Contact lenses are thin transparent plastic discs that sit on the cornea. Just like eyeglasses, they correct refractive errors such as myopia (nearsightedness) and hyperopia (farsightedness). With these conditions, the eye doesn’t focus light directly on the retina as it should, leading to blurry vision. Contact lenses are shaped based on the vision problem to help the eye focus light directly on the retina. Some contact lenses are less about seeing than being seen.
Contact lenses are closer to natural sight than eyeglasses. They move with the eye/
Normal glasses can get in the way of the line of sight. Contact lenses don’t. They can be worn several days at a time.
Contact lenses stay in place by sticking to the layer of tear fluid that floats on the surface of the eye and by eyelid pressure. The eyes provide natural lubrication and help flush away any impurities that may become stuck to the lens.
Originally, all contact lenses were made of a hard plastic called polymethyl methacrylate (PMMA). This is the same plastic used to make Plexiglas. But hard lenses don’t absorb water, which is needed to help oxygen pass through the lens and into the cornea. Because the eye needs oxygen to stay healthy, hard lenses can cause irritation and discomfort. However, they are easy to clean.
Soft contact lenses are more pliable and easier to wear because they’re made of a soft, gel-like plastic. Soft lenses are hydrophilic, or “water loving,” and absorb water. This allows oxygen to flow to the eye and makes the lens flexible and more comfortable. More oxygen to the eye means soft contact lenses can be worn for long periods with less irritation.
Daily-wear lenses are the type of contacts removed every night before going to bed (or whenever someone decides to sleep). Extended-wear lenses are worn for several days without removal. Disposable lenses are just what the name implies: they are worn for a certain period of time and then thrown away. Cosmetic lenses change the color of a person’s eyes. Ultraviolet (UV) protection lenses act as sunglasses, protecting the eyes against harmful ultraviolet rays from the sun.
Corneal reshaping lenses are worn to reshape the cornea and correct vision. Rigid, gas-permeable lenses have both hard and soft contact lens features. They are more durable than soft lenses but still allow oxygen to pass through to the eye. They don’t contain water, so are less likely to develop bacteria and cause infection than soft lenses. They are also hard enough to provide clear vision.
Contact lenses are frequently customized for athletes, computer operators and other applications. Many contacts don’t just correct vision problems but improve it.
Sunglasses provide protection from harmful ultraviolet rays in sunlight. Some sunglasses filter out UV light completely. They also provide protection from intense light or glare, like the light reflected off snow or water on a bright day. Glare can be blinding, with distracting bright spots hiding otherwise visible objects. Good sunglasses can completely eliminate glare using polarization.
Sunglasses have become a cultural phenomenon. In the fashion world, designer sunglasses make people look “cool,” or mysterious. They can also be ominous, such as the mirrored sunglasses worn by roughneck bikers and burly state troopers.
Cheap sunglasses are risky because although they are tinted and block some of the light, they don’t necessarily block out UV light. Cheap sunglasses are made out of ordinary plastic with a thin tinted coating on them.
There are several types of lens material, such as CR-39, a plastic made from hard resin, or polycarbonate, a synthetic plastic that has great strength and is very lightweight. These kinds of lens are usually lighter, more durable, and scratch-resistant. Optical-quality polycarbonate and glass lenses are generally free from distortions, such as blemishes or waves. The color is evenly distributed. Some sunglasses are very dark and can block up to 97 percent of light.
More expensive sunglasses use special technologies to achieve increased clarity, better protection, and higher contrast or to block certain types of light. Normal frames similar to prescription eyeglasses filter light but sometimes offer little protection from ambient light, direct light and glare. Wrap-around frames, larger lenses and special attachments can compensate for these weaknesses. Most cheap sunglasses use simple plastic or wire frames, while more expensive brands use high-strength, light-weight composite or metal frames.
The brightness or intensity of light is measured in lumens. Indoors, most artificial light is around 400 to 600 lumens. Outside on a sunny day, the brightness ranges from about 1,000 lumens in the shade to more than 6,000 lumens from bright light reflected off of hard surfaces, like concrete or highways.
