Very interesting, thanks!
I did that test some time ago :-)
Here I compared two 'identical' white XR-E's, one on an aluminium board with dielectric layer, one on a MT-Noctigon. On copper, at 2A, the output is 10% more than on aluminium, the current at maximum output shifts from 2A to 2.4A, on copper at 2.4A you get 20% more output than on aluminium at 2A.
So, even if the red XR-E is mounted on copper, it will be still way outperformed by the red XP-E
I thought I saw something like that done with a white XRE somewhere and there wasnt much to be gained, but I cant find it..
Anyways, I dont think the XRE will do much better on copper. Certainly not better than the XPE.
Btw., digikey, mouser, cutter etc. stock the XPE2 in red but shipping is too expensive. Maybe we can talk RMM into buying a few and then distributing them? :P
/edit: I thought it was djozz who did the XRE on copper test.. and it was him, he just posted the link. :D
Are you sure the die of the “XR-C” is bigger than XP-E ? Have you tried dedoming the XR-C to see if it can give anything interesting?
I was not precise enough, the die of the XR-C is bigger, as seen through the dome, which is the most relevant to its use, it is what a reflector or lens will 'see'. Upon dedoming the dies may not differ that much ( I think they will still be different though)
Ok 
I second the XP-E2 request. I’d be interested in them if anybody would sell them from say… Washington. :bigsmile: Especially now that the Gen 1’s are out of stock and I’m looking to make another order…
I have been taking notes. 
Thanks for posting your results djozz. Good work.
BTW, your lumen numbers are probably bogus. Lumen numbers are done by weighting light output by the human eye response curve. Your lumen conversion constant was probably derived from measurements of white LEDs. Lux meter readings are also weighted by eye response, but the conversion constant will probably be different for colored and white light.
Comparing your red “lumen” numbers between the LEDs should give a reasonable idea of their relative outputs, though.
Which way would you expect the results to be skewed?
I suspect that the true lumen numbers will be lower. Or maybe higher.
Fair enough.
Visually the red lights appear brighter to me than their lumen numbers suggest.
If anyone has amber to try, I’m always interested in those (for “low/no blue light” use in evening hours)
I give the lux/lumen readings to compare with other leds that are similar in colour, in that relative aspect it is useful , and although it may not be the absolute correct number for all single colour emitters, it does say something of how bright it is perceived (not completely 'bogus' IMO, but a little: yes). I know that it is much to ask of a cheap lux-meter to use the right correction curve so that for all measured wavelengths it is correct for the human perceived brightness (although it would be a simple software correction, I wonder if it is done right, or at all).
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Is it not safe to assume that all colors of XP-E are improved over their XR-C counterparts?
I must correct myself: I assume that light detection in a luxmeter is done with a single detection device. If it does not have the correct sensitivity for different wavelengths, according to the definition of the lux (not likely), any sensitivity correction must be done by optical filters before the light reaches the detector, after the detector the luxmeter does of course not know which part of the signal comes from which colour of light. For 24dollars shipped there's no way my luxmeter has such pre-detector correction (if it exists at all).
Pretty much all lux meters have a silicon photodiode sensor with a greenish filter in front of them. The filter is supposed to match the diode response to the human eye response.
The quality of that filter/match varies quite a bit. If you are looking at wide bandwidth white light, differences in wavelength/output tend to average out. Looking at a narrow band light (like a red LED) has greater potential for a mismatch at that wavelength being significant.
Nice to see that they survived that much current, as my red XP-E SK58 gets too hot to hold (on a 14500).