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Reference.org
First-surface mirror
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<p>A first-surface mirror or front-surface mirror (also commonly abbreviated FS mirror or FSM) is a <a href="/facts/Mirror/wMu4u6BB">mirror</a> 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 <a href="/facts/Glass/hezRsAGf">glass</a> or <a href="/facts/Poly(methyl_methacrylate)/CnCRBo5F">acrylic</a>. </p><p>Historically, the <a href="/facts/Bronze_mirror/Aj3w7ddQ">bronze mirror</a>, an FSM type, was standard from ancient times until relatively recent centuries. These were simply highly polished pieces of <a href="/facts/Bronze/YCYCVIKt">bronze</a> or other metals, usually small and round, and designed for a person to see their face. </p><p>First-surface mirrors are now made for applications requiring a strict reflection without a <i>ghosting</i> 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. <a href="/facts/Reflecting_telescope/5Y4aXFvF">Reflecting telescopes</a>, <a href="/facts/Rear-projection_television/7fmQXIlu">rear-projection televisions</a>, <a href="/facts/Periscope/jSCoMFnw">periscopes</a>, <a href="/facts/Non-reversing_mirror/JqbEEHCh">non-reversing mirrors</a>, high-quality <a href="/facts/Kaleidoscope/PTR2SCDm">kaleidoscopes</a>, and the animation process. </p><p>In cases where the mirror is subjected to extreme cold (as low as 33 K as in the <a href="/facts/James_Webb_Space_Telescope/w4GAWnWX">James Webb Space Telescope</a>), a polished pure <a href="/facts/Beryllium/dv295voP">beryllium</a> mirror is used without a first-surface coating in order to eliminate deformations caused by differing coefficients of <a href="/facts/Thermal_expansion/BP2vjX0N">thermal expansion</a>. </p><p>First-surface mirrors are more sensitive than back-surface mirrors and may be damaged by cleaning as the reflective surface is not protected by glass. </p>