About lenses and anti-reflective coatings: a call for stepping up the current technology

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Barkuti
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About lenses and anti-reflective coatings: a call for stepping up the current technology

The increase in output from single layer AR coated lenses is quite small. It is most benefitful for thrower applications, but not so much for flooders. Check the following threads:

There is also another related problem. It has to do with the performance of the usually used single layer λ/4 MgF2 coating itself, centered at 550nm:

 

 

The above image is from the following article: Anti-Reflection (AR) Coatings @ Edmund Optics

As you can see from the graph above, the difference in anti-reflective performance for single layer 550nm λ/4 MgF2 lenses (up to 0.4% for reds and up to 1% for violets) causes Duv increase and overall CRI distortion. And green Sick tint, by the way. 

As far as I see the VIS 0 and the VIS EXT are the best anti-reflective coatings we could use for flashlights. The performance of VIS 0 and VIS EXT coatings is in all respects far superior to the usual, common λ/4 MgF2 @ 550nm; be it for throwers, flooders or whatever.

So, how about asking lens manufacturers to coat their lenses with VIS 0 or VIS EXT? For affordable price, of course. Believe it, my dears. Thanks. Innocent 

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Thanks for posting this. Good information and something I’ve been thinking about lately.

VIS-EXT and VIS-0 look ideal. Even VIS-NIR or UV-VIS would be better.

I would weigh overall transmission rate equally or less than relative transmission. I like VIS-0 and VIS-NIR most for that reason.

I wonder if anyone knows what coatings FlashlighLens.com uses?

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Great idea! I almost always remove the AR coatings from lenses because of the negative impact on Duv and tint.

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Forget about Edmunds-coated lens in a production flashlight.
But there are many manufacturers that can provide quality multi-layer coatings, also in China.
I agree with you that it would be a worthwhile improvement.

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JaredM wrote:

VIS-EXT and VIS-0 look ideal. Even VIS-NIR or UV-VIS would be better.

I would weigh overall transmission rate equally or less than relative transmission. I like VIS-0 and VIS-NIR most for that reason. …

If we are to believe in the faithfulness of the anti-reflection coatings' graph, I cannot agree with the suitability of VIS-NIR or UV-VIS. They would work, I know, but they would induce some other sort of Duv and CRI distortions. Too many up and downs in the VIS-NIR or UV-VIS curves between 400 and 700nm, this is the reason. VIS-EXT and VIS-0, on the other hand, look more faithful, staying at ≈0.5% and below between 400 and 700nm.

Faithfulness matters. 

Agro wrote:
Forget about Edmunds-coated lens in a production flashlight.

But there are many manufacturers that can provide quality multi-layer coatings, also in China.

I agree with you that it would be a worthwhile improvement.

Edmunds coated? Does Edmunds posseses the patents of the above shown coatings? Patents expire, doesn't it?

Now, onto the “there are many manufacturers that can provide quality multi-layer coatings, also in China” thing, I agree.

I was also thinking in multi-layer coatings before I stumbled onto the Edmunds article. So, I've taken some additional time to check the following:

Anti-reflective coating: Guide to Solve Glare Problem on your Product @ WeeTect

 

 

I am no expert in this stuff, but if the above graph is indicative of the effectiveness of a multilayer λ/4 MgF2 coating, it certainly looks good. With three layers at 450, 550 and 650nm the result should be great. Four layers looks very neat. With five λ/4 MgF2 layers at the proper stepping the result would be gorgeous.

Also googled BBAR anti-reflective coating. In Anti-Reflective Coatings – VAR, BBAR, AR Anti-reflective coatings of vacuum optics @ Torr Scientific Ltd. there is a noteworthy coating, the FS-BBAR-397-727

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Average <0.5% reflection over the visible range at 0 degree is a kind of high-standard that many manufacturers reach.
I don’t know how many layers does it take to reach it but I know that each layer is a different material, so MgF2 doesn’t repeat (if it is present at all).

BTW a coating optimized for incidence angles a little higher than 0 degrees would work better with flashlights – because (manufacturing precision aside) 100% of light hits at least one lens surface at angle.

