USSLT LIGHTING INFORMATION
A foot-candle is a measurement of light intensity. This is a non SI unit, but it can be easily converted into more useful units of measure if you prefer including Lumens and Watts. By knowing a few simple calculations, it should be much easier to work out exactly what the foot-candle measurement really means.
If your light meter only calculates readings in food-candles, then all is not lost because you should be able to easily work out what this means in different units. The calculations are actually fairly easy to understand and use.
Step 1: Measuring the Light Intensity
The first thing that you need to do is use your light meter to measure the number of foot-candles that there are hitting the subject. Once you have this number, you can then use it to convert it into Lumens or Watts depending on what you want to do. For the case of this example, we will base our calculations on a measurement of 50 foot-candles.
Step 2: Converting to Lumens
One foot-candle is equal to 10.76 Lumens. This means that you need to take your foot-candles measurement and multiply it by 10.76. Using our measurement of 50 foot-candles, we get the following equation: 50 x 10.76. By working out this calculation, you will be able to get 538 lumens. A lumen is the same as Lux and is what many modern light meters will calculate their measurement in.
It's a good idea to write the measurement in Lux down because it's very easy to forget no matter how easy it seems to be to remember it.
Step 3: Converting into Watts
The next thing that you can do is to calculate the number of watts. This is very easy to do once you have converted into watts. One lumen is equal to 0.001496 watts. This means that you simply need to take your answer to step 2 and multiply it by 0.001496. In our example, 538 x 0.001496 equals 0.805 watts per square meter. Write this down again, and you can use it to work out anything you want to know.
Step 4: Doing your own Calculations
Now that you know how to convert foot-candles to lumens (Lux) and Watts. it is easy to do the same for your own measurements. Simply substitute 50 for your own measurements.
Calculating and converting is very easy and shouldn't take very long to do once you get used to it. As the number of lumens is just over 10 x foot candles, it's also very easy to estimate the lumens calculations in your head.