# Using a light meter



## QtrHorse (Jun 16, 2008)

I have a Ideal Sperry 61-681 digital lightmeter. What is the proper way to measure the output of a light? The reading fluctuates so much every inch away from the sensor. The meter only measures in foot candles and lux. What is the correct calculation to convert that to lumens? Everything I Google states foot candles equal lumens 1:1 if measured in a square foot but I'm not sure what that means. Do I need to have the light 1 foot away from the sensor, make sure it is lighting up one square foot of area or is there another correct procedure to measure the light output?

I'm trying to check the lumens output of a Ebay P60 drop-in that I bought for my Surefire 6P as well as a few other lights that I have.


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## Probedude (Jun 16, 2008)

I'm interested too! I just received an Amprobe LM631A on Friday and am itching to try it out.

Dave


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## darkzero (Jun 16, 2008)

Lux is usually measured at 1 meter. I always take two measurements, peak lux @ 1 meter, & peak lux @ 1 meter after the light has been running for 2 mins. I read that somewhere here.

Measuring foot candles is closer, like 1 foot or something I forget.

You can not convert Lux into lumens, they are different type of measurements. Lux is the measurement of whatever the light sensor is reading, generally just the hotspot. Lumens is a measurement of the entire light output as in the hotspot & spill. You need an integrated sphere to measure lumens.

So you could have a P7 that puts out way more lumens than a single P4 but the P4 could give you a higher lux reading since it can focus into a tighter hotspot.

I never really use my light meter to compare the brightness of two different lights (unless they're the same type). I mainly use it for measure the difference in lux before & after upgrading a light.

There's a sticky about light meter reading. Maybe it will be helpful? https://www.candlepowerforums.com/threads/94232


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## QtrHorse (Jun 16, 2008)

Ok, so I measured 4122 foot candles at one foot. What does that convert to in lumens if there is a calculation? Would it be around 190?


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## darkzero (Jun 16, 2008)

QtrHorse said:


> Ok, so I measured 4122 foot candles at one foot. What does that convert to in lumens if there is a calculation? Would it be around 190?


 
Read the post before yours.....


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## precisionworks (Jun 16, 2008)

> You need an integrated sphere to measure lumens.


You might want to build one ... not hard to do, under $50 in materials. See the thread in this section.


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## QtrHorse (Jun 16, 2008)

darkzero said:


> Read the post before yours.....


 
I did but obviously not completely or I would have seen the part about the sphere. Thank you for the explanation. I'm going to look for a lumen light meter now.

I also recorded a Lux reading of 5100 for that same light at 1 meter.


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## BVH (Jun 16, 2008)

When you find a Lumen measuring light meter, let us know. There's probably 4,000 of us here on CPF that would like to buy one.


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## QtrHorse (Jun 16, 2008)

BVH said:


> When you find a Lumen measuring light meter, let us know. There's probably 4,000 of us here on CPF that would like to buy one.


 
I'm assuming that means there is no such thing or it's almost impossible to find?

How do all the light manufactures get the lumen output on their lights? There are a lot of garage type drop-in makers who list lumen output as well. Are they just guestimating a lumen output?


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## darkzero (Jun 16, 2008)

QtrHorse said:


> I'm assuming that means there is no such thing or it's almost impossible to find?
> 
> How do all the light manufactures get the lumen output on their lights? There are a lot of garage type drop-in makers who list lumen output as well. Are they just guestimating a lumen output?


 
With an integrated sphere....

https://www.candlepowerforums.com/posts/2493028&postcount=4

https://www.candlepowerforums.com/posts/2484776&postcount=947

Lots of makers just list when the lumen output of the emitter is rated at or like you said, guestimating.


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## QtrHorse (Jun 16, 2008)

darkzero said:


> With an integrated sphere....
> 
> https://www.candlepowerforums.com/posts/2493028&postcount=4
> 
> ...


 
Thank you;

I just went back and read a lot of the sellers listings. Most of them do state the following or a variation of it, "This LED is rated at 240 lumens at 1000ma". I honestly never really thought much of it until now that I actually want to know what the lumens is and find out it's not as easy as going out and buying a $150 lightmeter.


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## Probedude (Jun 16, 2008)

darkzero said:


> Lux is usually measured at 1 meter. I always take two measurements, peak lux @ 1 meter, & peak lux @ 1 meter after the light has been running for 2 mins. I read that somewhere here.



Thank you!


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## precisionworks (Jun 17, 2008)

> There are a lot of garage type drop-in makers who list lumen output


As well as quite a few eBay sellers that haven't a clue about luminous flux.


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## QtrHorse (Jun 17, 2008)

precisionworks said:


> As well as quite a few eBay sellers that haven't a clue about luminous flux.


 
Correct, just like the Ebay listing for the Solarforce 290 lumen drop-in that I bought. I know it is no where near that output but I just wanted to check to see where it was at.


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