A different way to constrain the image irradiance equation: add another one.
Photometric stereo: use more images; same viewing direction, different illumination conditions (e.g., time of day).
Varying the direction of illumination changes the reflectance
map R. Suppose we measure 
for two illumination
conditions. We get two equations for 
:
The intersection of the isophotes in p,q-space determines the surface
orientation at 
.