Radiance is a physically accurate backward-raytracer. It's strenght lies with daylight simulations, but it does artificial lighting just as well.
Most people associate 'raytracing' with 'pretty pictures', but this is not necessarily always the case. Numbers can be just as informative. In fact, most of the time, I use Radiance not for churning out images, but for computing illuminance values and daylight factors. There are not many lighting calculations that Radiance can't do.