an apparatus for dividing light reflected by an illuminated surface into constituent parts of illuminating light and surface spectral reflectance of the surface . the apparatus includes a receiving device for receiving signals representing measured spectral power distribution im of the light reflected by the illuminated surface . the apparatus generates a weighting coefficient ε i for each of m illuminant basis functions eb i for describing the illuminating light . weighting coefficients σ j for each of n reflectance basis functions sb j are generated to describe the reflectance of the surface . the apparatus compares an approximation ia of the measured spectral power distribution im , ia being defined by the relation : ## equ1 ## where the set of product pairs eb i sb j is linearly independent . different weighting coefficients are regenerated until said ia and said im are generally equal . signals representing at least one of the constituent parts of illuminating light and said surface spectral reflectance are generated based on the weighting coefficients σ i and σ j which make said ia generally equal to said im . the apparatus has an output device for outputting the signals .