Polarised lenses don’t have the market penetration they perhaps deserve. April Petrusma breaks down the optics so more people can benefit from this lens option.
In today’s world, most would assume it to be common practise amongst eyecare professionals to recommend polarised filters for patients’ sunglasses with the aim of reducing disabling and blinding reflected glare. Unfortunately however, data shows that few optical dispensers fully understand the nature of polarisation and how polarised lenses can truly benefit the patient.
“The rows of iodine crystals contained within the polarised lens blocks the disabling horizontally vibrating polarised light waves and allows only the vertical vibrating light waves to reach the eye.”
According to Younger Optics – a global leader in prescription polarised eyewear – polarised Rx sunglasses make up just 7% of global prescription eyewear sales. This isn’t because only 7% of patients benefit from them, rather there is a lack of awareness from both eyecare professionals and consumers.
For eyecare professionals to fully understand how polarised lenses work, it helps to understand the basics of how light behaves.
Light itself is the interaction of both electric and magnetic fields travelling through space. The electric and magnetic vibrations of a light wave occur perpendicularly to each other. This means the electric field moves in one direction and the magnetic field in another. These electric and magnetic vibrations can occur in numerous planes but will always be at right angles to each other or 90 degrees apart.
To keep it simple when describing light behaviour in optics, we generally consider the directions to be horizontal or vertical as shown in Figure 1.
A light wave that is vibrating in more than one plane like this is known as “unpolarised light”. The light emitted by the sun, the stars, by a lamp or a torch are all unpolarised light sources.
Unpolarised and polarised light differences
The process of transforming unpolarised light into polarised light is known as polarisation.
Contrary to unpolarised light, polarised light waves are light waves in which the vibrations occur in a single plane only. This means plane polarised light consists of waves in which the direction of vibration is the same for all waves. We no longer have a horizontal and vertical direction of travel, only one or the other.
In Figure 2 you can see that when unpolarised light hits a flat horizontal surface, some of the light is refracted and absorbed beyond the surface and some bounces straight back off the surface and is reflected. The light that is reflected is on the same angle as the surface and is a single light wave only, which means it is now vibrating in only the horizontal plane and has become ‘polarised light’.
Unfortunately for us and our eyes, when light is reflected off shiny horizontal surfaces and the light is transformed into polarised light, that light produces blinding glare. This blinding glare can obscure or completely block our vision which is why we care about it so much.
If you’re wondering how this phenomenon occurs, it’s all to do with the exact angle the light hits the surface in the first place. When light strikes a surface so that there is a 90 degree angle between the reflected and refracted rays, this is when the reflected light becomes plane polarised.
The direction of polarisation will then always be parallel to the surface, so if we are talking about a horizontal body of water, the plane polarised light will also be vibrating horizontally. The angle of incidence that produces the 90 degree angle is a phenomenon called Brewster’s angle.
It was Sir David Brewster, a Scottish physicist, who first discovered this in 1811. Brewster’s law states that maximum polarisation happens when the reflected ray and the refracted ray are perpendicular to each other.
How do polarising filters work?
Polarising filters work just like an invisible picket fence or jail bars. To fully understand the process, if you picture yourself holding a broom handle horizontally and trying to pass it through the fence or the jail bars – it’s not going to happen, it’s impossible! But if you held it vertically, it would travel through the gaps nicely.
It’s the same analogy for polarised light waves. The vertical light we need, passes through the gaps no problem. But the horizontal polarised light is blocked and can’t penetrate through to the eye (Figure 3).
This is because polarised lenses have millions of parallel rows of tiny iodine crystals (so small they can’t be seen with the naked eye) and they act just like the fence. The rows of iodine crystals contained within the polarised lens blocks the disabling horizontally vibrating polarised light waves and allows only the vertical vibrating light waves to reach the eye.
Non-polarised sunglasses only filter ultraviolet (UV) rays and reduce the amount of general light transmission. They do not prevent glare as they do not stop unwanted horizontal polarised light from reaching the eye.
While polarised lenses offer numerous benefits in reducing glare and enhancing visual clarity, there are certain occupations or activities where they may not be suitable or recommended like in aviation and some medical or sporting professions. For the most part, however, as stated by Rodenstock’s Nicola Peaper “the most efficient solution for disabling and blinding glare is a polarising lens with a back surface anti-reflection coating that also reduces UV reflection”.
About the author: April Petrusma is the CEO of Optical Dispensers Australia and a Senior Lecturer at the Australasian College of Optical Dispensing. She is a qualified Optical Dispenser, RTO Trainer and Business Manager with a degree in Visual Communication.
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