Calculating

• 1
  One way to calculate illuminance is by starting with the total output of a known source.Jupiterimages/Goodshoot/Getty Images
  
  Determine the total light output of the light source. A 60-watt light bulb, for example, puts out about 800 lumens.
  
  
• 2). Evaluate the light distribution of the source. For example, a household light bulb illuminates pretty evenly, no matter from what angle it's viewed. So the 60-watt bulb in the example has an intensity of 800 lumens / (4 pi steradians). Steradians is a measure of "solid angle". Saying the intensity is 800 lumens / (4 pi steradians) is the same as saying that a light bulb in the center of a sphere would illuminate the entire sphere evenly, which is very close to true.
  
  
• 3). Select a distance at which to evaluate the illuminance.
  
  So, for the example, we could select a distance of 10 feet away.
  
  
• 4). Calculate the angle from the source that would subtend an area of one square foot at the distance selected. With solid angle, this is really easy: just divide the area by the square of the distance from the source. So for the example,
  
  (1 ft^2)/(10 ft)^2 = (1 ft^2) / (100 ft^2) = 1 / 100.
  
  
• 5). Multiply the source intensity by the angle calculated in the previous step. This gives lumens per square foot, which is another way of saying "footcandle." For the example: [800 lumens / (4 pi)] * (1/100) = 8 / (4 pi) lumens = 0.6 lumens into an area of 1 ft^2; so an 800-watt incandescent bulb has an illuminance of 0.6 footcandle 10 feet away.
  
  

Converting

• 1
  A lightmeter provides a direct measurement of illuminance, usually read out in lux.Tom Shaw/Getty Images Sport/Getty Images
  
  Measure the illuminance directly with a lightmeter. Lightmeters frequently provide the illuminance level in lux.
  
  
• 2). Convert lux to footcandles. One lux is defined as 1 lumen per square meter. One square meter is 10.76 square feet.
  
  
• 3). Divide the reading in lux by 10.76. That is the measured light level in footcandles.