# Lux to Lumens, again!



## AJ Botha (Jun 23, 2013)

Hi CPF guys, I just registered here because I am seeking some expert advice please! I have searched your Forum and could not really find the answer I am looking for. 

I am a student and my final project is to develop an LED light capable of producing 5000 lumens. It should also feature solar charging, motion activation and a bunch of other cool stuff. 

To my disposal I have a Lux meter, surprize surprize. I am using the Cree XM L2 LEDs running at 3A, 65 degrees. To pass my project I have to prove that I produce more than 5000 lumens, which I understand as the total amount of light produced. 
I can test my light in a dark-ish room, and 1m from the light source I measure around 1300 lux (4 LEDs). 
Now to convert this to lumens I have to calculate the surface integral of several rectangles, 
and sum these to find the total amount of lumens that my light produce, 
is this statement correct or do I miss understand the lux to lumen conversion?

Thanx in advance.


----------



## Harold_B (Jun 23, 2013)

Depending on the school, you might have access to an integrating sphere in your Physics Department or you might be able to gain access to a local lab with a sphere. You might try contacting Labsphere to see if they will put you in contact with one of their clients. Don't know until you try. One way or another you really should use an integrating sphere. The output of your device is not likely to be uniform which means you are likely to measure a peak or valley in the output profile resulting in over or under estimating the total output.

If all else fails, Google....

http://www.rapidtables.com/calc/light/lux-to-lumen-calculator.htm


----------



## RoGuE_StreaK (Jun 23, 2013)

You say you have 4 LEDs; according to the spec sheet the top-bin XM-L2's @3Amps will be putting out around about 1000 lumens. At the very maximum, that means you only have 4000 lumens to begin with, before losses from reflectors etc. I reckon you need to start with at least 6 LEDs to have any hope of reaching 5000 lumens OTF (out-the-front)


----------



## StarHalo (Jun 23, 2013)

Cooling and solar charging multiple emitters running 3 amps? This will be an interesting-looking light..


----------



## idleprocess (Jun 23, 2013)

Some time back I built a spreadsheet that could roughly calculate lumens based on emission profiles. It was messy and only worked with uniform beam profiles. If you're familiar with the geometry and units, it would not be difficult to devise something similar if you could take lux measurements at known angles.


----------



## AJ Botha (Jun 24, 2013)

Harold_B said:


> Depending on the school, you might have access to an integrating sphere in your Physics Department or you might be able to gain access to a local lab with a sphere. You might try contacting Labsphere to see if they will put you in contact with one of their clients. Don't know until you try. One way or another you really should use an integrating sphere. The output of your device is not likely to be uniform which means you are likely to measure a peak or valley in the output profile resulting in over or under estimating the total output.
> 
> If all else fails, Google....



Hi Harold_B.
I study at the North West University in South Africa, I asked our physics department and they do not have an integrating sphere unfortunately. I know by manually trying to calculate the amount of Lumens there will be some error, but as long as I can prove that I produce more than 5000 lumen I am in the clear, since that is my primary objective. 

Something like this calculator you provided might help me, I will discuss that later after experimenting with it a bit. 
http://www.rapidtables.com/calc/light/lux-to-lumen-calculator.htm


----------



## AJ Botha (Jun 24, 2013)

RoGuE_StreaK said:


> You say you have 4 LEDs; according to the spec sheet the top-bin XM-L2's @3Amps will be putting out around about 1000 lumens. At the very maximum, that means you only have 4000 lumens to begin with, before losses from reflectors etc. I reckon you need to start with at least 6 LEDs to have any hope of reaching 5000 lumens OTF (out-the-front)



I only bought 4 heatsink, turns out they work so well I wish I ordered a few more. Since I will most likely use 5 or 6 LEDs and cant get any more of these heatsinks without paying huge shipping fees, I think I am going to buy a large heatsink like 100mmX100mm and mount all of my LEDs on that.


----------



## AJ Botha (Jun 24, 2013)

StarHalo said:


> Cooling and solar charging multiple emitters running 3 amps? This will be an interesting-looking light..


To make it just that bit more interesting, the light must be IP-65 rated, that means dust and water proof, implicating that only passive cooling can be used, no active cooling since there will hardly be any air flow inside the light. Battery and solar charger will be external. Motion detector must be attached.


----------



## AJ Botha (Jun 24, 2013)

idleprocess said:


> Some time back I built a spreadsheet that could roughly calculate lumens based on emission profiles. It was messy and only worked with uniform beam profiles. If you're familiar with the geometry and units, it would not be difficult to devise something similar if you could take lux measurements at known angles.



This seems like the only option I really have to prove that I produce enough lumens. My current idea is to set the light up, say 2m from a wall, divide the wall into rectangles of say 50cm X 50cm, take the lux measure at the centre and each of the corners for each rectangle, interpolate using somthing like Matlab, convert to an area and then take the surface intergral of the area. That should get me close to what I want. 

Is this logic flawed or am I on the right track? 

Thanx for all the replies, great to have people that can help.


