So when you guys are talking about the efficiency or reflectance of a reflector your talking about how much light hits the reflector and how much goes out the front, missing the reflector.
I thought you were talking about the efficiency of the reflector coating which has to be like 97+%.
No, we’re talking about the reflectivity of the reflector surface.
It is 75-85% for the typical aluminum flashlight reflector.
A precision electroformed reflector with aluminum coating is 92%.
An extremely expensive and delicate silver coating is 97%.
A dielectric stack or cold mirror is even more expensive and approaches 99%.
Yikes! I see a mirror looking finish and just assume it’s more than 85%.
Nope, not even close 
Even the silver coating was unaffordable for me for my custom reflectors I’m ordering.
In order to not react with oxygen it also needs special protection coatings.
The problem with cold mirrors is that they need to be made of glass, so instead of $400 per reflector I would probably be paying $4000.
Another set of zoomie drawings.
First, in the zoomies I drew before I confused some optical parameters. Most importantly focal length with back focal length. Zomming action needs to be much shorter than I anticipated.
Second, I actually used a real lens in the drawings. It was very expensive, but I hoped there wouldn’t be a big difference if a cheaper one was used. I switched to a cheaper one now (though still not cheap, €49 when buying 1 piece).
As you can see, threads are way longer now and I stopped worrying about thermal transfer. The whole light is slightly shorter too.
Since focal lengths can be really short, I wanted to investigate a zoomie suggested before: where the precollimator is the only moving part.
But I haven’t acquired the skills to draw one yet….so instead I drew a non-zooming aspheric with even stronger lens
Side switch, thinner battery tube, ~13 mm longer than the zoomie above. With a pre-collimator it could possibly be even slightly shorter. As it is, even taking into account my drawing’s sloppiness, it should be shorter than B158.
The smaller F number lens you use, the bigger the spot and beam divergence will be.
If you want a tight beam you need to use longer focal length lenses.
Also, in your last image that is a double convex lens, you want to use plano-convex for collimation otherwise it will make a horrible looking spot.
I guess a wide beam is a good thing. After all, throw is about the same, just the hotspot larger.
With made-to-size collar it is not so clear benefit though….the collar collects less light, so intensity improvement is lower. On the other hand efficacy with shorter focal length is better as collar’s recycling efficiency ain’t great.
So it becomes a game of trading throw vs. body length and hotspot size.
OK, thanks for the info. Quick search yields info about spherical aberration, which I don’t know if is applicable to aspherics and stating that plano-convex is better without explaining. Could you give some pointers as to why is plano-convex better?
Also, do you think it’s going to be a big deal in such asymmetric lens?
I have bought aspherical double convex lenses and they had spherical aberration.
In order to collimate properly, you want all the light rays entering the flat side of the glass, because the asphere is already made to have a common focal point.
It seems like having a non-flat back surface counters that effect.
Also, you want the least possible incidence angle into the glass, so not only are small F number lenses bad for this, but having a convex back surface is even worse, and instead of entering the glass a lot of light will be reflected off of it.
Thanks for the info. These particular lenses are AR coated, so reflection shouldn’t be a big problem though.
The ideal incidence angle is 0 degrees regardless if they are AR coated or not.
When you have small F number lenses your angle is increasing a lot around the edges of the lens which causes total external reflection.
The bigger the angle, the less light that enters the lens.
I don’t know if you can even get an aspheric double convex lens to focus at a single point, you will probably need to ray trace the lens to figure out if that is possible because from my real world experience it has not.
Unfortunately I found no free software that can ray-trace aspherics.
You can add the BLF lantern to the BLF projects & donation list too if you wish. I will be making a donation towards its development & work towards its production. :sunglasses:
Zemax has a trial version for a month iirc.
You can probably find matlab code that can do it too, if you have a free license of matlab from a university.
An easy option is just use opticalraytracer and use the parabolic lens surface, it gives a pretty good estimate of the real world performance despite not being truly aspheric.
I’ll try opticalraytracer first and move to Zemax when I get more test cases accumulated. Thanks again.
Back to the topic of moving the precollimator:
If you defocus the usual way, by moving the (precollimator) lens closer to the LED, you end up with light hitting the body. You can try to fix it by turning the body to a mirror. In my ray-traces it didn’t work - I ended up with a high internal reflection in the main lens.