I mean: light coming from straight under the lens passes through the lens straight. But there is so little of it coming exactly straight from the single point that it’s 0% of the total output. If light goes straight towards the lens from any other point – it doesn’t refract on the
first surface but refracts on the second.

With strong aspheric lenses the incidence angles may be quite large even when the lens is focused.
When a light is out of focus, the average angle is large.

With TIRs it’s quite similar.

With windows it’s a bit different, the light going straight ahead from the entire led surface will come out straight as well as some that was bounced off the optics.

But we can’t hope for manufacturers to design custom coatings for us…even if some actually could do this.
And coatings designed to work best at moderate angles are quite rare.

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Quote:
I cannot agree with the suitability of VIS-NIR or UV-VIS. They would work, I know, but they would induce some other sort of Duv and CRI distortions. Too many up and downs in the VIS-NIR or UV-VIS curves between 400 and 700nm, this is the reason.

I agree those two coatings fall into a clear 2nd tier on that plot. I say that because I just assume that better performance = more cost or difficulty. Also, in a perfect world where LEDs didn’t have a tendency towards being green shifted and still having a blue spike, I wouldn’t say that.. but they do. I think the placement of the peaks in these coatings could help reduce the peaks in the output spectrum that deviate from the ideal BB spectrum

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Great post and replies.

WTB Titanium 4sevens 2xAA tube

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so can we really tell a difference between old and new coating with a naked eye?

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JaredM wrote:
Barkuti wrote:
… Too many up and downs in the VIS-NIR or UV-VIS curves between 400 and 700nm, this is the reason.

I agree those two coatings fall into a clear 2nd tier on that plot. I say that because I just assume that better performance = more cost or difficulty. Also, in a perfect world where LEDs didn't have a tendency towards being green shifted and still having a blue spike, I wouldn't say that.. but they do. I think the placement of the peaks in these coatings could help reduce the peaks in the output spectrum that deviate from the ideal BB spectrum

In my experience better performance quite often does not necessarily involves higher cost. For me, great performance and affordable cost is a very real thing. Things change and we evolve, its all in the amount of light we let in. Wink 

Concerning what you say about the peaks in led spectral power distributions, to reduce green and blue peaks the λ/4 MgF2 coating must not be set straight at these frequencies, i.e. not “tuned” for 455nm or 550nm. Maybe a multilayer coating set at 410, 500, 605 and 670nm, or sort  of. Heck, it's almost 10 AM here and I'm yet to go to bed, so we'll see later LoL.

alpg88 wrote:
so can we really tell a difference between old and new coating with a naked eye?

With a really good coating, pretty sure.

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alpg88 wrote:
so can we really tell a difference between old and new coating with a naked eye?

BTW, this is not necessarily a goal worth pursuing but when you look at a lens at angle, you see a colourful residue reflection. That’s how you tell that a lens is coated.

Some lenses have the reflection very weak and white which makes it pretty much invisible.
I remember Dale saying that when he showed his lights with UCL2 lenses, he would often get fingerprints on the lens…because people would feel an urge to check whether the lens was there.

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I question lens thickness too. Every AR coated lens from 64mm diameter and up are 3mm thick. I would prefer 1.5mm or less.

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I understood the UCL and UCLp lens from https://flashlightlens.com/ were superior in that they had less impact on duv and CRI - I think TK mentioned this a while back. Anyone know if that's still accurate?

They currently sell UCL (98-99%) and UCLv3 (98-99%) in glass, and UCLp (97%) in Acrylic. The v3 is AR coated Borofloat

Yes - the UCL and UCLp I've used in the past are super clear, but you can still catch a tinted color in a reflection off the lens.

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Agro wrote:

BTW a coating optimized for incidence angles a little higher than 0 degrees would work better with flashlights - because (manufacturing precision aside) 100% of light hits at least one lens surface at angle.

I mean: light coming from straight under the lens passes through the lens straight. But there is so little of it coming exactly straight from the single point that it's 0% of the total output. If light goes straight towards the lens from any other point - it doesn't refract on the first surface but refracts on the second.

