A method of selecting wavelengths of radiation for discriminating a first cell or tissue type from a different cell or tissue type is described. First and second sets of absorption spectra are obtained, each set comprising spectra obtained at a plurality of different spatial regions of the first cell or tissue type and of the different cell or tissue type, respectively. Sets of corresponding metrics are defined for the first and second sets of absorption spectra for each spatial region. Each metric comprises information corresponding to the absorption for at least two different wavelengths. The metrics in each set comprise different combinations of wavelengths. A characteristic value is generated for each metric. Distributions are generated for each metric using corresponding characteristic values for the first cell or tissue type and for the different cell or tissue type and compared to determine an extent of similarity. The metrics are ranked based on the extent of similarity and wavelengths associated with higher ranked metrics, having higher similarities, are selected.