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Thread: Anti-reflective coating question.

  1. #1

    Default Anti-reflective coating question.

    How do I know what side of a len is AR coated to put it in the right way after cleaning?

  2. #2
    *Flashaholic* Gunner12's Avatar
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    Default Re: Anti-reflective coating question.

    AR coating facing towards the emitter.

    The AR coating is usually delicate and can be damaged easily.

  3. #3

    Default Re: Anti-reflective coating question.

    Quote Originally Posted by Gunner12 View Post
    AR coating facing towards the emitter.

    The AR coating is usually delicate and can be damaged easily.

    Thanks! I meant after taking the light apart to clean and don't remember which way to put the len back in. If I put it in wrong way I'll scatch the coating but after looking very hard, even using 10x magnifier, I still don't know which side facing the emitter.

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    *Flashaholic* Gunner12's Avatar
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    Default Re: Anti-reflective coating question.

    Oh as in seeing which side is the AR coating side?

    I don't know much here.

    Maybe see which side reflects less light?

    I do recall reading somewhere that the AR coated side seems a bit oily compared to the non AR coated side.

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    Flashaholic* Lite_me's Avatar
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    Default Re: Anti-reflective coating question.

    I was about to ask the same question. After recently disassembling my EX10, I made sure I knew which side was where for reassembling. But I'm not sure it really mattered on this light. This is what it says on the NiteCore website for the D10 and EX10.

    "Impact-resistant optical lens with Dual-CoatingTM technique;"

    Says nothing about a scratch resistant coating on the exterior surface.
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    Default Re: Anti-reflective coating question.

    The best way might be to put a finger print on one side. If it's the AR coated side the finger print will be very visible. If it's the non-coated side the finger print will be much less visible.

    You will have to clean it again though.

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    Default Re: Anti-reflective coating question.

    single layer ar coatings have a characteristic blue reflection, where as double+ coatings usually have a greenish reflection. it also depends on the ar material and thickness as well. it can be titanium dioxide, silicon nitride etc, but mostly it is magnesium fluoride/single layer.
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    Flashaholic cdosrun's Avatar
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    Default Re: Anti-reflective coating question.

    I had the same problem; if you look at the lens with a light source on it at an oblique angle you will be able to identify two reflections, one coloured and the other not (colour based on the type of coating as described above). When you are looking at the AR coated side the coloured reflection will be closest to the light source and vice versa.

    The first reflection (the one nearest the light source) is coming from the top surface and the second reflection is coming from the far side and has obviously travelled further through the glass and appears further from the source. I bought a sapphire lens and had read that the AR coating comes off extremely easily so I wasn't prepared to touch it and above worked.

    Good luck.

    Andrew

  9. #9

    Default Re: Anti-reflective coating question.

    The AR coatings should be on both sides of the glass. Don't confuse single versus multilayers as single sided on the glass. You can have a single layer of coating on each glass surface. You can have multi layer coatings on both sides of the glass. It wouldn't do much good to just coat one side. Light reflects off of glass at each and every glass to air interface. So if they coated the surface facing the light source, when light passed through the glass to the interface with air facing the outside, there would still be a big bounce back of light. Typically 4% reflection versus less than 1% and possibly down to 0.4% per surface for a good multilayer coating.

    So it should not matter which direction you put the glass back in.

    You want to clean it with only Isopropyl Alcohol and putting fingerprints on it to see which side is which is not a good idea.

  10. #10
    Flashaholic* LED-holic's Avatar
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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by MrGman View Post
    The AR coatings should be on both sides of the glass. Don't confuse single versus multilayers as single sided on the glass. You can have a single layer of coating on each glass surface. You can have multi layer coatings on both sides of the glass. It wouldn't do much good to just coat one side. Light reflects off of glass at each and every glass to air interface. So if they coated the surface facing the light source, when light passed through the glass to the interface with air facing the outside, there would still be a big bounce back of light. Typically 4% reflection versus less than 1% and possibly down to 0.4% per surface for a good multilayer coating.

    So it should not matter which direction you put the glass back in.

