first surface mirrors (original) (raw)

Acronym: FS mirrors

Definition: metal-coated mirrors reality metallic mineral coating is on the front surface of the mirror substrate

Alternative term: front surface mirrors

Category: general optics

• mirrors
  • metal-coated mirrors
  • dielectric mirrors
  • dispersive mirrors
  • Bragg mirrors
  • crystalline mirrors
  • first surface mirrors
  • parabolic mirrors
  • variable reflectivity mirrors
  • deformable mirrors
  • laser mirrors
  • laser line mirrors
  • fiber loop mirrors
  • semiconductor saturable absorber mirrors
  • supermirrors
  • (more topics)

Related: mirrorsmetal-coated mirrorsinfrared optics

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DOI: 10.61835/ijx Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is a First Surface Mirror?

Common types of metal-coated mirrors, e.g. silver mirrors as are widely used in households, have their reflecting metal coating on the backside of a glass substrate (typically with an additional protection layer behind). In contrast to that approach, a first surface mirror, also called front surface mirror, has the reflecting coating on the front surface. This leads to various advantages over the first explained second surface mirrors, which are explained in the following section.

Figure 1: Comparison of second surface and first surface mirrors. In the former case, the incident light travels through the transparent mirror substrate, and a secondary reflection from the front surface can occur.

Ordinary dielectric mirrors can in principle also be considered as first surface mirrors, but the term is usually applied only to a metal-coated mirrors. These can be made with different types of substrates — not only with glass, but e.g. also with metallic substrates, e.g. with rectangular or circular front surfaces, and with flat or curved surfaces.

Consequences of Front Reflection

For a first surface mirror, the reflected light does not need to propagate through the mirror substrate. That has the following consequences for its performance, when compared with a second surface mirror:

• The mirror substrate cannot reduce the reflectivity by absorption in certain wavelength regions. Therefore, front surface mirrors are preferable in situations where wide low-loss reflection bands (e.g. in the infrared) are required.
• Wavefront distortions due to optical inhomogeneities of the substrate cannot occur. The substrate only needs to be sufficiently flat.
• For non-normal incidence of light, one avoids the transverse beam offset due to propagation in the glass substrate. In addition, in imaging applications one does not obtain ghost images due to the additional (weaker) front surface reflection of a second surface mirror.
• For normal incidence, strong chromatic dispersion based on interference of contributions from front and back surface reflections is avoided. (For details of such effects, see the article on Gires–Tournois interferometers.)

For many applications in optics, the mentioned advantages are vital; some examples:

• Laser mirrors (e.g. for CO2 lasers) generally need to be first surface mirrors.
• Imaging applications generally need them for avoiding ghost images.
• Broadband mirrors for spectroscopy are also usually made as front surface mirrors.

Therefore, first surface mirrors are quite common in professional optics. However, compared with back surface mirrors, they have the important disadvantage of being substantially more sensitive. The front surface may be touched, and a metal coating on the front surface is substantially more sensitive than a bare glass surface. For example, fingerprints can easily cause oxidation of the metal, and it may not be possible to clean such mirrors while retaining the full optical quality. Also, moisture or aggressive gases may cause oxidation of the mirror coating.

Protected and Enhanced Mirrors

In order to mitigate the sensitivity problem, and partly also for further improving the reflectivity, first surface mirrors are often made as protected or enhanced mirrors. This means that the metal layer is covered with a thin layer or multiple layers of a dielectric material such as amorphous SiO2 (silica) or Si3N4 (silicon nitride), which protects the coating particularly against oxidation (tarnish) and to a lesser extent against scratches.

Strictly speaking, the main reflection will no longer occur at the front surface, when there is a dielectric coating on the metal layer. That coating, however, is very thin, and therefore often not considered as providing an additional surface.

For enhancing the reflectivity, one often uses multilayer dielectric coatings; this leads to enhanced metal coatings e.g. for enhanced silver mirrors. One effectively combines the large reflection bandwidth of a metallic mirror with the higher reflectivity and damage threshold of a dielectric mirror. In some cases, the coating is also optimized for chromatic dispersion — mostly for use in ultrafast lasers.

