The most important measure of polarization performance is called the extinction ratio. This is a measure of how efficiently the polarizer separates out the parallel and perpendicular components of polarization. An extinction ratio of 1000:1 indicates that for every 1000 photons of one polarization, there is only 1 of the contrary polarization passing through the grid. In the case of a wire grid it is the radiation polarized perpendicular to the grid wires that is transmitted.
The extinction ratio of a wire grid polarizer varies by wavelength. It reaches a maximum at the longest wavelengths (λ) relative to the spacing of the grid (a). The ratio a/λ needs to be 0.5 or lower to have any polarizing effect, and ideally much, much less than this. When using a Specac holographic wire grid polarizer over the wavelength range 2.5 µm (4000 cm-1) to 25 µm (400 cm-1) the ratio a/λ varies from 0.1 to 0.01.
Another important measure is transmittance of the polarizer. This will be affected by the substrate that the grid is deposited on. Not all IR-transparent materials can transmit across the full mid-IR range. The thickness or diameter of the wires (d) relative to their spacing (a) also affects transmission. Increasing the ratio d/a will have the effect of lowering the overall transmission but tends to diminish the transmission of the ‘unwanted’ parallel component more. Up to a certain point, therefore, this causes an improvement in the extinction ratio.
