INDEX DATA

The luminous flux that can be transmitted through a polarization prism depends on its useful aperture and on the maximum solid angle of acceptance. To obtain maximum flux transmission both parameters must be maximized.

The free aperture of a prism may be defined as the diameter of the biggest circle perpendicular to the prism axis which will fit inside the entrance face of the prism.

The field angle, which is linked to the acceptance solid angle, is twice the maximum angle in relation to the prism axis under which a light ray will traverse the prism while still being completely polarized when the prism is rotated through a 180 angle.

The Length/Aperture ratio is the ratio between the length of the prism base, measured parallel to the prism axis (not the optical axis), and the minimum prism dimension, measured perpendicularly to the its base.

If the prism is used with point sources the maximum acceptable divergence cone of the incident beam will be determined by the prism field angle. However, if an extended source is used in a prism with the same L/A ratio, the maximum acceptable beam divergence angle is now determined by the L/A ratio. Thus, in specifying a calcite polarizing prism, three quantities are given: free aperture; field angle and Length/Aperture ratio.

If a polarizer is put on a linearly polarized beam and it is rotated through an angle around an axis parallel to the beam direction, the transmittance T will vary between a maximum value T₁ and a minimum value T₂ according to the equation

T = (T₁ - T₂)cos q² + T₂

where q is the angle between the polarization direction of the incident beam and the direction of maximum transmission T₁. T₁ and T₂ are known as the principal transmissions of the polarizer. If the polarizer is now put in randomly polarized light, the transmittance will be

T =¹/₂ (T₁ + T₂)

Assuming 1 and 0 as respectively the maximum and minimum values for T₁ and T₂, we have that the maximum transmission for an ideal polarizer will be 50% for a random polarization beam. In practice, commercially available components will show transmissions between 35 and 40%.

When two identical polarizers are mounted in series in linearly polarized light, with principal transmission directions parallel to each other, the transmission of the set will be

T _P =¹/₂ (T₁²+ T₂²)

When principal transmissions are perpendicular to each other we will get

T _P =T₁ T₂

The extinction ratio r of a polarizer is defined as the ratio of the minimum principal transmittance T₂ to the maximum principal transmittance T₁

r = T₂/T₁

In cemented prisms there is a bigger limitation to the maximum tolerable energy of a beam that can be transmitted through the prism, due to the absorption of energy by the cement and to the degassing of the polymeric materials used in mounting the prism in its cell. Thus, the maximum permissible in cemented prisms will be around 1W/cm² for continuous beams (CW). In the case of air-spaced prisms this limitation is set by scattering and residual absorption by the crystal itself. The maximum energy density for air-spaced prisms will go from 100W/cm² (CW) to 500MW/cm² (pulsed - 1 ns).

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