Special Lenses

The following types of lenses have been developed for special applications:

Plastic lenses: These lenses reduce the weight of eyeglasses where severe ametropia must be corrected. Another advantage is that these lenses are largely shatterproof, which is why they are preferred for children. However, they are easily scratched.

Absorption lenses: These lenses are indicated in patients with increased sensitivity to glare.

U Operating motor vehicles in twilight or at night with eyeglasses that absorb more than 20% of incident light is dangerous because of the resulting reduction in visual acuity.

Photochromatic lenses: These lenses darken in response to the intensity of ultraviolet light. The lenses become darker at low temperatures than at high temperatures; they lighten more slowly at low temperatures and more rapidly at high temperatures. Light attenuation ranges between 15 and 50% in some lenses and between 30 and 65% in others.

H Photochromatic lenses pose problems for patients operating motor vehicles. The lenses darken only slightly in a warm car with the windows closed due to the lack of ultraviolet light. Dark lenses lighten too slowly when the car enters a tunnel.

Coated lenses: Extremely thin coatings of magnesium fluoride can be applied to lenses to reduce surface reflection on the front and back of the lens.

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