 I better understand now.

The VIS-number thing. The number is the angle for which it is optimized. VIS-45 would be optimal then, doesn't it?

Tom E wrote:

I understood the UCL and UCLp lens from https://flashlightlens.com/ were superior in that they had less impact on duv and CRI - I think TK mentioned this a while back. Anyone know if that's still accurate?

They currently sell UCL (98-99%) and UCLv3 (98-99%) in glass, and UCLp (97%) in Acrylic. The v3 is AR coated Borofloat

I've taken a peek at the Flashlightlens.com website. That UCL™v3 seems great, any tests?

There's a ∅22.6 × 1.75mm available size, that conforms to the lens size listed for the Convoy S21A (∅22.8 × 1.5mm). However, although paying nearly €4 for a lens is a thing I could probably allow to happen, the shipping cost Facepalm to where I live is a whole different story. 

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Barkuti wrote:

Agro wrote:

BTW a coating optimized for incidence angles a little higher than 0 degrees would work better with flashlights – because (manufacturing precision aside) 100% of light hits at least one lens surface at angle.


I mean: light coming from straight under the lens passes through the lens straight. But there is so little of it coming exactly straight from the single point that it’s 0% of the total output. If light goes straight towards the lens from any other point – it doesn’t refract on the first surface but refracts on the second.



 I better understand now.


The VIS-number thing. The number is the angle for which it is optimized. VIS-45 would be optimal then, doesn’t it?


Yes, the number is the incidence angle at which the coating works best.

If you’re concerned about zoomie flood more than throw than yes, 45 would be about as good as it gets. Same with mule. If it’s zoomie with moderately strong (or weak) lens but you care about throw, the actual angles are going to be close to 0 degrees. Overall, something in between should work best.
If it’s a reflector light, beam is going to hit the lens (actually: the window) at near-0 angle. Spill – still less than 45 unless you have a very shallow reflector. I think for reflector lights the optimal option would be near 0. Considering that most coatings on the market are designed to work at exactly 0 – that’s not perfect but close.

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Not aware of any tests on UCL's in general. Not sure if/where there are AR lens tests here on BLF. With all the crazy testing equipment we got here, I can't recall seeing any AR lens tests to show the changes in duv and CRI.

I've only tested for lumens and throw. I've seen maybe 2% to 5% bumps. 5% more typical from regular non AR glass.

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Agro wrote:

Yes, the number is the incidence angle at which the coating works best. … Considering that most coatings on the market are designed to work at exactly 0 - that's not perfect but close.

0K but, does it really matters? Where are those VIS-x lenses, after all.

 

So there is a myriad of AR-coatings out there. 0K, maybe not a myriad  but a nice bunch.

The question is what can we aim to get a chance to see manufactured. Since I guess λ/4 MgF2 is public domain, we could probably get someone interested in manufacturing Question 4-layer λ/4 MgF2 coated lenses. As I see it that coating covers ±40nm very well, and so I would specify the 4 layers to be optimized for 430, 510, 590 and 670nm. That would deliver balanced performing lenses. Or it should, doesn't it? 

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That’s a good question. I’m truly not an optics expert and my intuition is relatively (on my personal scale) weak in this area. I don’t see how layering works honestly. I could only imagine its extremeley irradic and sort of an empirical method of discovery. What I mean is that my assumption is that the behavior of two layers is not an additive function of the two individuals, but rather something significantly different.

I’m going to buy some UCLv3 lenses and see how they look. I’ve only heard great things.

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JaredM wrote:
… I don't see how layering works honestly. I could only imagine its extremeley irradic and sort of an empirical method of discovery. What I mean is that my assumption is that the behavior of two layers is not an additive function of the two individuals, but rather something significantly different.

Irradic? Do you mean erratic?

Reality is created from above to below (as above so below). It all starts with an idea, a thought pattern which is energized and eventually materializes if all its requirements are met. Thus, when something “is not known” it is a lot better to stay in a positive mindset. It's extremely important if the aim is to succeed.

