Frontsurface

Frontsurface, in optics, refers to a front-surface mirror, a reflective element in which the reflective coating is applied to the very front surface of the substrate. Light reflects directly from the coated surface rather than after passing through the substrate, which eliminates the slight optical path and potential ghost reflections associated with back-surface mirrors. This makes front-surface mirrors preferable for high-precision imaging and beam steering.

Construction and coatings vary by application. Common substrates include float glass, fused silica, borosilicate glass, and quartz. The coating is typically aluminum for broad spectral performance or silver for high reflectivity in the visible range. For higher efficiency and wavelength specificity, dielectric multilayer coatings are used to achieve reflectivity above 99% in a defined band. A protective overcoat, such as silicon dioxide or magnesium fluoride, guards the reflective layer. Some designs employ additional protective windows or housings.

Optical performance and considerations. Front-surface mirrors minimize ghost images and focal shifts, enabling sharper images and accurate alignment. Because the coating lies on the surface, the mirror is more susceptible to contamination, scratching, or damage, which directly degrades reflectivity and scattering. They can be more sensitive to environmental factors and may require meticulous cleaning and handling. Dielectric coatings offer high laser damage thresholds and wavelength selectivity but are typically optimized for specific bands, whereas metal coatings provide broader coverage but with higher losses in some regions.

Applications. Front-surface mirrors are common in high-precision optical instruments, including telescopes, periscopes, laser systems, spectrometers, and photolithography. They are favored in laboratory optics, beam steering, and alignment applications where surface-level reflections must be controlled with minimal distortion.

See also: front-surface reflectors, back-surface mirrors, dielectric coatings.