The ideal distance between substrate and the U.V. light source for curing U.V. coatings.

determined by the strength of the lens; in plus lenses the greater the strength, the shorter the focal distance (this is why one needs to hold their reading material ever closer and closer as they get stronger and stronger adds in their lenses and also the reason for holding a magnifier close to one's face).

The distance light rays travel after refraction by a lens to reach a focus.