Luminous Flux
Luminous flux refers to the radiant power that the human eye can perceive. It is equal to the product of the radiant energy in a certain wavelength band and the relative luminous efficiency of that band. Since the human eye has different relative luminous efficiencies for different wavelengths of light, the luminous flux is not equal when the radiant power of different wavelength lights is equal.
The unit of luminous flux is "lumen" (lm). Luminous flux is a derived quantity of radiant flux evaluated according to the international standard visual characteristics of the human eye. It is represented by the symbol Φ (or Φr). Theoretically, its unit is equivalent to the electrical unit watt because visual perception is still related to light color. Therefore, the unit "lumen" is used according to the standard light source and normal visual acuity measurement units.
The lumen (lm) is the unit of luminous flux in the International System of Units (SI) and the American System of Units (AS). If light is considered as particles traversing space (photons), then the luminous flux of a light beam reaching a surface is directly proportional to the number of particles striking the surface within a certain time interval.
Illuminance
Illuminance refers to the visible light energy received per unit area, expressed in lux (lx). It is a physical term used to indicate the intensity of light and the degree to which the surface area of an object is illuminated.
In photometry, "luminance" is the density of luminous intensity in a specified direction, but it is often misunderstood as illuminance. The international unit of luminance is the candle per square meter (referred to as candela per square meter in Mainland China, Hong Kong, and Macau).
Illuminance on a surface illuminated by light is defined as the luminous flux incident on a unit area.
Luminous Intensity
Luminous intensity, simply referred to as intensity, is measured in candela (cd). Other units include the candle and the lumen per steradian (lm/sr). One candela (cd) is defined as the luminous flux emitted by a light source of monochromatic radiation (frequency 540 x 10^12 Hz) in a given direction, with a radiation intensity of (1/683) watts per steradian. This can be calculated using the Kirchhoff integral theorem.
From the nature of light, the intensity of light at a point in the light field refers to the average energy flow density passing through that point. A steradian is a solid angle with its apex at the center of a sphere, and the area it cuts on the sphere's surface is equal to the area of a square with the sphere's radius as its side length. When the radiation from a light source is uniform, the intensity is given by I = F/Ω, where Ω is the solid angle in steradians, and F is the luminous flux in lumens. For a point light source, I = F/(4π).
Luminous intensity is relevant for point light sources or situations where the size of the light source is small compared to the distance of illumination. It describes the converging ability of the light source in space, essentially indicating how bright the light source is.
Brightness
Brightness refers to the ratio of luminous intensity of a light source to the area of the light source as perceived by the human eye. It is defined as the luminance of the unit light source, i.e., the luminous intensity per unit projected area. The unit of brightness is candela per square meter (cd/m2). Brightness is the intensity of light perceived by humans, and it is a subjective quantity. Unlike illuminance, which is a physically defined objective quantity, brightness is a subjective perception. The terms brightness and luminance are often used interchangeably in everyday language. Brightness, also known as luminosity, indicates the degree of brightness of a color. The perceived brightness by the human eye is determined by the brightness of the color reflection or transmission.
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