A potential disadvantage of additional coatings arises from their deviating thermal expansion coefficients. This can cause problems particularly when mirrors need to be operated in wide temperature regions. That is the case for optical telescopes, for example.

Examples

Here are some examples of protected and enhanced metal-coated mirrors:

• Protected aluminum mirrors are relatively cheap mirrors for the visible and near-IR spectral region. They have a protective coating, e.g. made of silicon monoxide, but not particularly enhanced optical properties.
• There are UV-enhanced aluminum mirrors with particularly high reflectivity for ultraviolet light down to ≈250 nm. The enhancement of reflectivity, achieved with a dielectric multilayer coating, is typically obtained in a wide spectral range, i.e., also in the visible region. Besides the optical properties, the durability is also enhanced.
• Similarly, IR-enhanced gold mirrors are available, which have a high reflectivity in the near or even mid infrared, e.g. up to 20 μm.
• Ultrafast-enhanced silver mirrors have a very high reflectivity and low chromatic dispersion over the whole wavelength band of titanium–sapphire lasers (750–1000 nm). The coating also protects against the tarnishing of the silver, but it is still better to keep such mirrors in a dry environment.

It is also possible to apply laser line coatings, which can enhance the reflectivity at one or more specific laser lines.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a first surface mirror?

A first surface mirror, also called a front surface mirror, has its reflective coating on the front surface of its substrate. Unlike a common household mirror, light does not pass through a glass layer before being reflected.

What are the main advantages of first surface mirrors?

They prevent problems caused by light passing through the substrate, such as absorption losses, wavefront distortions, and ghost images. They also avoid transverse beam offsets when light hits the mirror at an angle.

Why are first surface mirrors so sensitive?

Their reflective metal coating is on the exposed front surface, making it vulnerable to damage from scratches, fingerprints, moisture, and oxidation (tarnish). In contrast, second surface mirrors have their coating protected behind a layer of glass.

What is an enhanced metal mirror?

An enhanced mirror is a first surface mirror with a multi-layer dielectric coating applied over the metal. This coating both protects the metal and increases its reflectivity and damage threshold.

Which metals are used for first surface mirror coatings?

Common coatings include aluminum for visible and UV light, silver for high reflectivity across the visible and near-infrared, and gold for infrared applications.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 53 suppliers for first surface mirrors. Among them:

⚙ hardware

Knight Optical has a range of first surface mirrors available. Coatings include UV enhanced aluminium (180–650 nm), enhanced aluminium (450–650 nm), dielectric (450–700 nm), NIR dielectric (700–1604nm), ion-plated silver (450–2000 nm), and protected gold (750 nm to far IR). We also offer a variety of different flatness options dependent on the application, from 2λ down to λ/10. Custom mirrors are also available if our stock optics are not a match for your requirements.

⚙ hardware

OPTOMAN is ready to develop cost-effective yet advanced, high accuracy and repeatability dielectric first surface mirror coatings optimized for your application. We concentrate on a single manufacturing technology — ion beam sputtering (IBS). Our dielectric mirrors are available in multi-wavelength or broadband configurations in spectral range from 193 nm up to 5000 nm, feature low absorption and are optimized for your application.

⚙ hardware

Edmund Optics offers a range of laser, flat, metal substrate, focusing, or specialty optical mirrors in a multitude of reflective coating options, including protected aluminum, enhanced aluminum, protected silver, protected gold, and dielectric. Optical mirrors are designed to reflect light for a variety of applications, including beam steering, interferometry, imaging, or illumination. Optical Mirrors are used in a wide range of industries, such as life sciences, astronomy, metrology, semiconductor, or solar.

⚙ hardware

UltraFast Innovations (UFI®) offers a range of first surface mirrors, including both dielectric and metal-coated versions. We particularly emphasize mirrors with broad bandwidth and well controlled chromatic dispersion.

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