Illumination intensity is a physical term referring to the flux of visible light received per unit area. It has great effect on photosynthesis of organisms. It can be measured by an illuminometer.
Illumination intensity refers to the energy of visible light received per unit area, referred to as illuminance, unit Lux (or Lx).  is a physical term used to indicate the intensity of light and the extent to which an object's surface area is illuminated. In photometry, "luminosity" is the density of luminous intensity in a given direction, but it is often misinterpreted as illuminance. The international unit of luminosity is candlelight per square meter (called candela in mainland China, Hong Kong and Macao).
The illumination on a surface illuminated by light is defined as the luminous flux irradiated per unit area. Let the luminous flux on the surface element dS be dΦ, then the illumination E on this surface element is: E=dΦ/dS. 1 lx=1 lm/㎡. When the luminous flux of an object uniformly illuminated by light is 1 lumen over an area of 1 square meter, its illuminance is 1 lux. Lumen is a unit of light flux.
Graph of light intensity and photosynthesis intensity
A point source with a luminous intensity of 1 candle has a luminous flux of "1 lumen "within a unit solid Angle (1 sphericity). Candela, "Candela." The concept of candle, which is a unit of Luminous intensity, was first invented by the British. At that time, the English defined the unit of candlelight as the light emitted from a foot-long candle made of a pound of ash. The definition has changed: a standard light source is 1/60 of the amount of light emitted by a cubic centimeter of black luminous material heated until it melts into a liquid, and a candle is the unit of light emitted by such a standard light source.