1. Irradiance
Irradiance is a measure of radiation. It is used to measure the electromagnetic radiation energy per unit area. Its unit is W/m’. In solar exposure, irradiance refers to the energy of sunlight hitting the ground.

2. Radiance
Radiance is a radiometric measure and its unit is W/s r.m². It refers to the energy radiated in a specific direction in a unit time per unit area on the surface of a radiation source.
It is the flux of radiation source radiation in unit projected area and unit solid angle. If dΦ represents the flux per unit projected area dA in the direction of dΩ, then its radiance is defined as dΦ/dAdΩ.

3. Luminous Intensity
Luminous intensity is a photometric quantity, its unit is candela (c d); it measures the luminous flux energy of a light source in a specific direction. Since the human eye is sensitive to different light waves, the luminous intensity depends on the wavelength of the emitted light. For example, the human eye is most sensitive to light with a wavelength of 555 nm, and the luminous intensity of light with the same radiation intensity but different wavelengths will be different.

4. Brightness
The term brightness is a photometric quantity; it is the luminous intensity of a light source per unit area. Its unit is c d/m².
Brightness can describe the emissive or reflective properties of a planar light source or a planar diffuse light source. It measures how much light power the luminous surface can receive from a particular angle to the human eye. Brightness is a measure of how bright a luminous surface is. In this case, the solid angle we study is the solid angle subtended by the pupil of the human eye. The brightness of the sun can reach about 1.6x109c d/m² at noon.

5. Luminous flux
The unit of luminous flux is lumen (lm). It is a measure of the energy of light that can be perceived by the human eye. It is wavelength related. One lumen is equal to the luminous flux produced by the luminous intensity of one candle in a unit spherical solid angle. It is used to describe the amount of light emitted by a light source. We can find this instruction on the bulb packaging in some countries. It tells us how high luminous efficacy a light source is.
The amount of light emitted by a light source is measured in lumens. The definition of lumens is based on the definition of luminous intensity candela.
1lm=1cd·sr
The spherical solid angle of a sphere is 4m, and a light source that radiates 1ed luminous intensity uniformly in all directions will emit a total luminous flux of lcd·4π·sr=4π≈12.57lm.
A candle emits about 1ed of light in any direction, and candela measures the luminous intensity of a light source of a particular wavelength. If you measure the luminous intensity in a cone per unit solid angle, the value is the luminous flux in lumens (lm). Lumens measure the optical power that the human eye can perceive, while radiant flux measures the total power (in watts) emitted by a light source.

6. Units for measuring lighting levels: Illuminance
Illuminance is a term used for the amount of light. Its unit in the International System of Units is lux (l x or lm/m²). It is wavelength related and is also used to measure the magnitude of luminous intensity. Lighting levels or illuminance allow us to measure how bright or dark an environment or work surface is. It depends not only on the type and power of the light source, but also on the distance of the light source from the target and its surface. Illuminance can also be measured in candles (symbol: ft c d, f c or f c d).
Illumination level or illuminance refers to the total luminous flux incident on the illuminated surface. Illuminance can be measured with an illuminometer. 1ftcd is 1 lumen per square foot; 1lx is 1 lumen per square meter.
1) 1lx=0.0929ftcd=1lm/m²
2) 1ftcd=10.764lx
3) 1lm/ft²=1ftcd=10.764lx.
Lighting level is a good measure of the efficiency of a light source. In terms of application, it usually starts from the needs of illuminance, finds a suitable lighting system and installs it according to the requirements.

7. Common levels of natural lighting outdoors
The indoor lighting level of natural light sources depends largely on the building’s architecture, location, size and number of windows, and other quantitatively describeable parameters. Similarly, for cloudy natural light weather, the description of lighting levels should be more variable depending on how much the sky is covered by clouds. These conditions require specific assessments for each situation.

8. Recommended lighting levels in different workspaces
Outdoor lighting levels under clear skies are suitable for almost all working conditions. The indoor illumination of the building is lower than that of the outdoor, which is related to the building structure and its lighting, so additional light sources are usually required for illumination. For most working conditions, a lighting level of 300 ~ 500lx is required; for more delicate operation, more than 2000lx is required.

9. Luminous efficacy
Luminous efficacy is the ratio between the luminous flux of visible light emitted by a light source and the electrical power (W) it consumes. Luminous efficacy, like luminous intensity, is based on the light sensitivity of the human eye, so it is wavelength-dependent. Its units are lumens per watt (lm/W). According to the definition, the maximum efficiency that the light source can achieve is 683lm/w, which is the most sensitive monochromatic green light of 555mm wavelength for the human eye. The luminous efficacy of the light source is the ratio of the output visible luminous flux to the input electrical power, and the luminous efficacy is described by the ratio of electromagnetic radiation, that is, the luminous flux to the radiant flux (lm/W)
Lighting or luminous efficacy (rather than efficacy) is a dimensionless parameter equal to the ratio of the luminous efficacy to the luminous efficacy of a light source.

10. Inverse Square Law
This law states that the illuminance of light radiated from a point light source is inversely proportional to the square of the distance from the light source (the same law applies to other linear waves).

So if an object is moved 2 times farther away from a point light, it will only receive 1/4 of the previous distance illumination, and if the observer is far enough away from the light source, the light source can be seen as a point light source.