2.5 Types of Spectacle Lenses
Description
This article is from the Vision and Eye Care FAQ, by grants@research.canon.com.au (Grant Sayer) with numerous contributions by others.
2.5 Types of Spectacle Lenses
+ Single Vision lenses
- spherical or with astigmatic correction
- in plastic or glass
+ Bifocal Lenses
- made of a main lens and an additional segment for near vision
- variety of shapes of near addition
+ Trifocal Lenses
- composed of a main lens and two reading ssegments; one for near
and one for intermediate distances (about arm lengths)
+ Multifocals
- composed of main lens and continuously variable addition for
various near reading positions
+ Aphakic Lenses
- high power plus lenses used for cataract patients.
- made with aspheric surfaces to minimise aberrations due to shape of
the lens
+ Photochromic lenses
- darken when exposed to short wavelength radiation (300nm - 400nm)
or ultra-violet light
- rate of darkening and final transmission of the lens (saturation
transmission) depends on the ambient temperature.
- lenses will not become as dark when driving due to UV absorption
by the car windscreen.
- available in plastic lenses now - Transitions+ and Spectralite are
two common lens names.
+ Coated Lenses:
- spectacle lenses may often be treated with a surface coating
(vacuum coating) to reduce unwanted reflections.
- the surface reflection is related to the material index so that
different lens materials will have different amounts of surface
reflectance.
- reflections are classified as 4 main types
(a) reflections visible by an observer from the front
(b) internal reflections of the lens itself
(c) reflections from behind the lens, eg overhead lighting
(d) reflections from the corneal surface.
+ Hi Index Lenses:
- manufactured from a higher refractive index material that enables
the lens thickness to be reduced whilst still maintaining the optical
properties. That is the optical power of a lens is a combination of
the two surfaces, front and back, and the thickness of the lens.
- increased index of lens also means that there is a reduction in the
base curve (back curve) so that there is greater thickness savings
when comparing two lenses of the same centre thickness.
+ Toughened Lenses
- Either heat or chemical treated to increase impact strength
- Used in industrial applications for added safety and protection.
+ Aspheric Lenses
- elliptical curve surfaces designed to minimise lens abberations
available as single vision and even some in progressive lens
designs.
+ Polarized Lenses
- available in glass, plastic and hi-index plastic (n=1.56)
Continue to:
My Books
