Table of Contents
Watts vs Lux
Watts measure electrical power consumed by a light source, while lux measures the illuminance (light intensity) received at a surface. The conversion depends on the luminous efficacy of the light source (lumens per watt) and the distance from the source to the surface.
A 10W LED can produce the same illuminance as a 60W incandescent bulb because LEDs have much higher luminous efficacy. Understanding this relationship is crucial for lighting design, photography, and workplace safety compliance.
Conversion Formula
For a point source radiating uniformly in all directions, the illuminance at distance d is the total luminous flux divided by the area of a sphere with radius d. Real light fixtures with reflectors concentrate light, increasing lux in the forward direction.
Illuminance Standards
| Environment | Required Lux | Foot-candles |
|---|---|---|
| Hallways, corridors | 50-100 | 5-10 |
| Office workspace | 300-500 | 30-50 |
| Detailed assembly | 500-1000 | 50-100 |
| Surgery / fine work | 1000-2000 | 100-200 |
| Direct sunlight | 100,000 | 10,000 |
Luminous Efficacy by Source
- Incandescent: 12-17 lm/W (most energy becomes heat)
- Halogen: 16-24 lm/W
- CFL: 45-75 lm/W
- LED: 80-150 lm/W (best commercial options exceed 200 lm/W)
- Theoretical max: 683 lm/W (monochromatic green at 555nm)
Frequently Asked Questions
Why can't I directly convert watts to lux?
Watts measure total power including infrared and other non-visible radiation. Lux measures only visible light weighted by human eye sensitivity. The conversion depends on how efficiently the source converts electrical power to visible light (luminous efficacy).
What is the difference between lux and lumens?
Lumens measure total light output from a source. Lux measures light intensity at a specific point. A 800-lumen bulb produces the same total light regardless of room size, but lux at the surface decreases with distance and increases in smaller rooms.
How many lux do I need for reading?
Comfortable reading requires 300-500 lux on the page surface. Task lighting should provide at least 500 lux for extended reading. Older adults may need 500-750 lux due to reduced pupil size and lens clarity.