UCL vs. mineral glass?

Is there going to be a noticeable difference in light output with a UCL lens vs. a mineral glass one?

The UCL is AR-coated on both sides. Appears about invisible at first glance, especially when mounted in a flashlight already. And my own tests gave something like 12-13% light output increase as measured on a light meter.

Unknown if plain mineral glass provides this kind of light pass-through ability.

Thanks! Using "logic" maybe without the ar coating some of the light would be reflected backwards,thus loosing a little output.What do ya think?

There is a 3-5% loss at each air to glass interface, which you have two of. Plus attenuation in float glass, and even borofloat, or borosilicate.

The UCL is made from borofloat and dual sided AR coating, I'm not sure of the quality that is used in the UCL, but typically you'd see ~2% loss total for this type of setup.

I've never looked up mineral glass, but you'd have ~8% loss minimum if the glass was perfectly clear.

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MR Bulk said:...And my own tests gave something like 12-13% light output increase as measured on a light meter...

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I always thought the 8% figure given on the website was conservative. They seem even better than that.

UCL is not borofloat, borofloat is much more heat resistant, closer to Pyrex than UCL.

I've popped UCL with the Mule concept light, but not borofloat, which was recommended by flashlightlens.com

Bill

You popped UCL? Cracked it?

Happened to me too...
Overtighten the bezel on the lens and turn on a light that makes some serious heat.

The bezel wasnt overtightened, in fact, at the time the lens was free floating a little as the reflector I was using was shaved a little. Overtightening didn't play any part, at least in the ones I popped.

Bill

I've never broken a UCL, but I've chipped several. They chip very easily on the edges.

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NewBie said:
There is a 3-5% loss at each air to glass interface, which you have two of. Plus attenuation in float glass, and even borofloat, or borosilicate.

The UCL is made from borofloat and dual sided AR coating, I'm not sure of the quality that is used in the UCL, but typically you'd see ~2% loss total for this type of setup.

I've never looked up mineral glass, but you'd have ~8% loss minimum if the glass was perfectly clear.

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Newbie, please confirm or refute my belief if you know, I am under the impression that most of the 3-5% loss per interface is due to reflection [hence AR surface treatment is useful]. I have some experimental data that suggests that some of the reflected light in a flashlight application gets out on its second pass after bouncing around inside the reflector/optic assembly.

Yes, typically it is 4% per surface, going in or out.

So, 4% gets reflected going in the first surface(and much greater if it is not perpendicular to the surface).

It arrives at the second surface on the way out, and 4% is reflected there.

Now this 4% gets shot back at the first surface, "going backwards".

This 4% that is "going backwards" to the first surface is then shot back where it came from. But 4% of the 4% is reflected forwards again (which at this point is 0.16% of the light).

So this 0.16% then arrives at the second face for the "second time", and 4% of this 0.16% is reflected back.

So we'd lost 8% but got 0.1664% back, which puts us at 7.8336%.

You can repeat this and get a slightly lower amount of loss.

But way back, we had about 4% bounce when we hit the first surface, going back to the source.

This initial bounce, goes back and hits the reflector for an 20-7% (7% loss for high end vacuum sputtered lab/telescope grade oxide overcoat to protect from oxidation). So lets say average is 10% (being low side for most reflectors) the 4% returning takes the hit in absorption, so we are get to 3.9%, on the way to the light source off the reflector.

Now we have a glass/quartz/saphire bulb surface we have to pass through, or the outside acrylic lens of the luxeon, then another interface from the acrylic lens to the silicone gel, and we finally get to the phosphor.

Then we hit this phosphor and I'd venture to guess see a 30% loss in absorption. Then it is outbound again.

The missing part is there is loss in the glass lens material, in the air, in the acrylic, in the silicone gel. I kinda look at it similar to the loss you see in a wire due to resistance.

Next, the surfaces are not perfectly perpendicular, but look like a mountain range under high power magnification. So you end up with a bit more loss than this simple "math".

The AR coating helps index match the materials, between the interfaces. From a EE perspective, its *alot* like impedance matching.

Lower index materials will have lower reflection, and higher
index materials will have a higher reflection loss. It's one of the reasons that diamonds sparkle so well. They have a fairly high index. Like in RF, where you often have a 50 ohm impedance, in optics, air has a Index of Refraction of 1.0.

Typically most AR coatings will reduce the reflection to under 0.5% This can drop to 0.2% with the skill of the coater, and the number of layers. High end coatings can drop below 0.1% reflection. Visible AR is typically a multitude of extremely thin layers.

From what I've seen, vacuum deposited AR tends to be much more durable than simple sputtered AR.

Silicon Dioxide AR tends to be a greenish hue in the reflection, where Magnesium Fluoride AR will have a purple hue. Other reflected hues are due to the lack of skill, and the coating will not be centered on the visible light spectrum, and for MgF(2), an improper coating will have a red or blue cast.

Just like the amount of reflection off a surface of glass increases as you move off perpendicular, then reaching the critical angle of 35-45 degrees, where you transition to a 100% reflection due to TIR, AR coatings also get more reflective off the perpendicular.

One bad thing about AR is you have to keep it clean. Unfortunately, even skin oil interfers with it's optimum properties. This can be significantly reduced by a final overcoating of hydrophillic/hygrophobic material. I cannot recall off hand which one helps with fingerprints and which one helps with fog.

Now remember that borofloat/borosilicate(crown glass) is water clear, looking edge on, and sodalime float glass (window glass) is green. You'll see additional losses to due the very low cost window glass. The green you see when looking edge on with sodalime, is an additional loss factor. And it significally reduces the multiple bounce light return.

There is a 0.2% typical penalty loss from absorption at each interface where you use a BBAR/HEA anti-reflective coating.

So, yup, I'd sure believe what you may have seen experimentally.

BTW, here is a quick and dirty on AR:
http://www.ocli.com/pdf_files/products/antireflection_coatings.pdf

Thanks for the detailed explaination!

Yikes, I guess it was of a minor length, sorry about that.

Just started thinking and writing, then hit the button at the end.

great info jar, thanks!

Anytime iddibhai!

As we each share a little bit of knowledge we have, the group becomes a little more knowledgeable and the "state of the art" improves.

I'd not be surprised if a number of flashlight manufacturer's monitor this forum for knowledge, which is fine by me. They'll learn also, and "the state of the art" improves, and new more advanced devices show up on the shelves, and since they all can learn, and they do compete, the price slowly drops.

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cy said:
I've never broken a UCL, but I've chipped several. They chip very easily on the edges.

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That's why I'm happy with Flashlighlens' tigerlight UCL lenses which come with a form fitting rubberlike surround. I've dropped Tigerlights with UCL lenses on carpet and outdoors on grass and dirt, with no problems. However I'm not quite prepared to drop mine on concrete, though it would be interesting to see the result.

Brightnorm