The past decade has seen considerable advances in many areas of astronomical and remote sensing optics. They have been motivated partly by the current activities of large telescope construction and ambitious spaceborne missions, and partly by technological developments promoted to increase performance sensitivity of mid-infrared instruments. The advantages of cooled and uncooled focal plane detector arrays, implementation of novel adaptive optics, and improved cryogenic performance, have all contributed to unprecedented performance from the current generation of optical instruments. Spectral measurements obtained from the mid-infrared wavelength regions are rich in unique absorption features that contain more data on the structure and composition of observations at these wavelengths than available in many other regions. This makes the infrared spectrum an important area of interest in the study of atmospheric sciences and astronomy.
Infrared multilayer interference filters have been used extensively in satellite-borne radiometers for over 40 years. The University of Reading has a space heritage of contributions of infrared optics in many of the most progressive scientific instrument programmes in the study of atmospheric and planetary science. At mid-infrared wavelengths, the fundamental radiance emitters of key molecules such as H2O, CO2, O3, CH4, N2O, N2O5, NO2, HNO3, HCN and many chlorofluorohydrocarbons (CFC) are responsible for climate change. By remotely taking infrared spectral measurements of temperature, pressure and chemical composition of molecular gases, the concentrations of elements that contribute to the effects of global warming and cloud formation theories can be determined. Characterization of the influences that affect atmospheric transparency is a complex branch of atmospheric physics with many spectral and temporal variables, all of which are affected by absorption in ground-based observations.
The optical design of the latest generation of astronomical telescopes have utilised high-performance mid-infrared filters and coatings to discriminate between wavelengths that are more accurately positioned and possess higher imaging performance than has ever been required in earlier instruments. This imaging performance is crucial to both space-flight and ground-based infrared telescopes which also demand narrowband isolation of discrete wavebands. These advances have found particular application to the design and fabrication of single-substrate filters in N-(λ=7.5-15 µm) and Q-(λ=15-28 µm) band mid-infrared atmospheric windows which are crucial to the study of interstellar and circumstellar environments for star and planetary formation theories.
Fourier transform infrared (FTIR) absorption studies of the vibrational modes of biochemical agents is a diagnostic tool for which mid-infrared passband filters can isolate spectral regions in which catalytic cycles occur. If low-frequency vibrational information can be obtained about the individual steps in the cycle, much progress can be made towards elucidating the step-by-step molecular mechanisms of critical biological processes. An example of this chemical process being Photosystem II, the enzyme responsible for the oxidation of water to dioxygen in algae and higher plants. In this application ultra-wide antireflective passband (λ=5-30 µm) filters were developed to facilitate reaction modulated infrared difference spectroscopy at cryogenic temperatures.