# Optical Forums > Ophthalmic Optics >  AR Coating

## Henchos

Since AR coatings increase transmission of light,i was wondering why we see a green hue(most the time) on the lens,is this due to the little amount of light being reflected is mainly green wavelength. And if so,should the green light be actully transmitted more so,since it falls within the photopic 550nm. And hence have other colour reflected instead????

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## For-Life

> Since AR coatings increase transmission of light,i was wondering why we see a green hue(most the time) on the lens,is this due to the little amount of light being reflected is mainly green wavelength. And if so,should the green light be actully transmitted more so,since it falls within the photopic 550nm. And hence have other colour reflected instead????


AR has still some light transmission, so manufacturers try to make it a more appealing colour, and a lot of them use green.  However, there are other ones out there.  There are a pastal blue, a more purple, and Zeiss even has a gold.

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## Darryl Meister

> And if so,should the green light be actully transmitted more so,since it falls within the photopic 550nm. And hence have other colour reflected instead????


This is indeed why SOLA engineered it's latest AR coating (Teflon) with an "arctic blue" reflex color, which also gives it an extremely low _luminous_ (weighted by the sensitivity of the eye) reflectance.

Best regards,
Darryl

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## For-Life

> This is indeed why SOLA engineered it's latest AR coating (Teflon) with an "arctic blue" reflex color, which also gives it an extremely low _luminous_ (weighted by the sensitivity of the eye) reflectance.
> 
> Best regards,
> Darryl


Darryl, what is the difference between UTMC PLUS (I think that is what it is called, the new one) and Teflon?

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## AEOC

How do AR coats increase the light transmission of a lens?  The only explanation that I've heard is that they reduce reflections off the lens surface, thereby allowing more light to pass through the lens.  My understanding was that the coats don't reduce the amount of reflection, but only reduce the visibility of the reflections (from an observer's viewpoint).

Also, are today's coats applied to both front and back surfaces?  I've been in the habit of specifying "backside AR" for sunglasses, but don't know if that's redundant nowadays.

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## Darryl Meister

For AR coatings, the transmittance is indeed _increased_ by essentially the same amount that the reflectance is _decreased_.

The amount of light reflected off the surface of the lens is reduced by an optical effect referred to as _destructive interference_. To make a long story short, some of the waves of light reflected off of the _coating_ are caused to align in such a manner with the waves of light reflected off of the _surface_ that they cancel each other out, effectively reducing the reflectance of the lens.

Conversely, some of these reflected waves of light go through _constructive interference_ in the opposite direction (because of a "phase change"), and eventually pass through the lens. These waves reinforce each other and become brighter, effectively increasing the transmittance of the lens.

AR coatings are generally applied to both surfaces of a lens, since both surfaces produce reflections. However, sunglass lenses represent a special case; after all, you generally don't want to _increase_ the transmittance of a sunlens. An AR coating is often applied specifically to the _back_ surface of a sunlens because the light passing _through_ the lens is reduced in intensity to the point that it becomes comparable (or at least nearly so) to the light _reflected_ off of the back surface.

Imagine trying to look out a window of your house at night... If there are lights on inside your house, the reflections off of your window make it difficult to see anything beyond it. However, if you shut your lights off so that your house is just as dark inside as it is outside the window, suddenly you can see much better. An AR coating on the back of a sunlens prevents you from seeing reflections of your face and your eye, which are made more readily visible because the lens absorbs much of the light passing through it.

Best regards,
Darryl

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## optowalt

thanks Darryl. You make some good explnations.You really know your stuff.  Regards  Walt

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## Darryl Meister

> Darryl, what is the difference between UTMC PLUS (I think that is what it is called, the new one) and Teflon?


To the best of my knowledge, we haven't introduced a "new" version of UTMC. Between UTMC and Teflon, Teflon improves upon most aspects of the coating. Teflon offers a lower reflectance, better antistatic properties, better oleophobic (grease resistance) properties, better hydrophobic (water resistance) properties, and better abrasion resistance. Additionally, Teflon is a highly recognized consumer brand-name in coating technology, which most patients will appreciate.

Best regards,
Darryl

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## Darryl Meister

> thanks Darryl. You make some good explnations


Thanks, Walt. I appreciate the feedback.

Best regards,
Darryl

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## AEOC

Thanks for the reply, Darryl.  So if AR increases the transmission of the lens, how does it reduce the perception of glare?  Is it by reducing off-axis reflections?  Seems counter-intuitive that a brighter image would be less "glary".

