Luminance describes the measurement of the amount of light emitting, passing through or reflected from a particular surface from a solid angle. It also indicates how much luminous power can be perceived by the human eye. This means that luminance indicates the brightness of light emitted or reflected off of a surface. In the display industry, luminance is used to quantify the brightness of displays.
There are a variety of units used for luminance. The SI unit for luminance is candela/square meter (cd/m2). In the U.S. one of the common units is the foot-lambert (fL); 1 foot-lambert (fL) equals 1/π candela/square foot, or 3.426 cd/m2. Professionals in the industry will be familiar with the term nit (nt). Nit is a non SI term used for luminance, and 1 nit is equivalent to 1 cd/m2.
Illuminance is a term that describes the measurement of the amount of light falling onto (illuminating) and spreading over a given surface area. Illuminance also correlates with how humans perceive the brightness of an illuminated area. As a result, most people use the terms illuminance and brightness interchangeably which leads to confusion, as brightness can also be used to describe luminance. To clarify the difference, illuminance refers to a specific kind of light measurement, while brightness refers to the visual perceptions and physiological sensations of light. Brightness is not a term used for quantitative purposes at all.
The SI unit for illuminance is lux (lx). In the U.S. people sometimes use the non SI term foot-candle when referencing illuminance. The term "foot-candle" means "the illuminance on a surface by a candela source one foot away". One foot-candle is equivalent to one lumen per square foot which is approximately 10.764 lux.