If you defocus by moving the lens farther from the LED, it’s about the same, the light hits the body again, this time by passing on the lens sides. You can make the precollimator larger, but it quickly gets counterproductive. But it can be fixed differently - by making a reflective precollimator assembly.
It will surely produce ring artefacts though…
Some traces:
Done optically right, it will make thermal path much longer as it needs the LED to stay on a long post. But the reflector doesn’t really need to have cover 360 degrees around the LED. We can make a couple of cut-outs that will transfer heat as well as reinforce the body.
BTW, I just checked the expected throw of dedomed SST-40 with a 50 mm diameter 40 mm focal length AR coated lens (99% transmission) using Enderman’s calculator.
I used 170 cd/mm² of the 173 peak that was measured by Köf3 @ TLF. And 2000 emitter lm.
Without collar it gets 268 kcd and 556 lm (all of that in the hotspot).
Scratch the rest, I did the calculations wrong. I’ll fix it in a later post.
OK, summary of calculations:
Without collar the light like noted above gets 268 kcd and 556 lm (all of that in the hotspot).
collar with cheap 75% reflective alu 490 kcd, 1017 lm
collar with electroformed alu 540 kcd, 1122 lm
collar with silver coating 555 kcd, 1152 lm
collar with dielectric coating 560 kcd, 1165 lm
strong precollimator (dropping focal length to 20 mm) 265 kcd, 1196 lm
precollimator + collar with cheap alu 330 kcd, 1488 lm
precollimator + collar with electroformed alu 345 kcd, 1555 lm
precollimator + collar with silver coating 349 kcd, 1574 lm
precollimator + collar with dielectric coating 351 kcd, 1582 lm
Now using uncoated 90% transmissive lens:
no collar 243 kcd, 506 lm
precollimator 219 kcd, 988 lm
precollimator + collar with cheap alu 272 kcd, 1229 lm
precollimator + collar with electroformed alu 285 kcd, 1284 lm
collar with cheap alu 445 kcd, 925 lm
collar with electroformed alu 491 kcd, 1020 lm
How about Osram Oslon Black Flat?
collar with dielectric coating 818 kcd, 495 lm
100 mm FL lens, collar with dielectric coating 1122 kcd, 142 lm
Synios P2720 DMLN31.SG?
100 mm FL lens, collar with dielectric coating 1222 kcd, 30 lm. It wouldn’t be fair to call it a BLF GT killer, would it? 
Notes:
I’m sorry if some feel I’m clobbering this thread…
but anyone for a 400+ kcd 18350 light?
Your lumen numbers don’t make sense to me. The collar will reduce the light output compared to a pre-collimator. You also can’t use both at the same time. The pre-collomiator serves no purpose when a collar is used (unless you put it on top of the collar when using a main lens with very long focal length).
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Detailed measurements regarding the Wavien collar can be found here (in German).
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The Wavien collar collects 75% of the light of a de-domed LED and lets 25% through the hole in the middle. Since the luminance is increased by 120% you get 0.25 * 2.2 * 0.95 = 52.5. Thats how much light exits the collar compared to the total lumens of the LED. Since it is made out of glass and has a dialectric coating, it has a reflectance of >90 (I assumed 95). After that you need to calculate how much of this the main lens catches (most likely all of it) and what its transmission rate is (uncoated glass => 92).
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Some more in-depth knowledge regarding the collar (this comes from mem, the inventor of the collar):
It’s actually not important what coating it has in terms of normal reflectivity raiting. The LED is only really excited by the blue part of the spectrum. So you need a collar that reflects as much blue light as possible in the 450-460nm range.
A few years ago some guys in the German TLF forum tried to build their own collars out of polished stainless steel. It didn’t work very well because stainless steel doesn’t reflect a lot of blue light.
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Some obervations regarding pre-collimators:
There are limits to what a pre-collimator can do. Generally you can expect a pre-collimator to roughly double your otf lumens with a typical aspheric lens without reducing the candela (usually it reduces the effect of imperfections in the main lens so that you don’t actually lose any otf luenens because of transmission losses in the pre-collimator). If you want the highest possible otf lumens you will also need an aspheric lens with very short focal-length, less than F/D=1. If the focal length of the pre-collimator is too short you will get a larger hotspot, but also less candela.
:money_mouth_face: :+1: