The technique is based on the measurement of birefringence. Birefringence is an optical property of those materials that have a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as well as plastics under mechanical stress.
When a birefringent material is irradiated with polarized light, the polarization of the transmitted light is modified. These modifications can be quantified through the measurement of a parameter, called «K». Then, the «Weighted Mean K» parameter is obtained by the summation of all the «K» of an image, divided by the sample area exhibiting birefringence.
This parameter is specific to each image; it notably reflects the density of the fibers, on a given field of view. The birefringence induced by the sample is measured qualitatively and quantitatively and then displayed on the screen in the form of a colorimetric map.
In the case of hair, the main component is keratin, which shows birefringence due to the semicrystalline, α-helical intermediate filament proteins (IF).
In this study, we determined the thermal protector properties of a commercial product using the weighted Mean K change (see figure 1)
Figure 1 Theoretical model for Kmax obtaining