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## Darryl Meister

I'm going to reprint some information (without the images, unfortunately) from a paper I wrote several years ago on AR and vision, which should address your question:

For the wearer, this reflected light often serves as a source of annoying glare and visual distraction. This glare can reduce the contrast of retinal images, making it more difficult to discriminate objects and fine detail. Both glare and the loss of reflected light can reduce visual performance and comfort, especially in low-light level conditions (such as night driving) or during visually intensive tasks (such as computer use).

Each surface of a spectacle lens acts as a curved mirror, and reflects a fraction of the incident light. The reflected image of an object is often referred to as a *ghost image*. While the size and clarity of the reflected ghost images vary with the power and form of the spectacle lens, the brightness (or intensity) of the ghost images increases with the refractive index of the lens material.

Specular surface reflections and ghost images produce visual noise, which degrades retinal image quality without contributing useful visual information. They reduce visual performance and comfort via two principal phenomena: visual disturbance caused by ghost images and reduced visual discrimination caused by veiling glare.

There are five unique specular reflections that the wearer may notice from spectacle lenses. These reflected ghost images could become visually disturbing to the wearer when the following conditions are met:

1. The reflected ghost image is bright (or intense) enough to stand out against the background.

2. The vergence (or power) of the reflected ghost image is similar to the focal power provided by the spectacle lens, or can be made similar through accommodation.

3. The reflected ghost image lies close to, but not necessarily in, the wearers line of sight.

If the ghost image is not bright enough to stand out against the background, it will be insignificant and go undetectedmuch like stars, which cant be seen during the day because of the suns bright light. At night, however, even the low intensity of the reflected image of a car headlight is still significantly brighter than the surrounding field of darkness.

If the ghost image is clearly in focus for the wearer, it is more likely to be visually distracting. This second condition may depend upon several factors, including the focal power (Rx) of the lenses, the form (front and back curve) of the lenses, and/or the distance of the original reflection source from the lenses. Ghost image V, for instance, is generally only in focus for lenses of plano- or low-minus powers.

If the ghost image is either superimposed upon the original object or lies out of the visual field, it wont be as noticeable to the wearer. In the presence of prism, the reflected ghost image is deviated considerably more than the refracted image of an object. If a small amount of prescribed or unwanted prism exists in the lens, the ghost image might be deviated away from the actual image just enough to annoy the wearer. Moreover, for any lens with power, prism is always induced when looking away from the center of the lens.

It has been conjectured that these disturbing ghost images may compete for the wearers attention and, if the wearer is cognizant of them, may also produce spurious stimuli to ocular accommodation and convergence. The eyestrain experienced by spectacle wearers performing visually intensive tasks may be due, in part, to the unnecessary adjustments in ocular alignment and focussing as the wearer tries to resolve the _figure_ (principal object of interest) from the spurious ghost images in its background.

Reflections back into the wearers eyes can have detrimental effects since they interfere with the light that forms images on the retina, thereby decreasing the _contrast_ of the retinal images. *Contrast* is a variation or change in _brightness_ (or luminance). When image contrast is reduced, our quality and sharpness of vision suffer. Consider viewing slides projected in a darkened room: If extraneous light hits the screen, the contrast in the projected image is reduced and the image becomes difficult to see.

*Visual acuity* refers to the ability of the eye to distinguish between two object points. The eye can only resolve two object points as separate when there is a sufficient amount of retinal image contrast between them (i.e., the *contrast threshold*). Consequently, visual acuity is very closely related to retinal image contrast and the observers *contrast sensitivity*which is the ability to detect variations in image contrast. As the contrast of the retinal image decreases, visual acuity also decreases because it becomes more difficult to discriminate image details. Mathematically, contrast is defined as the ratio of the difference in brightness between an object (_L_MIN) and its background (_L_MAX) to just the background.

How do specular reflections and ghost images reduce visual performance? These ghost images can serve as sources of glare within the visual field. Moreover, when the reflected glare source is large or defocused, it can produce a *veiling glare* over a large portion of the visual field. Since this reflected glare is added to the brightness of both the object of interest and its background, the difference in brightness between them remains constant. However, since the background brightness, which is the denominator of the contrast expression above, still increases, the contrast of the retinal image decreases.

Images with lower contrast are more difficult to resolve than images with higher contrast, which effectively reduces visual acuity. Hence, glare from lens reflections serves to reduce both contrast sensitivity and visual acuity. Veiling glare increases the _overall_ brightness of the distribution of light across the retina, without affecting the _difference_ between the brightness of the object points and their background. This diminishes the contrast between the object points and their background and makes it more difficult to distinguish the two points as separate.

