What is AR coating supposed to do?

[keiron99]

I have just got a Nite MX10 watch.

The crystal is apparently sapphire and also has several coats of AR. But whilst it is very clear, and appears to be very thick from the sound it makes when tapping it, is actually extremely reflective. In daylight, I have to adjust the angle of my wrist in order to be able to see the hands (which is not helped because the hands don't stand out enough against the dial with the model I chose).

Is this normal? I don't really have any watches of any quality to compare it against so don't really know whether I am expecting too much.

[verv]

in theory AR coating is supposed to stop reflections from the crystal.

some AR coatings are excellent.. example here (pic nicked from web, apologies)
well done AR's can make the dial look like theres no crystal present at all.



some are not so good.. i suppose it depends on inner/outer/both sides/quality of coating used etc.
im not an expert but on some watches ive owned, the AR has been present but about as useful as a chocolate ashtray.

i have no experience of your watch so cant give you a definitive.. but if the dial/hands are stealth it may be trickier to read anyway.
also ive found that if hefty sapphires are only coated on the inside, theres more glare from the sun catching the uncoated upper side of the crystal, particularly if theres a bit of a dome on it.

sorry i couldnt be of more use but i wouldnt worry about it too hard.

[lysanderxiii]

AR coating = Anti-Reflective coating.

Just what it says....

[keiron99]

Thanks.

I can only assume it isn't very effective on mine. In fact, it is rank poor to be honest!

[tim2012]

Just remember all glass is reflective, even with AR coating. So at certain angle to the light source it will reflect. But it should not happen looking straight down at the dial.

[keiron99]

Just remember all glass is reflective, even with AR coating. So at certain angle to the light source it will reflect. But it should not happen looking straight down at the dial.

I can see my face staring right back at me when i look at it head on. Not a pretty sight !

Does the fact that it is all back exaggerate this effect?

[Kiki Picasso]

I can see my face staring right back at me when i look at it head on. Not a pretty sight !

Does the fact that it is all back exaggerate this effect?

Your colour has got nothing to do with it. If you're ugly you're ugly

[robcat]

Breitling do a particularly good AR coating IMO - from some angles it really does look like there is no crystal.

[Seb d]

I can see my face staring right back at me when i look at it head on. Not a pretty sight !

Does the fact that it is all back exaggerate this effect?

I guarantee it'll only have numerous inner AR coatings, not outer, not at the sort of price that Nite watches retail for. Inner will reduce reflections a bit, but nowhere near as much as double AR, which is what the watches shown in this thread have. Does the crystal have a blue or purple tint when a light source hits it? If no, you only have single AR, as in internal, irrespective of the number of layers they apply.

[keiron99]

Does the crystal have a blue or purple tint when a light source hits it?

No, I can't see any sort of tint.

To be fair, this seems like a really well built and nicely finished watch. And I bought directly from Nite, who were absolutely superb to deal with online.

I think though, given I struggle to actually read the time on it (!) that I will change it for one less stealthy but with more of a contrast between the hands and the dial.

[Seb d]

No, I can't see any sort of tint.

To be fair, this seems like a really well built and nicely finished watch. And I bought directly from Nite, who were absolutely superb to deal with online.

I think though, given I struggle to actually read the time on it (!) that I will change it for one less stealthy but with more of a contrast between the hands and the dial.

So it's just inner, hence the reflections. Tag, Omega etc. only use inner coatings on numerous watches, so it's not a reflection of quality :) But it's relatively rare to come across double AR crystals outside of high end watches and companies like Breitling etc. Lum-Tec puts double AR on all its watches, Perrelet does and loads of others but Tag only applies it certain watches for example, so it really varies.

[Lammylee]

I think I would be wary of getting a watch with AR coating on the outside of the crystal as it can show scratches even though the Sapphire underneath would be unmarked.

[smalleyboy1]

My Fortis has AR on both inside and outside. In some light you would think there is no glass present at all. In other light it creates a blue tinge or sheen to the dial. Overall I'm very pleased with it.

