Chapter 9 303 the intensity of the specular reflection, or the shininess/brilliance of highlights, and it is normally used to describe medium-gloss surfaces. Sheen, on the other hand, is mostly used to describe low- gloss surfaces and is measured by illuminating the sample at grazing angles. Contrast gloss or luster describes low gloss materials, such as textile fibers and hair, and provides us with information about the difference between the specular areas with all other areas under investigation. Contrast gloss is the most frequently employed category of gloss to describe hair attributes. Absence-of-bloom gloss refers to the absence of hazy or opaque features adjacent to the specular reflection band. While this type of gloss normally would not be associated with hair, we find occasions when particular treatments, especially silicones and castor oil, result in haziness/ milkiness adjacent to the specular reflection band. Distinctness- of-image gloss refers to the sharpness of a mirror image in the substrate under investigation. Normally, this type of gloss would not be associated with hair however, certain polymeric treatments with extremely high refractive indices result in the visualization of reflecting components. Surface-uniformity gloss depends on surface asperities, which render texture to the sample. Due to its biological nature, hair has a very heterogeneous surface with many micro- features on the surface. Many of these elements on the surface cause impendent light to be scattered. On the other hand, treatment with certain cosmetic ingredients provides an overlying smooth layer with less light scattering. Historical background: During the 1970s and 1980s, very little was published on the topic of hair luster.10-12 Regardless, most of our current understanding of the optical properties of hair comes from the very first published studies by Stamm et al.10,11 During this time—and in some cases even nowadays—most laboratories performed luster measurements utilizing a goniophotometer.10-20 It is an instrument capable of measuring the spatial distribution of radiation—more specifically, the angular distribution of visible light reflected from a substrate. This very accurate technique can be employed to examine single hair fibers or even an array of fibers.