To determine lux, we need to know: the intensity of the light source (I) and the area of the surface (A) being projected upon. There are two generally accepted ways to measure lux: lumens per square meter and footcandles. Also, you may need to work in other units, so for these instances: 1 footcandle equals 1 lumen per square foot and 1 lux equals one footcandle x 10.76.
Calculating Lux for a Perpendicular Surface
To determine lux for the floor of a room, we must first measure its area. In this example, we find it to be 100 meters^2. Next, we need to find the intensity of the light source. The packaging of most light bulbs has this listed; in our example we'll use the value 1,500 lumens. Therefore, using the formula: lux = I / A = 1,500 lumens / 100 meters^2 = 15 lux.
Converting to Footcandles
If we need to convert to square feet or footcandles, we simply use the conversion factor: 1 footcandle = 1 lux / 10.76. So for our example, 15 lux / 10.76 = 1.39 footcandles or 1.39 lumens per square foot.