Polarizing lenses

kamal thakur 2nd year bsc opto

Polarizing lenses

Polarizing lenses reduce glare is commonly caused by reflections from water , snow , highways and metallic surfaces .

•They reduce light transmission

•They reduce reflected glare

•They increase contrast sensitivity

How polarizing lenses work

• Light reflected from surfaces such as a flat road or smooth water generally is horizontally polarized , instead of light being scattered in all directions . Reflected light generally travels in a more horizontally oriented direction.

Polarized lenses contain a special filter that blocks this type of intense reflected light , reducing glare.

• When light strikes a horizontal reflecting surface , it becomes partially polarized with the major direction of vibration being in the horizontal plane .

• If light strikes the surface of a refracting material , such as water or glass , most of the light will be refracted as it strikes the surface of the water and go on into the water. The rest of the light will be reflected . There is an angle of incidence of the light striking a surface where all of reflected light will be polarized . This angle is called Brewster,s angle.

Brewster,s angle

When light strikes a refracting surface , most light is refracted into the media , and some is reflected . The reflected light is partially polarized until it reaches a certain angle when it is completely polarized . This angle is where the reflected light and the refracted light are at right angle to one another. This angle is called brewster,s angle.

Manufacturing of polarized lenses

A filter that absorbs the horizontally vibrating components of light would be useful . Such a filter is available for ophthalmic use and is made from a sheet of PVA (polyvinyl acetate).

The PVA is first stretched to five times its normal length in one direction . Then dipped in iodine . The iodine is absorbed into the chains of molecules in PVA . These darkened lines create the polarizing filter .

The filter may be “sandwiched” between two layers of Cellulose Acetate Butyrate (CAB) .

This is how thin , plano polarizing lenses are made.

For prescription lenses , the polarizing sheet is mounted between hard resin or polycarbonate material . Alternately , it can be mounted on one layer of cellulose acetate butyrate material and molded directly into a plastic lens casting process.

If the polarizing lens is not oriented along 180-degree meridian , not all horizontally polarized light will be absorbed . And if a person is wearing a polarizing filter and tilts his head to one side , the filter will not absorb as much of the horizontally polarized light .

Advantages

• Decreased driving fatigue specially during day time.

• Fisherman can see more with polarizing lenses because it not only removes reflected or discomfort glare but enables the wearer to see below the surface.

• Comfort at the beach is increased because sand and water are both source of polarized glare marking these lenses especially appropriate.

• Color is not bleached.

Bright snowy rays are not blinding when viewed with polarizing lenses .

• Polarizing lenses are good alternative to sunglasses because they inhibit U.V radiation.

Application

Polarizing lenses are made in most lens style- not just single vision lenses , but bifocal , trifocals and progressive addition lenses as well.

they are available in glass , photochromic glass , plastic , photochromic plastic , polycarbonate and HI plastic.

Methods for demonstrating polarizing lenses

• It is possible to take two Plano polarizing lenses and by holding one before the other with their polarizing axis crossed at 900 eliminate all incoming visible light rays .

• Can be demonstrated by showing a glossy magazine .