2010 TRI/PRINCETON CONFERENCE 91 the image processing (~5–10s). The orientation image is then visualized. Different tools allow statistical analysis in regions of interest (ROI) in the image. The resolution of our imaging system permits to discriminate a single hair fi ber, i.e. one pixel corresponds to approximately 80–100 micron on the hair fi ber plane. This will al- low our simple model to be valid. EXPERIMENTAL VALIDATION OF THE CONCEPT We proceed to two basic experiments to validate the concept of the measurement: Birefringence measurement (extraction of the sinusoidal signal over a region of interest • and measurement of the angle of orientation) Evaluation of the angle measurement precision • Figure 8. (a) Intensity images in the visible spectrum for tresses of various shades (from left to right: black, dark brown, medium brown, dark brown with red dye, blond, bleached, grey 10%–90%). (b) Corresponding images in the NIR region.

JOURNAL OF COSMETIC SCIENCE 92 Figure 9. Photo of set-up. Figure 10. Signal versus the angle of the polarizer ϕ. BIREFRINGENCE MEASUREMENT We are using two hair tresses (blond and dark). Figure 10 shows the intensity I (a.u.) versus the angle of the polarizer ϕ, in a region of interest where the orientation of the hair is constant, for three orientations of the hair tress (θ = −10°, 0°, and 10°).