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Luminance vs. Illuminance
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In the lighting world, many lighting terms often sound similar but have vastly different meanings. This can lead to confusion and a misunderstanding of lighting terminology overall. In fact, two of the most used terms, illuminance and luminance are also the terms mixed up most often.

So, what’s the difference between “luminance” and “illuminance”?

Luminance

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.

Luminance is quantified using a spectroradiometer, luminance meter or a chroma meter.

Illuminance

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.