Measuring light....

[Cthelite]

If I understand lumens and lux correctly, it's the measurement of "brightness". Is this what is referred to as density...?

Is there a measurement for volume...?

 

[Doug Owen]

[ QUOTE ]
Cthelite said:
If I understand lumens and lux correctly, it's the measurement of "brightness". Is this what is referred to as density...?

Is there a measurement for volume...?

[/ QUOTE ]

Well, seems to me at least one of us is a mite confused here.

Lumens and lux are two very different measures. Lumens is a measure of total light output, lux a matter of intensity (at a given distance of course). One could say its a matter of density for lux I guess (photons per square inch per second), how that relates to volume I've no idea.

FWIW, when I was in school, volume was measured in dB......

Doug Owen

 

[phyhsuts]

Lumens is the measure of light quantity. Like lbs for force. Lux is a measure for illuminance, not unlike PSI for pressure. It is a measure of how many Lumens of light falls on a square meter (about 10 sq feet). So, Lumens is a measure of how much light there is and Lux is how brightly lit a surface is. Hope that helps. /ubbthreads/images/graemlins/grin.gif

 

[Atomic6]

If you wander over to the genearl discussion forum here, a page or two back I posted two different reference links. One is a Photonic Dictionary and the other is a link to a website discussing site/light/photons etc. Don't want to cross post-so not giving a link here.

 

[Cthelite]

I'm still confused and I'm sure not for lack of good explanations...

How are lumens and lux seen? Or, can the difference between the two be seen...?

And what ever happened to candlepower...?

I use an example of a garden hose size, pressure and water to visualize the difference between voltage and amps, can the same be done to show lumens and lux...?

Sorry guys, sometimes it takes a bit for some of these things to sink in... if ever... /ubbthreads/images/graemlins/icon23.gif

 

[TCG]

I will give it a shot.

Picture a wax candle. The total light output of an average
wax candle is 1 candle. Now picture a sphere of radius one meter with the candle in the center. The light from the candle that passes through 1 square meter area on the surface of this 1 meter in radius sphere is 1 lumen.

1 lumen = 1/683 watts of radient power. 1 watt = 1 joule/sec

The surface area of a sphere of 1 meter radius is 4pi meters^2. So the total light output from one wax candle is 12.57 lumens.

Lets take the case of a 1 watt ls led that produces 25 lumens of light output. We actually get out 25/683 watts of light energy. So the efficency of the led is only 3.7% The rest of the 1 watt is lost as heat. 96.3%!

1 lux = 1 lumen/meter^2 This is a measure of the intensity of the beam. Lumens is a measure of the total light output (power of the beam (energy/second))

If you integrate the lux reading over the area of the beam you get the total lumens of output.

So a wax candle produces 12.5 lumens of total light output, and at one meter has a beam intensity of 1 lux. At 2 meters, the intensity is 0.25 lux, but the total output is still 12.5 lumens.

Does that help?

If you want a garden hose analogy picture a sprinkler with a large circular patern. Lumens would be like the total water comming from the hose in gallons/minute. Lux would be like the water hitting you in the face. Gallons/minute hitting your face divided by the area of your face.

Todd