    You want to clean it with only Isopropyl Alcohol and putting fingerprints on it to see which side is which is not a good idea.
    Thanks for the info. Is this how eye glasses are coated as well, on both sides?
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    Flashaholic* Yoda4561's Avatar
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    Default Re: Anti-reflective coating question.

    No, normally only single sided. The primary purpose of the AR on eyewear is to prevent reflections light reflected off your face or behind you from obscuring visual clarity, single sided AR coatings do that fine. Double sided coatings do increase light transmission slightly but aren't necessary in most cases. Surefire also only coats their pyrex lenses on one side.

  12. #12

    Default Re: Anti-reflective coating question.

    Yes, there is no point in coating one side only. Any thing that requires AR coatings usually has it on all glass to air interfaces.

    The AR coatings on plastic are much easier to remove than on glass, not because of the coating so much as the plastic lens.

    Isopropyl Alcohol and cotton or polyester wipes are the only way to go. G

  13. #13

    Default Re: Anti-reflective coating question.

    Duplicate post, System locked up and was not showing it going through but yet I get 2 for the price of one.
    Last edited by MrGman; 07-05-2008 at 11:33 AM. Reason: Duplicate post

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    Default Re: Anti-reflective coating question.

    I've also used the reflection technique described above to identify the coated side(s). I've seen both single sided and fully coated lenses, my SF L2 only has the coating on the inside.

    Actually, IMO, there is little reason to have an AR coating on the outside as it will eventually rub away without delicate handling and (IIRC from some testing done here on CPF) really has little or no effect on out-the-front lumens as light is primarily lost through reflection as it enters the lens as opposed to refraction as it exits.

  15. #15
    Flashaholic* Yoda4561's Avatar
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    Default Re: Anti-reflective coating question.

    Pretty much, the only time you really need double sided AR is with multi-lens optics (cameras, scopes, binoculars, etc). Some prescription eyeglasses use doubled sided as well but that's mostly for cosmetic reasons (prevents glare from obscuring the face).

  16. #16

    Default Re: Anti-reflective coating question.

    Quote Originally Posted by AvidHiker View Post
    I've also used the reflection technique described above to identify the coated side(s). I've seen both single sided and fully coated lenses, my SF L2 only has the coating on the inside.

    Actually, IMO, there is little reason to have an AR coating on the outside as it will eventually rub away without delicate handling and (IIRC from some testing done here on CPF) really has little or no effect on out-the-front lumens as light is primarily lost through reflection as it enters the lens as opposed to refraction as it exits.


    I know a whole bunch of optical engineers who would disagree with you. Doesn't matter which way light comes through the glass, when it hits an interface to air without a good AR coating, 4% reflects back. It is the difference in the refractive index of the glass to that of air that causes the reflection, that doesn't change or matter as to whether it comes through the glass first to the air or the other way around.

    Also, Most AR coatings on glass actually improve its scratch resistance as well. It doesn't wear off all that easily but certain chemicals will attack the bond and remove it. And as some one who has actually had to polish AR coatings off of glass because the Op eng's decided to bond the surface to another optical piece and didn't want and need the AR coating there anymore, its not easy, and I was using diamond suspension polishing compound. Couldn't use chemicals because it would unbond the individual pieces that made this one optical component.

    Acetone is one of the worst things to clean AR coatings with at it will remove it. And of course this dissolves a lot of plastics as well.

    It is a 4% loss at each air/glass interface without a coating. If the vendor doesn't care about the loss it saves them money.

    If you have seen glass windows with only one coating they went cheap. You may not notice it in a flashlight but there is a loss.

    So I won't say that there is no single side coated glass pieces out there, but its not optimum, just saves money.

  17. #17

    Default Re: Anti-reflective coating question.

    Thanks for your inputs! Wow, I didn't expect this complicated about coating and the tricks to identify the AR side. I'll try the ways posted here and hope I'll "see" the difference.

  18. #18
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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by MrGman View Post


    I know a whole bunch of optical engineers who would disagree with you. Doesn't matter which way light comes through the glass, when it hits an interface to air without a good AR coating, 4% reflects back. It is the difference in the refractive index of the glass to that of air that causes the reflection, that doesn't change or matter as to whether it comes through the glass first to the air or the other way around.
    So what's your point? I don't see how this means that there is no advantage to having a single-sided AR coating.