[Thorien]

But having an outer AR coating isn't necessarily always a great idea, as you can end up with scratches on the coating (whilst the crystal isnt scratched) and it'll not look great!
You may even have to change the crystal, or remove the whole AR coating from outside.

[Qatar-wol]

My Fortis has AR on both inside and outside. In some light you would think there is no glass present at all. In other light it creates a blue tinge or sheen to the dial. Overall I'm very pleased with it.

I had a Fortis Cosmonaut Chronograph a few years ago, and I was teaching a class when a girl reached across and tapped the glass - she couldn't believe there was a crystal in place! And yeah, Breiting do good AR, too!

[Huertecilla]

AR coating = Anti-Reflective coating.

Just what it says....

Since sapphire has the highest light refraction of the three options it nééds at least an internal AR coating to not be outclassed by even simple glass never mind acryllic.

As to what AR can achieve, one should note that the refraction of double sided AR coated sapphire still comes a wee bit short of acryllic.

[keiron99]

Since sapphire has the highest light refraction of the three options it nééds at least an internal AR coating to not be outclassed by even simple glass never mind acryllic.

As to what AR can achieve, one should note that the refraction of double sided AR coated sapphire still comes a wee bit short of acryllic.

Very interesting. So mineral and acrylic are not without their merits then.

[onlynik]

The first generation Omega x-33 has an AR inside and out, but the outside was removed for the second generation, probably some astronaut complaining about it getting scratched.

[gonzomantis]

Anti-reflective coatings on the outer portion of the crystal are used to highlight any fingerprints or smudges. :)

[B Caine]

The AR coating on my Breitling seems to be very good, not too sure if it has it on the outside too, the dial is very clear though.

[Glen Goyne]

Does the crystal have a blue or purple tint when a light source hits it? If no, you only have single AR, as in internal, irrespective of the number of layers they apply.

Can you expand a little on colors and inner/outer? I have a watch with blue tiny that supposedly only has inner coating. It is a dome samphire. And does the color say anything for the amount of layers?

Personally I am not too happy with the blue tint. The dial is black and the blue refraction makes it a softer black.

[neillp]

My steinhart OVM has a AR coating and it does make a difference, to be honest though I just like the blue tint it gives :)

[Qatar-wol]

Since sapphire has the highest light refraction of the three options it nééds at least an internal AR coating to not be outclassed by even simple glass never mind acryllic.

As to what AR can achieve, one should note that the refraction of double sided AR coated sapphire still comes a wee bit short of acryllic.

Who wants refraction? Or did you mean reflection? I know acrylic refracts - the distortion from a curved acrylic crystal is much greater than that from a sapphire. But to say that a good AR still reflects more than acrylic doesn't fit in with my experience.

[NickF]

anto reflective?! looks good!

[robcat]

It's the reflectivity of the surface that matters, be it a watch crystal or camera lens/filter. It's the difference in refractive indices divided by the sum of them, squared - per surface. So the higher the RI, the more reflection from each surface.

Corundum has quite a high RI (1.7ish) so there is a lot of reflection per surface about 7-8%, so you lose 15-16% transmission. Plastics/normal glass tend to be about 1.5, so you lose only 8% in total. They will therefore look much less shiny.

Cilla's exaggerrating about non-coated glass/plastic being better than AR coated crystal though, as any half-decent AR coating will give you high 90's% transmission.

The blue hue can be distracting though.

[Dave O'Sullivan]

I did some comparison shots of a couple of watches I've got with AR coatings a while ago.

No AR


Not bad for a cheapy


Getting better


Typical Breitling, awesome


What I don't get is watches that have AR on the underside only. It makes sense if the dial or hands are reflective but some have it when they are not. Don't know how effective that is.

[dkpw]

The best AR I've seen was on my PO 8500, it really did look like there wasn't a crystal there until of course a smudge appeared, which could only be removed successfully with a microfibre cloth.

I recall reading somewhere that Rolex don't generally use AR, except under the cyclops on some models, as a deliberate aesthetic look, they want their watches to flash out to the world.

