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Why Polarized
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. Luckily, most of these problems can be resolved through the use of polarized sunglasses. Polarized lenses enhance contrast and visual clarity, increase depth perception, eliminate dangerous glare and reduce eye strain.
The Technology behind the Lens
The easiest way to describe a polarized lens is to describe its filter as 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 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 less effective with glare.
Polarized lenses are composed of thin sheets of polyvinyl alcohol that are stretched over a clear lens surface. The film is then typically passed through an iodine solution. Once in the solution, the iodine crystals align themselves parallel to each other, creating a window Venetian blind effect. 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 that are then laminated together to form one solid lens.
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. This was especially true for glass polarized lenses. Plastic polarized lenses are created when the CR-39 lens material, which is a thick viscous liquid, 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.
Over the Rainbow
There are a variety of polarized lens choices available today. There are a multitude of colors available as well as color densities. Each has its own unique attributes and recommended circumstances. What follows is a list of these lens choices and their suggested application:
Gray, brown, G-15: These are the typical colors for polarized lenses. They are the classic sunglass shades and utilized for general wear. The gray C and G-15 choices are general purpose colors and densities. The classic brown C shade has some contrast enhancement in addition to the general purpose density.
Dark Gray- 25% light transmission; truest color recognition and is ideal for bright sunny days and high glare activities such as deep water fishing and driving.
Light Gray- 55% light transmission; ideal for being used with tints to create a custom colored polarized lens.
Dark Brown- 25% light transmission; a true blue blocker balanced lens - it is ideal for improved contrast since it eliminates much of the blue wavelength, it also enhances visual acuity and depth perception, and is useful in activities such as driving and shallow water fishing.
Light Brown- 45% light transmission; ideal for being used with tints to create a custom colored polarized lens.
Melanin- 25% light transmission; a relatively new lens to the market, it contains the pigment that normally occurs in the body and is ideal for anyone with macular degeneration and works well in bright conditions and activities in which the light varies such as golfing.
Red- 55% light transmission; a high contrast lens that is ideal for low light fishing and as a shooting lens in bright light.
Orange- 50% light transmission; blocks blue light, increases contrast and is most commonly used for clay target shooting, biking, and skiing. Can inhibit color perception.
Green- 60% light transmission; true color balanced lens that works well in varying light conditions such as golf and tennis.
Violet- 55% light transmission; increases contrast and darkens certain backgrounds so it works well with shooting in average to bright, golfing and for winter sports.
Blue- 60% light transmission; lets in the maximum amount of blue light so it works well with sports activities against green backgrounds, for example tennis and golf. However, it does block much of the yellow wavelength, so it will have quite a darkening effect to the wearer.
Yellow- 80% light transmission; eliminates the blue wavelength of light so it is good for contrast enhancement. It truly makes images stand out from their background so it is effective in early morning hunting situations and for some target and skeet shooting. This lens will definitely inhibit color perception since it eliminates the blue wavelength.
Finishing Touches
Anti-Reflective Coating
Light bouncing off of a dark lens creates a window pane, mirror-like effect that causes distortion and eye fatigue. To prevent this, an AR coating can be applied to the back of the lens. Consisting of metal oxides, AR coating is a multilayer process. Each layer is a thin film that is designed to block a specific wavelength of light from reflecting. The more layers of the film that are applied to the lens, the more light wavelengths that are blocked from reflecting. Working by the destructive interference principle, AR causes the light that reflects from the inner and outer surfaces of each film layer to become equal and thereby canceling each other out and eliminating the reflection in the process.
The newer anti-reflective coatings are excellent for outdoor enthusiasts. They repel dirt, dust, water and fingerprints through the use of anti-static, hydrophobic and oleophobic coatings. Hydrophobic layers are thin layers of silicone that are placed on top of the AR. It provides a smooth surface that decreases the wetting angle of the lens. The result is that liquids form small beads and can be easily removed from the lens. Oleophobic layers are important with the multilayer coatings because the more effective the coating is in easing reflections, the more likely it is to show smudges. An added benefit is that when oleophobic and hydrophobic layers are combined, they create a seal that acts as a secondary scratch coat.
Mirror Coating
Mirror coatings are highly reflective coatings that are applied to the front surface of the lens. Applied using a vacuum process, mirrors can come in flash, solid, gradient, double gradient and triple gradient. It can help reduce eyestrain by blocking an additional 10 to 60 percent of light transmission to the eye. This can be especially helpful when participating in highly reflective outdoor activities such as water or snow skiing, driving, mountain climbing at high altitudes or at the beach. However, be mindful of the total amount of light you are blocking. If you are using a polarized lens with 80% light transmission (20% light blockage), you would most likely not want the mirror to exceed 60% since the combination would equal 80% light absorption or 20% light transmission. These are the levels of a standard sunglass. In extreme cases naturally, you could go with greater light blockage.
Polarization is the ultimate combination of form and fashion. When properly used, polarized glasses reduce dangerous glare, dramatically reduce eyestrain, provide a fashion statement and enhance performance.
With contributions from: Brian A. Thomas, P.h.D, ABOM
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