A mirror is an optical element that works based on the law of reflection and is mainly used for beam redirection, interferometry, imaging or illumination, among other things. The properties of mirrors depend on the optical coating, substrate and surface quality. The standard process is to coat a highly polished substrate material, and the properties of the optical mirrors produced vary depending on the substrate material and the materials and processes used for the coating. Common substrate materials include N-BK7 optical glass, fused silica, float glass, etc. The optical coating determines the reflectivity and stability of the mirror, and is the most critical component of the mirror, and the mirror film is usually made of metal or dielectric materials.
Mirrors can be divided into three types: planar mirrors, spherical mirrors and aspherical mirrors according to their shapes; According to the degree of reflection, it can be divided into total reflective mirror and transflective mirror (also known as beam splitter). We will briefly describe the two main types of mirrors, metal film mirrors and dielectric film mirrors, and today we will mainly talk about metal film mirrors.
The real part of the permittivity corresponds to the proportion of the amplitude of the electric field during light propagation, while the imaginary part corresponds to the energy loss of light. The dielectric constant of a metal can be described by the Drude dispersion model, also known as the free electron gas model. According to the Drude dispersion model, the relative permittivity of a metal is: the decay constant of the velocity of the electron, the frequency of the incident light, the oscillating natural angle frequency of the plasma formed by the free electrons, which can be expressed as the charge of the electron, the density of the electron, the dielectric constant in the vacuum, and the mass of the electron. The real part and imaginary part are respectively: and the complex refractive index of the metal can be expressed as: where the real part is the refractive index of the metal, which is determined by the propagation speed of the light wave in the absorbing medium; The imaginary part determines the attenuation (absorption of light energy) when the light wave propagates in the metal, which is called the extinction coefficient, in which the general metal film has a large extinction coefficient. When the beam of light is incident on the surface of the metal from the air, the amplitude of the light entering the metal is rapidly attenuated, so that the light energy entering the metal decreases correspondingly, and the reflected light energy increases. At the same time, the larger the extinction coefficient, the greater the absorption of light energy into the metal, resulting in a decrease in reflectivity, which is also the reason for the absorption of metal mirrors. Generally, metals with large extinction coefficients and stable optical properties are selected as metal film materials. 
Fig.1. Schematic diagram of reflection of metal mirrors.
Metal film mirrors, that is, a metal film layer is coated on the substrate to achieve the reflection of light. Metal films have a very wide reflectance band, but are often accompanied by some light absorption. The most optical metal film mirrors in the market are mainly aluminum film mirrors, gold film mirrors and silver film mirrors. 1.Aluminum film mirrors are generally evaporated on a high-precision polished flat substrate, and the use of this type of mirror is not limited by the angle of incidence of light. According to the actual use scenarios, the aluminum film mirror will be made subtle adjustments in production, such as: only evaporated aluminum film, coated with a protective film to prevent damage, coated with a protective film to improve the reflectivity in the ultraviolet band, and coated with a protective film on a parallel plane substrate. It has a relatively high reflectivity (>90%) from ultraviolet to infrared wavelengths, and the optical coating is relatively strong compared to other metal materials. JCOPTIX provides aluminum films with a protective layer with a reflectivity greater than 93% in the wavelength range of 450 - 700 nm, as shown in Figure 2. In addition, JCOPTIX offers UV-enhanced planar concave aluminum mirrors with reflectivity greater than 90% in the operating wavelength range of 250 - 600 nm. 
Fig.2. Aluminum film mirror.
2.Gold film mirrorsGold film mirrors are generally used in the near-infrared and infrared wide spectral regions, and silicon-based substrates or float glasses with high thermal conductivity, strong gold film adhesion and good heat resistance are selected in the preparation of gold film mirrors. In the actual use process, the gold film has a high reflectivity of infrared and near-infrared light, and the light without reflection will be absorbed by the gold film and will not pass through the substrate, and the strength and stability of the gold film mirror are better than those of the silver film mirror. Nanyang Jingliang Optoelectronics JCOPTIX offers gold film with protective layer covering, at 650 nm - 20The average reflectance is greater than 96% in the 0 m operating wavelength range, and different sizes of round and square mirrors are available. 
Fig.3 Gold film mirror.
3.Silver film mirrors.
The silver film has good reflectivity from visible to infrared wavelengths. Compared with aluminum film mirrors, silver film mirrors have a higher reflectivity in the visible to infrared band; In contrast to dielectric film mirrors, they are not affected by the angle of incidence of light. The adhesion between the silver film and the optical glass substrate is poor, and a protective film will be coated on the silver film during preparation to delay the oxidation of the silver film and increase its service life.
Fig.4. Silver film mirror.
JCOPTIX offers silver film mirrors with a silica protective layer at 450 nm - 20The average reflectance is greater than 96% in the 0 m operating wavelength range, and round, square, and D-shaped mirrors are available. JCOPTIX also offers right-angle prism mirrors with beveled silver coating with an average reflectance greater than 97 in the wavelength range of 450 - 2000 nm5. The average reflectance is greater than 96% in the wavelength range of 2 - 20 m; It has very low intrinsic group delay dispersion (GDD) over the operating wavelength range, making it ideal for redirecting femtosecond lasers.
Fig.5. Silver film right-angle prism mirror.
Metal film mirrors are economical and can be used in a wide range of wavelengths and at all angles of incidence, but the metal film is easily damaged due to absorption and slightly reduced reflectivity, so special care must be taken when cleaning. Metal film mirrors are mainly used in simple optical systems, optical systems that use low-output lasers, white light illumination systems or imaging systems, and infrared optical systems (gold film mirrors). For practical use scenarios, metal film mirrors can be divided into planar mirrors, spherical mirrors, and aspherical mirrors according to their shapes. Plane mirror, that is, the reflective surface is flat, and the reflective surface can be either the front surface or the back surface; Spherical mirrors, that is, the side that reflects the light source is spherical, which is divided into two types: convex and concave, and the parallel incident rays can converge to one point through spherical reflection; Aspheric mirrors, that is, mirrors with non-planar and aspherical surfaces, among which parabolic mirrors, hyperboloid mirrors and ellipsoidal mirrors are the most used.