I didn't create the λ/4 MgF2 coating or any of the other ones. I think multiple layers of such coating side by side results in a “flat” reflectivity curve at ≈1.3% reflectance, which is the lowest reflectance value delivered by the coating. It may be argued that ≈1.3% isn't the best thing, but everything between 400 and 700nm at ≈1.3% is excellent consistency; to preserve Duv and CRI consistency matters. For this reason I was speaking about the VIS coatings above, and also about the FS-BBAR-397-727 from Torr Scientific Ltd:

 

Now that I look at it more carefully, if the graph is accurate I'm not that sure. Performs best “in the green”, around 515nm. Despite performing better overall, green is not what we want. 

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I have noticed myself that AR causes change in output tint, never knew it will cause such a shift in duv.
Very useful info, lets get rid of AR coating and use sapphire lenses Cash

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Anti-reflective and lens loss are new domains and I’m trying to understand / calibrate my lightbox.

Just so finished making my own lightbox and I’m using a shutter diaphragm to simplify the flashlight’s different diameter insertion. But as I’m calibrating and checking against some lights, the lens’ loss varied from 8.8 to 14.9%

Some pics:

I find the loss quite high and too much disparity.

Is it normal for such losses? As I was taking the pictures, I saw that at any incident angle the shutter diaphragm wouldn’t show – too much reflectiveness. Without a backlight and some fumbling, I did get that one picture.

And as for the variations of loss, although I take care to position the light dead center and open the diaphragm just a 1mm or so around the light’s window diameter; yet I get varying losses from Convoy S2+ (4500ºK) to Sofirn’s SP33 (V2 – CC driver) (5000ºK).

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Yes. Erratic. Thanks for the correction.

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How do you take care of keeping the reflector tight up against the LED centering piece, or MCPCB without a lens?

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Tom E wrote:

How do you take care of keeping the reflector tight up against the LED centering piece, or MCPCB without a lens?

Huh? I don’t take the light’s lens off. What I call the light’s window is what is colloquially called the ‘lens’ but as this has no focusing properties but a retainer I prefer the term ‘window’.

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Oh, so you are talking about a lens for your fixture, not the lens of a flashlight in: "the lens’ loss varied from 8.8 to 14.9%"

About terminology, I'll still follow lens rather than window, as expressed here: https://flashlightwiki.com/Lenses and here: https://flashlightlens.com/glass-lenses/

 

 

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AR coating basics are simple from the optical theory point of view.

When light passes between 2 materials, part of it reflects and part of it refracts.
How much? If one is interested, Fresnel equations let you calculate that. But I won’t explain that to you as it’s not necessary.
What you need to know that at 0 degree, there are only 2 things that affect the amount reflected:

  • refractive index of the first material
  • refractive index of the second material

Furthermore, reflection is directly proportional to the square of their difference.

So the idea is to introduce an intermediate material with intermediate index.
Light doesn’t reflect between air and glass, but first between the air and coating and later again between the coating and glass.
But magnitude of each reflection is much smaller and the total is smaller as well.

Multilayer coating simply adds more layers to the stack, dividing the difference between refractive indices to even smaller parts.

And there’s no trial and error in coating design. Coating designers know what materials they can use, they know the refractive indices and can easily calculate the final performance.

There are no multiple layers of MgF2. MgF2 has always the same index so adding more layers doesn’t help. You need to use more material to make a multilayer coating.

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Thanks, Agro – detailed explanation of refractive indexes.

But my query is why do I have such differing lens loss through the shutter diaphragm? And the upper value of 14.9% is out of the realm. Yes, there could be as much as 4% going in as well as out (total of 8%).

I looked over the shutter lens and doesn’t seem to have any AR – surprising as this was taken out of a camera lens attachment. Also, I had taken the time to clean all the lenses (shutter & flashlight) with alcohol prior to any testing. My test is rather simple: measure the lux without the shutter and then with the shutter. As for the shutter opening, as the glass plate is some 1mm above, I allow about 2mm larger opening than the flashlight aperture.

Could the difference in LED CRI affect the difference in light transmission? Or for that matter, the tint?

