Structural investigations of human hairs by spectrally resolved ellipsometry.

Human hair is a biological layered system composed of two major layers, the cortex and the cuticle. We show spectrally resolved ellipsometry measurements of the ellipsometric parameters Psi and Delta of single human hairs. The spectra reflect the layered nature of hair and the optical anisotropy of the hair's structure. In addition, measurements on strands of human hair show a high reproducibility of the ellipsometric parameters for different hair fiber bundles from the same person. Based on the measurements, we describe a dielectric model of hair that explains the spectra in terms of the dielectric properties of the major parts of hair and their associated layer thicknesses. In addition, surface roughness effects modeled by a roughness layer with a complex refractive index given by an effective medium approach can be seen to have a significant effect on the measurements. We derive values for the parameters of the cuticle surface roughness layer of the thickness d(ACu)=273 to 360 nm and the air inclusion fA=0.6 to 5.7%.