Luminous Intensity Converter
Convert luminous intensity between candela, millicandela, kilocandela, lumen per steradian, candlepower and Hefnerkerze. Choose any two units for an instant, exact result.
Luminous Intensity Formula
All units on this page are expressed as multiples of the candela (cd), the SI base unit of luminous intensity. To convert, multiply the value by the factor of the source unit and divide by the factor of the target unit. Modern candlepower is identical to the candela, but the historical candlepower (the old international candle) is slightly smaller at 0.981 cd, so the tool lists them separately. The Hefnerkerze is an old German standard and its value of about 0.920 cd is approximate by nature.
What Is Luminous Intensity?
Luminous intensity is the amount of visible light a source sends in one particular direction, counted per unit of solid angle. Its SI unit is the candela (cd), one of the seven SI base units. One candela equals one lumen per steradian (lm/sr). Since the 2019 SI revision, the candela is defined by fixing the luminous efficacy of monochromatic green light of 540 × 10¹² Hz at exactly 683 lm/W.
Intensity tells you how concentrated a beam is. A lamp with a reflector may have the same total light output (lumens) as a bare bulb but a much higher candela rating in the beam direction. A source that radiates 1 cd equally in all directions emits a total of 4π lumens, about 12.5663706144 lm.
Before the candela, light sources were compared with real flames and lamps. That left a family of historical units, such as the candlepower and the German Hefnerkerze (HK), which was based on the Hefner lamp. Old catalogues, books and standards still use them. For brightness of surfaces see the Luminance Converter, and for light falling on a surface see the Illumination Converter.
Supported Luminous Intensity Units
Every unit below is converted through its definition in candela.
| Unit | Symbol | 1 unit equals (in cd) | Basis |
|---|---|---|---|
| Candela | cd | 1 cd | SI base unit |
| Millicandela | mcd | 0.001 cd | Exact |
| Kilocandela | kcd | 1,000 cd | Exact |
| Lumen per steradian | lm/sr | 1 cd | Exactly 1 cd |
| Candlepower (modern) | cp | 1 cd | Modern usage: 1 cp = 1 cd |
| Candlepower (historical, pre-1948) | cp (old) | 0.981 cd | 0.981 cd (international candle) |
| Hefnerkerze (Hefner candle) | HK | 0.92 cd | About 0.920 cd (historical) |
How to Use the Luminous Intensity Converter
- Enter the luminous intensity in the input box. Decimals and scientific notation such as 0.02 or 2e-2 work.
- Select the unit to convert from and the unit to convert to.
- The answer appears immediately, together with the value in every other supported unit. You can also press Convert Now.
- Use Swap Units to reverse the conversion, Copy Result to copy the answer, or Reset to clear everything.
Choose Candlepower (modern) when a source quotes candlepower as a synonym of candela, and Candlepower (historical) for figures from before the 1948 redefinition.
Practical Examples
- Indicator LED: a 20 mcd LED equals 20 × 0.001 = 0.02 cd.
- Torch beam: a flashlight rated at 12,000 cd is 12 kcd.
- Hefnerkerze: a lamp rated at 10 HK has an intensity of 10 × 0.920 = 9.2 cd.
- Historical candlepower: 50 old candlepower = 50 × 0.981 = 49.05 cd.
- Candela to HK: 1 cd = 1 ÷ 0.920 = 1.08695652174 HK.
Luminous Intensity – Common Values
Quick reference for frequently used conversions
| Candela (cd) | Millicandela (mcd) | Candlepower (old) | Hefnerkerze (HK) | Lumen/steradian |
|---|---|---|---|---|
| 0.001 | 1 | 0.00101936799185 | 0.00108695652174 | 0.001 |
| 0.01 | 10 | 0.0101936799185 | 0.0108695652174 | 0.01 |
| 0.1 | 100 | 0.101936799185 | 0.108695652174 | 0.1 |
| 1 | 1,000 | 1.01936799185 | 1.08695652174 | 1 |
| 5 | 5,000 | 5.09683995923 | 5.4347826087 | 5 |
| 10 | 10,000 | 10.1936799185 | 10.8695652174 | 10 |
| 25 | 25,000 | 25.4841997961 | 27.1739130435 | 25 |
| 100 | 100,000 | 101.936799185 | 108.695652174 | 100 |
| 1,000 | 1,000,000 | 1,019.36799185 | 1,086.95652174 | 1,000 |
Real-Life Applications of Luminous Intensity Conversion
- LED and lamp data sheets: indicator and signal LEDs are rated in millicandela, while spotlights and torches use candela.
- Vehicle and bicycle lighting: headlamp, brake light and signal regulations set minimum and maximum intensities in candela.
- Aviation and marine signals: navigation lights and obstruction lights have required intensities.
- Lighting design: photometric files and fixture catalogues give intensity distributions in candela.
- Historical documents: older German, British and American texts quote candlepower or Hefnerkerze.
- Education and labs: physics courses use candela, lumen and steradian together.