1. A method of producing a compact representation of spectrum data, the method comprising the following steps: (a) receiving or accessing the spectrum data comprised of pairs of wavelength and reflectance values; (b) interpolating a spline curve to the spectrum data, the spline curve having a set of control points, a knot vector, and representing wavelength and reflectance values as functions of an independent parameter; and (c) removing one or more knots from the knot vector that minimize a cost function in a parameter domain of the spline curve.