    Quote Originally Posted by MrGman View Post
    Also, Most AR coatings on glass actually improve its scratch resistance as well. It doesn't wear off all that easily but certain chemicals will attack the bond and remove it. And as some one who has actually had to polish AR coatings off of glass because the Op eng's decided to bond the surface to another optical piece and didn't want and need the AR coating there anymore, its not easy, and I was using diamond suspension polishing compound. Couldn't use chemicals because it would unbond the individual pieces that made this one optical component.

    Acetone is one of the worst things to clean AR coatings with at it will remove it. And of course this dissolves a lot of plastics as well.

    It is a 4% loss at each air/glass interface without a coating. If the vendor doesn't care about the loss it saves them money.

    If you have seen glass windows with only one coating they went cheap. You may not notice it in a flashlight but there is a loss.

    So I won't say that there is no single side coated glass pieces out there, but its not optimum, just saves money.
    I wasn't speaking in absolutes, simply relaying my understanding of this issue from what I've read on CPF. My recollection of college physics is admittedly a little fuzzy, but I was never professing to be an expert. The fact remains that its quite common to find flashlight lenses with AR coatings only on the inside. I gather you would claim that this is to save cost, but I have to question how much cost is actually saved and whether or not durability is also a factor. As a materials engineer, I appreciate the difference between hardness and durability. IN MY OPINION, hardness means pretty little if a common solvent can easily remove the coating.

    For example, my McGizmo S27PD was built with a cost-no-object approach to creating a durable lighting tool. The window is sapphire and has a single-sided AR coating. I don't see any reason for Don to have made this engineering decision apart from durability. The same holds true for my Surefire L2, not a cheap light either.

    My point was made in response to your claim that, "It wouldn't do much good to just coat one side". I didn't feel that to be an accurate statement, so unless you can back it up with some form of data I'd suggest you change your thinking on this. The fact is, although it may not be optimal, in many cases this design choice is quite practical and therefore common. My question to you would be: if there is little reason to use a single-sided AR coating, then why are they so common?

  19. #19

    Default Re: Anti-reflective coating question.

    "My point was made in response to your claim that, "It wouldn't do much good to just coat one side". I didn't feel that to be an accurate statement, so unless you can back it up with some form of data I'd suggest you change your thinking on this. The fact is, although it may not be optimal, in many cases this design choice is quite practical and therefore common. My question to you would be: if there is little reason to use a single-sided AR coating, then why are they so common? " (AVIDHIKER)

    You are speaking of feelings and your opinions as to what are the reasons vendors do what they do and what is common or not. My inspection of prescription eyeglasses with AR coatings is that they are on both sides. My discussions with the people who are in the business is that eyeglasses are coated on both sides. People don't normally splash acetone around their faces, but they are always wiping their glasses off on something they should not.

    My inspection of the glass windows in front of flashlights that have coatings is that they are on both sides. That doesn't mean I have inspected every one out there. If you have seen some that are not on both sides, that doesn't prove that it is "common" occurrence. Regardless of how you feel and your opinion; without an AR coating on each glass to air interface 4% of the light is reflected back per side. That is a fact, not subject to how you feel, If the vendor who makes a certain type of window didn't think it was necessary to coat both sides for a flashlight source, I am not disputing it, I am just saying it would have been more efficient to coat both sides.

    Here is an example.


    Sapphire Optics can be customized for critical applications.

    Meller Optics, Inc.
    Providence, RI 02940
    Dec 18, 2007 Featuring 9 Moh surface hardness, Custom Sapphire Windows are unaffected by corrosive fluids, abrasive particles, temperatures up to 1,000°C, and pressures up to 10,000 psi. Units range from Ľ-10 in. in diameter and from ˝ mm to 1 in. thick, making them suited for viewing inside pipes, pumps, vessels, or instruments containing caustic fluids. Windows provide 270 nm to 4.7 µm range of transmission, with up to 85% transmission uncoated and up to 99% with A/R coatings on 2 sides.

    Why would they waste the money, time, and effort, to coat both sides of the window if it wasn't an improvement? There are many more examples.

