# Some assistance with LUX measurements please?



## vestureofblood (Nov 1, 2011)

Hi all,


I recently picked up a lux meter and I want to find out if I am using it correctly.


Here is a basic picture of my setup with the meter set stationary and the light positioned 1 meter away.





I know ideally that light over the table would be off, but on the 200k setting the meter doesn't even register that. 



I let the light run a while and then adjusted it up and down till I got around the number I was expecting.






The meter was set to x100 and this reading sat very stable with no spikes or falls. Later with a few more slight adjustments I was able to peg the meter out to max.


Here are my concerns.

In order to get this reading @ 1M I had to adjust the focus of the lens out of the position where it would actually reach the farthest.

In the position where its the most laser like far away, the reading on the meter is much lower.

So my question is did I just cheat the meter to get 200k or is adjusting the focus like that what your suppose to do?


Another thing is I thought that if I moved back to 2 m I should get 1/2 of the same reading I had @ 1 M correct? Well I tried 2m 5m and 10m and did not find this to be the case it lost more than 1x Lux for each meter. Am I doing something wrong?


My light meter is a Mastech LX1330B

I measured 2400 lux @ 1 M from my Quark AA2 XPG R5 on turbo if anyone else has this light to compare the readings with.

Thanks.


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## xul (Nov 1, 2011)

vestureofblood said:


> I measured 2400 lux @ 1 M from my Quark AA2 XPG R5 on turbo if anyone else has this light to compare the readings with.


I get 2400 beam candlepower. If you know the beam angle we can work backward to bulb lumens and wattage, etc.

With care I still think a digital camera will work as a reflected light lux meter.

For some reason Maxabeam uses 83' vs. 1 meter as their measurement distance and I guess I'd imitate that. You can convert with very high accuracy using 

lamp illuminates a spot with	
2400	lux, E
at	
1	meters, R
or	
1.09	yards
with candlepower, I, is E(R^2)	
takes	
2400	cp


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## Al Combs (Nov 2, 2011)

vestureofblood said:


> Another thing is I thought that if I moved back to 2 m I should get 1/2 of the same reading I had @ 1 M correct? Well I tried 2m 5m and 10m and did not find this to be the case it lost more than 1x Lux for each meter. Am I doing something wrong?


The change in lux has an inverse square relationship with changes in distance. So at 2 meters you should be reading 1/4 the lux value you got at 1 meter.


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## vestureofblood (Nov 2, 2011)

Thanks guys this definitely helps some.

Al, that is a huge load off my mind right there. I was definitely doing it wrong LOL I just assumed I should be getting 50% lux at 2M LOL!

Could you give an example of this for me farther out so I am sure I get it right?

If I am getting 200,000 lux @ 1M then I should be getting 50,000 @ 2M correct? How do I determine what the Lux should be another yard out, I am guessing its not just another division of 4?


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## easilyled (Nov 2, 2011)

vestureofblood said:


> If I am getting 200,000 lux @ 1M then I should be getting 50,000 @ 2M correct? How do I determine what the Lux should be another yard out, I am guessing its not just another division of 4?



I assume the inverse square law means: lux reading = 1/ {distance} * {distance} 

therefore if the distance is 1 meters, the lux reading is in scale
if the distance is 2 meters, the lux reading will be a quarter what it was at 1 meter
if the distance is 3 meters, the lux reading will be a ninth what it was at 1 meter
if the distance is 4 meters, the lux reading will be a sixteenth what it was at 1 meter etc.


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## Al Combs (Nov 2, 2011)

+1 to what easy said about 1/(d^2).

In practice you would multiply your lux meter reading by the square of whatever the distance is to give you the 1 meter equivalent. As an example, say the wall of your shop is 5 meters away from your bench. The lux meter reading on your 2*AA Quark is 96 at 5 meters, so (5*5)*96= 2,400 lux.


