Sheet polarizers, dichroic

Although laboratory polarimeters generaUy use Nicol prisms as polarizers and analyzers, dichroic crystals (such as tourmaline) or dichroic sheet polarizers (such as Polaroid) may be used in the construction of special apparatus.f... 

Figure 11.5 Schematics of common types of polarizer. Top low-power dichroic sheet polarizer (for linear TT-polarization). Bottom high-power caldte Gian polarizer, splitting unpolarized light into orthogonally linear-polarized components for the extraordinary through and the ordinary reflected beams...

A second class of polarizers uses dichroism to produce linearly polarized light. Techniques to produce dichroic polarizing sheets were pioneered by Land [52], These are made by dissolving a strongly dichroic, small molecule into an amorphous, transparent polymer. The dichroic molecules are then oriented by a uniaxial stretching of the polymer matrix. Since this is accomplished below the polymers glass transition temperature, this ori-... 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

Bradbury et al. (1961) have described in detail the experimental techniques used in a study of films of DNA polymers and have recorded polarized infrared spectra of oriented sheets of the sodium and lithium salts of DNA, in both undeuterated and deuterated states, over a range of humidities between 0 and 94% relative humidity (r.h.). They found two forms of sodium DNA, one at humidities greater than 90% r.h. where the bases are perpendicular to the helix axis (B-form) and the other at humidities between 70 and 80% r.h. where the bases have tilted by an angle not less than 13° to the normal to the helix axis (A-form). Lithium DNA exists in the B-form for humidities greater than 66 % r.h., and in a second form, different from the A-form of sodium DNA, between 44 and 56% r.h. where the bases are tilted by about 4° from the perpendicular (C-form). These workers have discussed the orientation of the phosphate groups, which they inferred from dichroic effects. 

In the case of visible light, studies of polarization are dominated by the use of dichroic filters (e.g., the Polaroid sheet), for which photons with polarization parallel to a... 

The polarizing microscope requires a linear polarizer between the light source and the sample and an analyzer - another linear polarizer - after the sample. They should be rotatable. The polarizers used presently work by absorbing light of one polarization state and allowing the other to pass that is, they are dichroic filters. The material used is a Polaroid sheet, a highly oriented film of doped poly(vinyl alcohol). Other features desirable in a polarizing microscope are as follows ... 

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