What is radiance?

The radiation density, or radiance, describes the radiation of a radiation source in relation to the emitting area and the radiation direction. A smaller surface leads to a higher radiance for the same radiated power. Likewise, radiating this power into a smaller solid angle leads to a higher radiance. Accordingly, the unit of radiance is the radiated power per emitter area and solid angle, W/(m²*sr).

By applying the eye’s brightness sensitivity curve, a given radiance can be converted to visible luminance. UV and IR sources, being invisible, have no luminance but a radiance.