A first surface mirror or front surface mirror (also commonly abbreviated FS mirror) 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.
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, and high quality kaleidoscopes use this type of mirror.
The "silvering" on a front surface mirror is usually aluminium.
The "silvering" on a front surface mirror in an infrared instrument is usually gold.