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Accurate infrared transmittance measurements on optical filters using an FT-IR spectrometer.

The accurate measurement of transmittance characteristics of optical filters in the IR region of the spectrum is a demanding application for an IR spectrometer. We have demonstrated that the well known advantages of the Fourier transform IR spectrometer (FT-IR) can be utilized for this application. Measurements using FT-IR may be made faster and with better SNR than with dispersive instruments. In addition, the measurements have an optical precision that is superior in wavelength accuracy and equal to, or better, in transmittance accuracy. Instrumental parameters and test procedures necessary to obtain reliable and reproducible data are discussed in detail. The spectral characteristics of filters in both the near-and mid-IR regions are presented, covering a total range from 1-50 microm (10,000-200 wavenumbers). The types of optical filters examined included those with narrow and wide bandpass, and shortpass characteristics.