Optical spectra acquired on skin depend greatly on skin content in chromophores especially on photons absorbers (melanin) and on fluorescent molecules such as collagen and elastin. Such skin content in chromophores varies from one person to another one even within the same phototype class. Therefore, optical spectra must be standardized in order to be as independent as possible of inter-individual variability. Such a standardization should help increase the diagnosis accuracy of optical spectroscopy. In this study, we aim at defining the anatomical site that would allow best standardization. Standardization is evaluated through the ability of spectroscopy to discriminate pairs of histological classes such as BCC, SCC, AK and normal skin. Bimodal spectroscopy is used combining AutoFluorescence (AF) and Diffuse Reflectance (DR). Three anatomical sites are compared for reference spectra acquisition: non-lesional (NL) skin sites (i.e. the same anatomical sites as for BCC, SCC or AK), inner hand and inner wrist. AF raw data show a high (40 %) inter-individual variability of reference spectra intensity acquired on any of the three anatomical sites. Standardization using reference spectra allow best discrimination of histological classes when reference spectra acquired on NL skin sites and on inner hand are used for standardization of AF spectroscopy and DR spectroscopy, respectively.