A foot-candle (ft-cd) is the standard unit of illuminance, or light intensity, in the imperial system.
It's a fundamental concept used in lighting design to ensure spaces are adequately lit.
A foot-candle measures the amount of light that falls onto a specific surface.
To be precise, one foot-candle is the illuminance on a one-square-foot surface from a uniform light source of one candela that is exactly one foot away.
This direct relationship makes the foot-candle an intuitive unit for creating lighting plans in the imperial system.
It's important not to confuse foot-candles with lumens. While lumens measure the total light output from a source (like a lightbulb), foot-candles measure how much of that light actually reaches a surface.
The foot-candle is the imperial counterpart to the lux (lx), which is the standard unit for illuminance in the SI or metric system.
Since many lighting specifications are international, understanding how to convert between them is essential.
The conversion is straightforward:
- 1 foot-candle is approximately equal to 10.764 lux.
- 1 lux is approximately equal to 0.0929 foot-candles.
For quick reference, here's a simple comparison table:
| Feature | Foot-candle (ft-cd) | Lux (lx) |
|---|
| System | Imperial | SI / Metric |
| Definition | 1 lumen / square foot | 1 lumen / square meter |
| Conversion | 1 ft-cd ≈ 10.764 lx | 1 lx ≈ 0.0929 ft-cd |
Using foot-candle measurements ensures that different environments are lit correctly for safety, comfort, and function.
Lighting standards often provide recommendations for various spaces. Here are a few common examples:
- Hallways & Corridors: 5-10 foot-candles
- General Office Lighting: 30-50 foot-candles
- Task Lighting (e.g., Reading): 60-75 foot-candles
- Hospital Operating Room: Up to 2,000 foot-candles