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Sidney Stratton wrote:
Thanks, Agro – detailed explanation of refractive indexes.

But my query is why do I have such differing lens loss through the shutter diaphragm? And the upper value of 14.9% is out of the realm. Yes, there could be as much as 4% going in as well as out (total of 8%).

I looked over the shutter lens and doesn’t seem to have any AR – surprising as this was taken out of a camera lens attachment. Also, I had taken the time to clean all the lenses (shutter & flashlight) with alcohol prior to any testing. My test is rather simple: measure the lux without the shutter and then with the shutter. As for the shutter opening, as the glass plate is some 1mm above, I allow about 2mm larger opening than the flashlight aperture.


It would take refractive index of 2.15 to get this kind of loss. There are also transmission losses, so maybe a little less…but why would they use a glass of this high index? This does not seem like a good explanation.
Maybe there are 2 pieces of glass?

Sidney Stratton wrote:
Could the difference in LED CRI affect the difference in light transmission? Or for that matter, the tint?

Here we get into advanced stuff…
More advanced coating exploits wave phenomena to improve performance at certain beam wavelengths.
I have basic understanding of it but I can’t explain it better than the documents Barkuti linked to.

Yes, coatings actually have different performance at different wavelengths, therefore changing LED spectrum affects transmission. But higher CRI? Higher CRI has the blue peak smaller. If coating is relatively good near this peak, higher CRI will perform worse. If coating is relatively bad there, higher CRI will perform better. CCT should have higher effect on coating performance though.

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Thank, Agro.

I understand basic refractive indexes, and how due to the square in the equation, more smaller steps equals less overall reflection. I was sorta clueless as to the additive properties of the reflectance vs wavelength curves. I’ll give this topic a read sometime.. Currently overwhelmed with other things at the moment.

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So looking at all angles the shutter lens doesn’t show any tint – but the reflectiveness is considerable. I haven’t taken the assembly apart but as you have mentioned, there could be a double layer glass lens. Although the weight is low.

Still seems strange that an XML-2 / 4500ºK @ ~900lm (Convoy S2+) gives 21300 Lux w/out shutter and 18700 Lux w/ shutter. A 12.2% loss.

If I test an SP33v2 (Constant Current buck/boost driver) with an XHP50.2 (6V) @ 5000ºK and driven on High (~1000lm), I get 25500 Lux w/out shutter & 21700 w/ shutter – a 14.9% loss.

I find these values rather high and too much discrepancy to use the shutter diaphragm within the lightbox. I had done other tests and my lowest loss was at 8.8% – reasonable but again, not consistent.

So in conclusion, the shutter fixture won’t be used for my measurements.

There is much learned and referenced here.

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Sidney Stratton wrote:
Anti-reflective and lens loss are new domains and I’m trying to understand / calibrate my lightbox.

Just so finished making my own lightbox and I’m using a shutter diaphragm to simplify the flashlight’s different diameter insertion. But as I’m calibrating and checking against some lights, the lens’ loss varied from 8.8 to 14.9%

Some pics:

I find the loss quite high and too much disparity.

Is it normal for such losses? As I was taking the pictures, I saw that at any incident angle the shutter diaphragm wouldn’t show – too much reflectiveness. Without a backlight and some fumbling, I did get that one picture.

And as for the variations of loss, although I take care to position the light dead center and open the diaphragm just a 1mm or so around the light’s window diameter; yet I get varying losses from Convoy S2+ (4500ºK) to Sofirn’s SP33 (V2 – CC driver) (5000ºK).


.
I had a similar experience with my lumen tubes using a non-coated glass lens, of course on the tubes.
So I use .5mm wire to stand lights on while testing.
Almost impossible to calibrate for low and high lumen measurement.
So I have a 6” × 4” calibrated with ‘maukka’ lights for lights under 3,000 lumens.
And another straight 8” lumen tube for lights over 3,000 lumens that is calibrated roughly with a MT09R and a Manker U21.
There is probably some disagreement about the 8”, but it works ok, not perfectly, but at least ball park figures. LOL
.
6” lumen tubes.


.
8” lumen tube.


.

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