Luminance (symbol:$L$) is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected off a particular surface. Luminance characterizes the human visual perception of “brightness,” which is determined by the source's luminous efficacy and the reflective or transmissive properties of the surface.

Figure 1: Schematic illustration of luminance

(Image source: https://en.wikipedia.org/wiki/Radiant_flux#/media/File:Photometry_radiometry_units.svg)

Mathematical Expression:
$$L = \frac{I_\theta}{S \cdot \cos\theta}$$ where:

• $L$ is the luminance ($\mathrm{cd/m^2}$);
• $I_\theta$ is the luminous intensity in the specified direction ($\mathrm{cd}$);
• $S$ is the actual surface area of the source ($\mathrm{m^2}$);
• $\theta$ is the angle between the surface normal and the specified direction ($\mathrm{rad}$ or $^\circ$); the term
• $S \cdot \cos\theta$ represents the projected area in that direction.

Unit
The SI unit for luminance is the candela per square meter ($\mathrm{cd/m^2}$), also referred to as the nit (non-SI unit).