halfsilvered

A half-silvered mirror, also commonly called a beamsplitter, is a partially reflective optical element that divides incoming light into a transmitted portion and a reflected portion. It is usually a glass or dielectric substrate coated with a thin metallic layer or a dielectric multilayer stack chosen to achieve a desired split between reflection and transmission. In ideal terms, a half-silvered surface aims for equal reflection and transmission (approximately 50% each) at a specified wavelength and angle of incidence, but real devices exhibit some loss due to absorption and scattering and may be designed for other ratios.

Construction and varieties: half-silvered mirrors are made by depositing coatings on a flat plate, or by fabricating a cube beam splitter from two prisms with a partial coating at the interface. Metallic coatings (such as silver or aluminum) provide broad reflectance with higher absorption, while dielectric coatings use carefully engineered interference to tailor the split and wavelength response, often enabling low polarization dependence and reduced losses. The angle of incidence, substrate quality, and coating stack strongly influence performance.

Uses: they are widely used to split light paths in optical experiments and instruments, notably in interferometers such as Michelson and Mach-Zehnder setups, imaging systems, and certain camera or projector configurations. They can also form part of devices that route light to multiple sensors or viewports, or create reference and signal paths in metrology and scientific instrumentation. The specific splitting ratio and polarization behavior are selected to suit the application's requirements.