The Infrared Multilayer Laboratory technical library is a repository for useful information about infrared filters and coatings.
- Infrared materials data
- Substrate optical theory
- Infrared material absorption theory
- Thin film theory
- Filter types
- Effects of the space environment on infrared filters and materials
Infrared materials data
There are many high quality crystalline substrate materials that now exist for optical filtering that possess good transparent properties over a wide range of infrared wavelengths. This page outlines the properties for the most common materials used in the manufacture of infrared optical filters.
Substrate optical theory
The bulk optical properties of crystalline substrate materials are represented by the complex refractive index as a function of wavelength and temperature. Materials for the visible spectrum change comparatively little with temperature or wavelength. However, in the infrared every material is strongly influenced by the effects of lattice absorption and dispersion, with additional effects caused by pronounced changes with temperature. The use of predictive modelling of these properties can therefore be of considerable benefit in understanding the limitations, or advantages, in the filter performance, which can be achieved from knowledge of the materials prior to the multilayer design.
- Absorption and extinction coefficient theory
- Loss-free incoherent internal reflection
- Incoherent multiple internal reflection
- Reduced substrate-temperature effects
- Angle of incidence effects
- Total internal reflection
- Coherence of multiple internal reflections
Infrared material absorption theory
The design of any infrared filtering system requires the selection of materials based upon knowledge of the optical, mechanical and thermal properties available. Frequently, the selection of suitable materials results from compromises between these various properties as no single material will possess the ideal characteristics required to suit the wide variety of applications. A study of the infrared material characteristics, particularly the absorption and dispersion processes is therefore essential for the selection of suitable materials for use both as substrates and evaporated layer materials.
- Electronic absorption
- Lattice absorption
- Single phonon absorption
- Multi-phonon absorption
- Dielectric dispersion
- Thermal vibrations
Thin film theory
The origins of thin film theory lie in multilayer interference using the wave nature of light and superposition of coherent waves. The phases and amplitudes of modulated waves determine if the resultant propagation creates constructive or destructive interference and subsequent change of transmission or reflection to the incident light. This section describes the background theory and nomenclature of thin film interference phenomena.
There are many different filter types. This section outlines the generic types of filter that we can manufacture here at the University.
- Narrow bandpass filters
- Wide bandpass filters
- Longwave-pass edge filters
- Shortwave-pass edge filters
- Broadband antireflection coatings
- Single wavelength antireflection coatings
Effects of the space environment on infrared filters and materials
With a continuously increasing demand for improvements to spaceflight optical instrumentation, for both high optical system performance and lengthening operational lifetimes, an investigation into the effects of the space environment on the durability and spectral stability of infrared filters and materials on the NASA Long Duration Exposure Facility (LDEF) was opportune.