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Luminance

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Luminance is a popular term if we consider the intensity of light. It can be explained as the intensity per unit area. Light sources have extended area. It can be divided in to small luminous areas. The point source approximation is only valid if the size of the source are very much smaller than the distance of the observer from the source. Luminance is the quantity or amount of light output from a source as similar the intensity of light output of a source. Luminance intensity and luminance bear in photometry plays similar role as electric charge and surface charge density in electrostatics.

Definition

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Luminance is the illuminance per unit solid angle, measured in lm/m2/Sr. In other words, luminance is the density of visible radiation in a given direction. It is a measurable quantity similar to the brightness, but they are different. It is denoted by Lv.
Luminance depends on the nature of the substance and luminance flux. In simple words, luminance is the light intensity per unit area.

Luminance Lighting

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Luminance lighting is nothing but the light produced by a body. Sun, stars are natural luminous body. Since they can produce light by itself. There are many artificial sources also present like bulb, candles and fluorescent lambs etc.Sun and Stars can be called as primary light sources. Because they can generate light with out any help from external agency.

Measurement

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Luminance measurement is done using light meter. It is nothing but to measure the light intensity of a luminous object. By using this equipment, we can measure luminous intensity, luminance flux, luminous efficacy, illuminance and luminous emittance etc.

The quantities and their respective units related to luminance is summarized in the given table.

Quantity
SI unit
Luminance flux Lumen
Luminous efficacy Lumen per watt
Luminous energy Lumen second
Illuminance lux
Luminance intensity Candela

Luminous flux is explained in terms of monochromatic radiation at 555 nm. It is denoted by Fv and is measured in lumens. This is applicable for both scotopic and photopic photometry. In the case of other wavelengths, For other wavelengths, the luminous flux must be calculated by the appropriate scotopic or photopic luminous efficiency function.

Efficacy

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Luminous efficacy is defined as the ratio of luminous flux to radiant flux. In other words, it can be explained as the luminous efficacy describes the absolute eye response of the normalized efficiency function. It is denoted by $K_{\lambda}$.