Consider ghost image V: Light from a source in front of the wearer is first reflected from the back surface of the lens, then the front surface, and finally into the eye. The brightness of this reflection is small with respect to the transmitted light of the lens. For example, in an uncoated polycarbonate lens, the total lens transmittance is 90.2% (100 - 9.8) whereas the reflectance of ghost image V is less than 0.25%. This reflected light is only 0.3% (0.003) as bright as the primary image being formed on the retina.

Although the percentage of light reflected back into the eye is small by comparison to the amount of light reflected back into the environment, it is not necessarily trivial as far as vision is concerned. It can be significant whenever there are disparities of brightness within the visual field. For example, the brightness of a blue sky outside a window viewed from inside a room can easily be 1,000 times brighter (more specifically, the _luminance_ is 1,000 times greater) than the wall next to the window.

In this situation, the specular reflection of the sky has a luminance that is 3 times (0.003 × 1000) greater than the luminance of the wall, so the reflected light entering the eye will noticeably decrease the contrast of objects around the window. Another example is trying to see into a dark cafe under a bright overhead sky. The specular reflections of the bright sky interfere with the lower luminance images inside of the cafe. The magnitude of the reflected light relative to the light in the retinal image can be even greater in a night driving situation.

Another common source of veiling glare is reflections from the face (cheeks) of the wearer (ghost images I and II). These reflect off the front and rear lens surfaces and back into the wearers eyes. In the polycarbonate example above, nearly 10% of the light leaving the cheek and going towards the lens will be reflected back into the eye. Facial reflections rebounding into the eye can interfere with visionespecially in situations where the luminance of the face is greater than the luminance of the scene being viewed. This occurs commonly when a person is viewing into a darker environment from a brighter environment, such as when the face is illuminated by oncoming headlights while driving at night.

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## AEOC

Thanks again, Darryl.  Excellent info.

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## AEOC

I was told by our lab that today's AR coats block UV.  To what extent do they do this?  As good as a deliberate treatment?  And if they do, how do coated transitions still work?  I read that old thread on ARC and transitions, but (it was old) and it didn't mention how the lenses still change, just that they do.

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## rinselberg

I read that some ARs can be formulated NOT to block UV. I read that in some cases, ARs have been formulated NOT to block UV, specifically for the purpose of being used on photochromic lenses. Perhaps some of our AR experts will come in here with more exact details.

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## Jim Schafer

Early plastic photchromics (1980's early 90's) were affected by the AR coatings because the AR process (either in the AR coating stack or the hardcoat base needed for adhesion) blocked or partially blocked the UV wave length band that activated the photochromic molecules. Recognizing this, we worked with the AR providers and identified what hardcoats and AR materials least affected the original Transitions and Transitions Plus lenses. Being global and doing well in Europe, we knew we had to ensure the product worked with AR. 
Around 1994, two things happened at the same time, AR process evolved for and with the new lens materials and new photochromic dyes that were being developed for Transitions III were tweaked to work very well with the new AR processes. From the mid 90's, every Transitions Lens product has went through premium AR validations before it was launched. 

In short...todays AR may block UV but not at the narrow wave length band needed for activation of 21st century photochromic dyes. 
have a great week,
Jim

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## coda

Jim, that was a very lucid and complete answer to the AR-Transitions interaction, thanks.  

I only want to add that while AR coatings will and do block certain portions of the UV spectrum they *do not* block enough UV for you to consider them to be a replacement for a decent UV treatment.  In many AR coatings actually enhance the transmission of certain portions of the UV spectrum.  Play it safe, either vend UV treated lenses or lenses which inherently block UV for people who spend any significant time outside.

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## Darryl Meister

My understanding from our thin film techs is that it is much easier to just add UV inhibitors to a lens or hard coating resin than it is to engineer an AR coating stack with significant UV attenuation properties.

Best regards,
Darryl

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## AEOC

So I guess the premise that AR coats block UV wasn't entirely accurate to begin with (at least not to the extent of a UV treatment or polycarbonate lens)  Thanks for clearing that up.

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## Darryl Meister

> So I guess the premise that AR coats block UV wasn't entirely accurate to begin with


It's _possible_, just not necessarily _practical_. You'd probably have to find out for certain from the individual AR vendor.

Best regards,
Darryl

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## Chris Ryser

> *It's possible, just not necessarily practical. You'd probably have to find out for certain from the individual AR vendor*.


Being one of the first UV treatment manufacturers, 1982, I do have some expierience in that matter. You can see a lot of it on my website at http://optochemicals.com or can give many answers on that subject.

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