Re: POLARIZATION

[Hugh Logan <hlogan@ix.netcom.com>]

kifer/belk wrote:
> Maybe I missed an earlier installment on this idea that answers this
> question, but just exactly what is the standard linear polaroid
> material?  How is it made, what chemicals are involved, how are they
> "aligned" in the production-line situation?  How about circular
> polaroid?  How is it different in composition and manufacture?  If
> anybody can help me with this I thank you.
In case you missed this part of my earlier message, (Sunday, 6:01), I
repeat it here:

> The most descriptive explanation of how the "strands" in Polaroid
> filters work is the following comparison eith the microwave wire grid
> polarizer (in _Introduction to Optics_ by Pedrotti and Pedrotti,
> Prentice Hall, 1987): After mentioning that the conduction paths must
> be closer together for visible light than for microwaves, he states,
> "When a sheet of clear, polyvinyl alcohol is heated and stretched, its
> long, hydrocarbon molecules tend to align in the direction of 
> stretching. The stretched material is then impregnated with iodine
> atoms, which become associated with the linear molecules and provide
> 'conduction' electrons to complete the analogy to the wire grid." 
The page number in Pedrotti should have been p. 360. The above
description of Polaroid H-sheet, invented by Edwin H. Land in 1938.
It is still the most common form of Polaroid (at least as of 1987).
There is a similar description of Polaroid H-sheet on p. 505 of
_Fundamentals of Optics_, 4th ed. by Jenkins and White. (I referred to
Pedrotti, because of the more complete description of how it works).

Stretching of the polyvinyl alcohol films during manufacture seems
crucial to the alignment of the molecules in Polaroid H-sheet.
Jenkins and White also discuss the original Polaroid invented by Land 
in 1932: "These films consist of thin sheets of nitrocellulose packed
with ultramicroscopic polarizing crystals with their optic axes all
parallel." The polarizing crystals are quinine iodosulfate
(herapathite).

Jenkins and White also describe another Polaroid film known as 
H-Polaroid: "Land and Rogers found further that when an oriented
transparent film of polyvinyl alcohol is heated in the presence of an
active dehydrating catalyst such as hydrogen chloride, the film darkens
slightly and becomes strongly dichroic. Such a film becomes very stable
and, having no dyestuffs, is not bleached by strong sunlight." (p. 505).

Jenkins and White refer to a paper by Edwin Land summarizing the
development of sheet polarizers:
  J. Opt. Soc. Am., 41:957 (1951) .

> After discussing naturally occurring dichroic materials, the Pedrotti's 
> go on to say, "In non-metallic materials, the electrons acting as dipole
> oscillators are not free. In this case the wave they generate is not out
> of phase with respect to the incident wave, and complete cancellation of
> the forward wave does not occur. The energy of the driving wave,
> however, is gradually dissipated as the wave advances through the
> absorber, so that the efficiency of the dichroic absorber is a function
> of the thickness." I presume this applies to Polaroid sheets, so that
> the analogy with the wire grid polarizer is less than exact. 

The Pedrotti's go on to discuss this absorption in terms of the equation
for attenuation I=I(0)*e^(-alpha*x). Ideally, the attenuation
coefficient should be independent of wavelength. In reality, this is not
quite true for Polaroid H-Sheet. If white light is intercepted by 
Polaroid H-sheets crossed at 90 degrees, some blue light gets through.

Circularly polarized light can be produced by placing a quarter wave
plate in the path of of linear polarized light at an angle of 
45 degrees to the plane of the incident polarized light. The quarter
wave plate can be thin sheets of birefringent crystal such as mica or 
quartz cut parallel to the optic axis. The thickness is such that there
is a 90 degree phase shift between the ordinary (O) and extraordinary
(E) vibrations. (Jenkins and White, p. 567).

The "circular polaroid" that you refer to is probably a quarter wave
plate made from a birefringent plastic sheet. According to Jenkins and
White (p.567), "Quarter wave plates can also be made in large sheets of
plastic formed by _extrusion_. These sheets are double refracting and by
careful control of the thickness can be made to produce a phase
difference of pi/2 rad, or any other value."

Hugh Logan