Surefire U2A modification question

Hi all. Long time member infrequent poster. So I bought a new P4 Surefire U2A off ebay for $77 (nice score!) specifically to use as a camping light. In my research I liked the level spacing particularly in the low end as well as the use of a fairly large fairly deep reflector. My hope was it would be a light with generous spill that would also throw reasonably well. I also wanted long run time so I didn't feel I needed to take a ton of spare cells for a weekend trip. The light is fantastic and did satisfy all of my goals save one. With the cornucopia of emitters available I have become a bit of a tint snob. My favorite at the moment is the 219 with its high Kelvin temp and high CRI goodness. THe P4 in the U2A produces a fantastically smooth beam which I love but lets face it... it is shifted way to the blue end of the spectrum. This brings me to my question. Which emitter would be best to swap to without effecting the beam profile or output but could be had in a better tint? I want same beam, same output, same run time, better color.

I've used both Cree XP-G and XM-L LEDs with excellent results for beam quality. Run times are slightly better because the LED Vf is lower for the given drive current, giving slightly lower power draw. For the XM-L, I used a neutral tint (4A, 4B, 4C, 4D group), which IMO looked very good. To get a good beam, you have to get the LED mounted at the right height relative to the reflector. IIRC, I reflowed the XM-L onto an 0.8mm thick MCPCB from datiLED and thermally glued that to an 0.045" thick copper shim. For the XP-G, I used a 2mm thick, 10mm diam MCPCB trimmed down to 8mm diam (MCPCB thickness was untouched).

Justin Case said:

I've used both Cree XP-G and XM-L LEDs with excellent results for beam quality. Run times are slightly better because the LED Vf is lower for the given drive current, giving slightly lower power draw. For the XM-L, I used a neutral tint (4A, 4B, 4C, 4D group), which IMO looked very good. To get a good beam, you have to get the LED mounted at the right height relative to the reflector. IIRC, I reflowed the XM-L onto an 0.8mm thick MCPCB from datiLED and thermally glued that to an 0.045" thick copper shim. For the XP-G, I used a 2mm thick, 10mm diam MCPCB trimmed down to 8mm diam (MCPCB thickness was untouched).

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Thanks for the reply! In both cases how did the new LED impact output? I thought about XP-G but being more efficent then the P4 I am concerned it would boost output at all levels. I know it sounds wierd but this is something I don't want to do.

Seoul P4 T2 bin is about 80 lumens at 350mA drive. U2 bin is 100 lumens at 350mA drive. I forget what the drive current is when the U2 selector ring is at the min setting. Let's try bracketing things.

For the Seoul LED, a relative luminous flux of 0.25 looks it corresponds to a drive current of about 75mA. That would give either 80/4 or 100/4 lumens, or 20-25 emitter lumens.

For an XP-G, 75mA drive looks like it gives the same relative luminous flux of about 0.25. If you have a T5 bin XP-G at 139 lumens at 350mA drive, then 75mA drive would give 139/4 or about 35 emitter lumens.

Then factor in typical losses (say 1/3) and OTF lumens for the Seoul could be about 13-17 lumens vs. 23 lumens for the XP-G.

If the drive current at the lowest selector ring setting is less than 75mA, then the differences between the P4 vs XP-G would most likely be even smaller.

I have a stock U2 vs an XP-G T5 mod'ed U2 and I don't notice much of a difference in output at the lowest setting. I'd have to run a side-by-side comparison again to refresh my memory, but certainly any differences are not memorable.

Justin Case said:

Seoul P4 T2 bin is about 80 lumens at 350mA drive. U2 bin is 100 lumens at 350mA drive. I forget what the drive current is when the U2 selector ring is at the min setting. Let's try bracketing things.

For the Seoul LED, a relative luminous flux of 0.25 looks it corresponds to a drive current of about 75mA. That would give either 80/4 or 100/4 lumens, or 20-25 emitter lumens.

For an XP-G, 75mA drive looks like it gives the same relative luminous flux of about 0.25. If you have a T5 bin XP-G at 139 lumens at 350mA drive, then 75mA drive would give 139/4 or about 35 emitter lumens.

Then factor in typical losses (say 1/3) and OTF lumens for the Seoul could be about 13-17 lumens vs. 23 lumens for the XP-G.

If the drive current at the lowest selector ring setting is less than 75mA, then the differences between the P4 vs XP-G would most likely be even smaller.

I have a stock U2 vs an XP-G T5 mod'ed U2 and I don't notice much of a difference in output at the lowest setting. I'd have to run a side-by-side comparison again to refresh my memory, but certainly any differences are not memorable.

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I very much appreciate the detailed analysis! You are a bit over my head but I think I get the point. It seems like the XP-G doesn't impact the output, particularly in the low range, as much as I expected. This is good news indeed. just so I understand all of the analysis by "relative luminous flux of .25" do you mean 1/4 lumen (I know that is the unit of luminous flux). Also I did not undrstand the 80/4, 100/4 fractions. Why do you divide lumens by 4 to get emitter lumens?

jsmitty1967 said:

I very much appreciate the detailed analysis! You are a bit over my head but I think I get the point. It seems like the XP-G doesn't impact the output, particularly in the low range, as much as I expected. This is good news indeed. just so I understand all of the analysis by "relative luminous flux of .25" do you mean 1/4 lumen (I know that is the unit of luminous flux). Also I did not undrstand the 80/4, 100/4 fractions. Why do you divide lumens by 4 to get emitter lumens?

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The LED datasheets allow you to estimate lumens output vs drive current by providing a graph of relative luminous flux vs forward (or drive) current. The relative luminous flux is a multiplier value that you apply to the baseline output spec (which for the Seoul P4 and Cree XP-G is the lumens rating at 350mA drive current).

Thus, by definition, the relative luminous flux at 350mA drive for a P4 or XP-G is 1.0. The relative luminous flux at 75mA drive looks like about 0.25 (i.e. 1/4), giving an estimated output of 0.25*baseline, or baseline/4. For the T bin P4, the baseline is 80 lumens. Thus, the output of a T bin P4 at 75mA drive is about 80/4 = 20 lumens.

A similar calculation applies to the XP-G. A T5 bin XP-G is rated at 139 lumens at 350mA drive. That's the baseline. Thus, 75mA drive, with its relative luminous flux of also about 0.25, has an approximate output of 139/4 ~35 lumens.

You can apply the same approach at the high end. For example, the XP-G at the max drive current of 1.5A has a relative luminous flux of about 3.3 (~330%). Multiply that by 139 and the estimated output at max drive is 3.3*139 ~460 lumens. To estimate out the front lumens, take about 2/3 of that figure to account for various losses, giving you about 300 OTF lumens.