Suppose we have a uniform triangular light source, and we wanted to compute how much energy per area is falling on this surface. We can project the light source onto the hemisphere, and project that onto the base plane. We can multiply the area of the projection by the radiance of the light source to get the irradiance.

Suppose we have a uniform triangular light source, and we wanted to compute how much energy per area is falling on this surface. We can project the light source onto the hemisphere, and project that onto the base plane. We can multiply the area of the projection by the radiance of the light source to get the irradiance.