    My point is simple if the vendor truly wants maximum light efficiency out of the glass, they would AR coat both sides. If you need more proof google it.

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    Default Re: Anti-reflective coating question.

    You do get better performance (light transmission) with AR coatings on both sides of the lens. I'm not sure why we're arguing about this.

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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by Stereodude View Post
    You do get better performance (light transmission) with AR coatings on both sides of the lens. I'm not sure why we're arguing about this.
    I don't think this is really much of an argument, more of a derailment.

    The fact is, early on in this thread, MrGman made what I interpreted as a blanket statement suggesting that it was essentially pointless to AR coat one side of a flashlight window, and I simply disputed that statement and offered 2 fairly strong examples to illustrate. Yoda4561 had already been making the very same point.

    It would be nice if more people could calmly discuss things around here, but all too often it seems that their pride prevents that. As a professional engineer, I routinely call people out on such statements made without any references or data to back them up. Yes, it tends to rub certain people the wrong way... but that's their problem. I readily admit when I'm wrong since I am here, in part, to learn from other members - aren't we all?

    It appears that MrGman would now like to distract (ie, derail) this thread away from his original statement by focusing on the benefits of coating both sides, something which nobody was disputing. In my first post, I made it clear that I thought I had read something to the contrary related to flashlight windows, but now that would appear to be an error on my part. However, I chose my words carefully as I was unable to find the thread to back it up. Further, given my choice of words, his "thumbs down" (and condescending) response to my first post was, IMO, simply rude and overly defensive. After reading my post carefully, this is not the way I would expect an intelligent person to continue the conversation (but, perhaps that's a bit presumptuous of me).

    Clearly, coating both sides is preferrable for optics in general, but not necessarily for a flashlight application. I asked for evidence supporting his original claim that, "It wouldn't do much good to just coat one side". I've seen no evidence, it's as simple as that.

    Additionally, I still insist that single-sided coatings are relatively common for flashlights (no, obviously I haven't inspected every flashlight either, but I'm not new around here and know from experience that it's not uncommon). If this is a practice of Surefire, one of the premier US flashlight manufacturers (and which I have seen for myself when disassembling my L2), I highly doubt that it was a cost consideration and challenge anyone to find evidence to the contrary. As we all know, Surefire prides itself on its strong R&D. Thus, I feel it can be safely assumed that significant research supported this decision.

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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by AvidHiker View Post
    Clearly, coating both sides is preferrable for optics in general, but not necessarily for a flashlight application. I asked for evidence supporting his original claim that, "It wouldn't do much good to just coat one side". I've seen no evidence, it's as simple as that.
    I wouldn't go that far. An uncoated lens will transmit about 93% of the light through it. A lens that is coated on a single side will transmit about 96% of the light through it it, and a lens that is coated on both sides will transmit about 99% of the light through it.
    Additionally, I still insist that single-sided coatings are relatively common for flashlights (no, obviously I haven't inspected every flashlight either, but I'm not new around here and know from experience that it's not uncommon). If this is a practice of Surefire, one of the premier US flashlight manufacturers (and which I have seen for myself when disassembling my L2), I highly doubt that it was a cost consideration and challenge anyone to find evidence to the contrary. As we all know, Surefire prides itself on its strong R&D. Thus, I feel it can be safely assumed that significant research supported this decision.
    I'm sure that Surefire and whoever else only uses a single sided coated lens has a good reason for doing so. That reason however, is not because there's no benefit to coating both sides as opposed to one.

    My opinion is that it's likely due to durability and ease of cleaning.

    However, should you feel so inclined you can get double sided coated lenses from http://www.flashlightlens.com/ should you want to get that extra 3%.

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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by Stereodude View Post
    I'm sure that Surefire and whoever else only uses a single sided coated lens has a good reason for doing so. That reason however, is not because there's no benefit to coating both sides as opposed to one.

    My opinion is that it's likely due to durability and ease of cleaning.
    That's exactly my point as well, I think we're on the same page here.

    Quote Originally Posted by Stereodude View Post
    However, should you feel so inclined you can get double sided coated lenses from http://www.flashlightlens.com/ should you want to get that extra 3%.
    Yes, I have 2 maglite lenses from them - coated on both sides - work great (but don't see the regular abuse of my Ti-PD, so I can't speak to the durability of the exterior coating).

