An optical chemical analyser comprises a source of a first amount of radiation (46), an optics module configured to direct the first amount of radiation such that it is incident on or passes through a target (14) at a target location, the optics module further being configured to receive a second amount of Raman scattered radiation from the target and direct the second amount of radiation (206) to a Spatial Interference Fourier Transform (SIFT) module, the SIFT module including a first dispersive element (216) and a second dispersive element (218), the SIFT module being configured such that a portion of the second amount of radiation is received by the first dispersive element and interferes with a portion of the second amount of radiation received by the second dispersive element to form an interference pattern; the SIFT module further comprising a detector (48) configured to capture an image of at least a portion of the interference pattern and produce a detector signal (226) based on the captured image; and a processor configured to receive the detector signal from the detector and perform a Fourier transform on the detector signal to thereby obtain a frequency spectrum of the second amount of radiation.