JOURNAL OF COSMETIC SCIENCE 156 camera and a cylinder on which the sample is positioned (Figure 3). Using both polariza- tion camera and polarized illumination allows the independent measurement of polarized light and unpolarized light. Combining a camera with a cylinder allows recording the angular distribution of the sample without any moving parts. Acquiring an angular distribution requires a change of geometric confi guration of the group illumination-sample-observation. This change of confi guration is created by the cylinder. The orientation relatively to the illumination direction and observation direction changes according to the point on the surface of the cylinder that is considered (Figure 4). The angle of the measurement is the angle between the direction of the specular light and the direction of observation. The angle between specular refl ection and direction of obervation is the type of angle measured in goniopho- tometers. Imaging setup has the advantage of acquiring images that can be used as a di- rect visual control of the numerical data. Images allow understanding better how a change of certain parameters like a darkening of the diffused light or reduction of shine band width affect the visual appearance of the sample. The use of both polarized illumination and polarization camera allows recording three types of images (Figure 5): O A normal intensity image representing what a human eye would see. O A specular image representing the light that is polarized. This polarized light shows only the refl ections (fi rst and second.) O A diffused light image representing the light that is unpolarized. This unpolarized light shows only the light scattered inside the hair fi ber. It is the background color of the hair. Diffused light and refl ections are physically separated without using any fi ts or mathe- matical decompositions. The computation of the angular profi les is done by averaging the images along the transverse direction (Figure 6). The system is angularly calibrated with its geometric properties so that the real angles are known for each line of the image. These angular profi les are similar to those provided by goniophotometers. From these profi les and images, relevant parameters characterizing the light distribution are computed. These parameters include: integral of the curve, maximum, width… Figure 3. (a) Optical setup of the polarization imaging system. (b) Commercial system.

2008 TRI/PRINCETON CONFERENCE 157 Polarization imaging provides the profi le of intensity, specular and diffused light, as well as the corresponding images. These come from a direct measurement of a different phys- ical property of light. Goniophotometric measurements only gives the profi le of the in- tensity of light. Several methods like deconvolution and curve fi tting allow extracting specular and diffused light from goniophotometric measurement. The results obtained are very similar but also show some small difference. For instance, deconvolution or curve fi tting methods usually consider the specular light to be zero outside of the main specular peak. Polarization shows that it is not always the case. Having this physical separation allows better understanding of the effect of treatments. For instance, as specular light is considered to be zero outside of the main peak, a gonio- photometric method will attribute a decrease in the edges of the intensity light distribu- tion to a decrease of diffused light (meaning a darkening of the hair). Polarization meaurement allows saying if it comes from a decrease of the specular light outside of the Figure 4. (a) Sample positioned on the cylinder. (b) Complete angular distribution with a single image. Figure 5. Three images are acquired: an intensity image showing the normal view of the hair, a specular image showing only the refl ections (shine and chroma) and a diffused light image showing the diffused light only.