Image Analysis Tools to Quantify Visual Properties of Hair Fiber Assemblies 322 b correspond to yellow and blue, respectively. Typically, several parameters are calculated from L, a, and b values and include the total color difference (DE), chromaticity difference (DC), and yellowness index (YI). The total color difference takes the square root of the total summation of each difference component squared. DE = (DL)2 + (Da)2 + (Db)2 Eq. 11 Similary, the chromaticity difference is calculated as: DC = (Da)2 + (Db)2 Eq. 12 The yellowness index—often reported as DYI and corresponding to a difference between before and after samples—is used to measure changes in the yellowness of wool samples that have undergone textile processing as well as hair subjected to environmental stresses, such thermal degradation.51 It is calculated as: YI =100 1.28X -1.06Z Y Eq. 13 where the CIE tri-stimulus values X, Y, and Z are defined as: Y = 0.01L2 Eq. 14 X = 0.9804 0.01L2 + aL 175 Eq. 15 Z =1.181 0.01L2 - bL 70 Eq. 16 In addition to X, Y, and Z, color may also be characterized according to its hue and chroma. Both techniques—tri-stimulus colorimetry and diffuse reflectance spectrophotometry—are extremely useful for the measurement of hair color. Based on studies in the published literature, colorimetry appears to be the more popular of the two