Abstract: A Fourier transform spectrometer for use, among others, for sensing gases or liquids includes two polarizers, a birefringent optical component interposed between the two polarizers and a detector. Light from an extended source is substantially polarized by a first of the two polarizers, split into divergent beams by the birefringent optical component, and again caused to converge by the birefringent optical component so that, after passing through a second of the two polarizers, the light forms an interferogram in a plane behind the birefringent optical component where the detector is situated. The Fourier transform spectrometer is thereby provided with a way of forming an interferogram of the light from the extended source without a lens or a second birefringent optical component. This improves the field of view of the spectrometer and is likely to reduce its cost.