The glow induced in a material when bombarded by light. Brewster windows of fused silica fluoresce in UV light, increasing absorption of laser radiation and degrading laser mode and output.

The radiant, or luminous, power of a light beam; the time rate of the flow of radiant energy across a given surface.

The distance from the secondary nodal point of a lens to the secondary focal point. For a thin lens imaging a distant source, the focal length is the distance between the lens and the focal point.

Also Focus (noun).  The point toward which radiation converges or from which radiation diverges or appears to diverge.

Noun.  The point where rays of light meet which have been converged by a lens, giving rise to an image of the source. Verb: To adjust focal length for the clearest image.  Also Focal Point.