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Recently it has been a topic of some discussion that the phase associated with part of the self-emission of a Fourier-transform spectrometer may differ neither by 0 nor by π rad from the phase of an external target if there is absorption in the beam splitter. The conventional interpretation of this has been to separate the self-emission into three terms: instrument emission from the input port, in phase with the external target; emission from the secondary input port, π rad from the target; and emission from an absorbing beam splitter with an anomalous phase (neither 0 nor π rad with respect to the target). There is another necessary condition that has not received much attention, that the instrument not be isothermal. For polarized radiation there is an additional condition that suppresses the anomalous phase, and for unpolarized radiation there is a way to produce an anomalous phase without beam-splitter absorption.

We show that an artificially grown diamond plate serves as a very broadband beam splitter from approximately 7 μm, where the intrinsic diamond absorption becomes important, to the submillimeter region (a 250-μm wavelength or longer).

We show that room-temperature, crystalline quartz is a useful material for a beam splitter for spectroscopy in the far infrared, ∼60-µm wavelength and longer. We compare such a beam splitter with the traditional far-infrared candidates: Mylar, polarizing, and lamellar beam splitters.

We show that uniform time sampling of both the reference and the target channels in a continuous scanning Fourier transform spectrometer is a simple and versatile way of extending the Nyquist limit shorter than the wavelength of the reference channel. We also discuss the benefits of recording the reference channel when intensity calibrating the target data.

A cryogenic Fourier transform spectrometer has been built to measure thermal emission of the earth’s limb from a balloon-borne platform. Liquid nitrogen cooling of the spectrometer and liquid helium cooling of the detectors has provided sufficient sensitivity to detect, at 5–15 μm, fifteen molecular species relevant to stratospheric ozone chemistry. The spectral resolution achieved, 0.022 cm^−1, is the best yet attained for emission mode data at these wavelengths. The philosophy behind the design of the optical and electronic systems is presented, followed by an analysis of the performance achieved during balloon flight.

We show that an artificially grown diamond plate with an improved surface flatness serves as an infrared beam splitter to wavelengths as short as 2.2 µm (4500 cm^-1).

Rapid automated frequency response of IR detectors can be measured to get responsivity, noise, and NEP using multiple harmonics of the IR detector response.