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Polarised Lenses
How Do Polarised Lenses Work?
Light reflected from surfaces like a flat road or smooth water is generally horizontally polarised. This
horizontally polarised light is blocked by the vertically oriented polarizers in the lenses. The result: a
reduction in annoying and sometimes dangerous glare. The principle of polarised light reduction is best
illustrated by thinking of a polarised lens as a Venetian blind. The blind blocks light at certain angles
while allowing light to transmit through selected angles. Polarizing filters are aligned 90° to the angle
of the polarised light. As spectacle lenses are designed to eliminate the polarised light in the horizontal
plane, the filter is placed vertically in the eyewire. This means that the filter must be properly aligned
during surfacing and edging layout, otherwise the filter will not work properly.
Effect of a polarizer on reflection
from mud flats. In the picture on the
left, the polarizer is rotated to
transmit the reflections as well as
possible; by rotating the polarizer by
90° (picture on the right) almost all
specularly reflected sunlight is
blocked.( From Wikipedia)
The effects of a polarizing filter on
the sky in a photograph. The picture
on the right uses the filter. (From
Wikipedia)
Tint vs. Polarised
Although darkly tinted sunglasses may reduce brightness, they do not remove glare like a polarised
lens. In addition, dark sunglasses without added ultraviolet protection may cause more damage to the
patient’s eyes than not wearing sunglasses at all. The darkness of the lens can cause the pupil to
dilate, letting more ultraviolet rays into the inner parts of the eye. Polarised lenses solve both
problems by eliminating glare and filtering out harmful ultraviolet light because the filter reduces the
polarised glare and also has ultraviolet absorbing properties.
Benefits of Polarised Lenses
Reduces eyestrain, greater comfort, improves visual acuity, provides safety, eyes feel rested, realistic
perception, reduces reflections and enhances visual clarity, certain lens materials can be darkened and
provides limitless colour options.
Who would benefit?
Light-sensitive people such as post-cataract surgery patients or by those exposed to bright light through
windows, boaters and fishermen, outdoor sports enthusiasts as well as by drivers and general use
wearers, skiers, golfers, bikers, and joggers, who enjoy a clearer view and elimination of glare.
Warning:
NB: Not always for use in aircraft cockpits or cars with liquid crystal displays (LCD). The problem with
LCDs is that when viewed through polarised lenses from a certain angle, they can be invisible.
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Polaroid
The original material, patented in 1929 (U.S. Patent 1,918,848) and further developed in 1932 by Edwin
H. Land, consists of many microscopic crystals of iodoquinine sulfate (herapathite) embedded in a
transparent nitrocellulose polymer film. The needle-like crystals are aligned during manufacture of the
film by stretching or by applying electric or magnetic fields. With the crystals aligned, the sheet is
dichroic: it tends to absorb light which is polarised parallel to the direction of the crystal alignment,
but transmits light which is polarised perpendicular to it. The resultant electric field of an
electromagnetic wave (such as light) determines its polarisation. If the wave interacts with a line of
crystals as in a sheet of polaroid, any varying electric field in the direction parallel to the line of the
crystals will cause a current to flow along this line. The electrons moving in this current will collide
with other particles and re-emit the light backwards and forwards. This will cancel the incident wave
causing little or no transmission through the sheet. The component of the electric field perpendicular
to the line of crystals however can cause only small movements in the electrons as they can't move
very much from side to side. This means there will be little change in the perpendicular component of
the field leading to transmission of the part of the light wave polarized perpendicular to the crystals
only, hence allowing the material to be used as a light polariser.
This material, known as J-sheet, was later replaced by the improved H-sheet Polaroid, invented in
1938 by Land. H-sheet is a polyvinyl alcohol (PVA) polymer impregnated with iodine. During
manufacture, the PVA polymer chains are stretched such that they form an array of aligned, linear
molecules in the material. The iodine dopant attaches to the PVA molecules and makes them
conducting along the length of the chains. Light polarised parallel to the chains is absorbed, and light
polarised perpendicular to the chains is transmitted.
Another type of Polaroid is the K-sheet polariser, which consists of aligned polyvinylene chains. This
polariser material is particularly resistant to humidity and heat.
Other applications
Polaroid sheets are used in liquid crystal displays, optical microscopes and sunglasses. Since Polaroid
sheet is dichroic, it will absorb impinging light of one plane of polarisation, so sunglasses will reduce
the partially-polarised light reflected from level surfaces such as windows and sheets of water, for
example. They are also used to examine for chain orientation in transparent plastic products made
from polystyrene or polycarbonate.
The intensity of light passing through a Polaroid polariser is described by Malus' law.
Polaroid is also used as a trade name for a variety of products sold by licensees of the Polaroid
Corporation, including consumer electronics, sunglasses based on Polaroid polarisers, and instant-print
photographic film and cameras. In February 2008, Polaroid announced that it is discontinuing
production of its instant film and will close its factories in the United States, Mexico and the
Netherlands.
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