Chapter 9 309 Several different approaches to calculate luster have been suggested over the years, for both image analysis and goniophotometric measurements. Some of these calculations have been utilized interchangeably between both imaging and goniophotometric approaches. Based on the definition of luster, the Nickerson contrast gloss (N c ) parameter was developed to provide a comparison of the specular and diffuse regions of a substrate. Eq. 3 While this parameter has been employed to measure contrast gloss in many different materials, it was found not to give such satisfactory results for hair in both goniophotometric and image analysis studies.10,24 One of the first luster equations utilized for hair—proposed by Stamm et al.—takes the difference between specular and diffuse reflection normalized to the amount of specular reflection:10 Eq. 4 In this case, the integrated diffuse reflection (D Stamm ) is the area under a straight line obtained by connecting the first and last points of the specular reflection curve. More than a decade later, Reich and Robbins proposed a modification to the equation originally proposed by Stamm et al., which took into account the width of the specular reflection band.20 Eq. 5 The model proposed by Reich and Robbins provided good differentiation between hair samples (e.g. different pigment types, cosmetic treatments, sebum, etc.) and correlated well with subjective panel evaluations.20 Another model, employed by Guiolet et al., simply takes the ratio of specular to diffuse reflection.12 Eq. 6