How to measure light intensity

Written by lindsay haskell | 13/05/2017
How to measure light intensity
Cinematographers and photographers, as well as physicists, often need to measure light intensity. (Justin Sullivan/Getty Images News/Getty Images)

Film and photography require specific amounts of light intensity. A light meter, also known as a footcandle meter, can be used to measure the "footcandle" or "lux," "lumens" and "candlepower" of any light. These measurements will help you adjust your flash, camera and lighting aperture for how much light you want in your picture. If you don't have a light meter, you can measure light intensity using a meter stick, as light intensity depends on the shape of the light source and the directions toward which it radiates light.

Control the amount of light that enters your area. Sunlight can vary and give you inaccurate, unbalanced results.

Use your light meter to measure the incident light, which consists of any light that falls on an object directly from the light source. Measure the distance from your light source to your main object by holding your light meter in the path of light as close to your main object as possible.

Measure for reflected light, which consists of external light that indirectly lights an object. Hold your light meter as close to the object as you can. The reading on your meter will tell you how illuminated and light-reflective the object is.

Determine the wattage of the light source of which you need to measure the light intensity. This is typically on the bulb.

Use a meter stick to determine the distance between your light source and an object in metric units. Convert this value to meters. For example, if the light intensity is 64 centimetres from the light source, the light intensity is 0.64 meters. This number is the radius of the sphere of light surrounding the light source.

Determine the surface area of the sphere of light, now that you have the radius of the sphere. The surface area of a sphere is 4(pi)r^2, where r = radius. In this example, squaring the radius of 0.64 meters yields approximately 5.15.

Divide the bulb's wattage by the value you got in Step 3. This will give you the light intensity.

Things you need

  • Light meter
  • Meter stick
  • Calculator

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