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 Radiance:
 Electromagnetic energy flux,
the amount of energy traveling
 at some point x
 in a specified direction ,
 per unit time
 per unit area perpendicular to the direction
 per unit solid angle
 for a specified wavelength
 denoted by
 Spectral Properties:

Total energy flux comes from flux at each wavelength
 Picture:
 For the indicated situation
is
 energy radiated through differential solid angle
 through/from differential area dx
 not perpendicular to direction (projected area is )
 during differential unit time dt
 Power:
 Energy per unit time (as in the picture)
 Radiosity:
 Total power leaving a surface point per unit area



(integral is over the hemisphere above the surface point)
 Bidirectional Reflectance Distribution Function:

 is a surface property at a point
 relates energy in to energy out
 depends on incoming and outgoing directions
 varies from wavelength to wavelength
 Definition: Ratio
 of radiance in the outgoing direction
 to radiant flux density for the incoming direction
 Energy Balance Equation:


is the radiance
 at wavelength
 leaving point x
 in direction ,

is the radiance emitted by the surface from the point

is the incident radiance impinging on the point

is the BRDF at the point
 describes the surface's interaction with light at the point
 the integration is over the hemisphere above the point
CS488/688: Introduction to Interactive Computer Graphics
University of Waterloo
Computer Graphics Lab
cs488@cgl.uwaterloo.ca