270 JOURNAL OF COSMETIC SCIENCE Figure 1. Optical micrograph of virgin hair ( x4 50) showing weak colored patterns of iridescence produced by the cuticle sheath. eterious than beneficial to the optical properties of hair. The reason is that the colored patterns are microscopic and incoherent at the macroscopic level and they may act rather as an incoherent light scattering process. Understanding the phenomenon of light scattering by the cuticle sheath is, therefore, crucial for modeling and improving the visual perception of hair (9,10). In fact, it is the cuticle sheath the part of hair that acts as a gate for the incoming and outgoing light from the hair shaft. Excessive light scattering from the cuticle cells may impair shine. It can also inhibit the amount and quality of the incoming white light penetrating into the cortex and also affect the quality of the outgoing colored light produced within the cortex by the melanin gran- ules. In this paper an analysis of the light interference patterns (LIPs) appearing in damaged cuticle cells will be presented. The analysis will show that the LIPs appearing in damaged cuticle cells are produced by a combination of two phenomena, namely: thin film light interference and light diffraction. Both phenomena arise from the damaging action of mechanical and swelling stresses as they induce micro-structural changes in the cuticle cell layers, in particular, in the density of the cement layer and endocuticle. The changes in density appear to be produced by the appearance of numerous micro-voids that coalesce into larger porous defects creating the conditions for cuticle cell lifting, thin film interference, and light diffraction.

2006 TRI/PRINCETON CONFERENCE 271 Figure 2. Optical micrographs (x350) (2a and 26) of hair fibers with cuticle cells showing strong patterns of light interference after subjected to 20 cycles of tension and retraction. Figs. 2c and 2d are SEM micrographs (x350) showing cuticle lifting and de-cementation of areas shown in Figs. 2a and 26, respec- tively. Circles and lines in captions help ro identify position of same cuticle cells in both pictures. EXPERIMENT AL The hair used in the experiments was Premium Grade Brown Caucasian from Interna- tional Hair Importers. In order to induce de-cementation and buckling of cuticle cells, single hair fibers were subjected to 20 cycles of mechanical extension and retraction at room temperature conditions. Each cycle consisted in applying a tensile deformation of 20% and allowing 1 second for deformation recovery. In all consecutive applied cycles the percentage of extension didn't exceed 20% of the original fiber length. In the past it has already been shown that this type of protocol leads to the production of a large number of de-cemented cuticle cells (12). After the fibers were damaged they were analyzed by optical microscopy using a Hi-Scope Advanced KH-3000 from Hirox LTD. When needed the hair fibers were subjected to cycles of thermal or torsion stresses using the protocols already described elsewhere (13,14). In order to asses the effect of solvent penetration on the LIPs various hair fibers, either, before or after mechanical damage were immersed in water or isopropyl alcohol. Areas of hair fibers presenting LIPs were also analyzed by SEM. RES UL TS AND DISCUSSION MAIN CHARACTERISTICS OF LIGHT INTERFERENCE PATTERNS Before making a detailed description of the results it should be mentioned here that the strong LIPs were only observed on hair fibers subjected to damage. Virgin undamaged hair fibers were always absent of strong LIPs and instead they showed weak colored patterns of iridescence (see Figure 1). As it will be discussed later the strong LIPs