Luminance is the measurable amount of light which is emitted by or reflected from a source. Luminance meter is an alternate term for exposure meter . Luminescence is the visible light produced from a surface submitted to invisible radiation such as UV, X rays and son on. Unlike fluorescent light it continues to be emitted after the existing source is removed, gradually fading away. Luminosity is the brightness of either a light source or a reflective surface.
Luminous flux is the intensity of a light source, measured in lumens .