Study on signal-to-noise ratio of two-layered media

For some non-invasive measurement on human skin surface or fingernail, the information of dermis is useful and usually the epidermis or fingernail disturbs the light signal that carries information of dermis. In this paper, a two-layered media model was developed to simulate human finger. The first layer simulates the epidermis or fingernail and the second layer simulates dermis and tissue under dermis. A beam of light normally hits on the two-layer model, some photons travel through the first layer and return to the surface, some photons travel through the first and the second layers and then return to the surface. The light from the first layer is defined as noise and the light from the second layer is defined as signal. The intensity distributions of light from the first layer and the second layer were experimentally studied on the model surface. The distribution of SNR on the surface was obtained and the results show that there is an optimal source-detector distance where the SNR is greater than that at other source-detector distance. The results are useful for non-invasive measurement on skin.