Luminous Flux Converter: Lumens to Watts and More

Luminous flux is the total perceived power of light emitted by a source in all directions, measured in lumens (lm). It accounts for the sensitivity of the human eye at each wavelength. Luminous intensity, by contrast, is the flux emitted per unit solid angle in a specific direction, measured in candelas (cd). A lamp with a reflector and a lamp without one can have the same total luminous flux but very different intensities in any given direction. For a point source emitting uniformly in all directions, total flux equals 4π steradians multiplied by intensity in candelas.

Lumens measure visible light output; watts measure power consumed. To convert, you need the luminous efficacy of the light source in lumens per watt (lm/W): Watts = Lumens / Efficacy. A typical incandescent bulb produces about 15 lm/W, a halogen around 20 lm/W, a CFL around 60 lm/W, and a standard LED around 100 lm/W. High-efficiency LEDs can reach 150 lm/W or above. The same 800 lumens requires about 53 W from an incandescent but only 8 W from an LED.

Luminous efficacy (lm/W) is the ratio of luminous flux (in lumens) to power consumed (in watts). It measures how efficiently a light source converts electrical energy into visible light. Higher efficacy means less electricity is wasted as heat. The theoretical maximum efficacy for white light is approximately 240 to 350 lm/W depending on the colour spectrum, set by the limits of human vision. Current commercial LED technology typically achieves 100 to 200 lm/W, while incandescent bulbs typically achieve only 10 to 18 lm/W.

A standard 60 W incandescent bulb produces approximately 800 lumens. When replacing it with an LED, look for an LED rated at 800 lm. At a typical LED efficacy of 100 lm/W, this requires approximately 8 W. At 150 lm/W (high-efficiency LED), approximately 5.3 W. For other rooms, rough guides: a 40 W incandescent produces about 450 lm; a 75 W produces about 1,100 lm; a 100 W produces about 1,600 lm. The US Department of Energy (energy.gov) provides additional guidance on bulb selection.

Lumens and candela are related through solid angle. One lumen equals one candela emitted over a solid angle of one steradian (1 lm = 1 cd sr). For a uniform point source, total luminous flux in lumens equals the intensity in candelas multiplied by the solid angle in steradians over which light is emitted. A full sphere subtends 4π steradians (approximately 12.566 sr), so a uniform 1 cd source emits approximately 12.566 lm total. In practice, luminaires are not point sources and emit non-uniformly, so photometric data is used.