The Nature of Light Dr. Alan Doolittle, Ga Tech The total power density emitted from a light source can be calculated by integrating the spectral irradiance over all wavelengths or energies of interest. However, a closed form equation for the spectral irradiance for a light source often does not exist.

*Unlike radiant flux density, the definition of radiance does not distinguish between flux arriving at or leaving a surface. Another way of looking at radiance is to note that the radiant flux density at a point on a surface due to a single ray of light arriving (or leaving) at an angle θ to the normal to that surface is dΦ/dA * cos θ.*