Terms on LED

Published:2011/5/18 1:15:00 Author:Phyllis From:SeekIC

The luminous intensity of a small element of surface area in a given direction is divided by the orthogonal projection of this area onto a plane at right angles to the direction. Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle.

In geometric optics, the luminance is invariant. In an ideal optical system, the output luminance is as same as that of the input. As a matter of fact, in passive and optical systems, the output luminance is at most equal to the input. Fro example, if one demagnifies an image with a lens, the luminance will be concentrated into a smaller area, so the illuminance is higher at the image . But the light at the image plane fills a larger solid angle so the luminance comes out to be the same assuming there is no loss at the lens. The image can never be brighter than the source.

Illuminance is the total amount of visible light illuminating on a surface from all directions above the surface. Therefore illuminance is equivalent of irradiance weighted with the response curve of the human eye. The standard unit for illuminance is Lux. In photometry, illuminance is the total luminous flux incident on a surface per unit area. It is a measure of the intensity of the incident light and wavelength that weighted by the luminosity. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

In the past we often called Illuminance as brightness, but this leads to confusion with other uses of the word. Brightness should never be used as quantitative description but nonquantitative references to physiological sensations and perceptions of light.

Luminous intensity is the radiating power of a source of light in a particular direction, weighted with the spectral sensitivity of the eye. The light-current in lumens can be obtained by measuring luminous intensity in candela over the entire solid angle in Sterad. The related integrated circuit is MC2832.

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