The term 'brightness unit' refers to a unit of measurement used to quantify the amount of light emitted from a source. The three most common brightness units are lumen (5 letters), lambert (7 letters), and candela (7 letters). A lumen is a unit of luminous flux, which measures the total amount of visible light emitted from a source. A lambert is a unit of luminance, which measures the intensity of light in a given direction. Finally, a candela is a unit of luminous intensity, which measures the amount of light emitted from a point source in a particular direction.