could be modified to include a plurality of lasers each for a specific wavelength or wavelength range preferably on the same monolithic device the microcontroller 8 calculates the glucose concentration by a least square calculation referring to reference spectra such as shown in fig2 or 3 for known glucose concentrations the calculated concentration is displayed on display 9 alternatively the glucose concentration could also be calculated from an average of concentrations obtained from the absorptions at each wavelength relative to a reference absorption for a reference glucose concentration determined beforehand preferably the microcontroller 8 calculates also the error of the least square calculation i e the root of mean square error and makes a selection of only portions of each acoustic pulse and a selection of those acoustic pulses which originate from laser beam pulses of selected wavelengths so as to minimise the error the selection can be chosen by trial and error among a number of pre prepared selections until the error is smaller than a certain value thereby the measurement of the glucose concentration focuses on an area within the tissue 1 where the measurement is most reliable for example a blood vessel the measurement principle of the present embodiment has been tested in the measurement of glucose concentrations in milk milk has been used as a testing solution instead of blood because it is readily available and resembles blood in that a number of substances are present which could potentially disturb the measurement these substances are e g lactose proteins and fat the absorption spectra of milk with 100 mg glucose per dl but different amounts.