Abstract: A Fourier transform spectrometer comprising an interferometer comprising a fixed mirror, a movable mirror and a detector, wherein: the mirrors are positioned parallel to each other and form an optical cavity; the movable mirror is operable to move relative the fixed mirror to change the size of the optical cavity; a first mirror of the mirrors is positioned such that light received from a sample enters the optical cavity through a back of the first mirror; the detector is positioned and operable to detect light which has exited the optical cavity through a back of the second mirror of the mirrors and output an interferogram; and the mirrors are distanced from each other such that the movable mirror is at or can move to a position where at least two paths to the detector for received light reflected within the interferometer are equal length.