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A Wollaston prism, named for William Hyde Wollaston, is a polarizing beam splitter formed from two birefringent optical prisms with crossed crystal-axis orientations. The two linear polarization components encounter different effective refractive indices and leave in different directions. Both output beams are retained, with orthogonal linear polarizations. A manufacturer describes approximately equal and opposite deviations and explicitly defines its splitting angle as the full angle between the emerging beams.^[ref]

Calcite and quartz implementations illustrate why material matters. The manufacturer uses cemented calcite elements and optically contacted quartz elements; its quartz designs provide smaller splitting angles because the birefringence is smaller. A prism's material, wavelength and cut therefore belong with a stated angular separation, rather than treating that separation as a universal property of the name.^[ref]

The ideal intensity division follows projection onto the prism's polarization axes. For incident linear polarization at angle $\theta$ to one axis and total optical power $P$, a lossless model givesFor an original2milliwatt beam at $\theta =30^{\circ }$, the outputs carry1.5and0.5milliwatts. Equal powers occur at45degrees; they are not guaranteed for every input. Unpolarized light gives equal average powers in this ideal model. Real transmission loss and unequal detector responses require separate characterization.

For an original geometric example, suppose a particular prism produces a symmetric full separation $\Delta =1^{\circ }$. At distance $L=0.50\mathrm{m}$ from a common effective exit point, the separation of beam centres isUsing $\tan \Delta$ for each beam would mistakenly treat the full splitting angle as a one-sided deviation. Finite beam diameters and any exit-face displacement also affect detector placement.

A bench demonstration rotates a polarizer before the Wollaston prism while recording both output powers. Their complementary variation tests the polarization decomposition. An analyzing polarizer after each exit can check the two output directions independently. Unlike the virtual copies produced by an optical biprism, orthogonally polarized beams have no mutual intensity cross term on a polarization-insensitive screen. Projecting coherent beams onto a common polarization can permit interference after suitable spatial overlap; changing direction alone does not supply that projection.