Figure 2.

The interaction of reflectance and illumination in generating spectral returns. A “surface” with a particular reflectance efficiency function, illustrated here as a simple sine wave, is “illuminated” by spectra that are the same (A) or the opposite (B) or are shifted toward long (C) or short (D) wavelengths. The spectral returns given by the product of the reflectance efficiency function and the illuminant vary systematically relative to the return generated by the same surface under an equally intense illuminant whose power is uniformly distributed (dotted line) (see text for further explanation).