The field of infrared microspectrometry has become, over the past three years, an important analytical technique, used in conjunction with Fourier transform infrared spectrophotometry (FT-IR). The theoretical limitations of the technique have not before been published. Proper application of the theory can have a significant impact on the attainment of optimal performance from infrared microscope systems. The major limiting issue for the successful application of this technique is the diffraction theorem. This theorem as manifested in the infrared region is explained in detail in this paper, insofar as it is relevant to infrared microsampling.
Several other issues pertinent to the design and use of infrared microspectrometers are also discussed. Specifically, the choice of on-axis mirrors as opposed to lenses or off-axis mirrors in the critical areas of the instrument is explained. Also, specimen-based optical effects are identified and discussed.
Finally, two schemes to deal with the adverse effects of diffraction are presented—ultramicroscopy and redundant aperturing. These have the effect of improving the spatial resolution of the instrument. A spectrum obtained from a 5-µm spot of a polystryrene film is included to demonstrate this improvement.