Prescription Sunglasses

Prescription sunglasses are often the best solution for clear, comfortable vision outdoors or when driving in sunny conditions. Though it can be an advantage to wear contact lenses, and then use cheap commercially-available sunglasses over them, for most people the clarity of vision through laboratory-quality lenses is superior. And some environments, ones that are dusty, salty, or windy, can dry contacts out, making them less ideal.

The purpose for tinting lenses is to enhance vision and comfort by filtering out unwanted or damaging light, mostly in the blue and ultraviolet part of the visible spectrum. There are several main features preferred in tinted lenses, so below is a discussion of the most common features, and some theory to explain how they work.

Ultraviolet (UV) Light

This is a wavelength on the electromagnetic spectrum which is shorter than violet, but longer than the X-ray. Ultraviolet light is the most damaging kind of light to the human eye, and even blue and violet exposure is known to cause some damage.

The UV spectrum is divided into A, B, and C, with A being the longest and least damaging of them. On the opposite side, UVC (200-280nm) is extremely damaging to all life on this planet, but does not pass through the earth’s atmosphere. (The sun gives off a great deal of UV - about 10% of its total power). 324nm is the wavelength where our skin begins to burn, and this is within the ‘B’ part of the UV spectrum.

For the eye, the most damaging rays are in the range above 400nm, and lenses marked ‘UV400’ protect against these. Though all optical plastic lenses absorb some UV light from 270-360nm in order to keep them from yellowing over time, adding UV coatings increase the depth and range of protection for your eyes. (Keep in mind that UV lens coatings are transparent to the human eye, and do not change the color of the lenses.) Since snow and water environments increase UV exposure by reflecting UV rays from below, and high altitude environments increase it because there is less of the earth’s atmosphere to filter UV out, these are situations where extra UV protection is essential.

Polaroid Lenses

Though there is still scientific dispute about the nature of light, it is safe to say that light acts as both wave and particle. Polarized light is a kind of light in which individual light waves are aligned so that they travel parallel to one another. This is why polarization is considered to be an example of the wave property of light.

Light normally travels as a transverse wave in free space, with the direction of polarization being perpendicular to the direction of travel. This angle can rotate as it travels (circular or elliptical polarization), but when light is being reflected off of something (like a road, water, glass surface) its polarization is made more uniform (linear)—like it is flattened out. Polaroid lenses only allow light through that is travelling at a certain polarization angle, so when these lenses are oriented to screen out light that is traveling in the direction of the reflected light, it greatly lessens the glare off of reflected surfaces. This can really increase safety and comfort when driving a car or a boat, making polaroid sunglasses superior to other kinds. (Note: Aviators tend not to use polaroid lenses because they interfere with seeing their LCD cockpit displays.)

Light coming through the earth’s atmosphere is polarized, so Polaroid camera filters are used to block out the polarized portion coming from the blue sky to darken it, which increases the contrast between the sky and the clouds in a photograph. They are also used in photography to reduce glare in water scenes.

The Role of Common Tint Colors

It is very helpful to first understand the concept of chromatic aberration - bear with me!

Though ‘chromatic aberration’ might sound like a weird kind of avant-garde music, in optics this involves how a lens focuses different wavelengths (colors) of light. When light passes through a clear material (called a medium), you could think of it as though it were being slowed down: the denser the medium, the slower the light passes through. Think of a prism, a wedge-shaped piece of material that has two flat surfaces but with more material toward one end: if light rays traveling in a straight line have to pass through this, they will be bent toward the thicker end. This alters their path of travel. But now remember that white light is made up of many different colors (wavelengths) of light: if it were to pass through this prism, each wavelength in it will be bent to a slightly different degree depending on how densely the light rays are packed together (their wavelength/color.) This causes the colors to separate, with the amount of this separating power determined by how dense the medium is. (Think of a piece of leaded crystal put in a window which makes a ‘rainbow’ on the wall when sunlight hits it.) All lenses work this way as well: light is bent toward where the thick part is, causing images to be shifted in where they become clear. The trouble is that this focused image, made up of many colors of light, may not come into focus in exactly the same place because some parts of the light are being focused ahead of others. This is chromatic aberration. Optical materials are engineered to minimize this scattering, keeping images from being blurry or having halos around them.

Tinting a lens simply screens out certain wavelengths of light, enabling you to choose which wavelengths of light pass through. Generally, neutral colors like gray or brown are preferred by most people for sunglasses because they dampen all colors most evenly, allowing them to be seen more naturally. Sometimes, however, dampening most colors and leaving only one is preferred because it limits the scattering caused by chromatic aberration, making images appear clearer. Since the human eye is most sensitive to the wavelength 555nm (perceived as amber), this is the best choice. This is why amber is the color preferred for sports like cycling, hunting, shooting, skiing, snowboarding, snowmobiling, indoor basketball, handball, racquetball, and tennis. It is also why amber is the colore used in foggy environments, either in the street lights or on vehicles (‘fog lights’). Blue is a poor choice because the human eye processes shorter wavelengths (like blue), more slowly than the rest of the visible spectrum.

Though every effort has been made to be accurate, all articles contained within this website are for strictly informational purposes, and not to be used as a sole source for healthcare training and instruction. Neither does Family Vision Center of Tacoma, P.S., assume responsibility for actions undertaken by someone using information presented on this website to make healthcare decisions.


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