A lens is an optical system with two refracting surfaces, the simplest lens has two spherical surfaces
Lenses are made of transparent materials like plastic or glass, each lens has two faces and is part of a sphere and can be a convex or concave
CONVEX: If a lens is thicker at the center than the edges, it is a convex lens or converging lens, this is due to the fact that parallel rays passing through them will converge to meet at a focus point, these lenses are also called as + ( plus ) lenses, These lens causes magnification of an object while viewing through them.
CONCAVE: If a lens is thinner in the center and thicker at the edges, it is a concave or diverging lens since parallel rays of light passing through them will spread out .concave lenses are also called - (minus)lenses, concave lenses cause minification of an object when viewed through it.
Lens Materials: Include
- Mineral Glass
- MR 8 lenses
- CR 39 lenses
Made of silica and is highly scratch resistant and has excellent optical qualities, however since is not impact resistance can lead to injuries to eyes while worn as spectacle lenses, not in use for optical lenses, however due to its optical qualities are still used in medical instruments.
MR 8 LENSES
These are also called as MITSUI RESIN lenses and have a refractive index of 1.60, these lenses are lighter, stronger and clearer optical lenses, MR 8 lenses made of thiourethane resin make it possible to build thin eyeglasses with high impact resistance, these lenses resist breaking and chipping and are suggested for rimless frames making them safer to use.
PLASTIC OR CR 39
Made of cellulose acetate is superior to mineral glass in terms of softness, less weight and can be tinted or colored for cosmetic appeal, CR stands for Columbia Resin and its 39th formula of a thermosetting plastic, these material are highly impact resistant and carries high transparency, multiple coating is possible, however the only drawback with these lenses are easily prone to scratches.
These are derivatives of petroleum and plastic polymer and is 10 times more impact resistant than CR 39 lenses, The refractive index of poly-carbonate are 1.59 and provides an extra level of protection for the users and hence the choice of lenses for children and for sportswear and in rimless frames, all poly-carbonate lenses are 100 % UV proof .
These are impact resistance lenses with a refractive index of 1.532 and similar to poly-carbonate but higher in impact resistance and better optical quality and can provide clearer vision, although it is lighter material it is thicker than poly-carbonate lenses hence cannot be used for high prescription.
Lens Thickness and Refractive Index
The refractive index of refraction or refractive index is a measure of how efficiently a lens material can bend light , the index of refraction of eyeglass lens material usually ranges from 1.50 (regular plastic) to 1.74 of high index or compressed plastic , the higher the index the thinner the lens material.
A poly-carbonate lens with an index of 1.59 would be 40 % thinner than a regular 1.50 plastic lenses and the extremely high index lens of 1.74 would be 65 % thinner than plastic lenses.
The different kinds of lens category which can be used as eyeglasses , These depends on several factors including lifestyle , job , preference and visual needs , based on these they are broadly classified as:
- Single Vision Lenses
SINGLE VISION LENSES
This lens type has one prescription that covers the whole lens. They’re typically for people with long or short sight that only need glasses for things like driving or reading – so they have Clear vision for seeing up close and far away.
Bifocal lenses are divided into two parts to accommodate a combination of any two different prescriptions into one lens. One area of the lens will have one prescription (usually distance) and a segment in the lower half will have the other (usually near vision).
Progressive or Varifocal lenses allow you to see at any distance, because they have different sections for viewing close up and far away and everything in between. So whether you are driving, using a computer, shopping or reading, you can wear one pair of varifocals to accommodate all your vision needs.