Do you know the difference between luminous flux, illuminance, intensity, and luminance. The terms sound similar, but they represent very different things. These are all photometric measurement terms, meaning they relate to the measurement of visible light. Understanding the differences in these terms is critical in optical design, analysis, and measurement.
Luminous flux is the total amount of light emitted from a source into all directions. For visible light luminous flux is measured in lumens.
Illuminance is lumens per unit area. Illuminance is commonly measured in lux, or lumens per square meter, though other units are sometimes used, such as lumens per square foot, also called foot-candles. Illuminance on a surface can be thought of as the illumination pattern of a source or luminaire.
Intensity is flux into a given direction or solid angle. For visible light, intensity is measured in candela, which is lumens per steradian. Intensity can also be considered the angular distribution of the light source.
Luminance is flux per unit area per solid angle. This is light emitted from an area into a certain angle or angular pattern. The unit for luminance is the candela per square meter, also called nits. Luminance is what your eye sees when you look at a computer display, a traffic light, an LED, or any other device that emits light that is intended for someone to see or look at. As you can expect, luminance is a critical component of many types of optical systems.