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## ma_sha1 (Nov 2, 2011)

vestureofblood said:


> Thanks guys this definitely helps some.
> 
> Al, that is a huge load off my mind right there. I was definitely doing it wrong LOL I just assumed I should be getting 50% lux at 2M LOL!
> Could you give an example of this for me farther out so I am sure I get it right?
> If I am getting 200,000 lux @ 1M then I should be getting 50,000 @ 2M correct? How do I determine what the Lux should be another yard out, I am guessing its not just another division of 4?



I am glad you finally got a lux meter! 

Generally speaking, The higher the CP of the light, the further distance you should use to measure the lux & then convert to [email protected] 1 meter

I usually measure at 0.1X meters if the light is about X Klux. For example, If the light is 10K lux or less, measure at 1 meter directly. 
If light ~50K lux or less, measure at 5 meters. 100K lux, measure at 10 meters, > 1 million Lux, measure at 50-100 meters or more. >10 Million lux, measure at 100 meters or further. 

*Equation* to convert CP from lux reading @ X meters: *CP= "[email protected] meter" times square of X*
Example: Lux meter reads 10,000lux @ 10 meter distance, CP=10,000 x10x10= 1 Million CP

Make sure to check out my lux collection thread, a lux meter will only lead to modding of higher throw lights 
http://www.candlepowerforums.com/vb...ated-Short-Arc-amp-HID-spotlight-Lux-readings

Try not to correlate reading at different distance too much. Some lights, for example aspherical light, converge & then spread, if measure too close, at its beam converging point, it'll get falsely high readings at that distance.


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## vestureofblood (Nov 2, 2011)

Ok I think I have the formula for the inverse square now, believe it or not the part that was escaping me was what to "square" LOL. So it D x D x Lux. 3m x 3m x 20k lux = 180k @ 1m.

I guess if I had ever seen the inside of an algebra book knowing the meaning of * or ^ etc would have caused me to arrive there more quickly 


ma_sha1,

So basically your saying that yes, focusing the beam in super tight @ 1m is just cheating the meter and the way to get a more accurate reading is to focus the light to max length and then take a reading beyond where the beam converges, in the case of this light 10+ meters?


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## xul (Nov 2, 2011)

If you list measured vs. calculated lux readings on Excel and use their correlation function you will probably get an r better than 0.9.
http://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient


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## ma_sha1 (Nov 3, 2011)

vestureofblood said:


> ma_sha1,
> 
> So basically your saying that yes, focusing the beam in super tight @ 1m is just cheating the meter and the way to get a more accurate reading is to focus the light to max length and then take a reading beyond where the beam converges, in the case of this light 10+ meters?



Correct 
However, 10 meter read + conversion might get higher [email protected] 1meter numbers, as lux meter might saturate out at high end when reading very high numbers.
Also, a lot less distance error to read at 10 meters vs. read at 1 meter.


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## Al Combs (Nov 3, 2011)

Excel would definitely be better than a calculator for something like this. Especially if the distance from the flashlight to the lux meter squared happens to be a number not easy to remember. Using Excel is much easier than rearranging the furniture.:naughty: If cell A1 represents the distance in meters between the light and the lux meter and B1 represents the actual lux meter reading, then the formula =(A1^2)*B1 pasted in C1, or any other cell, would give the equivalent 1 meter lux value. Ma_sha1 touched on an interesting point in that at 1 meter, candela or candle power and lux are all basically the same thing. No doubt that has something to do with why most lights have a 1 meter lux rating. Or the point of converting a lux measurement back to its 1 meter equivalent.

Some lights have exactly the same intensity at 1 meter as they do at 100 meters, after you multiply by 10,000 of course. Vestureofblood's Quark would probably fall into this category. If it is just a matter of the beam intensity being beyond the range of your lux meter, neutral density filters are fairly cheap and can achieve the same result as changing the distance. But vestureofblood mentioned a light that could be refocused to give a higher reading in a 1 meter measurement with his lux meter. Normally a flashlight's beam is adjusted to give its tightest focus in the distance where you need extra light the most. Basically if you can refocus the beam to get a higher lux reading, that's how you know you need to test it farther away.



xul said:


> If you list measured vs. calculated lux readings on Excel and use their correlation function you will probably get an r better than 0.9.



