EVALUATION OF HAIR DYES USING PHOTOELECTRIC COLORIMETRY By J. CALLISON, C. SCHMIDT, R. PIEL, S. GRANT and W. HOLLAND* Presented May 8, 1962, New York City IN THE manufacture of hair coloring products, the importance of control cannot be over-emphasized. To produce hair dye colors which will not vary from one production batch to another and which will con- sistently duplicate the desired shade on a subject's hair presents com- plex problems. The method of standardization employed at present by most manufac- turers is a visual evaluation by an experienced colorist. Generally, in the laboratories of hair dye manufacturers, the accepted method of hair color evaluation is the dyeing of human hair swatches and color assessment by an expert technician. The trained colorist is not only able to determine color difference in terms of strength, but he is also capable of estimating the quantitative magni- tudes of these differences. This skill is learned by experience and careful tutelage forming the traditional basis for color assessment in all the in- dustries concerned with colors. It is understandable that even an expe- rienced colorist cannot reproduce his judgments perfectly. Color tech- nicians, since they are individuals with characteristic visual differences, are often not in agreement on their observations. They are handicapped because they occasionally must work with increments of color that are just within their powers of discrimination, and the latter are not always capable of faithfully reproducing their decisions. This difficulty, coupled with the fact that all hair is different in texture and in its affinity for color pick-up, made it obvious that a better method of evaluation should be employed. This paper concerns a new application of an old system to solve this problem. Color standardization utilizing instrumental methods has been accepted in the paint, plastics, ceramics and textile industries. In the dye industry the use of colorimetry, or transmission spectrophotometry, has been applied in the control of the dye bath and the manufacture of the * Helene Curtis Industries, Inc., Chicago 39, Ill. 449

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