# How do I measure LUMENS? What light meter to use?



## Free (Jun 2, 2003)

I need a light meter that will measure LUMENS. I am finding meters that show footcandles or Lux but haven't seen anything that will measure in LUMENS.

I am not exactly sure I understand the forumula for converting to Lumens or if it would be accurate to do so.

My main purpose for this would be to measure the light output of video projectors but it would be nice to use it on my flashlights as well.

When measuring Lumens, would the correct procedure be to have all of the light from the source fall within the confines of the measuring sensor. In that case the light would have to be very close to the sensor?

If anyone has a recomended meter that will do what I am looking for I would appreciate a link.

Thanks very much /ubbthreads/images/graemlins/smile.gif

Phil


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## shiftd (Jun 2, 2003)

Phil. The device to measure lumens output is several times bigger in MAGNITUDE than the ones to measure lux and ftcd. Dunno why. But they are totally different device, altogether. The device (or one of them) to measure lumen are called integrating sphere, and that was around ~3000, I think. Also, there is no way to correlate lumens and lux. Lux depend on lumen, how you shape the beam, how tight it is, but lumen depend on the buld and etc. It is internally dependent on the source of light while lux depend on internal as well as external factor.


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## PaulW (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

Hi Phil,

I use a Wavetek LM631. I believe most of the meter owners here do also. Try a search on posts going back a year for "LM631."

It will measure lux, the amount of light per unit area. Converting lux (or candlepower) to lumens requires knowing the beam pattern of the light, using a little bit of integral calculus, and having a lot of patience. /ubbthreads/images/graemlins/smile.gif


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## Free (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

Thanks Paul, I will check out that meter.

Now where do I find a calculus professor to help me do the conversion /ubbthreads/images/graemlins/grin.gif

Shiftd, I did a search on Integrating Sphere and WoW! that is a little more extensive than I was planning on but very cool. Here is a link:

http://eetd.lbl.gov/btp/lsr/facilities/integratesphere.html


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## keithhr (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

I did a search a while back on the LM631 that seems to be so popular here and I was wondering if a simple instruction manual comes with the meter? So many times, manufacturers include as little info as possible.


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## shiftd (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

Wow, it really is a sphere afterall
thanks for the link /ubbthreads/images/graemlins/thumbsup.gif

Btw, the LM631 is really a good one. Get one, it is definitely worth it for us flashaholic /ubbthreads/images/graemlins/cool.gif


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## shiftd (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

Keithr, yes. The LM comes with small, yet complete instruction manual.


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## asdalton (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

[ QUOTE ]
*Free said:*
Now where do I find a calculus professor to help me do the conversion /ubbthreads/images/graemlins/grin.gif


[/ QUOTE ]

The method involves numerical integration, with is probably the easiest kind of calculus to do. (You just have to calculate the areas of a lot of little rectangles or trapezoids, and then add them up. Excel works great for this.) The spherical geometry adds a little complication, but not much.


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## Gransee (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

You can measure lumens without an integrating sphere but the accuracy is not the same. Lumen is a measurment of total photon output. To capture all rays, the light source is placed in middle of the sphere. There is still a good amount of math involved (you can't just read the output from the edge of sphere). 

I just posted some pics  of a sphere we commissioned to test our lights:


Peter


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## Free (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

That is really cool Peter, and I bet its not cheap /ubbthreads/images/graemlins/smile.gif


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## The_LED_Museum (Jun 2, 2003)

*Re: How do I measure LUMENS? What light meter to u*

I think just the sphere alone costs around $9,000.00 or thereabouts. Then you have the measuring sensors & other measurement equipment and the computer they need. /ubbthreads/images/graemlins/eek.gif /ubbthreads/images/graemlins/eek.gif /ubbthreads/images/graemlins/eek.gif

My ProMetric can't do Lumens, so no help in that department. /ubbthreads/images/graemlins/blush.gif


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