Now, the computation of the radiance H in W/sr/cm must be computed where cm is the emitting area. Next, emitting area must be converted into a projected area and the solid angle ; this product is often called the etendue. This step can be obviated by the use of a calibrated source, where the exact number of photons/s/cm is known at the detector. If this is unknown, it can be estimated using the black-body radiation equation, detector active area A d and the etendue. This ultimately converts the outgoing radiance of the black body in W/sr/cm of emitting area into one of W observed on the detector.