a procedure for calculating the skin thickness calibration model 16 heretofore described two alternative experimental methods for realizing the calibration set are provided below in the first method spectral measurements of a target area of human skin are obtained using a nir reflectance instrument biopsies of the scanned region are then obtained and examined histologically the thickness and chemical composition of the key indicators specific to each tissue layer were included in the calibration set using multivariate regression analysis techniques a calibration model can then be developed to relate the spectral skin measurements known as predictor variables to the known skin layer thickness and chemical compositions known as response variables this technique uses a priori information regarding the general physiology of skin and exploits the inherent difference between skin layers and their compositions to develop a model that predicts skin layer thickness and composition noninvasively the second approach is to develop a tissue model that adequately represents the fundamental absorbing and scattering characteristics of an in vivo tissue system although living tissue is a highly complex system the transform from an in vivo system to a tissue model is made possible by an a priori knowledge of the primary absorbing and scattering species present in the living tissue system becasue the model also includes a known thickness for each tissue layer and since the concentrations of absorbing and scattering components are known a monte carlo simulation may be used to simulate the photon propagation of light through the tissue model the result of the monte carlo simulation is a diffuse reflectance measurement that is comparable to an actual reflectance measurement obtained experimentally the tissue model must be validated in order to confirm that the model mirrors the complexity of the living tissue with sufficient accuracy to.