Selecting the Appropriate Mirrors in Optical Systems

Mirrors are a crucial component in optical systems. They can be used to focus and direct light, reject specific wavelengths, and combine wavelengths in imaging and other applications. Several factors should be considered when selecting mirrors.

Materials

• Metal Mirrors: These provide a combination of absorption and reflectance (and if thin enough, transmittance). They can be used as neutral density filters, neutral beam splitters, or broadband reflectors. The type of metal used determines its spectral characteristics. These mirrors are mostly used outside the angle of incidence.

• Dielectric Mirrors: Composed of fine layers of non-absorbing materials (usually fluorides and oxides) with varying refractive indices. The composition and thickness of the layers are configured to produce reflection or transmission within a specified wavelength range as per application or customer requirements. These materials absorb very little light, so dielectric mirrors can often be used as dichroic mirrors (where some colors of light pass through while reflecting different colors). The angle of incidence and wavelength range must be determined during the design phase.

Function

• Imaging: Requires λ/10 or better flatness to reduce image distortion. Beam control and non-imaging applications do not have strict flatness requirements.

• Wavelength Combination: Uses dielectric dichroic mirrors to combine different laser beams onto one axis. This application requires 1/4λ or better flatness per inch.

• Wavelength Splitting: Dielectric dichroic mirrors can also reflect desired wavelengths. Applications include excluding IR and NIR light, transmitting emission light, and reflecting excitation light while using multiple detectors to identify alternating bands of hot mirrors. For such applications, the reflection and transmission wavelengths must be thoroughly defined. These are typically used at a 45° angle of incidence.

• Wavelength Suppression: In some cases, researchers may want to exclude specific wavelengths from the system. Examples include sorting filters (reflecting unwanted wavelengths), cold mirrors (reflecting shorter wavelengths while transmitting longer wavelengths, usually used in lamp assemblies), and hot mirrors (reflecting infrared or near-infrared light).

From a functional perspective, these are dichroic mirrors applied to different aspects. They are typically used at near-normal to normal incidence.

Angle of Incidence

Mirrors are mostly configured to be used at a specific angle of incidence (AOI). Hot mirrors are typically used at zero or near-zero AOI, while dichroic mirrors are often used at 45°. The AOI is determined by the optical design of the system. When the AOI exceeds about 25°, polarization differences should be explored. Read more about the angle of incidence for additional information.

Physical Environment

Durability requirements should be determined based on the physical environment the mirror is exposed to. Temperature cycling is crucial for space applications. For outdoor applications, temperature and humidity cycling, abrasion resistance, condensation, and salt spray may need to be considered. Radiation flux (when filters are exposed to high energy or intense light beams) may cause performance degradation over time. Air-conditioned laboratory spaces or protected laboratory instruments have limited environmental requirements.

Wavelength Range

• UV (180-400 nm): While traditional metal mirrors can work over a wide wavelength range, other metals may perform better in specific wavelength ranges. First surface aluminum mirrors protected with magnesium fluoride are typically recommended for wavelengths below 430 nm. Bena Optics also produces custom dielectric mirrors for this range, including fine layers of transition metal oxides or silicon dioxide, magnesium fluoride, and lanthanide fluorides for lower wavelengths.

  

• Visible (400-700 nm): Visible light mirrors are traditionally made of silver, placed on the top (first surface) or back of a glass substrate. They are often shielded with an additional layer of silicon dioxide (for the first surface) or opaque plastic material (for the back). Dielectric mirrors are composed of non-absorbing materials in alternating layers to enhance reflectance at specific wavelengths and angles while excluding others. Enhanced metal mirrors use both dielectric and metal layers to optimize reflectance.

  
  

  
  

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• NIR – IR (700 nm - 10 µm): In the IR and NIR ranges, gold mirrors are commonly used, which absorb some visible wavelengths but also have high reflectance (over 95% above 1,500 nm). Another option is transparent conductive oxide mirrors (e.g., ITO), which provide transparency at shorter (visible) wavelengths and high reflectance at longer wavelengths.

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• Broadband: Applications may sometimes require high reflectance over many of the wavelength ranges mentioned above. These applications include astronomy, solar thermal or photovoltaic, and hyperspectral imaging.