Luminescent material absorbs energy, acts on it and then emits this as light, either immediately or over a longer period, in excess of thermal radiation.

This light emission, or exitation energy (in contrast to thermal radiation), does not first contribute to the thermal energy of the compound.

Light emission from a substance during the time when it is exposed to exciting radiation is called fluorescence.

Phosphorescence

The afterglow (glow in the dark) that can be detected by the human eye after the cessation of excitation is called phosphorescence.

Brightness

Brightness is another expression for the photometric term luminance.

It describes the emitted luminous intensity per unit area of a light source in millicandella per square meter or foot-lamberts (1mcd/m2 = 0.2919 mft.L.)