# How Can I check Lumens Myself?



## Ken_McE (Aug 14, 2005)

I just picked up some "Commercial Electric" brand compact fluorescent bulbs at the Home Depot. The package states that they use 23 watts and produce "100 watt light output". Elsewhere they define 100 watts as 1,600 lumens, which is wrong,(100 watts =1,700 lumens in the US) but I think I'm supposed to let that pass.

However, in service I don't think I'm getting 1,600 lumens, I think it's more like the 1,275 lumens I'd get out of a 75 watt bulb. A direct comparison is difficult because the color of the light is different. How can I check?


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## The_LED_Museum (Aug 14, 2005)

To measure light in lumens, you'd need a large and expensive instrument called an "integrating sphere".
They cost approximately $20,000.00. /ubbthreads/images/graemlins/jpshakehead.gif

As far as I know, nobody here at CPF owns or has regular access to one.


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## Lynx_Arc (Aug 14, 2005)

First thing it fluoros usually have to warm up for a short time to get up to full brightness. Secondly most twisters that equate equivelent wattages to incans list the lumens they output and you can compare that. Lastly they are not IDENTICAL wattages but mostly they fudge and say it is closer to 100 watt than 75 watt so lets call it 100.

I have seen 23 and 26 watt twisters both claim 100watt equivelents and you and I know the 26s are probably brighter. I figure they don't want to admit one bulb at 91 watts and another at 88 watts comparably as selling them would be more difficult.


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## Ken_McE (Aug 14, 2005)

>First thing it fluoros usually have to warm up for a 
>short time to get up to full brightness. 

Good point.

>Secondly most twisters that equate equivelent wattages 
>to incans list the lumens they output and you can compare that. 

Er, they do list it. However I suspect they are factually incorrect. 
What my grandmother would call "lying". How can I tell?

>Lastly they are not IDENTICAL wattages but mostly they fudge 
>and say it is closer to 100 watt than 75 watt so lets call it 100.

When I buy the bulb, can I give them .89 cents and say "let's call it a dollar" ?

>I figure they 
>don't want to admit one bulb at 91 watts and another at 88 watts
>comparably as selling them would be more difficult. 

I have caught bulb manufacturers lying. I don't care for it. 
I think getting what is listed on the label is reasonable. 
However I don't want to go after them if they don't deserve it.


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## Lynx_Arc (Aug 14, 2005)

I have found when it comes to CFLs I tend to get the brighter one than I am replacing, IE replace a 60watt incan with a 75 watt equiv CFL, a 75watt incan with a 100watt CFL. The savings will still be there and usually they are noticably brighter even though they are not necessarily quite as bright as *advertised* I just wish they had 200 and 250watt CFL twisters that fit standard bases (not 3way) for similar pricing. I have a 150 watt equiv CFL twister and it is bright but the 200 watters use Utubes instead and I have yet to see a 250 watt CFL you didn't have to special order.
I typically look at the wattage of the twister and multiply by 4 to get a close estimate of wattage.


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## Zelandeth (Aug 15, 2005)

Hmm, I guess you could get a ballpark figure though. You'd need a moderately large (very) white box (not shiny, just white), a light meter, and your lamp holder (preferably painted white too). 

Effectively you're creating a mini integrating sphere. You'd not be seeing a huge level of accuracy, but it could be used for comparison purposes (you could use high quality bulbs you believed the lumen ratings of for calibration). It'd be good enough that you could prove that your CF lamp is in fact giving out less lumens than your 100W incan.


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## winny (Aug 16, 2005)

[ QUOTE ]
*Ken_McE said:*
Elsewhere they define 100 watts as 1,600 lumens, which is wrong,(100 watts =1,700 lumens in the US) but I think I'm supposed to let that pass.


