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Polarized Lenses

Polarized Lenses & How They Work
 
Click for: Frequently Asked Questions About Polarized Sunglasses
 

 

Sun and Light Polarization:

The glare of the sun on the sea (and other surfaces) is highly polarized! Polarized lenses are used in sunglasses to reduce glare from reflective surfaces such as the surface of a lake or the hood of a car. They accomplish this feat through a process called polarization, much like a venetian blind controls sunlight through a window.


Sunlight itself is not polarized - light from the sun will either be absorbed or reflected horizontally, diagonally or vertically. The problem this causes for fishermen and drivers lies in the horizontal reflectivity. Sunlight bouncing off a horizontal surface will strike the viewer's eyes at a similar angle. This means the glare from the surface of the water or the road or the car hood will be strong. Polarized lenses have a laminated surface containing vertical stripes. These stripes only allow vertically-polarized light to enter the wearer's eyes. Glare is eliminated because the horizontally-polarized lightwaves cannot bypass the polarized filter.


Polarized lenses do not provide universal protection from glare, however. If the wearer tilts his head past 45 degrees or so, some of the horizontal light can enter and cause some bright spots. Some wearers who use polarized lenses in snowy areas may find that they lose some contrast between snow and shadows. Fishermen and drivers seem to derive the most benefit from polarized lenses because they deal with mostly horizontal surfaces. A fishermen wearing polarized sunglasses can see below the surface of the water to search for schools of fish or hidden obstacles. Drivers are no longer affected by a constant reflection of light from their hoods or the road ahead.


There are some limitations to the use of polarized lenses, however. Because the polarizing stripes reduce the amount of light entering the eye, polarized lenses cannot be used for regular eyewear with clear lenses. Prescription sunglasses can be tinted a very light gray to accommodate the polarized laminate, but most contact lenses cannot. Some drivers discover that polarized lenses can cause distortions in liquid crystal displays (LCDs). Onboard clocks and other instrument displays may be temporarily unreadable.


Both horizontally and vertically-polarized lenses are used to view three-dimensional movies. Some of the light reflected from the movie screen is vertically polarized, which means only a lens with a similar vertical laminate will receive it. The other lens only receives horizontal lightwaves. Your brain takes both of these images and combines them to create a realistic sense of depth.

 

What is Polarization?

When light strikes a surface, it becomes polarized. This means that the photons in the light are all vibrating in one direction, instead of in random directions. The light waves travel from "pole" to "pole" along that plane. This phenomenon is what causes the glare reflected from a car window or chrome, or from a wet pavement. This glare distorts the true color of objects and makes them harder to distinguish. It also causes a mirror-effect on wet surfaces, so that objects below the water's surface cannot be clearly distinguished.

 

What Does a Polarized Lens Do?

Polarized lenses filter the waves of light by absorbing some of the reflected glare while allowing other light waves to pass through them. The most common illustration of how a polarized lens works to reduce glare is to think of the lens as a Venetian blind. These blinds block light that strikes them from certain angles, while allowing light from other angles to pass through. A polarizing lens works when it is positioned at a 90-degree angle to the source of the glare. Polarized sunglasses, which are designed to filter horizontal light, are mounted vertically in the frame, and must be carefully aligned so that they will properly filter the light-waves.

 

How Polarized Sunglasses Are Made

The first polarized lenses were made of a polarizing film that was sandwiched between two flat sheets of glass. Because glass lenses are so heavy, however, the advent of plastic and polycarbonate materials, which are lighter and thinner, made the glass lenses less popular.
One of the problems with early polarized sunglasses was delamination, when the polarizing film separated from the pieces of glass or plastic. This problem has been solved with modern lenses, as the plastic can be melted and poured into a mold in which the polarizing film has been suspended. The plastic then hardens around the film, creating a solid material, rather than a layered one. Polycarbonate lenses are created in a different manner, since polycarbonate lenses are injection molded and the heat from the process would destroy the polarizing film. For polycarbonate lenses, the polarizing film is applied to the front of the lens and covered with a scratch-resistant coating. This process means that polarized polycarbonate lenses are the thinnest and lightest polarized lenses available.

 

What Colors are Available?

All polarized lenses have some color, as it is possible to polarize light only when the film is colored. Darker colors provide higher levels of polarization. Depending on the material used, polarized lenses are available in a variety of colors. The following colors are available in each of the following lens materials:

  • Glass: light or dark gray, classic gray, light or dark brown, red, orange, yellow, green, blue, violet, copper.
  • CR-39 (a plastic polymer -- allyl diglycol carbonate): light or dark gray, classic gray, light or dark brown, red, orange, yellow, green, blue, violet, copper, melanin.
  • Plastic 1.56: light or dark gray, classic gray, light or dark brown, red, orange, yellow, green, blue, violet, copper, melanin.
  • Polycarbonate: dark gray, dark brown.
  • High-Index Plastic 1.60: light or dark gray, light or dark brown.

 

How Do I Choose the Right Color?

Select your polarized lenses carefully, as lens color does have an impact on what you see. It is important to choose the right color lens for the activity in which you will be wearing the sunglasses. Each color affects vision in different ways.

