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The Luminous Intensity Converter converts between units of luminous intensity, which measures the luminous power emitted by a light source in a particular direction per unit solid angle. The SI base unit is the candela (cd), one of the seven SI base units.
Since 2019, the candela is defined by fixing the luminous efficacy of monochromatic radiation at 540 THz (green light) at exactly 683 lumens per watt. This connects the photometric quantity (candela) to the radiometric quantity (watt) through the sensitivity of the human eye.
Luminous intensity describes how concentrated a light source's output is in a given direction. A bare light bulb emits light in all directions with relatively uniform intensity, while a flashlight or spotlight concentrates light into a narrow beam, producing high candela in the beam direction despite potentially having fewer total lumens.
Historical light standards include the candlepower (cp), based on a standard candle and approximately equal to 0.981 candela, the Hefnerkerze (HK) used in Germany (0.903 cd), and the carcel used in France (9.74 cd). These were replaced by the international candela for consistency.
Modern applications: a typical standard LED produces 10–50 mcd, a high-brightness LED 1,000–20,000 mcd (1–20 cd), a household light bulb about 100–130 cd uniformly in all directions, and a powerful spotlight or searchlight can reach millions of candelas in its beam.
All values are normalized to candelas (cd). Key conversions: 1 candlepower = 0.981 cd (approximately), 1 Hefnerkerze = 0.903 cd, 1 carcel = 9.74 cd. SI prefix: milli = 10⁻³, kilo = 10³.
Luminous intensity and lumens are related by the solid angle: lumens = candelas x steradians. A point source emitting 1 cd uniformly in all directions (4π sr) produces 4π ≈ 12.57 lumens total.
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5 cd = 5000 mcd
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100 cp = 98.1 cd
The candela (cd) is the SI base unit of luminous intensity. It measures the luminous power emitted per unit solid angle in a particular direction. It is defined via the luminous efficacy at 540 THz = 683 lm/W.
A candela measures directional intensity (power per solid angle). A lumen measures total light output. A 1-candela isotropic source produces 4π ≈ 12.57 lumens total.
Candlepower (cp) is an older unit approximately equal to one candela (1 cp ≈ 0.981 cd). It was based on the light output of a standard spermaceti candle.
A typical household flashlight: 1,000-10,000 cd. A tactical flashlight: 10,000-100,000 cd. A powerful spotlight: 1,000,000+ cd. The candela rating depends on beam concentration.
Millicandelas (mcd) are used to rate LED brightness. Standard indicator LEDs are 10-200 mcd. High-brightness LEDs for backlighting or signaling are 1,000-20,000 mcd.
The Hefnerkerze (HK) was the German standard unit of luminous intensity from 1884, based on the Hefner lamp burning amyl acetate. 1 HK = 0.903 candela. It was used until about 1940.
Candela = Lumens / (beam solid angle in steradians). For a uniform beam with half-angle θ: solid angle = 2π(1-cosθ). Narrow beams produce more candelas per lumen.
Beam candela depends on total light output (lumens) and beam concentration (optics). A 1000-lumen source focused into a 10° beam produces about 42,000 cd. The same lumens in a 60° beam produce about 1,400 cd.
The carcel was a French unit of luminous intensity based on a standard Carcel oil lamp. 1 carcel = 9.74 candela. It was used in France before international standardization.
The candela is an SI base unit because photometry (measurement weighted by human eye sensitivity) cannot be derived from purely physical (radiometric) units without the human visual response function.
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