I'm measuring directly off the light source. Its diffuse color is (0,0,0) so that it doesn't pick up any extra light.Also take into account what reflectance the surface (-s) have, the size of the space.
I'm not sure what you mean by that? Surely it's just a scaling factor and the illumination is even across the plane?Keep in mind that the calculations use the centre of the light source as origin, thus using a surface of 1x1m may not be accurate.
