In photometry, illuminance (symbol:E)is the total luminous flux incident on a surface per unit area. It serves as a measure of how intensely a surface is illuminated.

Figure 1: Definition of Illuminance ( Source: https://en.wikipedia.org/wiki/Illuminance )

The mathematical relationship is expressed as:
$$E = \frac{\Phi}{S}$$

where:

• $E$ is the illuminance, measured in lux (lx);
• $\Phi$ is the luminous flux incident on the surface, measured in lumens (lm);
• $S$ is the surface area of the illuminated object, measured in square meters ($m^2$).

The magnitude of illuminance is determined by the luminous intensity of the source, the distance to the surface, and the angle of incidence, according to the inverse-square law and the cosine law. As a measure of incident light, illuminance is independent of the surface's material properties, such as color or reflectance.

Unit
The SI derived unit of illuminance is the lux (symbol: lx). One lux is defined as one lumen(lm) of luminous flux uniformly distributed over an area of one square meter(). This relationship is expressed as: $$1\ \mathrm{lx} = 1\ \mathrm{lm/m^2}$$

Measurement
Illuminance is typically measured using a luxmeter. The device employs a photodiode integrated with optical filters to match the CIE photopic luminous efficiency function $V(\lambda)$,ensuring the output aligns with human visual perception. Furthermore, the detector must incorporate cosine correction to maintain measurement accuracy regardless of the light's angle of incidence.

Figure 2: luxmeter