How to Calculate Light Intensity

Written by ethan schwartz
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How to Calculate Light Intensity
Light intensity decreases with distance. (light bulb image by sheldon gardner from

Calculating light intensity at a particular point is a basic lab exercise that students encounter in physics class. This calculation is slightly more difficult than other calculations involving light because there are several different ways to evaluate light intensity. The light intensity at a particular point depends on the configuration of the light source and the directions in which it radiates light. The simplest example of calculating light intensity deals with the intensity of light around a bulb that radiates light equally in all directions.

Find the wattage of the bulb. Your lab worksheet may give you this information or you may have to find it yourself. The wattage is usually printed on the bulb.

Measure the distance between the light source and your point of interest. Use metric measurements.

Convert the distance that you measured into metres. For example, if the point at which you want to calculate the light intensity is 81 cm away from the light source, report your answer as 0.81 metres. This value represents the radius of a sphere surrounding the bulb.

Square the value from Step 3. You will use this number to calculate the surface area of the sphere. The surface area of a sphere is equal to 4(pi)r^2. In this example, squaring the radius of 0.81 metres gives you 0.656.

Multiply the answer from Step 4 by 4. In this example, multiply 0.656 by 4 to get 2.62.

Multiply your answer from Step 5 by pi. This answer is the surface area of your relevant sphere of light intensity. In this example, multiply 2.62 by pi to get 8.24. If you have a scientific calculator, use the pi key to do this problem. If you are using a four-function calculator, you can approximate pi as 3.14.

Divide the bulb's wattage by the answer from Step 6. This final answer is given in watts per meters squared. This answer tells you that the light intensity at your point on the sphere is equal to the number of watts that the bulb radiates divided by the surface area of the sphere. If you had a 60-watt bulb in the centre of this sphere, you would divide 60 by 8.24 to get 7.28 watts per metres squared as the light intensity at your point of interest.

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