Polarised eyewear has widespread use in Australia today, but where did it originate, how does it work and how is it made? This article looks at the history and science behind polarised safety glasses and sunglasses.
What is Polarisation?
To understand how polarised sunglasses and safety glasses work, we need to understand polarisation. To understand polarisation, we need to understand a few basic things about light.
‘Normal’ light from the sun or a lamp consists of a series of waves of electromagnetic radiation, made up of electric waves and magnetic waves, that travel in the same direction, or trajectory. However, despite travelling in the same direction, the waves don’t all ‘wave’ in the same direction: some wave up and down, others side to side, and at a several different angles. This light is known as ‘unpolarised’ light.
Polarisation occurs when light waves hit a filter that is composed of long chain molecules organised in the same direction. These, only let light waves of a particular direction through. Therefore ‘polarised’ light consists of light waves whose vibrations occur on a single plane, rather than several. In this way, polarised sunglasses work by reducing the total amount of light coming through the ‘polarising’ lens to the eye, reducing brightness.
However, the second type of polarisation and the real value of polarised sunglasses and safety glasses compared to non-polarised eyewear concerns glare. In this instance, natural polarisation occurs when light hits a non-metallic reflective surface, such as a lake, windscreen or road. When light hits the horizontal reflective surface, the light is split into refracted light, which tends to be vertically planed and passes into the material, and reflected light, which is predominantly horizontally polarised. As polarised sunglasses are designed only to allow vertically polarised light through, the horizontally polarised reflected light, or glare is blocked.
How Can You Tell if Your Glasses are Polarised?
A good way to check if your glasses are polarised is to hold the lens up to an LCD screen or a reflective surface and rotate the glasses 90%. If the glasses are polarised, the glare will increase or decrease as you turn the lens. Alternatively, if you put two polarised lenses together and rotate them so they are perpendicular, you will find no light comes through, as both the vertical and horizontal light waves are blocked.
The Discovery of Polarisation
The concept of polarisation actually dates back to the Vikings, who used light and crystals for purposes of navigation. However, scientifically, the concept of polarisation began in the 17th century and was part of a series of findings regarding the properties of light. It was discovered that a single light ray could be split into two, and that there was more than one wave in a light beam. Then, in 1808 the concept of polarisation was discovered by Étienne- Louis Malus, a French Physicist, and further developed by Sir David Brewster
From Polarisation to Polarised Sunglasses
The use of sunglasses can be traced back to the prehistoric era when Inuit people wore flattened walrus ivory glasses to block harsh sun rays. However, the earliest historical references date the use of sunglasses back to around the 12th century in China. Sunglasses were made from smoky quartz to protect eyes from glare and to hide facial expression in court. From there, various incarnations of sunglasses were developed, but widespread use didn’t occur until the early 1900s with the rising of popular entertainment and Hollywood.
Polarised sunglasses were a later invention, following the invention of Polaroid in 1932 by Edwin H Land. He succeeded in aligning submicroscopic crystals of iodoquinine sulphate and embedding them in a sheet of plastic, naming it a ‘Polaroid J sheet’. He joined forces with George Wheelwright iii, a Harvard Physics instructor, and in 1936 they developed and used several types of Polaroid material in sunglasses and other optical devices like camera filters. Thence came the birth of the polarised sunglasses and the Polaroid camera. From there, the use of polarised lenses in safety eyewear was an easy stretch.
How are Polarised Lenses Made?
Now that we understand how polarisation works, we can understand the properties required by a material to be effective: it needs to include long-chain molecules arranged in a consistent direction. However, there are several different approaches used to make polarised lenses, with varying results.
The cheapest way is to laminate a polarised acetate or acrylic-based film between thin sheets of acrylic or acetate, then use heat to make a curved shape and cut the lens shape out. However, this often results in distortion of the lens and the material scratches easily.
Another method is via polycarbonate injection moulded lenses, where the polarised film is put into a mould and polycarbonate is injected around it. This is the most common approach used by sports brands like Bollé Safety, who favour polycarbonate because it is more impact resistant than other materials and makes lighter-weight lenses. However, polycarbonate can scratch easily, so an anti-scratch coating like Bollé Safety’s Platinum coating is useful for durability.
A third method involves inserting a layer of polarised film between layers of CR39, or ‘Columbia Resin #39’. CR 39 has an optical clarity almost as good as glass but is less likely to shatter. However, CR 39 is not as impact resistant as Polycarbonate, which is important in eyewear designed for safety and sport. CR 39 lenses also tend to be thicker, heavier and, unlike polycarbonate, are not inherently UV-protective and so must be treated with an additional UV-protective coating.
Glass can be used in the same way as CR-39 for optimum clarity and scratch resistance but does not have UV protection. It is also the least shatter-resistant, so should not be used in safety glasses designed for impact protection.
What is the Difference Between Sunglasses and Safety Glasses?
When choosing glasses for safety use, specialised safety glasses are recommended above normal sunglasses as they are made to specific safety standards that normal sunglasses are not held to. However, polarised safety glasses, such as Bollé Safety’s Baxter Polarised, have the benefit of being safety glasses that also act like sunglasses, so are particularly useful.
Who should Wear Polarised Sunglasses or Safety Glasses?
Polarised sunglasses and safety glasses have the same properties as their non-polarised counterparts, like UV protection and increased comfort and visibility. However, they perform the added service of improving visibility and clarity when there is reflected light. Therefore, polarised eyewear is particularly useful when reflective surfaces are more common: maritime environments, water or snow sports, or when working outside in mines or on construction sites. However, lots of reflective surfaces are found in everyday settings; from roads or concrete or when driving, due to reflections from glass windscreens. As such, Polarised eyewear is useful to pretty much everyone.
There are a few instances where polarised eyewear is not ideal. In most settings, not seeing reflective light is desired, but in some cases, like when driving on roads that might be icy, being able to see the reflection of light off the ice can prevent hazards.
Moreover, aeroplane windscreens are polarised, and so pilots should not wear polarised sunglasses as visibility will be reduced.