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Spectral Characterisation of Infrared Optical Materials and Filters

G J Hawkins

PhD Thesis - The University of Reading, Department of Cybernetics (1998)


The optical and semiconductor properties of the materials used in the design and manufacture of infrared interference filters play a vital role in defining the spectral performance achievable from a multilayer filter design. This thesis examines the theoretical basis of the behaviour of absorptive and dispersive mechanisms in optical materials and derives methods of determining values for their complex optical constants.

By applying these properties to the multilayer filter design, a predictive model for the filter performance has been constructed to determine if a chosen design can achieve the specified spectral performance requirements, prior to manufacture. Examples are given demonstrating the convergence of prediction with practice.

This predictive model approach has then been expanded to develop a method for determining the spectral design requirements for the individual filters and coatings integrated into an atmospheric radiometer instrument. This process uses an integrated systems approach, by which the characteristics of all the contributing elements provide a predicted spectral model of the instrument. By then applying reverse synthesis to this model, the particular spectral requirements of the individual filters can be determined. Examples are given of particular spectral design requirements for filters derived using this method.

The effects of the space environment on the spectral and physical properties of infrared filters and materials is also presented. This includes a description of the radiation environments to which filters are subjected in low Earth orbit. A quantitative analysis of the effects of this environment on the spectral characteristics of exposed filters and materials is made, together with an assessment of the physical degradation mechanisms that affect filter performance.

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