Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Measuring Energy Output 
Name: John
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
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


Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory