The invention concerns the compact representation of a reflectance spectrum of a material. For example, for in compression, identification and comparison of reflectance spectrum data of multiple materials. The compressed representation interpolating a spline curve to the reflectance spectrum data, the spline curve having a set of control points, a knot vector, and representing wavelength and reflectance as functions of an independent parameter (42). Then removing one or more knots from the knot vector that minimise a cost function in a parameter domain of the spline curve based on the wavelength function (44). Aspects of the invention include a method, software, a computer system and the compact representation itself.