----------



## AJ Botha (Jun 24, 2013)

https://www.dropbox.com/s/mnr05q69sh2ryr6/IMG_2013062124317.jpg





Cant seem to insert the photo, hope this link to my Dropbox works.


----------



## SemiMan (Jun 24, 2013)

AJ Botha said:


> I only bought 4 heatsink, turns out they work so well I wish I ordered a few more. Since I will most likely use 5 or 6 LEDs and cant get any more of these heatsinks without paying huge shipping fees, I think I am going to buy a large heatsink like 100mmX100mm and mount all of my LEDs on that.




100mm * 100mm is not big at all when you are trying to dissipate 50+ watts, think along the lines of something 4 times that size or 200 * 200mm

Semiman


----------



## SemiMan (Jun 24, 2013)

I would see if your professor will allow you to "calculate" the output:

- Measure the drive current
- Use the minimum flux for the given LEDs from the data sheet
- Measure the solder point temp and calculate the junction temp in order to determine the loss in output at that temperature
- Estimate the optical losses, say 10% if you are just using a simple glass plate and the interior is reflective


This is likely to be much more accurate that a direct measurement without the proper tools.


Are you planning to put optics in front of the XMLs?

Semiman


----------



## SemiMan (Jun 24, 2013)

AJ Botha said:


> This seems like the only option I really have to prove that I produce enough lumens. My current idea is to set the light up, say 2m from a wall, divide the wall into rectangles of say 50cm X 50cm, take the lux measure at the centre and each of the corners for each rectangle, interpolate using somthing like Matlab, convert to an area and then take the surface intergral of the area. That should get me close to what I want.
> 
> Is this logic flawed or am I on the right track?
> 
> Thanx for all the replies, great to have people that can help.




Yes on track if no optic is used. If you use an optic, there will be too much variance from spot to spot and you will need a smaller measurement grid and just sum up the grid values.

Of course, you realize 2M from the wall without an optic, you will need a wall that is 7 meters * 7 meters (works much better on the floor).

Of course, you also need to ensure reflections from the wall do not reflect onto other walls and back. The light suspended over the floor of a very large room (auditorium, gym, arena) would be best for doing this.

Also ensure you are not close to the meter when measuring or reflections off you will cause error.



Semiman


----------



## AJ Botha (Jun 26, 2013)

Thanx, when I have it set up in a few days I will post some pictures to ensure I am doing it right! I don't use optics yet and I do have access to a large room with a high roof.


----------



## AJ Botha (Jul 21, 2013)

Hi guys!I finally came around to do the lumen measurement test, please feel free to comment on what you think the accuracy of the test might be!I found a really dark room with no windows, but with a rather low ceiling unfortunately.

Test Procedure:1. Set up LEDs about 45cm from one wall, about 1.5m off the ground
2. Switched the LEDs on and the room light off to decide on a large enough area to conduct the test on, and marked it off.
3. After drawing the outlines for the test, which was about 2.5m high and 2.5m wide;
4. I divided the test area into blocks of 50cm X 50cm, with the center block sub-divided into 9 blocks for increased accuracy at the brightest spot,
5. measured and recorded the lux reading for each block at the centre off the block, staying well out of the way for reflections, 
6. averaged the lux readings over the total area per block, resulting in 512 lux per block, over an area of 6.25 squared meters, resulting in a lumen output of 3200 lumens for 4 LEDs at 3A each, thus giving around 800 lumen per LED.

Some thoughts:1. I was not able to measure all of the light, since the grid would become impractically large,
2. I only took measurements in the center of each block, possible leading to slight over estimation, that should average out nicely with point 1,
3. I kept the lux meters face flat to wall, to ensure that the test is repeatable, but causing additional losses since the lux meters measuring dome shadowed itself, overall I think I produce close to 900 lumens per LED, 

but I am happy to be able to prove that they produce at least 800.

Grid result in lux:




Test:








Photo with camera flash:




Test set up:


----------



## AJ Botha (Jul 22, 2013)

BUMP!

Opinions please gentlemen? 
Can I present this to my lecturer as proof that the LEDs produce at least 800 lumen at 2.95A?


----------



## SemiMan (Jul 22, 2013)

I would like some black cloth to block reflections. I would do an error analysis ... The prof probably will.

Semiman

Sent from my Nexus 4 using Tapatalk 4 Beta


----------



## Fink (Jul 22, 2013)

Subscribing just because this is cool. 

Fink


----------



## AJ Botha (Jul 23, 2013)

Hi Semi Man! 
Where would you place the black cloth to absorb reflections? On the ceiling perhaps? 
And the error analysis to determine if the test would be more accurate if the blocks were smaller or the area larger? And perhaps if the face of the lux meter is always held perpendicular to the light source? 

Flink, haha hope you enjoy!


----------



## znomit (Jul 23, 2013)

AJ Botha said:


> BUMP!
> Opinions please gentlemen?


I would also do a really fine linear measurement across the wall, assume rotational symmetry, calculate lumens(and errors) from that.


----------



## AJ Botha (Jul 23, 2013)

znomit said:


> I would also do a really fine linear measurement across the wall, assume rotational symmetry, calculate lumens(and errors) from that.


 
That sounds like an excellent idea!


----------

