Introduction
Luminous intensity measures light output in candela (cd), the SI unit (formerly 'candle'). One candela is roughly the light output of a single wax candle. A 60 W incandescent bulb emits ~100 cd; an LED flashlight emits 200-1,000 cd focused into a narrow beam. The unit is tied to human vision — monochromatic green light at 555 nm weights strongest.
Why luminous intensity units exist and how they diverged
The candela was originally the intensity of a standard whale-oil candle (1881). Redefined several times; now defined as the light intensity of a monochromatic 540 THz source producing 1/683 watt per steradian. The 683 factor ties candela to watt via the peak sensitivity of the human eye at 555 nm.
Luminous intensity (cd) is power per solid angle. Luminous flux (lumen, lm) is total light output — a 60 W bulb outputs ~800 lm, and if that were uniform over all directions, the intensity would be 800/(4π) ≈ 63.7 cd. A flashlight with the same 800 lm focused into a 1-steradian beam gives 800 cd.
How to convert luminous intensity
Candela and candlepower are nominally the same. Old 'candlepower' ratings on 19th-century gas lamps are approximately candela. Modern flashlight 'candlepower' marketing is often peak beam intensity in candela.
Units supported by this luminous intensity calculator
- Candela (cd)
- Lumen/steradian
- Millicandela (mcd)
- Kilocandela (kcd)
- Candlepower (intl.)
- Hefnerkerze
- Bougie décimale
- Violle
Common luminous intensity conversion mistakes
- Lumens vs candela. Lumens is total output; candela is intensity in a specific direction. A bare bulb and a flashlight can emit the same lumens but very different peak candela.
- Old candlepower ratings. 1 candlepower ≈ 1 candela approximately; not exact.
- Watts as brightness. Watts is power consumed, not light emitted. A 10 W LED bulb emits more light than a 60 W incandescent.
Real-world luminous intensity examples
- Candle flame: ~1 cd.
- 100 W incandescent bulb (isotropic): ~130 cd.
- Tactical LED flashlight: 200-1,000 cd (focused).
- Car headlight high beam: 75,000+ cd (regulated max).
- Searchlight: 100 million+ cd.
- Sun at Earth's surface: 1.6 × 10⁹ cd/m² luminance.
Tips for accurate luminous intensity conversion
- For flashlights, candela sells 'throw' (how far the beam reaches); lumens sells 'flood' (total brightness).
- For room lighting, lumens matters more than candela.
- Luminous efficacy (lm/W) measures bulb efficiency — modern LEDs hit 100-200 lm/W, incandescents 10-17 lm/W.
Related: Illuminance Converter · Energy Converter · Power Converter.