Science Buddies: "Ask an Expert"

Hey! I'm doing my science fair project on which type of light bulb (incandescent, compact florescent, halogen, LED, ext) is the most cost efficient. I was planning on just measuring the number watts that the light bulb uses over a period of time, but as I'm doing my research, its saying a lot lumens. Should I measure lumens in my experiment? If so, why? Thanks for clarifying a little about electricity for me.

\Hi,

Here is some information on Lumens:

http://en.wikipedia.org/wiki/Lumen_(unit)

http://www.theledlight.com/lumens.html

It seems to me you want to measure something like lumens per watt so you report the amount of useful light per unit of electricity consumed.

Best Regards,

Barrett Tomlinson

seayoshi wrote:Should I measure lumens in my experiment? If so, why?

Measuring the lumens produced tells you how much light was produced (at a given instance). Measuring how many watts tells you how much electricity was used (at a given instance). The ratio of lumens to watts is a way to compare two different bulbs in their conversion of electricity to light. The reason why you want to measure both the input and the output of any conversion process is to have a means of "normalizing" a comparision. For example, if you compare a 40 watt incandecent bulb with an 80 watt incandecent bulb, the 80 watt bulb will use twice as much power but will it produce more or less light than twice the amount produced by the 40 watt bulb? If you are comparing different technology light bulbs, you can't fairly compare their light output without normalizing them to a lumen per watt basis.

Even then, there are difficulties. If you compare spot light bulbs that have reflectors to concentrate the light intensity in some pattern with A type bulbs that are unidirectional except for the area of their base, you have to find some way of fairly comparing them either against some intended purpose or in total light output. If you use an incident meter to measure the light intensity, the room, its shape, its color, the placement of the bulb, and everything else in the room can affect the reading.