The relationship between LED luminous flux and illuminance and light intensity

- Feb 02, 2021-

The luminous flux of the LED is an indicator of the overall brightness of the light source. The unit is lm (lumens). It is often used when expressing the brightness of the lighting source. It refers to the sensitivity of the human eye (visual sensitivity) to represent the physical quantity of the light emitted by the light source. The specific value is that a light source with an isotropic luminous intensity of 1cd (candela) emits a luminous flux of 1lm within a solid angle of 1sr (stereoradian). Here, sr is the unit of solid angle, which means the apex angle of a cone whose area taken from the spherical center to the center of the sphere is the second power (r2) of the radius (r).

  Light intensity is an index indicating the solid angle density of luminous flux. The unit is cd. It is often used to indicate the glare of LEDs for display. It is defined as a light source emitting 540×1012Hz (wavelength 555nm) frequency monochromatic light, the light emission intensity in the specified direction is 1/683W/sr, and the light intensity in this direction is defined as 1cd.

  Brightness is an indicator of the intensity of light emitted from a light source, secondary light sources such as a reflective surface and a transmissive surface to the observer. The unit is cd/m2. Like luminous flux, it is a physical quantity expressed in conjunction with the sensitivity of the human eye. It is often used to indicate the brightness of display screens such as liquid crystal panels and PDPs.

   Illuminance is an index indicating the brightness of light irradiated on a plane. The unit is lx (lux), sometimes also marked as lm/m2. It refers to the luminous flux per unit area of the luminous flux emitted by the light source to the planar object. Used to compare the brightness of lighting fixtures irradiated on a flat surface.

Luminous flux, as the name implies, is the amount of light passing through, in units of lumens Lm.

Now it is the basic unit in optics, defined (simplified): the wavelength is 555nm, the radiation is 1 watt-hour, and the luminous flux is 683 lumens.

(Light) Intensity: The luminous flux emitted by a point light source in a given direction in a unit solid angle, in candela (Cd).

(Light) Illuminance: The luminous flux projected on a unit area, in lux, Lx.

(Light) Brightness: the luminous flux per unit solid angle and unit projected area when leaving, arriving or passing through a certain surface, unit nit.

The relationship between luminous flux and illuminance and light intensity

   The relationship between luminous flux, luminous intensity, brightness and illuminance is simply summarized as follows: luminous flux divided by unit solid angle equals luminous intensity; luminous flux divided by unit area equals illuminance, luminous intensity divided by unit area equals luminance.

Common lighting parameters: introduction of luminous flux, illuminance, light intensity and other parameters

In 1967, the 13th International Conference on Metrology in France stipulated that candela, candela/square meter, lumens, and lux were used as the units of luminous intensity, luminosity, luminous flux and illuminance, respectively, 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. Definition of candlelight, international candlelight, candela

Under the standard atmospheric pressure of 101325 Newtons per square meter, an absolute "black body" (that is, an ideal object that can absorb all external light without reflection) with an area equal to 1/60 square centimeters, at the solidification temperature of pure platinum (Pt) (about 2042K) At 1769℃), 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 equal. In terms of quantity, 60 candela is equal to 58.8 international candles, and 1 candela of Hefner lamp is equal to 0.885 international candles 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 in 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 point light sources, I=F/4. Luminous brightness refers to the brightness of the light-emitting surface, which refers to the ratio of the luminous intensity of the light-emitting surface in a specified direction to the area of the light-emitting surface perpendicular to the 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 this diffuse scattering surface, so the image from all directions has the same sense of brightness.

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

Sun: 1.5*10; fluorescent lamp: (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 radiates in all directions, and the light energy passing through a certain area in a unit time is called the radiant energy flux passing through this area. The frequency of each color light is different, and the sensitivity of the eyes to each color light is also different. Even if the radiant energy flux of each color light is equal, it cannot produce the same degree of brightness visually. In each color light, yellow and green light can stimulate The greatest sense of brightness. If green light is used as the level and its luminous flux is equal to the radiant energy flux, then for other colors of light, the ability to inspire a bright feeling is smaller than that of green light, and the luminous flux is also less than the radiant energy flux. The unit of luminous flux is lumens, which is the transliteration of English lumen, abbreviated as lm. Absolute black body at the solidification temperature of platinum, the luminous flux radiated from an area of 5.305*10³cm² is 1lm. To show the relationship between luminous intensity and luminous flux, a point light source with a luminous intensity of 1 candela emits 16 luminous flux per unit solid angle (1 steradian). The output luminous flux of a 40W fluorescent lamp is about 2100 lumens.

① Unit: Lumen (lm)

  First light = the luminous flux emitted by a light source of one candle (1cd) within a three-dimensional arc angle.

  One candlelight = the light emitted by burning 7.776 grams of standard candles per hour.

  The luminous flux can also be defined as the luminous flux whose wavelength is 555nm in the air, which is equal to 1/683W radiant flux.

② Relation

  φ=4 π I

  (1 candlelight = 4 π lumens = 12.56 lumens)

  (1cd = 4 π lm = 12.56 lm)

4. Illumination and Lux

The illuminance can be directly measured with an illuminance meter. The unit of illuminance is lux, which is the transliteration of English lux and can also be written as lx. When an object is uniformly illuminated by light, when the luminous flux obtained on an area of 1 square meter is 1 lumen, its illuminance is 1 lux. Sometimes, in order to make full use of the light source, a reflection device is often added 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 environmental illuminance values: unit lux

Dark night: 0.001-0.02; moon night: 0.02-0.3; cloudy indoor: 5-50; cloudy outdoor: 50-500; sunny indoor: 100-1000; illuminance under the sun at noon in summer: about 10*9 power Illumination required when reading books: 50-60; standard illuminance for home video cameras: 1400.


    • Lux (lux)=lumens/square meter

    • Amplitude (phot) = lumens/square centimeter

    • Meter-candle: The illuminance on the surface of the illuminated object at a distance of one meter from the light source of one candela.




    r is the distance from the light source to the surface of the illuminated object



    dA is the unit area of the illuminated object

    When the subject is tilted and forms an angle θ with the beam:


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