This was very informative. I was thinking more of my 4Sevens QuarkAA2. Even though they changed their advertising, I haven't seen an upgrade in a while. It uses a Cree XLamp XML. I am wondering if a Cree XLamp XML2 would give me brighter light for the same run time....................
Or am I asking a question incorrectly? From the sounds of it, I am attacking the issue from the wrong perspective. Can you make an assumption and help me out here?
Brighter light? Yes. Enough to matter? Well, not especially. A 20% increase in output for the same current is nice, but I would prefer 20% lower current for the same output and +20% runtime myself. That's rarely an option since an LED swap is easy in production, but a driver circuit change is difficult.
The human eye perceives relative brightness just about logarithmically. In theory, this means that you need 4x the output (4x the current, 1/4th the runtime) to appear twice as bright. In practice, it's not quite so simple. But carrying this theory tells me that 20% is noticeable, but not really something to shout over. With a good camera, you can tell the difference. And a camera light that's 20% brighter can make a useful difference in exposure time, or allow slightly better aperture choice.
In practice, human perception of brightness is complicated by a few things:
1. Relative light output. If you have too many lumens at your feet, you won't see the few that get down-range. But we see lux (Lumens per square meter), not lumens. Think of 'lux' as brightness, and 'lumens' as total output (Brightness x area lit). The technical interpretation is, 'Relative lux matters more than lux.' This is why a 20 lumen mini mag lite incandescent is plenty good in the woods, but a 200 lumen Quark might not be enough to see into machines in daylight: Relative lux.
2. Pupil contraction. Outside scotopic vision (Super dark) and the lower range of mesopic vision, pupil contraction is the main 'brightness adjustment' of your eyes. Bright lights make your pupils contract, and dim ones let them expand. This allows less or more light in.
So, in pitch dark with your flashlight: Allow a sudden increase in brightness,
without changing beam pattern (Keeps relative lux the same), will let you see a bit further. Now, quadruple output (Twice apparent brightness) lets you see about 1.4x farther, for a bit, in theory. In practice it will vary a lot. But then your pupils contract if foreground light is enough to require it. So you are draining your batteries extra fast, and maybe not seeing very much better. If there is mixed lighting (Daylight and a dark crevice), +20% output gives +20% lux. This might be enough to get the too-dark area bright enough to see... Or it might not.
There are many caveats and exceptions to these rules of thumb, but it comes out to "+20% output isn't a big deal." So I usually prefer +20% runtime. However, the raw output is not all that has changed with the XP-G2 and XM-L2. Some change in the light distribution has happened, which in the XP-G increases 'throw' (center lux) quite a bit. There, it matters. The effect is less of a bump with the XM-L. And finally, it's not throw to beat a light that is built for throw. But it helps some.
So for a simple LED swap, I'd say, 'Look at reviews comparing them.' And learn how to interpret the numbers some of our experts record. But usually it's more of an enthusiast action than a large technical improvement.
