Polaroid is sometimes misused for glasses in the same way as Hoover is when referring to vacuum cleaners. “Polaroid” is actually a trade name, the property of the Polaroid Corporation of the U.S.A., the first company to market sunglasses incorporating the polarising principle.

 

Several spectacles lens manufacturers now produce such glasses lenses and some are available for prescription glasses  in plastics.  They are commonly known as polarised lenses but should be more accurately described as polarising as it is the light which gets polarised, not the lens or the spectacles themselves.

 

Polarisation occurs when light passes through a doubly refracting medium such as certain crystals, Iceland spar, mica, tourmaline and others.  They produce two refracted rays , the ordinary and the extraordinary.  The light travels along these two rays with different speeds and the light vibrations in one of them take place along a direction 90 degrees to the vibrations in the other.  The reader will of course completely understand all this. (Perhaps he will sometime explain it to the writer).

 

Just how this incredible phenomenon can be reproduced in a thin plastic lens is a mystery and understood, one suspects, by only a few.

 

So what are the practical applications of the polarising lens? 

 

Many readers will be aware that polarised sunglasses considerably reduce horizontal reflections.  This can be observed by looking through a polarised sunspec at a surface which is reflecting light.  The reflections will all but disappear.  Now if you rotate the sunspec through 90 degrees, the reflections will magically reappear.  This property makes polarised sunglasses very popular with anglers and small boat sailors.

 

How can you tell if you recently purchased sunglasses, that you have genuine polarised lenses?  Simple,  go out and buy another pair; hold one pair up to the light; hold the other in front of it, with one lens of each co-axial and rotate one pair through 90 degrees, keeping the two lenses co-axial, then if the two co-axial lenses appear to go completely black, then not only have you bought genuine polarised lenses, but also you clearly understand the meaning of the word co-axial.

 

Do polarised sunglasses have any disadvantages over other types of glare protectors?  Actually, yes, they do tend to expose the stress patterns in toughened glass, as  used in car windscreens.  Some people find this disturbing, others not.

 

This phenomenon has practical applications in the optical business.  In rimless spectacles where the mountings are actually screwed to the lens, it is possible to strain the lens by over tightening the screws.  If this happens, it can be detected with the aid of  a strain tester.  This is a device consisting of two pieces of polarising material mounted a few centimetres apart with axes at 90 degrees.  (This means they are producing the same effect as our two sunspecs mentioned above).  The view through the strain tester will be almost black but if a strained lens is viewed by placing it between the two pieces of polarising plastic, the strained area will show up with a bright “starring” effect around the offending screw.

 

A strain tester is also used to check that toughened glass lenses (still demanded in some industries) are actually toughened.  The toughened lens is inserted in the strain tester and it will be observed as being light with a strong dark, almost black cross over it .

 

There is a notice on the door of my shop which reads “Polaroid sunglasses available here”.  Although it is sometimes obscured by another which reads “Gone fishing”.          

Tags: fishing glasses, Polarized, Polarized Glasses, Polarized Lenses, polarized spectacles

This entry was posted on Wednesday, June 17th, 2009 at 2:52 pm and is filed under Lenses. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.