The only source of information here is the image irradiance
equation that ties intensity 
to the reflectance map 
:
This is a 1st-order PDE with two unknowns: 
and 
.
The reflectance map R depends on surface orientation 
,
illumination, and surface reflectance properties.
R can be derived analytically from known 
, or measured
empirically, with a calibration object.
Must make assumptions to solve the image irradiance equation.