worsted, obtained from Test Fabrics, Incorporated, New York City, was found to give excellent and reproducible results. Samples of the cloth were dyed, employing the same controlled conditions that are used in the dyeing of a test subject's hair. Great care was taken to maintain the following conditions constant: dyeing time, dye bath