I don't think statistical analysis would help give a better correlation to a 1 meter lux value in this case. A PPMC coefficient is only appropriate to linear relationships. Any variance in this equation is caused by whether or not the beam of the light in question is fully collimated at the distance it's being tested. There is no way to predict that with a simple equation. Just test the light as far away as practical.


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## xul (Nov 3, 2011)

Al Combs said:


> A PPMC coefficient is only appropriate to linear relationships.


Yes, the linear relationship is measured values vs. predicted values. That should plot as a straight line. 
The underlying formula can still have all kinds of exponentials, factorials, etc..


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## Al Combs (Nov 4, 2011)

The non-linearity is not that distance squared is a logarithmic expression. The unpredictable variable is whether or not the light source is fully collimated at the distance being tested. If you will a statistical anomaly.


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## xul (Nov 4, 2011)

Al Combs said:


> The non-linearity is not that distance squared is a logarithmic expression. The unpredictable variable is whether or not the light source is fully collimated at the distance being tested. If you will a statistical anomaly.


So what kind of correlation can we expect between measured and calculated values for these lights? I guess this is why Maxabeam makes their measurements at 83'.


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## Al Combs (Nov 5, 2011)

xul said:


> So what kind of correlation can we expect between measured and calculated values for these lights? I guess this is why Maxabeam makes their measurements at 83'.


I don't believe there is any direct correlation between the two. It depends on the light being tested. A bare LED with no optic would follow the inverse square relationship between beam intensity and distance perfectly. Well if it was infinitely small it would. In an actual flashlight the optics change all that. The diameter of the reflector/optic, its focal length and the physical size of the light source all play a role. EasyB's SST-50 spotlight is an excellent visual representation of this fact. For inverse square to hold true at every distance, the beam would have to begin diverging the moment it leaves the light. But you can clearly see in the last three pictures the narrowest part of the beam is perhaps 5 meters in front of his spotlight. The image of the LED die appears sharp on the tower so it's not a matter of being out of focus. If you were to place a lux meter at this 5 meter node, the reading would be too bright to infer accurately by inverse square the intensity level at the tower. The question becomes, how far away do you have to test before this 5 meter node isn't a significant percentage of the result? Perhaps better suited to trial and error than theory. And yes, Maxabeam with its 1° spot and huge reflector is another good example of a light you can't properly test in your basement.


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## ma_sha1 (Nov 5, 2011)

Well said, Al Combs, I couldn't have explained any better :thumbsup:


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## Al Combs (Nov 5, 2011)

Thanks dude. How's the new job going?


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## ma_sha1 (Nov 5, 2011)

Al Combs said:


> Thanks dude. How's the new job going?



Thanks for asking. It's been a while but getting settled down, I found a job in DC, after 2 month, family didn't want to move,had to quit & get another job. Just started another job in CT at beginning of Sept, at least able to commute to home for the weekend.


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## Al Combs (Nov 5, 2011)

Hope all goes well...


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## xul (Nov 5, 2011)

Al Combs said:


> But you can clearly see in the last three pictures the narrowest part of the beam is perhaps 5 meters in front of his spotlight.


Then supposedly the light acts as if it's really a point source 5 m forward of where it actually is. 

I saw an online link on figuring the apparent location of the source because the actual position is modified by the optics. Shoulda' kept it.


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## Al Combs (Nov 7, 2011)

xul said:


> Then supposedly the light acts as if it's really a point source 5 m forward of where it actually is.


Not so much a point source as a greater concentration of rays in the center of the beam. The EasyB beamshot while it graphically illustrates the point, has an element of trick photography to it. The light does diverge continuously from the Fresnel lens to the tower 250 meters away. But the light at the very edge of the beam being relatively faint, falls below the threshold level of pixels assigned by the camera to represent the beam. The same way you sometimes you see a beamshot pointing straight up in the air. It will abruptly end in the middle of nowhere instead of gradually fading away. Not to pick on LightSward, but he came to mind in that context because people keep telling him he needs a new camera. The third shot in his 36" Gorilla thread is a good example of digital "level cramping".


