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Name: John
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Question:
Hi How can I measure the total joules, per minute, from an led floodlight bulb? I realize it changes as the distance grows. Would a regular light meter help with this?



Replies:
John,

It is Joules per minute per square-meter that changes, not Joules per minute. The Watt rating of the bulb is the Joules per SECOND. Divide this by the area at a specific distance over which the light falls. For a standard light bulb, the light is emitted in a spherical pattern: divide by 4(pi)(radius^2). If the floodlight emits in only one direction, you will have to estimate what portion of the spherical surface is involved. A light meter usually measures such a quantity. As color and light reflected from walls can affect what a light meter reads, the meter only provides an estimate. Of course, the calculation is also an estimate.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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