Which layer does the beam splitter belong to

The beam-splitting action occurs in the thin-film coating layer applied to the optical interface of the beam splitter.Detailed ExplanationIn most beam splitters, the layer responsible for dividing the incident light into reflected and transmitted beams is a specialized thin-film coating. This coating can be dielectric, metallic, or partially reflective, depending on the type of beam splitter and its intended application .Dielectric coatings: These consist of multiple alternating layers of high and low refractive index materials. The thickness of each layer is carefully designed to exploit interference effects, controlling the proportion of light reflected versus transmitted at specific wavelengths . Dichroic beam splitters, for example, use this alternating layer structure to separate light by wavelength .Metallic coatings: In half-silvered mirrors, a thin metal layer (commonly aluminum or silver) is deposited on a glass or plastic substrate. The thickness of this metallic layer determines the splitting ratio, typically reflecting part of the light while transmitting the rest .Adhesive layers in cube beam splitters: In cube-type splitters, two prisms are cemented together with an adhesive layer (epoxy or polyester). This layer can also contribute to the splitting effect through frustrated total internal reflection (FTIR), but the primary splitting is still controlled by the thin-film coating applied to the hypotenuse of one prism .Pellicle or membrane splitters: These use an ultra-thin membrane with a reflective coating, where the coating itself is the functional layer performing the split, while the substrate is negligible . In summary, regardless of the beam splitter type—plate, cube, or pellicle—the layer that actually performs the splitting is the thin-film or reflective coating, while substrates and adhesives mainly provide mechanical support and alignment .