Luminous Intensity Converter

Luminous intensity is how bright a light source is in one particular direction: the light it sends into a given solid angle. Its SI unit is the candela (cd), one lumen per steradian. An ordinary wax candle gives off about 1 cd, which is where the unit comes from.

The candela replaced the older national candles in 1948. This converter covers those historical units: the candle, the German and UK candles, the Hefner candle and the French Carcel unit.

How to convert luminous intensity units

Type a value into any field, and every other unit is calculated at once. For example, a lamp of 100 cd:

1. In Hefner candles: divide by 0.9.

   100 / 0.9 = 111.1 HK

2. In Carcel units: divide by 9.61.

   100 / 9.61 = 10.41 Carcel units

From candelas to lumens: for a source that shines equally in every direction, the total light is lumens = candelas * 4 * π, about 12.57 lumens per candela. For a spotlight, multiply by the solid angle of its beam instead.

Common conversions

Luminous intensity units

• Candela (cd): The candela is the standard unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction.
  Conversion Factor: 1 cd = 1 cd (Base Unit)

• Candle (c): The candle (or candlepower) unit is an older unit of luminous intensity. In 1948, the unit candela replaced candlepower. One candlepower unit is about 0.981 candela but in general modern use, a candlepower now equates directly (1:1) to the number of candelas.
  Conversion Factor: 1 cd = 1 c

• German Candle (c): Similar to the standard candle, the German candle is another historical unit of luminous intensity. It was used in Germany before the adoption of the candela.
  Conversion Factor: 1 cd = 0.95 c (german)

• UK Candle (c): The UK candle is yet another variant used in the United Kingdom. Like other candle units, it has been largely replaced by the candela in modern scientific and technical contexts.
  Conversion Factor: 1 cd = 0.96 c (UK)

• Hefner Candle (HK): The Hefner candle, named after Friedrich von Hefner-Alteneck, is a historical unit used in Germany. It was based on the light emitted by a lamp burning amyl acetate.
  Conversion Factor: 0.9 cd = 1 HK

• Carcel Unit: The Carcel unit is an obsolete French unit of luminous intensity, defined by the light produced by a Carcel lamp burning a standard quantity of oil.
  Conversion Factor: 9.61 cd = 1 Carcel unit

• Lumen per Steradian (lm/sr): An equivalent unit to the candela. The total luminous flux of a light beam is 1 lumen when its intensity is 1 candela, and its space angle is 1 steradian. Therefore, 1 lumen per steradian is equivalent to 1 candela.
  Conversion Factor: 1 cd = 1 lm/sr

Frequently asked questions

What is the difference between candelas and lumens?

Lumens measure all the light a source gives out, in every direction. Candelas measure how much of it goes one way. A narrow beam can have a very high candela figure from few lumens, which is why torches quote both.