Anti Glare Coats?

Anti Reflection lenses or Anti Glare?

This is a remarkably big topic but we do get asked this a lot so we shall try our best to explain without descending into anything too horrendously complex!

Is there any such thing as an anti glare coat? Not in the sense that some advertising campaigns might have you believe. In fact, many of the coatings that are touted as being anti glare are actually anti reflection (AR) coatings, something quite different indeed!

Reflections and Coatings

corning-gorilla-glass-1

The centre of this sheet has had a circle of anti reflection coating applied.

The surface of every lens reflects some light, with this amount varying according to several factors such as materials and even pre existing coatings like hard coats. Typically uncoated or hard coated lens surfaces reflect around 7% of light, which of course means that only 93% of the available light is making it to your eye. For thinner, higher index lenses this number drops to as low as 85%. The result of this is that we now have lenses that not only show reflections, but they are also not allowing all of the light through, impeding better vision.

What can we do about this? The solution to reducing reflections and increasing light transmission is rather ingenious and makes use of  destructive interference. By placing a coating on the surface that makes the reflected light out of phase with the incident light we can get the ‘peaks’ of the incident waves to interact with the troughs of the reflected waves and vice versa causing the reflection to be cancelled. To do this the coat needs to be 1/4 of the wavelength thick, or odd multiples of this number to maintain the out of phase relationship.  Because visible light has a range of wavelengths an average value roughly around yellow is often used, which while not perfect, does allow for optimal results. Modern AR coats will also have multiple layers to reduce the issues caused by the averaging even more, plus there are extra coatings that are added for resilience, hence another name for AR coats is ‘multi-coats’.

constructive-destructive-wave1

Click image for link to original at www.tutorcircle.com

If that sounds excessively complex then think of waves at the beach, sometimes two waves can arrive at the same time, resulting in a bigger wave as their energy combines. Other times a wave will arrive at the same time as a gap in the swell so it is fallen into the ‘gap’ and is cancelled out, it’s this cancelling out effect that we want to achieve.

Now that all of that is out of the way we can think about what it means. When we apply an AR coat to a lens it is actually increasing the transmission of light to effectively reduce the effect of the lens on light transmission. What we want to achieve is a situation where the prescription can do it’s work with as little awareness of a lens being there as possible for the wearer. That’s pretty much the opposite of anti glare!

content_tratam_solcrystal_trafic

The glare from these headlights is caused in part by the lens, which is why AR coats can be beneficial for night driving by actually increasing light transmission.

What do you see on an AR coated lens? As a wearer you will get more comfortable vision as the lens will let through as much light as possible. For situations where you need to concentrate for extended periods this is a tremendous advantage. A similar effect is observed driving at night and particularly in the rain, halos from streetlights are reduced and general comfort is increased. For people looking at you there will always be some slight reflection as we cannot cancel every wavelength simultaneously, because of this AR coated lenses will tend to have a slight green or blue reflection which we refer to as the bloom.

A downside to the extra clarity is the fact that AR coated lenses may show dirt and marks more easily. They don’t necessarily get any dirtier or scratch easier than other lenses, in fact they are designed to be tougher than ever, but they do show marks simply as a result of the more refined surface. The good news is that modern coatings are constantly improving and are now easier to clean than uncoated lenses so you can have the best of both worlds.

There are some circumstances in which a lens itself will create reflections but it is important to realise that this is not ‘glare’. These can be troublesome reflections, ghost images or even halos around lights that are a result of light interactions inside the lens. Night driving and driving in the rain is a perfect environment for these kinds of halos (see the image above), thankfully AR coats are very useful in reducing these effects to.

One cosmetic benefit of AR coats is the decreased awareness of the lens when looking at the wearer, meaning that we all get to see your eyes. This is especially useful for film and television where coatings have long been used to eliminate reflections, I often look for the AR coats on actors spectacles in films and try to identify the company that made them. And yes, I realise how nerdy that sounds.

Reducing Glare

If we consider glare as originating from a light source or surfaces like sand or water then we can begin to understand the techniques needed to reduce its impact. Glare by definition is a strong or dazzling light source so when we consider this in the context of AR coats we should readily see that increasing light transmission will do very little to reduce discomfort caused by glare or bright light, if it is performing as intended then it will have no effect on the glare at all.

To reduce glare we need to reduce the amount of light reaching the eye. This can be in the form of a tint, polarisation or very specific lenses like the new blue control types of lens. In any event what we see for glare reduction is an attenuation or blocking effect. Lenses with this in place will not look as clear as an AR coated lens, (obviously in the case of sunglasses!) but even the new generation of blue control lenses will have a light tint to them.

Screen Shot 2016-12-19 at 12.10.11 pmYellow tints have been cited anecdotally as reducing glare at night but their actual effect is somewhat more complex that this. By design yellow filters will reduce some effects of glare by blocking a percentage of the blue light from headlights for example. However, the trade off is that a percentage of the overall light level has been blocked in the process, so we have a reduction in glare but also a reduction in transmitted light. Consider the effect of that at night when driving where it is generally desirable that we get more rather than less light to the eye, if only for the sake of safety. Because of this tints for night time driving are rarely recommended.

What should I use?

So know you know the difference between reducing reflections and reducing glare the big question is ‘what do I want to actually achieve?’. Consider your symptoms and what may be causing them. Is it fatigue or glare? Do you want to increase or decrease the amount of light coming through the lens?

The easiest way to work out your requirements is to talk to a qualified professional like those at Hannaford Eyewear. Why not contact us today?