1. A computer method operable on a computer comprises: retrieving by the computer 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 by the computer 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 by the computer the first irradiance approximation and the second irradiance approximation for a point having a third distance from the light sources; and generating from the interpolation of the first irradiance approximation and the second irradiance approximation a third irradiance approximation for the point having the third distance from the light sources.