Plate beamsplitters are used to break incident, monochromatic light beams into two beams, each with a specified intensity ratio. Plate beamsplitters are optical windows with a semi-transparent mirrored coating, which passively breaks the beam into two or more separate beams. One of the beams is reflected incident energy, of which a very small portion is absorbed, while the second beam is fully transmitted. In addition to the task of dividing light, beamsplitters can be employed to recombine two separate light beams or images into a single path.

Plate beamsplitters are available in three distinct shapes; circular, square and rectangular. Circular and square plate beamsplitters are generally designed to function at 45 degree incidence, while rectangular plates usually function at 0 degrees of incidence. However, beamsplitters of all three shapes are available that can function at either or both angles of incidence.