The invention relates to a device for measuring a spectrum of a light beam, in a previously selected range of wavelengths, said spectrum being generated by a sample to be analysed, said optical measurement device including at least one light source (12), a measurement cell (9) and a measurement detector (17) placed on an optical measurement path, said optical measurement path being travelled by an optical measurement beam emitted by the light source (12) and hitting the measurement cell (9), a self-calibration means suitable for taking into account the possible deflection of the light sourcesdue to environmental or operating conditions, regardless of the presence or absence of a sample to be analysed in the measurement cell, said self-calibration means including a means (15) for creatingan optical reference path travelled by an optical reference beam, emitted by the light source and not hitting the measurement cell, and a reference detector (18). Said measurement device is remarkable in that it is designed to be installed onboard an automobile and comprises a means (30, 31) for selectively mixing the optical measurement beam and the optical reference beam, the measurement detector (17) also acting as a reference detector (18).