The Future of Polarization
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Polarized sunglasses are an important part of the ECP’s arsenal of protective eyewear.
Many popular summer activities such as fishing, bicycling, sports, even relaxing beside the pool, can be visually demanding for our patients. Glare, eyestrain, and depth perception problems can all result from the bright summer sun. Ocular conditions such as iritis and cataracts can make the effects of the sun even more devastating to our patients. Luckily, most of these problems can be resolved through the use of polarized sunglasses. But, the use of polarization doesn’t end there. Understanding polarization will become even more important as technology progresses.
What is Polarization?
Light is a series of electromagnetic waves. The placement and rate at which these waves oscillate, or vibrate, is the light’s polarization. Direct light, such as sunlight, vibrates in all different directions simultaneously and is therefore
unpolarized. Once light hits a highly reflective, bright, flat surface however, it bounces off the surface and becomes polarized. Polarized light occurs when all of the light, except for one vertical and one horizontal angle of light, is either scattered or absorbed. The plane of polarization of reflected light is parallel to the surface that is reflecting the light.
Since most reflective surfaces are on a horizontal plane, i.e. water, the road, car hoods, snow, the polarized beams that cause the most glare problems is the horizontal polarized light. The intensity of this horizontal polarized beam can make it difficult for the patient to obtain accurate information about color, texture or depth of an object leading to discomfort or a safety hazard. Therefore, to eliminate the horizontal polarized light from reaching the eye, a filter is fixed at an angle that only allows vertical polarized light to enter the eye. This completely eliminates the horizontal waves from reaching the eye.
The Polarized Filter
The easiest way to describe a polarized filter is to compare it to a micro Venetian blind. Like the Venetian blind on a window, a polarized filter blocks light coming in at a certain angle, while allowing light to be transmitted through the lens at an angle 90 degrees away. The blinds are positioned so that the horizontal light enters the room while the vertical light is blocked. The polarized filter is horizontally aligned so that the horizontally reflected light is absorbed. This can be demonstrated by viewing light reflected off of a flat surface through a polarized lens. Rotate the lens 90 degrees so that the changes in the intensity of the reflection can be observed. The horizontal alignment of a polarized lens is crucial. Any misalignment will cause the lens to be ineffective. Worse yet, if one lens is aligned horizontally and the other is not, the patient will find their vision disorienting.
Polarized lenses are composed of thin sheets of polyvinyl alcohol that are stretched within a clear lens surface. The stretching causes the molecules to form long chains that run parallel to the direction that the sheet was stretched. The film is then typically passed through an iodine solution. Once in the solution, the iodine crystals align themselves with the long molecules, creating a window blind effect as described above.
Because there are multiple ways of creating a lens, there are multiple ways of applying the polarized filter. The oldest form of polarization is called lamination. This is when a polarized film is placed between two pieces of lens substrate. Although not considered a significant problem today due to improved manufacturing methods -
delamination, or the separation of the polarizing film from the lens material, was a major issue for eye care professionals. Glass ophthalmic lenses are still done with the lamination process.
Plastic lenses were also done with lamination, but the newest technology has all but eliminated that procedure for plastic lenses. Plastic polarized lenses are created when the CR-39 is poured into a mold in which the polyvinyl film has already been placed. Due to the heat created by the injection molding process of polycarbonate manufacturing, the polarized filter is placed on the lens blank and then coated with a scratch resistant coating. This helps protect the polarized film from heat degradation.
Customizing the lens
Although tinting ophthalmic lenses is always an option, for superior glare protection there is no comparison to polarized sunglass lenses. The many benefits of Polarized lenses include:
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Enhanced contrast – especially with brown polarization
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Eliminating dangerous glare
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Reduced eyestrain
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Increase depth perception, particularly around water
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Enhanced visual clarity
That being said, there are many ways to enhance a polarized lens for the patient. Not all polarized lenses are dark
grey! There are a variety of colors available and there are a variety of densities available. Most polarized lenses are available in shades A, B and C with C being the traditional dark sunglass shade and A and B being significantly lighter in density. Other techniques can improve the polarization performance as well. The main way to achieve this is through coating selection. The most popular coatings include mirror coating and anti-reflective coating.
Mirror Coating
Mirror coatings are a highly reflective coating that is applied to a lens to help reduce the amount of light that enters a lens. The concept is to reflect the light off of the lens surface as opposed to absorbing the light inside the lens. Composed of various metal oxides, the color and density of the coat is based on the amount and ratio of oxides that are placed on the lens. Applied using a vacuum process, mirrors can come in flash, solid, gradient, double gradient and triple gradient.
An important side note is to be mindful of the combination of mirror and polarization to avoid blocking too much light. A 50% mirror with a 50% polarization filter would effectively block all light. The average sunglass protection is 80% blockage of light. Naturally, you can exceed the 80% blockage if the circumstances warrant, but for an average person in average circumstances the 80% rate is a good rule of thumb to follow.
Anti-reflective Coating (AR)
Reflections off of the back surface in a dark lens can be very distracting for the patient and pose a safety hazard. To prevent this, an anti-reflective coating is applied to the lens. There is some debate as to whether or not to apply AR coating to the front surface of the lens. One thought is that the AR coat, when applied to the front of the lens, interferes with the way the polarization process works. Another belief is that AR coat gives the same benefit outdoors as it does indoors and that the reflection and glare elimination is superior with AR applied on both surfaces. On a practical note, many ECPs refrain from applying anti-reflective coatings to the front surface because of the wear and tear most sunglasses receive. It is up to the eye care professional to determine which would best benefit his or her patient.
The Future of Polarization
With the recent increase in popularity of 3D feature films and the introduction of 3D television and computer games into the home, prescription polarized glasses for entertainment purposes are more likely to become standard.
In the past, a small number of 3D movies were presented in anaglyph format. Anaglyph is when 3D images are projected on the screen and then filtered through the eye using red/green or red/blue glasses. The colors would suppress the complimentary colors rendering the images black and causing a 3D image to enter the eye. Although easier to use and create than modern imaging systems, the red/green and red/blue system does not work well with most color films and results in reduced color saturation and ghost imaging.
Polarized systems have been around since the 1950s but they were often expensive and only comprised a niche within the entertainment business. Today, polarization for 3D movies is becoming more reliable and standardized. The way it works is that two images are superimposed onto the screen using different polarizing filters. Then glasses are worn with the polarizing filters in the opposite direction. The filters are either linear (horizontal vs. vertical) or circular (clockwise vs. counter clockwise). This allows the image to be seen differently by each eye. Previously, the different types of polarization made it cost prohibitive to have a prescription pair of glasses made for 3D viewing. Today, with polarization standardization becoming easier and 3D entertainment becoming commonplace, prescription polarized 3D glasses are becoming more likely.
Polarization is an excellent tool in the eye care professional’s tool box. When properly used, polarized glasses reduce dangerous glare, provide a fashion statement and enhance performance. By understanding the method by which polarization works, ECPs may one day enhance the way individuals enjoy their multimedia tools and entertainment.
With contributions from: Brian A. Thomas, P.h.D, ABOM
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