What Is a Lumen?
A lumen (lm) is the SI unit of luminous flux, measuring the total amount of visible light emitted by a source as perceived by the human eye. Unlike watts (which measure total radiant power), lumens account for the eye's sensitivity curve, giving more weight to green-yellow wavelengths where human vision is most sensitive. One lumen is defined as the luminous flux emitted by a light source of one candela intensity over a solid angle of one steradian.
Lumens have become the primary metric for comparing light bulb brightness, replacing the old watt-based system. A typical 60W incandescent bulb produces about 800 lumens, while an LED achieving the same brightness uses only 8-12 watts, demonstrating the importance of separating light output from energy consumption.
Lumen Formula
Where Φ is luminous flux (lumens), I is luminous intensity (candelas), Ω is the solid angle (steradians), and θ is the beam angle.
Light Source Comparison
| Source | Lumens | Efficacy (lm/W) |
|---|---|---|
| Candle | 12 | ~0.3 |
| 40W Incandescent | 450 | 11 |
| 60W Incandescent | 800 | 13 |
| LED (10W) | 800 | 80 |
| T8 Fluorescent | 2,800 | 90 |
Frequently Asked Questions
What is the difference between lumens and lux?
Lumens measure total light output from a source, while lux measures illuminance (lumens per square meter) at a surface. A 1000-lumen flashlight focused into a narrow beam creates high lux in a small area, while a 1000-lumen room light creates lower lux spread over the entire room. Lux = lumens / area.
What is luminous efficacy?
Luminous efficacy measures how efficiently a light source converts electrical power into visible light, expressed in lumens per watt (lm/W). Incandescent bulbs achieve about 12-17 lm/W, fluorescents 50-100 lm/W, and modern LEDs 80-200+ lm/W. The theoretical maximum is 683 lm/W for monochromatic 555 nm green light.
How many lumens do I need?
General room lighting typically needs 300-500 lux. For a 20 m2 room, this requires 6,000-10,000 lumens total. Task lighting (reading, cooking) needs 500-1000 lux locally. Outdoor security lighting typically uses 700-1300 lumens per fixture.