|
There are at least two general categories of absorptive-type glass polarizers which involve silver metal in their polarizing mechanism. Glasses of the first category, permanent polarizers, contain elongated, submicroscopic particles of silver metal aligned along a common axis. Their polarizing capacity is due to a resonant absorption of the silver conduction electrons, the spectral location of which depends on the extent of silver particle elongation and the particle orientation with respect to the applied electromagnetic (light) field. These polarizers have been made with dichroic ratios (optical density ratios) in excess of 40. The best performance is obtained in the long wavelength visible and near infrared regions. They are useful in applications similar to those of plastic sheet polari-zers. They have better optical quality and better thermal, mechanical, and chemical durability. Polarizing glasses of the second category are optically alterable. That is, their color and degree of polarization can be changed by intense visible or near infrared irradiation. These glasses owe their polarizing properties to small, nonspherical specks of silver metal in contact with silver halide crystallites within the glass. The change in color and polarization is believed due to the growth and dissolution of differently shaped and oriented specks during irradiation. This second category of glass polarizer shows much lower dichroic ratios than the first, generally less than 5, but has the advantage that the orientation of the polarizing axis can be changed, or made to vary from one region of the glass to another, by suitable high intensity irradiation.
|
