The luster perceived in the hair is a combination of two different physical interactions of light on the fiber: diffuse spreading (D) and specular reflection (S). Measuring the intensity of these two interactions, it is possible to quantify the values of luster (L) for a determined type of hair, according to the equation presented by Equation 1. This equation not only produces the results observed by instrumental methodologies for the evaluation of luster, but also has a good correlation with the luster perception presented by the consumers and with the sensory evaluation done by trained panelists.
Equation 1 also demonstrates that the bigger the luster, the bigger is the reflected specular light (S) in comparison with the diffuse spreading (D). And that the spreading increase reduces luster perception.
L = S - D / S
Figures 1 and 2 present the curves of the specular reflections for different hair colors and Figure 3 presents an illustrative image identifying the different peaks: brightest band first specular reflection; least bright band sum of the other specular reflections.
The diffuse spreading widens and reduces the peaks, increasing the contributions of the other specular reflections that is not the first one, increasing the sensation of hair opacity.
Figure 01 Comparison of the specular reflections for the blond hair (dotted curve) and dark-brown hair (solid curve).
Figure 02 Scanning done with the goniophotometer in three different fibers illuminated with the non-polarized white light.
Figure 03 Visual analysis of shine for different hair colors. Black ellipsis: first specular reflection. Blue rectangles: other specular reflections
The factors that may contribute for these two interactions and, consequently, in the perception or not of hair with luster are the most diverse. However, the specular reflection depends, mainly, on the more external part of the hair fiber or exocuticle, as it is more commonly known. Diffuse spreading, on the other hand, is the sum of optical discontinuity observed on the surface and the more internal parts of the fiber and their disarray when observing the hair, which is actually a set of fibers piled up one over the other.
Improving the homogeneity and uniformity of the surface of the hair is the objective of products that are known as good providers of luster (it is the case of oils and silicones). However, some products that are produced with this purpose do not promote the expected luster, especially when compared to shampoos with a high cleaning power. This happens because the structure of the hair fiber itself already presents a high rugosity, what contributes that the optical discontinuities that increase the contribution of diffuse spreading. Depositing a product on the hair fiber in such a way that the rugosity increases instead of making the surface of the fiber uniform, what is observed for these products is a luster reduction.
Thus, the technique for proving luster takes into account the difference of a clean natural hair and the same damaged hair before and after the application of a determined product. The closer the luster is, after a product application, to a natural clean hair luster, the more luster this product presents.