Comfort levels are around 3,500 lumens. Brightness above this level produces glare. Squinting is the natural way to filter such light. In the 10,000 lumens range, prolonged exposure to light of such intensity can cause temporary or even permanent blindness. A large snowfield, for instance, can produce more than 12,000 lumens, resulting in what is commonly called, “snowblind.”
Three kinds of light are associated with sunglasses: direct, reflected, and ambient. Direct light is light that goes straight from the light source (like the sun) to the eyes. Too much direct light can wash out details and even cause pain. Reflected light (glare) is light that has bounced off a reflective object to enter the eyes. Strong reflected light can be equally as damaging as direct light, such as light reflected from snow, water, glass, white sand and metal.
Ambient light is light that has bounced and scattered in many directions so that it is does not seem to have a specific source, such as the glow in the sky around a major city. Good sunglasses can compensate for all three forms of light.
Sunglasses use a variety of technologies to eliminate problems with light: tinting, polarization, photochromic lenses, mirroring, scratch-resistant coating, anti-reflective coating, and UV coating.
The color of the tint determines the parts of the light spectrum that are absorbed by the lenses. Gray tints are great all-purpose tints that reduce the overall amount of brightness with the least amount of color distortion. Gray lenses offer good protection against glare. Yellow or gold tints reduce the amount of blue light while allowing a larger percentage of other frequencies through.
Blue light tends to bounce and scatter off a lot of things; it can create a kind of glare known as blue haze. The yellow tint eliminates the blue part of the spectrum and has the effect of making everything bright and sharp. Snow glasses are usually yellow. Tinting distorts color perception are tinted glasses are not very useful with there is a need to accurately see color. Other colors include amber, green, purple and rose, all of which filter out certain colors of the light spectrum.
Light waves from the sun or even from an artificial light source such as a lightbulb, vibrate and radiate outward in all directions. Whether the light is transmitted, reflected, scattered or refracted, when its vibrations are aligned into one or more planes of direction, the light is said to be polarized.
Polarization can occur naturally or artificially. On a lake, for instance, natural polarization is the reflected glare off the surface is the light that does not make it through the “filter” of the water. This explains why part of a lake looks shiny and another part looks rough (like waves). It’s also why nothing can be seen below the surface, even when the water is very clear.
Polarized filters are most commonly made of a chemical film applied to a transparent plastic or glass surface. The chemical compound used will typically be composed of molecules that naturally align in parallel relation to one another. When applied uniformly to the lens, the molecules create a microscopic filter that absorbs any light matching their alignment. When light strikes a surface, the reflected waves are polarized to match the angle of that surface. So, a highly reflective horizontal surface, such as a lake, will produce a lot of horizontally polarized light. Polarized lenses in sunglasses are fixed at an angle that only allows vertically polarized light to enter.
Sunglasses or prescription eyeglasses that darken when exposed to the sun are called photochromic, or sometimes photochromatic. Because photochromic lenses react to UV light and not to visible light, there are circumstances under which the darkening will not occur.
A good example is in the car. As the windshield blocks out most of the UV light, photochromic lenses will not darken inside the car. Consequently, many photochromic sunglasses are tinted. Photochromic lenses have millions of molecules of substances, such as silver chloride or silver halide, embedded in them. The molecules are transparent to visible light in the absence of UV light, which is the normal makeup of artificial lighting. But when exposed to UV rays in sunlight, the molecules undergo a chemical process that causes them to change shape.
The new molecular structure absorbs portions of the visible light, causing the lenses to darken. Indoors, out of the UV light, a reverse chemical reaction takes place. The sudden absence of UV radiation causes the molecules to “snap back” to their original shape, resulting in the loss of their light absorbing properties.
With some prescription glasses, different parts of the lens can vary in thickness. The thicker parts can appear darker than the thinner areas. By immersing plastic lenses in a chemical bath, the photochromic molecules are actually absorbed to a depth of about 150 microns into the plastic. This depth of absorption is much better than a simple coating, which is only about 5 microns thick and not enough to make glass lenses sufficiently dark.