[jegger]

In my experience I prefer none or only on the inside, smudges far too much when on the outside, I didn't realised I touched the face of my watch so much until I had a watch with AR coating on the outside.

[robcat]

What I don't get is watches that have AR on the underside only. It makes sense if the dial or hands are reflective but some have it when they are not. Don't know how effective that is.

It still helps with transmission from the dial to you, as the reflection is per surface/change in refractive index. It's much harder to see the bloom on the inside though, so you have to take the manufacturer's word for it. It's a compromise between improving transmission, and durability.

[Seb d]

It still helps with transmission from the dial to you, as the reflection is per surface/change in refractive index. It's much harder to see the bloom on the inside though, so you have to take the manufacturer's word for it. It's a compromise between improving transmission, and durability.

The best way to see the inner AR is to hold the watch under a light source and then put a finger between the crystal and the light source. At the right angle you will see a a slight, second parallel reflection of your finger but in blue or purple, indicating the presence of the inner coating.

[keiron99]

Anti-reflective coatings on the outer portion of the crystal are used to highlight any fingerprints or smudges. :)

Yep, as well as the reflection on my Nite, the smudges are absolutely terrible. I'm looking at it the watch now, and all I can see is a mass of reflections and smudges. I can't see the hands or dial to tell the time!

[oymd]

Is it my eyes or the IWCs have a very bluish AR?

[robcat]

The best way to see the inner AR is to hold the watch under a light source and then put a finger between the crystal and the light source. At the right angle you will see a a slight, second parallel reflection of your finger but in blue or purple, indicating the presence of the inner coating.

I've not tried that. You can sometimes see a blue hue at the edges, but also if you look through water, this reduces the relection from the outer surface and you can then see a blue bloom from the inner coating. This is particularly apparent on my VC Overseas, which is coated only on the inside.

[Seb d]

Can you expand a little on colors and inner/outer? I have a watch with blue tiny that supposedly only has inner coating. It is a dome sapphire. And does the color say anything for the amount of layers?

Personally I am not too happy with the blue tint. The dial is black and the blue refraction makes it a softer black.

The best explanation I have ever read is the one I have quoted below from here: http://rick_oleson.tripod.com/index-166.html The whole thing is very interesting but I have highlighted the section relating to colour, and I think it does an excellent job of explaining why we see a certain colour reflected by the AR coating.
Is your crystal single or double domed? This will make a big difference to how the AR coating is perceived due to the way in which the two crystals reflect light differently.


Why do coated lenses look purple (or blue or green or whatever)?

First, let’s get the most obvious misconception out of the way:
Lens coating has NO COLOR. It is a water-clear material throughout the visible spectrum. It has only 2 meaningful characteristics that make it appear colored: (1) Refractive Index, and (2) Thickness.

To understand why coated lenses look colored, you have to understand what the coating is there for, and how it works. The purpose of coating on a lens is to reduce reflections from the lens’ surface.Light reflects from the surface of the glass because the refractive index of the glass is different from that of the air, and the reflection is formed at this interface of different refractive indices.

Light (for the purpose of this discussion at any rate) behaves as a wave: when a light wave strikes the glass surface, some of it bounces off as a reflection – the reflected ray still has the same wave characteristics as the incident ray, just moving in the opposite direction.


Wave reflection from a single glass surface (reflection shown in red)


This reflection has no apparent color, it reflects all wavelengths equally and has a greyish or silver appearance.


Because the reflection is a wave,though, it has an important characteristic: it can be cancelled out by another wave, if that other wave can be made to be of equal frequency, direction and magnitude and opposite phase. In the sketch above, you can see that a second wave spaced 1/2 wavelength out of phase would have its highest point aligned with our wave’s lowest point; if this can be made to happen, the 2 waves will cancel each other out.

