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Home > Articles > English > LED knowledge > What is cd and lm?

LED knowledge

What is cd and lm?
From: The Site Author: LIGHTUP LED DateTime: 2013-7-12

 What is cd and lm? What are the units of these LED parameters?


To understand the LED products, you need to know what these parameters are first. Common use of LED parameters:

  • Luminous Flux
  • Luminous intensity
  • Luminance
  • Total Luminance
  • Color temperature
  • Light Effect
  • Color rendering 


(lm)-Luminous Flux

Luminous flux is a quantitative expression of the brilliance of a source of visible light, which is electromagnetic energy within the wavelength range of approximately 390 nanometer s (nm) to 770 nm. This quantity is measured in terms of the power emitted per unit solid angle from an isotropic radiator, a theoretical point source that radiates equally in all directions in three-dimensional space. The standard unit of luminous flux is the lumen (lm).


(cd) -Luminous Intensity

Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian . The candela was defined and adopted as the standard unit of luminous intensity. One candela (1 cd) is the magnitude of an electromagnetic field ( EM-field ), in a specified direction, that has a power level equivalent to a visible-light field of 1/683 watt (1.46 x 10 -3 W) per steradian at 540 THz.


(cd/m²) - Luminance

Luminance is the luminous intensity, projected on a given area and direction. Luminance is an objectively measurable attribute. The unit is 'Candela per Square Meter' (Cd/m²). So, different monitors can be adjusted to the same luminous intensity by measuring the luminance in Cd/m².


Total Luminance

Total luminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception.


(lm/W)-Light Effect

Light Effect is the rate of the electronic power turning into the light power; it is unit as “lm/W”.


(K)-Color Temperature

Color temperature is a simplified way to characterize the spectral properties of a light source. While in reality the color of light is determined by how much each point on the spectral curve contributes to its output, the result can still be summarized on a linear scale. Standard unit for color temperature is Kelvin (K).


(ra)-Color rendering

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant Standard unit for color temperature is Ra.


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