A method, system and reference target for estimating spectral data on a selected one of three spectral information types is disclosed. Spectral information types comprise illumination of a scene, spectral sensitivity of an imager imaging the scene and reflectance of a surface in the scene. The method comprises obtaining a ranking order for plural sensor responses produced by the imager, each sensor responses being produced from a reference target in the scene, obtaining, from an alternate source, data on the other two spectral information types, determining a set of constraints, the set including, for each sequential pair combination of sensor responses when taken in said ranking order, a constraint determined in dependence on the ranking and on the other two spectral information types for the respective sensor responses and, in dependence on the ranking order and on the set of constraints, determining said spectral data that optimally satisfies said constraints.