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

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.

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.

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 :money_mouth_face:

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).

Yes. Erratic. Thanks for the correction.

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’.

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/

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.

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?

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?

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.

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.

:beer:

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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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” x 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
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6” lumen tubes.


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8” lumen tube.


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I like your way of thinking. May just make a wire support instead of using different cardboard washers. Then using some white filler material from some old bedding (comforter) to pack around the light.

I see you have different light meters. Are they of equal measurement quality?

Bought the same Uni-T as your 1st pic. Modded the head to turn 180º. Using Bluetooth for sampling and data transmission. Also, want to experiment outside in frigid winter conditions - more like real-life situation.

So far the lumens conversion of my higher output lights (4000 to 6000 lm) concurs with my previous ceiling-bounce tests - although some 5 to 10% less.

Addendum;

The values I get are not absolute but for my own reference. If I publish any test results, I’ll use the Lux values.

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LOL, I did the same mod on my Uni-T that you did, also use the Bluetooth too.
The meters were tested in this thread. 5 luxmeters compared (april 2018)
The Uni-t did good for its cost.
The trick is to look at the test results and see the variation at the light your testings frequency 3000K, 4000K etc. and the add or subtract the % from your reading.
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Example: Say you are checking a 3000K light with your Uni-t, on the chart at ” Kaidomain K2 host led: Cree XM-L2 3000K 90+ CRI “, checked 116lm vs the bench-marked ” MobiLux Class A ” at 138lm. So that will be 16% added to your reading.
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I could go on a long talk about my dealings with lumen tubes, even tried to stir some input with no luck. LUMEN TUBES, pics, accuracy, opinions Please.
Hope this helps. :beer:

Invaluable info!

So to correct for the tint variation, Djozz’ calibrated standard (Mobilux), I can find a correction factor.

Pity he hasn’t any 5000ºK tested light - my standard.

Since I am not going to design any multilayer coating I guess this is irrelevant, but since MgF2 is tuned for a given frequency to me is obvious that multiple layers of MgF2 would show some combined behaviour, with each MgF2 layer tuned for a different frequency with a different thickness (the optical thickness of the optical coating must be an odd integer multiple of λ/4 - quoted from Edmund's article).

In any case, I think the goal of this thread was to lure or attract a trustworthy manufacturer of lenses willing to make and sell something to beat the usual lenses for sale, lenses which negatively affect tint (Duv) and CRI. Someone willing to sell internationally for affordable pricing, and of course willing to preserve/maintain the quality of its product (even if periodic reviews are necessary).

Expanded polystyrene lightbox, cool. Thanks for showing Sidney, guess that if I were to build a lightbox I'd choose that way (making every penny ;-) count LoL).

If you lay multiple layers of the same material, there’s no index change between them. So there’s no reflection and no refraction between the layers. So in effect it ends up working as a single thick layer.

One technique used to build dichroic mirrors is to have 2 materials alternating and playing with layer thickness, each layer pair tuned to some light frequency. Having many layers this way allows ultra-high reflectivity mirrors…but that’s precisely the opposite of what we want here.

it makes VERY little difference!

(AR coating)

they aren;t cameras!