1. A method for estimating spectral data on a selected one of three spectral information types comprising: illumination of a scene; spectral sensitivity of an imager imaging the scene; and, reflectance of a surface in the scene, the method comprising: obtaining a ranking order for a plurality of sensor responses produced by the imager, each of the plurality of sensor responses being produced from a reference target in the scene; obtaining, from an alternate source, data on the other two spectral information types; and, 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; in dependence on the ranking order and on the set of constraints, determining said spectral data that optimally satisfies said constraints.