> I saw an online link on figuring the apparent location of the source because the actual position is modified by the optics. Shoulda' kept it.


Was your article perhaps about principal planes?


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## xul (Nov 8, 2011)

Al Combs said:


> Not so much a point source as a greater concentration of rays in the center of the beam. The EasyB beamshot while it graphically illustrates the point, has an element of trick photography to it. The light does diverge continuously from the Fresnel lens to the tower 250 meters away. But the light at the very edge of the beam being relatively faint, falls below the threshold level of pixels assigned by the camera to represent the beam. The same way you sometimes you see a beamshot pointing straight up in the air. It will abruptly end in the middle of nowhere instead of gradually fading away. Not to pick on LightSward, but he came to mind in that context because people keep telling him he needs a new camera. The third shot in his 36" Gorilla thread is a good example of digital "level cramping".
> 
> Was your article perhaps about principal planes?


Thanks for the link. . . I'll have to bone up on this concept to know for sure.


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## AServantLeader (Jan 14, 2012)

I found this thread while web surfing for information about Lumens, Lux, and converting a light source that was rated at Lumens @ 2 meters back to Lumens @ 1 meter.

In addition I was trying to figure out how to determine the Lumen at 1 meter if the light source was farther away from the surface than 1 meter.

I believe I have an Excel spreadsheet all set but would love to have a person the knows this kind of math to verify my work.

I am helping our church set up it's new light requirements for the new location we are moving to. This spreadsheet I have (if correct) will help a ton.

So anybody wanna look it over? I can e-mail the file to them.

Thank you for your time,

Steve


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## Surya P (Mar 4, 2012)

HI,

I was searching for lux meter when I stumble on this post and also it seems LED light need different Lux Meter.
http://www.grainger.com/Grainger/AMPROBE-Light-Meter-6NZG0

Here is one article mentioning the use of the LED capable light meter :
http://lumeniquessl.com/category/general-commentary/

The content of the link :

Another item I have in my care is a GE Model DW-48 light meter, this one a 1940 model. It has a dual purpose. It reads footcandles for general illumination use, and provides F stop data for photographers. This was left to me by my father when he passed, and remains a favorite of my lighting instruments. Interestingly, this meter reads LED light levels just fine. Unlike some of the modern meters I have, this meter remains within a 5%+/- range of true readings. I recently purchased an Amprobe LM-200LED meter for a field device after discovering other meters were giving me odd readings. The Amprobe device is really good for the $100 asking price, and provides reading in line with what I see using a full blown sprectroradiometer. So does the DW-48, reading virtually the same as my Amprobe, and with less variability than my Minolta T10M and Testo meters. I also like the deco styling, metal and glass enclosure. However, its not very useful in low light conditions, as the meter face is not illuminated and black, making it more a curiosity than a functional meter for daily use. Besides, if it gets damaged, it would break my heart, since it is more than a meter to me personally.

There is one professional one but the price...yikes .....US$2,800ish
http://www.light-measurement.com/color-meter/

How much difference accuracy or how much drift can a light meter be if not NIST certified or after a year ?

Thanks guys....


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## yifu (Mar 5, 2012)

Surya P said:


> HI,
> 
> I was searching for lux meter when I stumble on this post and also it seems LED light need different Lux Meter.
> http://www.grainger.com/Grainger/AMPROBE-Light-Meter-6NZG0
> ...


The percentage uncertainty for cheaper lux meters is around 10% either. IMO it's not worth getting the more expensive ones, unless there 's a particular function you're looking for like peak hold for example. It's the same case as multimeters. A cheap multimeter reads the same as a Fluke 99% of the time, except for some current measurements. Not worth the 100 dollars if all you do is take basic li-on voltages or tailcap currents.


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