A new approach to the problem of dispersive windows in infrared microspectroscopy

Barium fluoride windows are commonly used in infrared microspectroscopy because of their good transmission and their nonhygroscopic characteristics. The latter feature is particularly useful for biological specimens. The difference in infrared focus from visible focus occurs particularly at low frequencies, due to differing indices of refraction at various wavelengths, causes experimental problems. The work described in this paper addresses this issue. One example of this is illustrated with recent studies of bone sections by microspectroscopy at small mask sizes that require working around this problem when estimating the contents of phosphate and carbonate which absorb at low frequencies. When using a single 2 mm thick BaF2 dispersive substrate it is necessary to adjust infrared focusing of the transmitted radiation onto the detector by maximizing the single beam response of the microspectrometer specifically for the band or bands being studied. At optimum infrared focus the projected image of the mask degrades in the visible view. Thus it was necessary to alternate between the visible focus for the image capture and the optimum infrared focus for scanning the spectrum. The problems and limitations imposed from dispersive substrates were summarized by Carr where the focus shift response by wavelength was calculated for several common substrate materials. The relative transmission resulting from extreme focus shifts which ranged from 20 to 140 μm was determined specifically for a 2 mm thick BaF2 disk at various wavelengths. Of the various nondispersive infrared transmitting materials that exist, the experimentation presently reported involves diamond windows of different dimensions. Whereas the benefit of using a diamond or another substrate with minimal dispersion is obvious, the limitations of size, cost and interference fringes warrant consideration. Single beam spectra as well as spectra of specimens obtained with these alternatives illustrate their utility in solving the problem of dispersive windows in infrared microspectroscopy. [Copyright &y& Elsevier]