XM-L2 and XP-G2 lux settings

nikosb

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Dec 11, 2014
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I have a Jax-Z1 flashlight and I am doing some modifications to increase the throw. So far I have tried a de-domed XM-L2 U3 and a de-domed XP-G2 S2 LEDs both on Noctigen copper sink pads. At a distance of approximately 26ft (8m) I get a lux reading of 1760 and 2000 for the XM-L2 and XP-G2 LEDs respectively. I was expecting that the XP-G2 would be better for throw but I am not sure about the mere 13% increase. The XM-L2 had a die size of 4mm2​ and at 3A that the Jax-Z1 driver operates it produces about 1000lumens. This results in an die emittance of 250 lumens/mm2​. The XP-G2 had a die size of 1.92mm2​ and at 3A it produces about 800lumens (see emitter test results) . This results in a die emittance of 416 lumens/mm2​ which is 66% higher than that of the XM-L2. If these numbers are correct should I expect to see an 66% increase in lux between a XM-L2 and XP-G2 at the same drive current? Why then am I seeing only a 13% increase?
 

TEEJ

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Jan 12, 2012
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I have a Jax-Z1 flashlight and I am doing some modifications to increase the throw. So far I have tried a de-domed XM-L2 U3 and a de-domed XP-G2 S2 LEDs both on Noctigen copper sink pads. At a distance of approximately 26ft (8m) I get a lux reading of 1760 and 2000 for the XM-L2 and XP-G2 LEDs respectively. I was expecting that the XP-G2 would be better for throw but I am not sure about the mere 13% increase. The XM-L2 had a die size of 4mm2​ and at 3A that the Jax-Z1 driver operates it produces about 1000lumens. This results in an die emittance of 250 lumens/mm2​. The XP-G2 had a die size of 1.92mm2​ and at 3A it produces about 800lumens (see emitter test results) . This results in a die emittance of 416 lumens/mm2​ which is 66% higher than that of the XM-L2. If these numbers are correct should I expect to see an 66% increase in lux between a XM-L2 and XP-G2 at the same drive current? Why then am I seeing only a 13% increase?

It could be because they don't have proportional increases at the same drive current.

IE: A X% current increase in one and an X% increase in the other, would NOT be expected to give the same % in output.

Part of the lux derivation will be dependent upon the ability to focus the collimated light. The larger the effective surface area, the harder to focus the beam tightly to optimize the resultant lux.
 

nikosb

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Dec 11, 2014
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It could be because they don't have proportional increases at the same drive current.

IE: A X% current increase in one and an X% increase in the other, would NOT be expected to give the same % in output.

Part of the lux derivation will be dependent upon the ability to focus the collimated light. The larger the effective surface area, the harder to focus the beam tightly to optimize the resultant lux.

The lumens increase in not proportional to drive current but the lumens numbers I gave are for 3A for both XM-L2 and XP-G2, that is at the same drive current. The XP-G2 has also a smaller surface area so it easier to focus and optimize the resultant lux. Do you mean the increase in lux is not proportional to the increase in die emittance, that is a 2x increase in die emittance does not mean a 2x increase in lux?
 

sven_m

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Jul 11, 2009
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The spot lux are not determined by the emitter size but only the emitter luminance ("surface brightness").
Two LEDs with the same luminance and different sizes yield the same spot lux with different spot sizes.
Measuring luminance would be the best way to find out, but it's not the easiest.

Can you measure the lux in a given distance without the lens?

Finding the cause: In your case it would be interesting to measure the actual current through the LEDs.
Perhaps different LED forward voltages result in different currents?
 
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