It turns out that we CAN make this happen. What we need to do is to create a second reflection, in a slightly different location from the glass surface that created the first. In order to space the 2 reflections exactly 1/2 wavelength apart, the 2 reflective surfaces need to be spaced 1/4 wavelength apart: 1/4 wavelength for the incident wave going in plus 1/4 wavelength for its reflection coming back out = 1/2 wavelength. For this reason,anti reflective coatings are sometimes referred to as “Quarter Wave”coatings.


Wave reflection from a coated glass surface, with reflections from glass and coating cancelling each other (reflection from glass shown in red, reflection from coating shown in blue)

Remember, though, that the reflection occurs where there is a difference in refractive index from one material to the next. So not just any coating material will work; if the coating has the same refractive index as glass (about 1.6), there will be no “red” reflection,only the blue one – and there will be no anti-reflective effect,the lens will behave just as if it was uncoated. Similarly, if the coating had the refractive index of air (1.0), there would be no“blue” reflection and the lens would appear to be uncoated. In order to make the red and blue reflections equal so that they can cancel each other out, the refractive index of the coating needs to be about halfway between that of glass and air so that the difference at each interface will be equal. The perfect single coating material would therefore have a refractive index of about 1.3. The material most commonly used for this purpose, Magnesium Fluoride, has a refractive index of about 1.37 – not a bad compromise, considering that it offers durability and other desirable properties.

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Okay, great, I get it. But why the color?

As you can see in the diagram above, for any given wavelength it should be possible to obtain nearly 100% cancellation of reflections by means of applying a 1/4 wavelength-thick layer of Magnesium Fluoride to the surface of the glass. But the trick is, 1/4 of WHAT wavelength? Light isn’t all one wavelength, there’s a different wavelength for every color from 400nm (violet) to 700nm (red). No thickness of coating can be 1/4 wavelength of ALL of them…. so the designer has to choose.Typically, they choose a wavelength that’s close to the center of the visible spectrum, so that the beneficial effects of the coating will be as uniformly distributed over the visible range as possible.The visible range being 400-700nm, the wavelength generally chosen as the target was in the neighborhood of 550nm, or right in the center.


(top)Wavelength vs color
(bottom) Wavelength vs color ofreflection

As you can see in the top chart above, the mathematical center of the visible spectrum, 550nm, corresponds to a color in the yellow-green range. A coating of 137.5nm thickness (550 divided by 4) would virtually eliminate any reflections of this color from the surface of the lens. It would reduce reflections of other wavelengths also, but the efficiency of its effect would become progressively lower as the wavelength deviates from 550. The net result is that the overall goal of suppressing reflections is achieved, but it is not achieved equally for all colors. We see this when we look at the reflections from the coated glass surface: there is no green light reflected at all, but there are reflections of the other colors. The overall effect is that of a reflection in the complement of the targeted color. These complements are shown in the lower chart in the figure above – 550nm corresponds to a blue-violet color ….. exactly what we see in a coated lens.
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So, what’s the difference between “single”and “multi” coating?

Multicoating is significantly more efficient, and slower and more expensive to achieve, than single coating, but the principles and the basic process are the same. The basic difference comes down to refractive index. A single coating must have a refractive index about halfway between those of glass and air, and it took a number of years to find a suitable material in Magnesium Fluoride. To apply multiple coatings with similar effect,they must be of materials with an even progression of refractive indices, calculated to correspond to the number of layers desired.For example, if you want to apply 5 layers, you would need 5 different materials with indices of roughly 1.1, 1.2, 1.3, 1.4 and 1.5 to span the range between air (1.0) and glass (1.6). From hereon, the process is the same as with the single coating, except that each layer is made a slightly different thickness from the others,with the thicknesses selected to correspond to several different wavelengths within the visible spectrum so that the suppressive effect applies not only at the 550nm center but across the whole range. A multi-coated lens does not appear as strongly colored as a single coated lens – its general tendency is to appear black by comparison to the blue-violet single coated or silver-grey uncoated lens. This is by design: the purpose of coating is to suppress reflections, and the purpose of multi-coating is to suppress ALL reflections.

So if I see reflections of multiple colors in a lens, that doesn’tmean it’s multi-coated?

