Illuminance
Illuminance is a physical quantity that characterizes the amount of luminous flux received per unit area of an illuminated surface, directly reflecting the degree to which the surface is illuminated. Its magnitude depends on the luminous intensity of the light source, the irradiation distance, and the angle (following the inverse square law and the cosine law), and is independent of the intrinsic properties of the illuminated object, such as its material and reflection characteristics.
Unit: The SI unit is the lux (symbol: lx), defined as a luminous flux of 1 lumen (lm) uniformly distributed over an area of 1 square meter (m²), i.e., $1\ \text{lx} = 1\ \text{lm/m}^2$

Mathematical Expression: $E = \frac{\varPhi}{S}$
where:

• $E $ is the illuminance (unit: lx);
• $\varPhi$ is the luminous flux passing through the illuminated area (unit: lm);
• $S$ is the surface area of the illuminated object (unit: m²).
  

Measurement of Illuminance
Illuminance is primarily measured using a luxmeter.
The core of illuminance measurement is to simulate the sensitivity of the human eye to light of different wavelengths. The key component is a photodiode equipped with an optical filter, which matches the spectral response of the detector to the CIE luminous efficiency function (the sensitivity curve of the human eye), thereby ensuring that the measurement results are consistent with visual perception. To ensure measurement accuracy, the detector's sensitivity must satisfy the cosine response characteristic, meaning that the measurement signal for incident light is unaffected by the angle of incidence, ensuring that light from different directions is correctly measured.

Difference between Illuminance and Irradiance?
Illuminance is a photometric quantity that takes into account the differences in the human eye's sensitivity to light of different wavelengths; whereas irradiance is the corresponding radiometric quantity, representing the total optical power per unit area (unit: W/m²), without being weighted by the visual sensitivity of the human eye.

Difference between Illuminance and Luminance?
Illuminance quantifies the light incident on a surface (unit: lm/m²), while luminance characterizes the visual brightness of the surface, describing the light emitted or reflected by the surface in a specific direction (unit: lm·sr⁻¹·m⁻²).

How does the distance from the light source affect illuminance?
For a small isotropic light source, illuminance attenuates with the square of the distance. This is because as the distance $d$ increases, the total luminous flux is distributed over a larger spherical surface area $4\pi d^2$.