Ultraviolet treatment is applied to lenses to protect against harmful UV sunrays that can accelerate the development of cataracts and macular degeneration. It is extremely important to protect eyes from the damaging effects of the sun. UV treatment is easy to apply to lenses and is often included with the purchase of eyeglasses.
Polarized lenses are usually used to make sunglasses. They are available most commonly in grey or brown tint but many other colors are available. Vertically polarized lenses decrease bright glare and reflections by blocking horizontal polarized reflected light. Polarized lenses have been used by fishermen for years to better deal with bright light being reflected off water and to see deeper into the water.
Photochromic lenses have a special chemical coating that makes them change to a dark tint in the sunlight and turn clear indoors. Photochromatic lenses are great for people who do not wish to carry a separate pair of prescription sunglasses. It is important to recognize that these lenses do not darken as well while driving a car. The windshield prevents most of the UV light from reaching the lens.
The axis in a prescription describes orientation of the axis of the cylindrical lens. The direction of the axis is in degrees measured anti-clockwise from the horizontal line through the centers of the pupils when viewed from front side of the glasses (i.e. when viewed from the point of view the person making the measurement). It varies from 1 to 180 degrees.
In the illustration below, viewed from the point of view of the person making the measurement, the axis is 20 degees if written in minus notation or 110 deg (at 90 degrees to the negative axis) if written in plus notation.
The DV portion of the prescription describes the corrections for distant vision. For most people under forty years of age, this is the only part of the prescription that is filled in. The NV or near-vision portion of the prescription is blank because a separate correction for near vision is not needed.
The NV portion is used in prescriptions for bifocals.
In younger people, the lens of the eye is still flexible enough to accommodate over a wide range of distances. With age, the lens hardens and becomes less and less able to accommodate.
This is called “presbyopia”; the presby- root means “old” or “elder”. (It is the same root as in the words priest and presbyterian.)
The hardening of the lens is a continuous process, not something that suddenly happens in middle age. It is occurring all along. All that happens around middle age is that the process progresses to the point where it starts to interfere with reading. Therefore almost everybody needs glasses for reading from the age of 40-45.
Because young children have a wider range of accommodation than adults, they sometimes examine objects by holding them much closer to the eye than an adult would.
This chart (which is approximate) shows that a schoolchild has over ten diopters of accommodation, while a fifty-year-old has only two. This means that a schoolchild is able to focus on an object about 10 cm. (4″) from the eye, a task for which an adult needs a magnifying glass with a rated power of about 3.5X.
The NV correction due to presbyopia can be predicted using the parameter age only. The accuracy of such a prediction is sufficient in many practical cases, especially when the total correction is less than 3 diopters. See also the following calculator for computing this correction.
When someone accommodates, they also converge their eyes. There is a measurable ratio between how much this effect takes place (AC/A ratio, CA/C ratio).