Irradiance Caching seeks to solve the computationally intensive problem of inter-diffuse reflectance. Irradiance is the power per unit square area incident on a surface. Given the irradiance at a point on a diffuse Lambertian surface, calculating the reflected radiance is as simple as first calculating radiosity by multiplying irradiance by the surface reflectance and then dividing by pi. The Irradiance Caching algorithm calculates irradiance sparsely across a scene, caches these values, interpolates cached values, and finally calculates reflected radiance for diffuse surfaces.