Talk about future projects and donation topic

http://candlepowerforums.com/vb/showthread.php?106790-bLu-vs-tLu-IS-confirms-65-conversion-factor

The reason it seems like less than 25% loss when you measure the flashlight lumens is because not all of the light is actually hitting the reflector, about 1/4 to 1/2 of the light exits the flashlight as spill, unaffected by reflector losses.

Maybe newer technology has changed this since then, like with the new nitecore reflectors and stuff like that.
It’s not going to be anywhere close to 90% though, you need electroforming for that.
Maybe like 80-85% at best.

The only real testing being posted so far.
But even their test is not perfect. They didn’t actually test the bulb without the reflector. Bulbs have manufacturing tolerances like everything else. Also, they are using a potted bulb. The base that it is pottet into obviously absorbs some of the light that the bulb reflects towards it’s legs.

So in general it doesn’t make much sense to compare the values the way they did. The should have tested the potted bulb without the reflector first.

Their 65% factor is probably accurate, but only for flashlights with bulbs and uncoated lenses! The lumens of lightbulbs are reduced more compared to LEDs.

Concerning Nitecore:
I would be rather suprised if they actually did something special. To me it sounds like they invented nice marketing terms for the same processes that 4sevens uses (aluminium pvd coating and afterwards a clear coat to prevent oxidation).

EDIT: I found another test. He tested a regulated, modded LED maglite with different reflectors, lenses, with and without bezel. He also tested for a baseline value. His numbers seem a bit off though. Borofloat glass should be around 94%, he measured 98.
The problem here is that one really needs to use a rather accurate luxmeter-sphere (if one doesn’t have access to a real calibrated IS) to get meaningful results. Otherwise you just never know if you measured something useful.

Point is, the reflectance of the reflector is not anywhere near 90%, it’s closer to 80 I would guess.
Electroforming doesn’t only make a more accurate shape, but a lower surface roughness also increases the reflectivity.
Absorption and scatter depend on how accurate the surface is, to fractions of a wavelength of light.

No matter what the actual values are, I think this topic deserves a more in depth Look.

Agree.

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 :frowning:
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. :frowning:

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.