A method for a computer system includes retrieving from a computer readable medium a first irradiance approximation for a plurality of light sources for a first distance in the midfield from the light sources; retrieving from the computer readable medium a second irradiance approximation for the plurality of light sources for a second distance in the farfield, where the second distance is greater than the first distance; and interpolating the first irradiance approximation and the second irradiance approximation to generate a third irradiance approximation for a point having a third distance from the light sources.