What are polaroid sunglasses?

These are plastic sunglasses (the new ones are glass) which have the ability to stop glare by cutting out the horizontal vibrating light waves. The plastic lenses also do not cut out the infra-red waves and hence are not advisable for tropical countries. For tropical climates, glass polaroid lenses are more effective though far costlier than plastic.

They are not suitable for driving in cars with ordinary toughened glass windscreens because they cause a matted pattern to appear on the screen and detract from vision. The more expensive laminated windscreens does not cause this effect. However, since all cars in India have the toughened glass variety, the blotches can be a cause of visual confusion leading to accidents.

What are safety glasses? Can ordinary glass be "toughened" to resist breakage?

Safety glasses are recommended for all cases where there is a risk from flying objects, as in industry or turning at a lathe, hammering and also for school children.

Safety lenses are of four types, two in glass and two in plastic.

(I) Glass

(a) Laminated glass

(b) Toughened glass

(II) Plastic

(c) Methylmethacrylate plastic

(d) CR 39 plastic

(a) Laminated glass: two layers of glass with a sheet of cellulose acetate between the two layers, sold under the trade name of Salvoc, only recently available in India. Since it is not powered, use is restricted to industries.

(b) Toughened lens: can be toughened by air treatment (easily available in India) and by chemical treatment; available, but only at top establishments.

Toughening reduces the risk of breakage. However, its greatest advantage lies in that, even if it breaks, the fragments have smooth edges and no sharp points.

(Hardened and toughened glass are used by all automobile industries).

Toughening of glass is a simple treatment and highly recommended for children and adults exposed to risk.

(c) Methylmethacrylate plastic: is extremely efficient (commercially available in Bombay for spectacle and sunglasses). Has a high impact resistance but can scratch eaisly.

(d) Plastic CR 39: has the dual advantage of impact resistance and resistance to scratches but is not freely available in India yet.

To summarise: in high risk conditions, plastic lenses are far superior and safer than glass lenses.

Dustin Flores

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