Candela
From Freepedia
- For other uses, see Candela (disambiguation).
The candela (symbol: cd) is the SI base unit of luminous intensity.
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Definition
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
SI multiples
| Multiple | Name | Symbol | Multiple | Name | Symbol | |
|---|---|---|---|---|---|---|
| 100 | candela | cd | ||||
| 101 | decacandela | dacd | 10–1 | decicandela | dcd | |
| 102 | hectocandela | hcd | 10–2 | centicandela | ccd | |
| 103 | kilocandela | kcd | 10–3 | millicandela | mcd | |
| 106 | megacandela | Mcd | 10–6 | microcandela | µcd | |
| 109 | gigacandela | Gcd | 10–9 | nanocandela | ncd | |
| 1012 | teracandela | Tcd | 10–12 | picocandela | pcd | |
| 1015 | petacandela | Pcd | 10–15 | femtocandela | fcd | |
| 1018 | exacandela | Ecd | 10–18 | attocandela | acd | |
| 1021 | zettacandela | Zcd | 10–21 | zeptocandela | zcd | |
| 1024 | yottacandela | Ycd | 10–24 | yoctocandela | ycd |
Origin
It may have been defined with reference to the luminous intensity of a single candle.
Explanation
The frequency chosen is in the visible spectrum near green. The human eye is most sensitive to this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye (called the V-lambda curve in the reference below).
A common candle emits about 1 cd. A 100 W lightbulb about 120 cd.
Historically, the candela was a fundamental unit of the SI. It was defined in terms of the black-body radiation emitted by 1/60 of 1 cm2 of platinum at its melting point. The modern definition is no longer fundamental because it is based on another SI unit of power, the watt. Traces of its history remain, however. The arbitrary (1/683) term was chosen such that the new definition would exactly match the old definition.
SI photometric light units
| SI light units | ||||
|---|---|---|---|---|
| Quantity | SI unit | Symbol | Notes | |
| Luminous energy | lumen seconds | lm · s | lumen seconds are sometimes called Talbots | |
| Luminous flux | lumen or (candela · steradian) | lm | also called Luminous power | |
| Luminous intensity | candela or (lumen / steradian) | cd | ||
| Luminance | candela / square metre | cd/m2 | also called Luminosity | |
| Illuminance | lux or (lumen / square metre) | lx | ||
| Luminous efficacy | lumens per watt | lm/W | maximum possible is 683 | |
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Reference
- "Unit of Luminous Intensity: Candela (cd)". Electro Optical Industries, Inc.
