Chapter 9 321 the trichochromes. While trichochromes contribute to the red hues in red hair, kynurenines give hair a yellowish tinge, which is readily apparent in non-pigmented hair types such as gray, Piedmont, and some shades of blonde. Conventional methods to measure hair color: The generally accepted techniques for measuring hair color are reflectance spectrophotometry and colorimetry. In both cases, light is reflected from the sample surface and compared to a white standard. While both instruments have a measurement sensitivity greater than the human eye, reflectance spectrophotometry is a more versatile, and more expensive, technique. Normally, reflectance spectrophotometers perform a wavelength-by-wavelength analysis over the entire visible spectrum and part of the UVA region, while colorimeters sample much larger bands and with the utilization of filters try to simulate the eye of a human observer. In the case of reflectance spectrophotometry, an integrating sphere is utilized in the illumination of the sample, which can measure diffuse light reflected from the sample. Once spectrophotometric plots are generated for a sample, the curves must be converted to tri-stimulus X, Y, and Z values—usually carried out by the instrument software. The CIE (Commission internationale de l’éclairage, or International Commission on Illumination) designed this mathematical model of color space in 1931 as an approximation to that viewed by the human observer. X, Y, and Z roughly correspond to red, green, and blue. Y is also indicative of the total luminosity of the sample. Most colorimeters, on the other hand, employ a tungsten incandescent lamp as the light source, which is reflected from the sample surface and passes through a diffuser. Tri-stimulus filters are placed in front of the photodetectors, providing a standard observer view of the sample (as defined by CIE). Data output is in the form of X, Y, and Z values or the more popular L, a, and b values, which correspond to the lightness (L) and color components (a and b) of the sample. The scale of L spans from L=0 for black to L=100 for white. Red and green are represented by a in the positive and negative directions, respectively. On the other hand, positive and negative values for