Luminance vs Illuminance

What is the difference between luminance vs. illuminance?

When talking about light and lighting design; two of the most confusing and often misused terms are luminance and illuminance.  They are often substituted for one another, interchanged and used incorrectly by people. The Fusion Optix BrightMinds® camera app measures luminance. Here’s a brief explanation of the two terms.

Luminance

Luminance is the density of luminous intensity in a given direction and falls within a given solid angle. It is measured in cd/m2 or nits.

Luminance is often used to characterize the emission from a diffuse surface. It indicates how much luminous power will be perceived by eye when viewing the surface from a particular angle. Luminance remains the same regardless of the distance from the light source.

Illuminance

Illuminance is the density of photons which fall within a given surface area. It is measured in lux, or footcandle(fc).

Illuminance can be measured with a lux meter. For a given light source, the closer to a light source the illuminated area is, the higher the Illuminance value.

Illuminance is used in lighting applications to measure the amount of light reaching an object such as a wall.

Fusion Optix Luminance vs Illuminance: the sky is illuminance whereas the light reflected of the causeway in the foreground is luminance

The sky is illuminance whereas the light reflected of the causeway in the foreground is luminance