Re: how do they measure the lumens of a light bulb?

Dear Dan, 

It sounds like you have a pretty good idea of what a "lumen" is.  It is, 
indeed, a measure of the intensity of  light.  Encyclopedia Britannica 
defines a lumen as "unit of luminous flux, or amount of light, defined as 
the amount streaming outward through one steradian (a unit of solid angle, 
part of the volume of space illuminated by a light source) from a uniform 
point source having an intensity of one candela (1/683 watt)."  This means 
that if you measure the amount of light on a given surface at a given 
distance from the light source, you can calculate the lumens for that light.  
Typically, a manufacturer will use a photometer to count the amount of flux, 
or photons (the smallest bits of light), hitting the photometer's 
photodetector.  They know how big the photodector is and they can measure 
the distance that the photodector was from the light source. So, let's do a 
thought experiment:  Suppose we have a photometer that is detecting a flux 
of 5 lumens.  The photodetector is a small square that is 1 cm x 1 cm and 
the detector is placed 10 cm away from the light bulb.  The portion of the 
solid angle covered by the detector is approximately the area of the 
detector divided by the surface area of a sphere with a radius of 10 cm.  
This would be (1 cm x 1 cm)/(10 cm ^2 x 4 x pi), which is approximately 1/
1200 of the total area.  Since we measured 5 lumens for that angle, we can 
now just multiply times 1200 and get the total lumens of the bulb as 5 X 
1200 = 6000 lumens - a very bright bulb, indeed!  Now, the reason that 
different bulbs of the same wattage have different lumens is that each 
manufacturer uses a different material for their lighting element.  The 
wattage only refers to how much energy is used by the light bulb - not how 
much light it emits.  That is why flourescent light bulbs, like you probably 
have in your classroom, put out a lot of light and use only 40 watts, where 
a 60 watt incandescent light bulb puts out much less light.  The 
incandescent bulb turns a lot more of the energy into heat which is 
basically wasted, and, in the summertime, must be removed by an air 
conditioner.  Can you figure out why most large buildings use flourescent 
light bulbs?  

If you would like to know more, just email me at tjamison@observera.com

Todd Jamison
Senior Scientist
Observera, Inc.