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 often referred to as mirror beamsplitters as the neutral color characteristics provided by their optical crown glass designs gives the appearance of a mirror. Having a partially silvered coating allows them to produce a desired transmission-to-reflection ratio. These ratios usually vary between 50:50 and 20:80, depending upon the application, when operated at 45 degrees of incidence. In most cases, plate beamsplitters have multiple layers of dielectric coating on one surface, while the second surface is coated with an antireflective material, which helps to, but doesn’t completely remove ghosted images. The second surface may also be wedged to eliminate internal fringe, which can help to cut down the incidence of ghosting even further. However, if the removal of ghosted beams is of primary concern, pellicle or cube beamsplitters should be used instead plate beamsplitters.
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.