SPECTACLES

The lenses fitted in a frame constitute the spectacles. It is a common, cheap and easy method of prescribing corrective lenses in patients with refractive errors and presbyopia. Some important aspects of the spectacles
are as follows:

Lens materials
1. Crown glass of refractive index 1.5223 is very commonly used for spectacles. It is ground to the appropriate curvature and then polished to await the final cutting that will enable it to   fit the desired spectacle frame.

2. Resin lenses made of allyl diglycol carbonate is an alternative to crown glass. The resin lenses are light, unbreakable and scratch resistant.

3. Plastic lenses are readily prepared by moulding. They are unbreakable and light weight but have the disadvantages of being readily scratched and
warped.

4. Triplex lenses are also light, they will shatter but
not splinter.

Lens shapes
1. Meniscus lenses are used for making spectacles in small or moderate degree of refractive errors. The standard curved lenses are ground with a concave posterior surface (–1.25 D in the periscopic type or –6.0 D in the deep meniscus type) and the spherical correction is then added to the anterior surface.

2. Lenticular form lenses are used for high plus and high minus lenses. In this type the central portion is corrective and the peripheral surfaces are parallel
to one another.

3. Aspheric lenses are also used to make high plus aphakic lenses by modifying the lens curvature peripherally to reduce aberrations and provide better peripheral vision.

Single versus multiple power lenses
1. Single vision lens refers to a lens having the same corrective power over the entire surface. These are used to correct myopia, hypermetropia, astigmatism or presbyopia.

2. Bifocal lenses have different powers to upper (for distant vision) and lower (for near vision) segments. Different styles of bifocal lenses are
shown in Fig. 1.

Fig. 1. Bifocal lenses. (A) two-piece; (B) cemented supplementary
wafer; (C) inserted wafer; (D) fused; (E) solid.

3. Trifocal lenses have three portions, upper (for distant vision), middle (for intermediate range vision) and lower (for near vision).

4. Multifocal (varifocal) or progressive lenses having many portions of different powers are also available.

Tinted lenses
Tinted glasses reduce the amount of light they transmit and provide comfort, safety and cosmetic effect. They are particularly prescribed in patients with albinism, high myopia and glare prone patients. Good tinted glasses should be dark enough to absorb 60-80 percent of the incident light in the visible part of the spectrum and almost all of the ultraviolet and infrared rays.
Photochromatic lenses alter their colour according
to the amount of ultraviolet exposure. These lenses
do not function efficiently indoors and in
automobiles.

Centring and decentring
The visual axis of the patient and the optical centre of the spectacle lens should correspond, otherwise prismatic effect will be introduced. The distance
between the visual axes is measured as interpupillary distance (IPD). Decentring of the lens is indicated where prismatic effect is required. One prism dioptre effect is produced by 1 cm decentring of a 1 D lens.
Reading glasses should be decentred by about 2.5 mm medially and about 6.5 mm downward as the eyes are directed down and in during reading.

Frames
The spectacle frame selected should be comfortable i.e. neither tight nor loose, light in weight and should not put pressure on the nose or temples of the patient, and should be of optimum size. In children large glasses are recommended to prevent viewing over the spectacles. Ideally, the lenses should be worn 15.3 mm from the cornea (the anterior focal plane of eye), as at this distance the images formed on the retina are of the same size as in emmetropia.