In a process for recognizing organic substances in solids by measuring the spectram of the radiation which is reflected or transmitted, radiation with a wavelength between 2-4 &mgr;m is used and, for radiation of solids, unmodulated radiation is used. The radiation source should have an output of at least 50 W and the distance between the illumination optics from the solid being investigated should be at least 10 cm. Recognition is based on absorption bands of the extended oscillations of C—H bonds and sometimes the presence of absorption bands of N—H bonds. Broad band radiation is preferable for radiation of the solids, and the reflected or transmitted radiation is directed through a narrow band filter with adjustable transmission frequency. This can be advantageously formed with an acousto-optical filter.