The invention relates to a portable and self-contained optical spectrometer for analyzing a light spectrum backscattered by an illuminated sample, in order to deduce the content of at least one compound constituting the sample, said spectrometer being arranged about an optical axis and including: a target area centered on said optical axis; a plurality of optical sensors trained on the target area; an opaque measurement chamber including: an opening centered on said optical axis; at least one diffusing filter blocking said opening; and an inner bottom capable of housing the plurality of optical sensors and a main illumination device capable of illuminating the sample. The invention further relates to a method for using such a spectrometer.