[/ QUOTE ]

I'm sorry to inform you, but there is NO correlation between power and luminous flux (or wattage and lumensage for US people /ubbthreads/images/graemlins/wink.gif). 
Take a 100 watt resistor, 0 lumen. Period.
If you think that's a bad example, take a 1000 watt halogen bulb and dim it to 60 watt and you will get 10 lumen max. 
Take a standard 1000 hour 60 watt light bulb and drive it to specifications, receive 710 lumen. 
Overdrive it to 150 %, receive 3000 lumen for 100 milliseconds. The last one was a guess, but a reasonable one.

This does not answer your question in any way but I just can't let things like that pass. 

Good luck on your integrating sphere hunt! Please inform us if you gain access to one as many CPFers would find it useful. /ubbthreads/images/graemlins/smile.gif

Kind regards
Andreas


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## Zelandeth (Aug 16, 2005)

I've been planning to try making the crude one suggested above for a while, just for comparative purposes, but as I don't yet have a light meter, it'd be a bit pointless!


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## Dae (Sep 8, 2005)

thanks for the thread, I've been wondering too. a Lux meter is much cheaper, mine is 70USD.


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## Ice (Sep 11, 2005)

Ther is an easy way to at least compare (!) light strenghts.
It won't work well if the light colour is different and it only shows the difference in one spot, but it's better then nothing:
Just take a sheet of paper and drop a little oil (normal salad oil or something like that) in the middle, so that a little part of the paper will become more or less translucent. Then take the two lights you want to compare and put one on each side of the paper. If the two lamps are equally bright, the tranparent surface will be about as bright as the surrounding area. Otherwise the "hole" will look darker than its surroundings on the side of the paper where the brighter light is.
Well, better then nothing, and if the lights have the same colour. it works surprisingly well.


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## balazer (Sep 11, 2005)

winny said:


> I'm sorry to inform you, but there is NO correlation between power and luminous flux


Actually luminous flux is a measure of radiant power in the visible light band. You can convert directly between lumens and watts.


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## dudeldam (Jan 13, 2006)

That´s true, Ice, it is called "Fettfleckphotometer" where I live, and has been one of the earliest methods of comparing light intensities. If you take a standard candle 1m away from the oil dot, you´ll have a source with approx. 1cd (candela=candle) on one side, which is illuminating a sphere with 1m radius. The light flow density through the oil dot is now 1 lux, which is the metric version of the american "footcandle" in a sphere with 1 ft radius. Now adjust your unknown source on the other side to equal brightness. Illumination Intensity is dropping with the square of the distance, so if you measure 8m distance between your light source and the oil spot, you have approx. 8^2=64 cd, but spread over a sphere with 64m^2 surface, so the light flow density through the spot is equal in both directions.

Unfortunately, "standard candles" (which make 1.015cd) or Hefnerkerzen (which have 0.9 cd) are not sold anymore, but a normal white stearine candle will do for home purposes.

More definitions of lightning units youll find under

http://www.schorsch.com/kbase/glossary/luminous_intensity.html

Regards

Dudeldi



Ice said:


> Ther is an easy way to at least compare (!) light strenghts.
> It won't work well if the light colour is different and it only shows the difference in one spot, but it's better then nothing:
> Just take a sheet of paper and drop a little oil (normal salad oil or something like that) in the middle, so that a little part of the paper will become more or less translucent. Then take the two lights you want to compare and put one on each side of the paper. If the two lamps are equally bright, the tranparent surface will be about as bright as the surrounding area. Otherwise the "hole" will look darker than its surroundings on the side of the paper where the brighter light is.
> Well, better then nothing, and if the lights have the same colour. it works surprisingly well.


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## Ice (Jan 13, 2006)

@dudeldam: Where I live it's called Fettfleckphotometer, too. Hallo Landsmann! 

That square law is a very good idea, you would need to know only the brightness of one light source to measure others.
However, there is one considerable drawback:
The square law only applies if the light source is omni directional. It does not apply e.g. to a directional, focused beam of a normal flaschlight. If it was for example perfectly focused (like a laser) the light intensity would not diminish with distance at all (in the vacuum ).
Unfortunately I have no idea how to solve that problem. Maybe you cuold use a brighter standard candle and change the distance between that light and the paper...


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