First-surface mirror
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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.[1]
First-surface mirrors 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. Reflecting telescopes, rear-projection televisions, periscopes, non-reversing mirrors, high-quality kaleidoscopes, and the animation process.
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 eliminate deformations caused by differing coefficients of thermal expansion.
Silvering
The "silvering" on a front-surface mirror is usually aluminum for visible light and gold for infrared radiation.
References
- ^ Paschotta, Dr Rüdiger. "First Surface Mirrors". www.rp-photonics.com. Retrieved 2022-08-30.