Lens

Lens MaterialsProgressive LensAnti-reflective LensPhotochromic LensSunwear

Lenses were originally made from glass, but many are now made from various types of plastic including CR-39 and polycarbonate. These materials reduce the danger of breakage and weigh less than glass lenses. Some plastics also have more advantageous optical properties than glass, such as better transmission of visible light and greater absorption of ultraviolet light. Some plastics have a greater index of refraction than most types of glass; this is useful in the making of corrective lenses shaped to correct various vision abnormalities such as myopia, thus allowing thinner lenses for a given prescription.

CR-39 lenses are the most common plastic lenses due to their low weight, high scratch resistance, and low transparency for ultra violet and infrared radiation. Polycarbonate lenses are the lightest and most shatter-resistant, making them the best for impact protection, yet offer poor optics due to high dispersion, and having a low Abbe number of 31.

Hi-Index lenses are the thinnest, flattest and most cosmetically appealing lens developed. They are geared towards those with very strong prescriptions — both nearsighted and farsighted. Glasses correct vision by bending the light that passes through them. A stronger prescription requires more bending than a weaker one, resulting in a thicker lens. High index lenses are made of denser material than conventional lenses, bending more light through less thickness, resulting in ultra-thin, lightweight glasses.

Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass. Hydrophobic coatings designed to ease cleaning are also available, as are anti-reflective coatings intended to improve night vision and make the wearer’s eyes more visible.

Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or radiation.

For additional information about lens manufacturing, see Answers.com — How is an eyeglass made?

Progressive lenses (also called progressive addition lenses (PAL), progressive power lenses, graduated lenses, no-line bifocals, and varifocal lenses) are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation. They are characterised by a gradient of increasing lens power, added to the wearer’s correction for the other refractive errors.

The gradient starts at a minimum, or no addition power, at the top of the lens and reaches a maximum addition power, magnification, at the bottom of the lens. The length of the progressive power gradient on the lens surface is usually between 15 and 20 mm with a final addition power between 1.00 to 2.50 dioptres for most wearers. The addition value prescribed depends on the level of presbyopia of the patient and is closely related to age.

Additional Notes:

The wearer can adjust the additional lens power required for clear vision at different viewing distances by tilting his or her head to sight through the appropriate part of the vertical progression.

The lens location of the correct addition power for the viewing distance usually only requires small adjustments to head position, since near vision tasks such as reading are usually low in the visual field and distant objects higher in the visual field.

Progressive addition lenses avoid the discontinuities (image-jumps) in the visual field created by bifocal and trifocal lenses and are more cosmetically attractive. Since bifocal and related designs are associated with ‘old age’, proponents have suggested the lack of segments on the lens surface of a progressive lens appears more ‘youthful’ since lenses associated with younger wearers [single vision] lenses tend to be free of segments or lines on the surface.

Photo credit: Stanistani

anti-reflective_coating_comparison

Comparison between a glasses lens without anti-reflective coating (top) and a lens with anti-reflective coating (bottom).

Anti-reflective or antireflection (AR) coatings are a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection.

Opticians dispense antireflection lenses because the decreased reflection makes them look better, and they produce less glare, which is particularly noticeable when driving at night or working in front of a computer monitor. The decreased glare means that wearers often find their eyes are less tired, particularly at the end of the day. Allowing more light to pass through the lens also increases contrast and therefore increases visual acuity.

Antireflective ophthalmic lenses should not be confused with polarized lenses, which decrease (by absorption) the visible glare of sun reflected off of surfaces such as sand, water, and roads. The term anti-reflective relates to the reflection from the surface of the lens itself, not the origin of the light that reaches the lens.

Many anti-reflection lenses include an additional coating that repels water and grease, making them easier to keep clean. Anti-reflection coatings are particularly suited to high-index lenses, as these reflect more light without the coating vs. a lower-index lens.

photochromicPhotochromic lenses may be made of either glass or plastic, and darken upon exposure to ultraviolet radiation. Once the UV is removed, for example by walking indoors, the lenses will gradually return to their clear state.

Typically, photochromic lenses darken substantially in response to UV light in less than one minute, and then continue to darken very slightly over the next fifteen minutes. The lenses fade back to clear along a similar pattern. The lenses will begin to clear as soon as they are away from UV light, and will be noticeably lighter with in two minutes and mostly clear within five minutes. However, it normally takes more than fifteen minutes for the lenses to completely fade to their non-exposed state.

oakleysunwearSunglasses are a visual aid, variously termed spectacles or glasses, which feature lenses that are colored or darkened to prevent strong light from reaching the eyes. Sunglasses can improve visual comfort and visual clarity by protecting the eye from glare.

Strong light aside, medical experts advise the public to wear sunglasses with UV protection whenever outside. UV rays can cause short-term and long-term ocular problems such as photokeratitis, snow blindness, cataracts, pterygium and various forms of eye cancer.

More recently, high-energy visible light (HEV) has been implicated as a cause of age-related macular degeneration. Some lens manufacturers are able to block HEV light. Sunglasses may be especially important for children, as their ocular lenses are thought to transmit more HEV light than compared to an adult (it is said that a human ocular lens yellows with age).

Some sunglasses pass ANSI Z87.1 requirements for basic impact and high impact protection.