Luminous Intensity Converter (candela, candlepower)

Understanding the luminous intensity converter — concept and principle

Luminous intensity tells you how bright a light source appears from one particular direction. It is the quantity behind flashlight "peak beam intensity" ratings and signal lamp specifications. This converter changes values between the SI unit, the candela (cd), the modern candlepower, and two historical flame-based standards: the Hefner candle and the pre-1948 international candle.

Candela measures light sent in one direction; old flame standards were slightly smaller or larger than 1 cd.

How it is calculated

Every unit is first converted to candelas and then divided by the size of the target unit: result = value × (cd per "from" unit) ÷ (cd per "to" unit). The factors used are:

1 candlepower (modern) = 1 cd exactly.
1 Hefner candle ≈ 0.903 cd.
1 international candle (pre-1948) ≈ 1.019 cd.

The historical factors are approximate, because flames could only be reproduced to within a few percent.

The principle

Light leaves a source spread over many directions. Luminous intensity measures how much luminous flux (lumens) goes into a small cone of directions, per unit solid angle: I = dΦ ÷ dΩ, so 1 cd = 1 lm/sr. Since 1979 the candela has been tied to a precisely specified radiation rather than a flame: a source emitting monochromatic light at 540 × 10¹² Hz (green light near 555 nm, where the eye is most sensitive) with a radiant intensity of 1/683 watt per steradian has an intensity of 1 cd. Earlier flame standards, such as the German Hefner lamp (an amyl acetate flame) and the international candle, differed slightly in size, so they do not convert at exactly 1.

Worked example

Inputs: value = 1,000, from candelas, to modern candlepower.
Modern candlepower equals 1 cd, so 1,000 cd × 1 ÷ 1 = 1,000 candlepower.
In Hefner candles: 1,000 ÷ 0.903 = 1,107.419712.
In international candles: 1,000 ÷ 1.019 = 981.3542689.

How to read the result

A bigger unit gives a smaller number. For a beam, intensity also tells you illuminance at a distance by the inverse-square law: E (lux) = I ÷ d². A 1,000 cd beam gives 1,000 lx at 1 m and 10 lx at 10 m. Flashlight makers using the ANSI/PLATO FL1 standard quote "beam distance" as the distance where illuminance drops to 0.25 lx, which is 2 × √I metres; 1,000 cd corresponds to about 63 m.

Common mistakes

Treating candelas and lumens as interchangeable. A narrow beam can have a huge candela rating from only a few lumens.
Assuming old catalogue candlepower figures match modern values exactly.
Applying the inverse-square law very close to a lens or reflector, where the beam has not yet formed.

Keep in mind: The Hefner and international candle factors are historical approximations. Use them for reading old documents, not for photometric calibration.

Frequently asked questions

How many lumens is 1 candela?

It depends on the beam. A source radiating 1 cd evenly in all directions emits 4π ≈ 12.57 lm. A focused beam with the same lumens can show far more candelas.

Is candlepower the same as candela today?

Yes. Modern usage defines 1 candlepower as 1 cd, so the numbers are identical.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · NIST Special Publication 811 (2008), Guide for the Use of the International System of Units · ANSI/PLATO FL 1-2019, Flashlight Basic Performance Standard

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.