In 1967, the 13th International Conference on Weights and Measures in France stipulated that candela, candela/square meter, lumens, and lux be used as the units of luminous intensity, brightness, luminous flux and illuminance, etc., which are important for unifying the optical measurement units used in engineering technology. significance. In order to make it easier for you to understand and use, the following will give a brief introduction to the relevant knowledge:
1. Candlelight, International Candlelight, Candela
Under the standard atmospheric pressure of 101325 Newtons per square meter, an absolute "black body" (that is, a satisfactory object that can absorb all external light without reflection) with an area equal to 1/60 square centimeter, at the solidification temperature of pure platinum (Pt) (about 2042K 1769°C), the luminous intensity along the vertical direction is 1 candela.
Moreover, the three concepts of candlelight, international candlelight and candela are different and should not be equivalent. Quantitatively, 60 candela is equal to 58.8 international candela, and 1 candela of a Huffner lamp is equal to 0.885 international candela or 0.919 candela.
2. Luminous intensity and brightness
Luminous intensity is referred to as light intensity, and the international unit is candela (candela) abbreviated cd. Lcd refers to the luminous flux emitted by a light source within a unit solid angle of a specified direction. When the light source radiation is uniform, the light intensity is I=F/Ω, Ω is the solid angle, the unit is steradian (sr), F is the luminous flux, the unit is lumens, and for a point light source, I=F/4. Luminance is the brightness of the luminous surface, which refers to the ratio of the luminous intensity of the luminous surface in the specified direction to the area of the luminous surface in the 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 the TV is similar to such a diffuse scattering surface, so the images are viewed from all directions and have the same sense of brightness. Give 1.3 billion Chinese people more hearing care!! The following are the brightness values of some light sources: unit: cd/m2
Sun: 1.5*10; Fluorescent light: (5-10)*103; Moonlight (full moon): 2.5*103; Black and white TV screen: about 120; Color TV screen: about 80.
3. Luminous flux and lumens
The light energy emitted by the light source is radiated in all directions, and the light energy passing through a certain area in unit time is called the radiant energy flux through this area. The frequency of each color light is different, and the sensitivity of the eye to each color light is also different. Even if the radiant energy flux of each color light is equal, the visual brightness cannot be the same. Maximum bright feeling.
If the green light is used as the level, and its luminous flux is equal to the radiant energy flux, then for other color lights, the ability to arouse a bright feeling is smaller than that of the 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 English lumen, abbreviated as lm.
At the solidification temperature of platinum, the luminous flux radiated from an area of 5.305*103cm2 for an absolute blackbody is 1lm. In order to show the relationship between luminous intensity and luminous flux, the luminous flux emitted by a point light source with a luminous intensity of 1 candela within a unit solid angle (1 steradian) is 16. The luminous flux output of a 40W fluorescent lamp is about 2100 lumens.
4. Illuminance and Lux
Illuminance can be measured directly with an illuminometer. The unit of illuminance is lux, which is the transliteration of English lux, and can also be written as lx. An object that is uniformly illuminated by light has an illuminance of 1 lux when the luminous flux obtained on an area of 1 square meter is 1 lumen. Sometimes in order to make full use of the light source, a reflective device is often attached to the light source, so that more luminous flux can be obtained in certain directions, so as to increase the illumination on the illuminated surface.
Such as car headlights, flashlights, camera lights, etc. Here are the various ambient illuminance values:
Unit lux night: 0.001—0.02; moonlit night: 0.02—0.3; cloudy indoor: 5—50; cloudy outdoor: 50—500; sunny indoor: 100—1000; power; illumination required for reading books and periodicals: 50-60; standard illumination for home cameras: 1400.
The brightness calculation formula and related derivation of the diffuse reflection surface are as follows:
L=ρE/π is: brightness = illuminance × reflectivity ÷ pi, so: E=Lπ/ρ
Example: Assuming that the reflectance of your reflective surface is 0.7, then to achieve 10×102cd/m2, the illuminance required for brightness is: E=Lπ/ρ=1000×3.14/0.7=4485Lux
