A device for measuring a spectrum of a light beam, in a wavelength range chosen beforehand, the spectrum being generated by a sample to be analyzed, the optical measuring device including at least one light source, a measurement cell and a measurement detector placed on a measurement optical pathway, the measurement optical pathway being taken by a measurement optical beam emitted by the light source, and encountering the measurement cell, a self-calibration unit allowing any drift of the light sources, due to environmental conditions or conditions of use, to be taken into account independently of whether a sample to be analyzed is present in or absent from the measurement cell, the self-calibration unit including elements for creating a reference optical pathway, taken by a reference optical beam emitted by the light source, and not encountering the measurement cell, and a reference detector.