Main articles: Corrective lens, Pinhole glasses, and Refraction error Corrective lenses are used to correct refractive errors of the eye by modifying the effective focal length of the lens in order to alleviate the effects of conditions such as nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. Another common condition in older patients is presbyopia which is caused by the eye's crystalline lens losing elasticity, progressively reducing the ability of the lens to accommodate (i.e. to focus on objects close to the eye).
Figure 39: Demonstrating how Corrective Figure 40: Eye glasses.
The power of a lens is generally measured in diopters. Glasses correcting for myopia will have negative diopter strengths, and glasses correcting for hypermetropia will have positive diopter strengths. Glasses correcting for astigmatism require two different strengths placed at right angles in the same lens.
Prescription lenses, made to conform to the prescription of an ophthalmologist or optometrist, are used to make prescription glasses, which are then verified correct using a professional lensmeter.
Figure 46: Pinhole glasses.
inhole glasses are a type of corrective glasses which do not use a lens and are claimed to help correct the eye's refractive error without introducing the image distortion of traditional lens-based glasses. Pinhole glasses do not actually refract the light or change the focal length, they operate by reducing the size of the blur circles in the retinal images. In blurry vision, every point on the object corresponds with a blur circle in the image. By reducing the size of the blur circles, they reduce the overlap of the blur
circles in the image, clarifying the vision somewhat. They do not heal the eye of refractive error, as sometimes claimed, and vision with pinhole glasses, although clearer than without them, is not as clear as with conventional lenses.
Correcting one's vision is done by using lenses to move the focal point on the retina accordingly with one's particular needs. The depth of the curve, the thickness of the lens, and the precise shape of the
lens can all be used to change the focal point.. Figure 47: Pinhole glasses.
Eyeglasses can normally correct and compensate for four types of vision deficiencies:
Myopia is a vision disorder that causes far objects to appear blurred but near objects are seen clearly. Individuals suffering from myopia are prescribed eyeglasses with concave lenses, which compensate for the refraction error by moving the image of the distant objects that cannot be seen clearly backward onto the retina.
Figure 48: Concave lenses. Figure 49: Demonstrating how Concave
Figure 50: Concave lens diverging rays.
Correcting hyperopia is normally done with eyeglasses with convex lenses. With this disorder, the patients can see distant objects clearly but they have trouble with seeing objects that are close to them. Eyeglasses with convex lenses compensate for the refraction errors by moving the image of a distant object forward onto the retina..
Figure 51: Convex lenses. Figure 52: Demonstrating how
Convex lenses works.
Figure 53:Large Convex lens.
Astigmatism is typically corrected with a cylindrical lens. This disorder is caused by a non-uniform curvature in the refractive surfaces of the eye, which leads to an abnormality in focusing the light rays on the retina. As a result, a part of the light rays are focused on the retina and the other part is focused behind it or in front of it.
Figure 54: Large Cylindrical lens.
Figure 55: Cylindrical lenses.
Figure 56: Demonstrating how Cylindrical lenses works.
Presbyopia is more frequent in people over 40 years old and it is corrected with convex lenses. These patients need reading or bifocal eyeglasses.
Corrective eyeglasses can significantly improve the life quality of the patient as they are helpful in both correcting vision disorders and reducing problems that appear when such lenses are needed, such as headaches or squinting.
Corrective lenses can also be added to work masks or eyeglasses which are used in sports.
Eyeglass lenses are commonly made from 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, allowing thinner lenses for a given prescription. Newer plastic lenses, called iZon, can also correct for the symptoms of the higher order aberrations that naturally occur on a person's optical system (including retina, lens, and cornea). These lenses create sharper vision for people who have problems with blurry or dull vision, as well as help reduce the halos, starbursts, and comet-tails often associated with night time driving.
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 reduce glare, improve night vision and make the wearer's eyes more visible.