    I'm not unaware or in denial that there are plenty of dual-sided AR coatings out there, I'm simply pointing out that the same holds true for single-sided coatings on flashlight windows. There must be a good reason for that.

    I suppose we can say that having just one side coated offers roughly half the improvement in transmission versus having both sides coated, right? I think that's a lot more accurate than claiming that single-sided AR coatings "wouldn't do much good".

    Depending on the requirements of your optical system, you may or may not want coatings on both sides - the point is, its very application-dependant.

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    Flashaholic* Yoda4561's Avatar
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    Default Re: Anti-reflective coating question.

    It's probably due to AR's lack of chemical resistance. Most of them aren't very resistant to saltwater/chemical exposure, and I believe you lose the benefit of AR when you smudge it with oil/fingerprints. I'm guessing that rather than deal with customers complaining about their AR coating being messed up most manufacturers choose to coat only one side. The coatings are getting better all the time, I'm sure eventually we'll get to the point where we can have fully multicoated flashlight lenses that resist smudging and fog buildup etc with little additional cost over a standard lens.

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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by AvidHiker View Post
    There must be a good reason for that.

    Lower costs while still being able to claim they're AR coated? Yes, I'm a cynic!

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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by Jarl View Post
    Lower costs while still being able to claim they're AR coated? Yes, I'm a cynic!
    Boy, then that McGizmo must be one serious penny-pincher. I dunno, this argument just doesn't float imo.

    Honestly though, does anyone have any clue how much (if any) cost is saved by coating one side only?

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    Default Re: Anti-reflective coating question.

    Ok, I've found the source of my original confusion regarding AR coatings. This is taken from Don's McLux FAQ (http://www.candlepowerforums.com/vb/...ad.php?t=85527)


    The lens is sapphire crystal with an internal AR coating ... extremely hard but sacrifices some output in the process. 4sevens has taken some "unofficial" measurements:

    Lux at 1 meter:
    1387 no lens
    1229 (88.6%) stock sapphire AR inside
    1222 (88.1%) stock sapphire AR outside
    1371 (98.8%) water-white glass double side AR for arc4+ from flashtlightlens.com

    The choice in this light is for maximum strength and durability of the lens with good light transmission, but there are other options. It is all a matter of choice between strength, transmission and possibly fragile outside coatings.


    One could surmise from this data that an interior coating is marginally more effective than an exterior one, but more data is needed to tell for sure. It also reminds me that, from what I've read, these relatively minor incremental improvements in transmission don't (in reality) translate to greater perceived brightness (at least for a flashlight beam) as the human eye doesn't easily distinguish such minute changes. IIRC, the rule of thumb is that you need something on the order of a 50% increase in brightness for a truly noticable improvement in the resulting beam.

    Guess this is a nit-picky discussion. But, for us flashoholics, tweaking to achieve the ultimate performance just comes naturally - whether it be brightness, durability or whatever.



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    Default Re: Anti-reflective coating question.

    Remember also that that is comparing sapphire with FLL's top grade optical glass. Sapphire is not very high up on light transmittance compared to other materials but is the most durable and scratch resistant.

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    Default Re: Anti-reflective coating question.

    Quote Originally Posted by Yoda4561 View Post
    Remember also that that is comparing sapphire with FLL's top grade optical glass. Sapphire is not very high up on light transmittance compared to other materials but is the most durable and scratch resistant.
    Right, thanks for reminding me - this data is for sapphire which, while highly transparent, has a significantly higher refractive index (1.77) when compared with glass (1.47) which I believe is the cause for the additional light loss (lost to internal reflection). The interaction between the coating and substrate will also be different for sapphire versus glass and the coating compositions may be different as well. I know its not the best example, but its all that I could find.

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    Default Re: Anti-reflective coating question.

    Wouldn't it actually be more expensive to only apply the AR coating to one side as opposed to just "bathing" the whole lens into the AR stuff? Also, subsequent handling would be more cumbersome (= more expensive for the manufacturer) since you'd have to distinguish which side is which.

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