The relationship between light intensity, brightness, illumination and luminous flux

　　In 1967, the 13th International Conference on Weights and Measures in France stipulated that candela, candela/square meter, lumen, and lux are used as units of luminous intensity, brightness, luminous flux, and illuminance, respectively, which is of great significance for unifying the optical measurement units used in engineering technology. For the convenience of understanding and use, the following is a brief introduction to the relevant knowledge:

　　1. Definition of Candela, International Candela, and Candela

　　Under standard atmospheric pressure of 101325 Newtons per square meter, an absolute "black body" (i.e. an ideal object that can absorb all external light without any reflection) with an area equal to 1/60 square centimeter, at the solidification temperature of pure molybdenum (Pt) (about 2042K or 1769°C), has a luminous intensity of 1 candela in the vertical direction. In addition, the concepts of candlelight, international candlelight, and candela are different and should not be equated. In terms of quantity, 60 candela is equal to 58.8 international candlelight, and 1 candela of a Hefner lamp is equal to 0.885 international candlelight or 0.919 candela.

　　2. Hair intensity and brightness

　　Luminous intensity is referred to as light intensity, and its international unit is candela (abbreviated as cd). Lcd refers to the luminous flux emitted by a light source within a unit solid angle in a specified direction. When the radiation of the light source is uniform, the light intensity is I=F/Q, where Q is the solid angle in steradian (sr), and F is the luminous flux in lumen. For point light sources, I=F/4. Brightness indicates the brightness of the luminous surface, which refers to the ratio of the luminous intensity of the luminous surface in a specified direction to the area of ​​the luminous surface in a vertical and specified direction, and the unit is candela/square meter. For a diffuse scattering surface, although the light intensity and luminous flux in each direction are different, the brightness in each direction is equal. The fluorescent screen of a TV is similar to such a diffuse scattering surface, so when you watch the image from all directions, you will have the same sense of brightness.

　　The following are the brightness values ​​of some light sources: unit: cd/m²

　　Sunlight: 1.5*10 ; Fluorescent lamp: (5-10) *10 3 ; Moonlight (full moon): 2.5*10 3 ; Black and white TV

　　Color TV screen: about 120; color TV screen: about 80.

　　3. Luminous flux and lumen

　　The light energy emitted by the light source is radiated in all directions. The light energy passing through a certain area in unit time is called the radiant energy flux passing through this area. The frequencies of different colors of light are different, and the sensitivity of the eyes to different colors of light is also different. Even if the radiant energy flux of different colors of light is equal, it cannot produce the same brightness visually. Among the colors of light, yellow and green light can arouse the greatest sense of brightness. If green light is used as a standard, and its luminous flux is equal to the radiant energy flux, then for other colors of light, the ability to arouse a sense of brightness is smaller than that of green light, and the luminous flux is also smaller than the radiant energy flux. The unit of luminous flux is lumen, which is the transliteration of the English word lumen, abbreviated as lm. At the solidification temperature of molybdenum, the luminous flux radiated from an area of ​​5.305*10 3  cm 2 by an absolute black body is 1 lm. To show the relationship between light intensity and luminous flux, the luminous flux emitted by a point light source with a luminous intensity of 1 candela in a unit solid angle (1 steradian) is 1 lumen. The luminous flux output of a 40W fluorescent lamp is about 2100 lumens.

　　4. Illuminance and Lux

　　Illuminance can be directly measured with an illuminance meter. The unit of illuminance is lux, which is the transliteration of the English word lux, and can also be written as lx. When the luminous flux obtained by an object evenly illuminated by light is 1 lumen on an area of ​​1 square meter, its illuminance is 1 lux. Sometimes, in order to make full use of the light source, a reflector is often attached to the light source so that more luminous flux can be obtained in certain directions to increase the illuminance on the illuminated surface. For example, car headlights, flashlights, photography lights, etc.

　　The following are various ambient illumination values: unit: lux

　　Night: 0.001-0.02; Moon night: 0.02-0.3; Cloudy day indoors: 5-50; Cloudy day outdoor: 50-500; Sunny day indoors: 100-1000; Illuminance under sunlight at noon in summer: about 10*9; Illuminance required for reading books and periodicals: 50-60; Standard illumination for home video cameras: 1400.

　　The brightness calculation formula and related derivation of the diffuse reflection surface are as follows:

　　L=p E/π

　　That is: brightness = illumination x reflectivity + pi

　　So: E = Lπ/p

　　Example:

　　Assuming the reflectivity of your reflective surface is 0.7, the illumination required to achieve 10X10 ²cd/m² brightness is:

　　E=Lπ/p=1000X3.14/0.7 = 4485 Lux

　　Light intensity/cdela, luminous brightness/cd/m², luminous flux/lumen, illuminance/lux