First surface mirror

A first surface mirror or front surface mirror (also commonly abbreviated FS mirror or FSM) is a mirror with the reflective surface being above a backing, as opposed to the conventional, second surface mirror with the reflective surface behind a transparent substrate such as glass or acrylic. In cases where the mirror is subjected to extreme cold (as low as 33 K as in the James Webb Space Telescope), a polished pure beryllium mirror is used without a first surface coating in order to obviate buckling caused by differing coefficients of thermal expansion.

Example of a second surface mirror (left) and a first surface mirror (right). In both pictures, the pen is touching the surface of the mirror. "Ghosting" (a faint reflection from the first layer) is visible in the left image (more obvious when this file is displayed at full-size).

Technicians assemble six of the 18 first surface mirrors used in the James Webb Space Telescope.

They are made for applications requiring a strict reflection without a ghosting effect as seen with a second surface mirror, where a faint secondary reflection could be observed, coming from the front surface of the glass. This includes most optics applications where light is being manipulated in a specific manner. Telescopes, rear-projection televisions, periscopes, non-reversing mirrors, high quality kaleidoscopes, and the animation process Spectrafocus use this type of mirror.