Not necessarily. A multi-coated lens may present multi-colored reflections, but so will many single coated lenses. The way to tell a multi-coated lens from a single one is to compare the brightness of the reflections rather than their color: the lens with brighter reflections will be the single coated one.

How much better is multi-coating than single coating?

The difference between single- and multi-coating is not as great as that between an uncoated and a single coated lens; the single layer of coating deals with the majority of the reflections, and multi-coating is a refinement to further improve its effectiveness. There are also, of course, different degrees of multi-coating with different levels of effectiveness: the more layers there are and the more different wavelengths can be optimized, the better the performance of the multi-coating will be.

[mantaray]

Breitling do a particularly good AR coating IMO - from some angles it really does look like there is no crystal.

Agreed - just had my Chronospace done with a new AR crystal, and it really makes an amazing difference ... nothing else in my collection like it .... I guess the down side is that, if (once!) you scratch it has the potential to drive you nuts ....

[Huertecilla]

Very interesting. So mineral and acrylic are not without their merits then.

Acrylic is the best scorer on all point save for hardness. This is a mixed disadvantage as you can also polish it very easily. Besides; it costs less than the AR coating...

[Barry]

Sometimes you can't see it, other times you get a flash of Blue or a lot of Blue

[Glen Goyne]

Is there a quality issue? Is bad AR or badly applied AR more blue compared to better AR?

[Seb d]

Is there a quality issue? Is bad AR or badly applied AR more blue compared to better AR?

Partly, yes, though it's not a case of being more blue, rather a case of how much of a reflection you get which dictates how blue it appears and the subsequent quality. The Parnis further up has very blue AR and it's nowhere near as good as that of the IWC and Breitling, so quality can and does vary. Ultimately having AR, regardless of the quality of the coating, is better than having none when it comes to reducing reflection, but Chinese watches like Parnis, replicas etc. have a coating that reflects very blue.

[100thmonkey]

The best AR I've seen was on my PO 8500, it really did look like there wasn't a crystal there until of course a smudge appeared, which could only be removed successfully with a microfibre cloth.I recall reading somewhere that Rolex don't generally use AR, except under the cyclops on some models, as a deliberate aesthetic look, they want their watches to flash out to the world.

Agree on the PO coating it is almost invisable at times, Rolex do use AR on the new Ceramic Subs Cyclops but I dare say the rest is urban myth

[robcat]

Partly, yes, though it's not a case of being more blue, rather a case of how much of a reflection you get which dictates how blue it appears and the subsequent quality. The Parnis further up has very blue AR and it's nowhere near as good as that of the IWC and Breitling, so quality can and does vary. Ultimately having AR, regardless of the quality of the coating, is better than having none when it comes to reducing reflection, but Chinese watches like Parnis, replicas etc. have a coating that reflects very blue.

It actually two factors. One is the number of layers, the other is the chosen thickness/coating RI. The whole point of the quarter wave coating is that the thickness relates to the wavelength of light that you want the coating to cancel out. Single layer coatings normally are chosen to leave a blue bloom, but although multi layer ones (like Breitling appear to use) can also result in a fainter blue bloom, theoretically by using various thicknesses, cancelling out various wavelengths, you can end up with a different colour bloom. Glasses and camera lenses/filters often have a greenish bloom. You will always have some bloom, but it will just be harder to see.

So the difference between a good coating and a bad coating is most likely to be the number of layers, and the choice of layer thicknesses. A poorly applied coating would result in patches of different colour residual refelctions, representing different coating thicknesses. (I've seen this on cheapo camera filters - the effect is more like thin layer of oil on water, which is the same principle).

[oymd]

Sometimes you can't see it, other times you get a flash of Blue or a lot of Blue

Nice comparison!

I feel that the IWCs are ALWAYS bluish...at any angle!

[yonsson]

For me, high grade hardlex, like on the mm300 is the way to go. I've only got one sapphire crystal watch, and it has more reflections and it smudges A LOT more than hardlex.

I don't know if it has AR, but reflections are minimal even if it sometimes gives a slight blue tint.