Lux vs Footcandle
Lux and footcandles are both units of illuminance, measuring the amount of light falling on a surface. Lux (lx) is the SI unit, defined as one lumen per square meter. Footcandle (fc) is the imperial unit, defined as one lumen per square foot. The footcandle is still widely used in North American lighting standards, while lux is used internationally.
Illuminance is distinct from luminance (brightness of a surface) and luminous flux (total light output). Understanding these distinctions is essential for proper lighting design, where the goal is to provide appropriate illuminance levels for different tasks and environments while minimizing energy consumption and glare.
Conversion Formula
Lighting Standards
| Space | Lux | Footcandles |
|---|---|---|
| Parking lot | 50 | 5 |
| Corridor | 100 | 10 |
| Office (general) | 300-500 | 30-50 |
| Detailed work | 500-1000 | 50-100 |
| Surgery | 10,000-100,000 | 1,000-10,000 |
| Direct sunlight | 100,000 | 10,000 |
Frequently Asked Questions
How do I measure illuminance?
Use a lux meter (also called illuminance meter or light meter), which contains a photodiode with a filter matching the human eye's sensitivity curve. Place the sensor at the work surface facing upward to measure the light falling on that surface. Take readings at multiple points and average them for a representative measurement.
What illuminance do I need for an office?
The Illuminating Engineering Society (IES) recommends 300-500 lux (30-50 fc) for general office work, 500-1000 lux for detailed tasks like drafting or accounting, and 100-200 lux for break rooms and lobbies. These guidelines help balance visual comfort, productivity, and energy efficiency.
Does more light always mean better visibility?
No. Excessive illuminance causes glare, visual fatigue, and wasted energy. The key is appropriate illuminance for the task, good uniformity (no dark spots or harsh contrasts), proper color rendering (CRI), and minimal direct and reflected glare. Quality of light matters as much as quantity.