Spectroscopy of light emission stimulated by scanning tunnelling microscopy (STM) was performed on metallic nanoclusters. Features in the spectrum relating to the metal's complex dielectric function can be used to identify them as An, Ag or Cu. However, another approach, less sensitive to tip effects, is preferred here. Normalising spectra to a reference metal produces quantitative data on the bulk optical properties of each individual cluster. The contrast with traditional methods of optical characterization of metal clusters where surface effects dominate measured dielectric properties as the size of the clusters reaches the nanometer scale is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.