  • Gray: The most popular color for sunglasses in the U.S., gray transmits all colors evenly. It makes a good everyday or all-purpose choice of sunglasses. It is recommended for ocean fishing, where the water tends to be blue. "Classic gray" is a color that provides near-natural vision and true color perception.
  • Brown: Because brown improves contrast and depth perception, it is best for driving and shallow-water fishing. Light brown is a good color for overcast days and is recommended for skiers and golfers. Brown is another popular color, especially among the fashion-conscious, because many fashion authorities believe that brown is flattering to the face.
  • Red: Red is primarily used by sportsmen working in bright, sunny places.
  • Orange: Like yellow, orange lenses filter out blue light and increase contrast sensitivity. Orange is a popular color among skeet shooters because it makes the targets more visible.
  • Yellow: This color filters out blue light. Because it provides the highest level of contrast, it is perfect for cloudy-day driving and for marksmanship practice.
  • Green: Green improves contrast slightly, and keeps a good balance among the colors of light. It is a popular color for tennis, golf, or driving.
  • Blue: Blue allows the most blue light to filter through, so it is good in partly cloudy weather. Blue is a popular lens color among tennis players and golfers.
  • Violet: Violet causes green to fade and enhances orange, green, and black. It is good for shooting under average conditions.
  • Copper: Copper provides maximum contrast and is best for any activity in which good vision is important.
  • Melanin: Melanin lenses are relatively new on the market. They are a golden-brown tint and provide high contrast. Melanin lenses are believed to help protect against macular degeneration. Melanin is a pigment that occurs naturally in the body and protects the body from harmful exposure to sunlight. Melanin lenses are made with a synthetic form of this pigment that never loses its potency.
 
 

Frequently Asked Questions About Polarized Sunglasses

 

How do I know if mine are polarized?

Very easily done. Just look through the glasses at the reflection of any object on a window panel (but not at yourself! you have to look at an angle to the glass). Then, turn the sunglasses around as if they were the hands of a clock facing you. If the intensity of the reflection doesn't change with respect to what you see through the window, they are not polarized.


I still don't understand, why are polarized sunglasses better than normal dark sunglasses?

Normal sunglasses decrease the intensity of everything by the same amount. Polarized sunglasses can selectively eliminate the reflection from light coming from above the water surface.


Then, could polarized glasses be crystal clear?

No, they will always decrease the intensity of un-polarized light by at least half. In practice somewhat more because of losses in the material.  They come in all shades from light gray (only polarizing) to very dark (polarizing plus black tint), and also light sensitive (photochromatic).  You can even find sunglasses with darkness adjustable by the user.


Do polarized sunglasses help see fish and rocks below the water surface?


Yes. First, if there is glare, they help a lot by reducing its blinding effect. Second, even without glare they selectively reduce other reflections from objects above water, including clouds and even the sky (the reflected sky gives most of its blue color to the sea). Finally, light coming from under water is slightly polarized in the vertical plane (polarized on transmission). The end effect is that the water seems darker but more transparent! But remember, it only works if you look at the water at some angle and not straight down.


Does it matter the time of the day?


Yes. Maximum polarization is obtained when the sun is at about 37 degrees from the horizon (in theory 100% polarization at the Brewster angle). If the sun is very low or very high the sunglasses will be of little help in filtering the glare in calm seas. A rule of thumb would be that polarized filters limit the glare from calm waters for a sun altitude between 30 and 60 degrees (but see next question). Anyway, it should be stressed that polarization won't help in looking directly at the sun (except in decreasing the overall intensity of everything by half).


Does it matter the choppiness of the sea?

Yes. When the sea is ruffled the sun reflection becomes the familiar glitter, an elongated pattern of shimmering water stretching towards the sun. Because different parts of the glitter are reflected from different wave slopes, the degree of polarization varies from place to place. In those conditions the sunglasses will also help for high or low suns and the benefit will depend on where you are looking.  As a side note, the width and length of the glitter together with the altitude of the sun can be used to compute the height of the waves without ever getting close to them!


Can polarized sunglasses help when driving a car?

Yes. Those pesky bright reflections of the sun on the cars ahead can sometimes be attenuated a good deal. They tend to be horizontally polarized, thus perfect for vertically polarized sunglasses.  The reason is that the surfaces that you see on the car in front of you (the back window, the trunk door, and even the roof) will be slanted towards you, while the sun will be more or less aligned in the vertical plane through both cars (if not you wouldn't see its reflection from those surfaces). However, if the sun is relatively low behind you, the sun rays will be near perpendicular to the reflecting (vertical) surfaces which won't polarize the light.  The sunglasses will help more with the reflections from the glass than from the metal as the former are polarized to a higher degree.  What about the brightness of the road itself?  That light is also partially polarized, but by scattering (as opposed to reflection).  The direction of polarization will change with the direction of the road with respect to the position of the sun.  The rule is that the polarization is tangential to a circle centered on the sun.  That means that if the sun is in front, behind, or high above, the road brightness will tend to be horizontally polarized and the filter of the sunglasses will provide some help.  However, if the sun is to one side, the polarization will rotate vertically, the more so the lower the sun happens to be.  Of course, if the road is wet you get the same anti-glare power than at sea.


Can polarized sunglasses help in skiing and other non-water sports?

Sometimes yes, sometimes no. The snow glare is not polarized, so they won't provide extra help in that regard (although often advertised for that purpose). But, with the sun high, the air-light (haze + sky) near the horizon is polarized by scattering and the polarized sunglasses can make features far away really stand out (this is used in fire detection). On the other hand, with the sun low the sunglasses could be detrimental looking south or north, as the air-light would be vertically polarized. Other situations can be thought were they would be useful, but remember that one reason they are so good in water sports is that the reflector always remains horizontal!


Can I use my 3-D polarized glasses for fishing?

Not a very good idea.  The filters on those glasses are oriented at 90 degrees to each other, almost always at +/- 45 degrees with respect to the line joining the temples (generally the transmission axis points down towards the nose).  Thus, horizontally and vertically polarized light will be attenuated equally.  In addition, you can get a bad case of dizziness: the left and right eyes may see polarized objects with very different intensities, confusing stereovision and resulting in vertigo (the same effect as produced by the glistening of the snow).  

 

 

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