A plane-parallel optical plate with a partially transmitting coating on one surface and a antireflection coating on the second surface. When the beamsplitter is mounted at 45 degrees with respect to a beam of light, it separates the beam into two orthogonal beams. The ratio of the power in the beams is determined by the coating. Typical ratios are 30:70, 50:50, and 70:30.
Plate beamsplitters can be highly sensitive to polarization, and ratios are typically stated for unpolarized or randomly polarized light. If the light is polarized, a nonpolarizing plate beamsplitter, tailored for a specific wavelength, can ensure the appropriate performance.