What is the relationship between illuminance and brightness?

Illuminance and luminance are two different optical concepts used to describe the properties of light, and they have a certain relationship, but they involve different aspects:

Illuminance: Illuminance is a measure used to describe the brightness of light incident on a surface. It is typically quantified in lumens per unit area (lux) and is expressed as candela per square meter (cd/m²). In simple terms, illuminance measures the intensity of light falling on a surface. The calculation formula for illuminance is as follows:

Illuminance (lux) = Luminous Flux (lm) / Area (square meters)

Illuminance is commonly used to assess the distribution and intensity of light produced by a light source within an area. For example, in indoor lighting, we often aim to achieve a certain level of illuminance to ensure adequate brightness for various activities and tasks.

Luminance: Luminance refers to the brightness of a light source or a reflecting surface within the observer's field of view. It is expressed in units of candela per square meter (cd/m²) and is contingent on the observer's viewing angle, meaning it depends on the perceived brightness of the light source or surface from the observer's line of sight. The calculation of luminance typically involves angles and the reflective properties of the surface.

The relationship between illuminance and luminance can be summarized as follows:

Illuminance measures the brightness of light incident on a surface and is typically expressed in lumens per unit area.

Luminance refers to the brightness of a light source or surface within the observer's field of view and is typically described using angles and reflective properties.

While they involve different units of measurement and concepts in describing the properties of light, both illuminance and luminance have important applications in the fields of optics and lighting. They are critical parameters when designing lighting systems or evaluating visual comfort.