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In SI base units: cd · m −2. The candela per square metre (symbol: cd/m2) is the unit of luminance in the International System of Units (SI). The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area. The nit (symbol: nt) is a non-SI name also used for this unit (1 nt = 1 cd/m 2). [ 1 ]
The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018. [10] [11] The new definition, which took effect on 20 May 2019, is: The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, [a] K cd, to be 683 when expressed in the unit lm W −1, which is equal to cd sr W −1 ...
Luminous intensity. In photometry, luminous intensity is a measure of the wavelength -weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.
1 candela (modern definition) Candlepower (abbreviated as cp or CP) is a unit of measurement for luminous intensity. It expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. The historical candlepower is equal to 0.981 candelas. In modern usage, candlepower is sometimes used as a ...
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. [1] It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. The procedure for conversion from spectral radiance to luminance is ...
The lumen (symbol: lm) is the unit of luminous flux, a measure of the perceived power of visible light emitted by a source, in the International System of Units (SI). ). Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to ...
The foot-candle is defined as the illuminance of the inside surface of a one-foot-radius sphere with a point source of one candela at its center. Alternatively, it can be defined as the illuminance of one lumen on a one- square foot surface with a uniform distribution. Given the relation between candela and lumen, the two definitions listed are ...
Luminous efficacy. Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power (electric power ...