An apparatus for measuring of material in a liquid through the absorption of light, the apparatus having a measurement chamber with a measurement window through which (a) a transmitted signal projects to a target point, and (b) an absorbed light signal is reflected from the target point, a coupling mass with piezoelectric transducers being connected to said measurement window to cause vibrations thereof to keep said measurement window clean, a source of power for said apparatus, a blank sample for calibrating said apparatus, the improvement comprising: a light source for generating said transmitted signal; a first beam splitter, wherein the first beam splitter is reflective below a cut-off wavelength and transmissive above the cut-off wavelength, wherein the first beam splitter receives light from the light source, wherein the light is split by the first beam splitter so that a first portion of the light with first wavelengths above the cut-off wavelength is projected to said target point as the transmitted signal, and wherein a second portion of the light with second wavelengths below the cut-off wavelength is reflected to a second beam splitter as a reflected light signal; the second beam splitter receiving said absorbed light signal from said target point and said reflected light signal from said first beam splitter to give an output of recombined light beams, said second beam splitter being highly reflective above the cut-off wavelength and highly transmissive below the cut-off wavelength; a detector for receiving said recombined light beams and generating a single spectral output; a light control module connected to said light source and said detector, said light control module compensating for deviations in intensity of said transmitted signal by: calibrating with said blank sample, by: calculating SR current by dividing a measured light intensity above the cut-off wavelength (sample LI) by a measured light intensity below the cut-off wavelength (base LI) for a current sample, and calculating SR OPPM by dividing sample LI by base LI for the blank sample, and thereafter calculating absorption by the following: Absorbance = Lo g 10 SR OPPM SR current. a light source for generating said transmitted signal; a first beam splitter, wherein the first beam splitter is reflective below a cut-off wavelength and transmissive above the cut-off wavelength, wherein the first beam splitter receives light from the light source, wherein the light is split by the first beam splitter so that a first portion of the light with first wavelengths above the cut-off wavelength is projected to said target point as the transmitted signal, and wherein a second portion of the light with second wavelengths below the cut-off wavelength is reflected to a second beam splitter as a reflected light signal; the second beam splitter receiving said absorbed light signal from said target point and said reflected light signal from said first beam splitter to give an output of recombined light beams, said second beam splitter being highly reflective above the cut-off wavelength and highly transmissive below the cut-off wavelength; a detector for receiving said recombined light beams and generating a single spectral output; a light control module connected to said light source and said detector, said light control module compensating for deviations in intensity of said transmitted signal by: calibrating with said blank sample, by: calculating SR current by dividing a measured light intensity above the cut-off wavelength (sample LI) by a measured light intensity below the cut-off wavelength (base LI) for a current sample, and calculating SR OPPM by dividing sample LI by base LI for the blank sample, and thereafter calculating absorption by the following: Absorbance = Lo g 10 SR OPPM SR current. calculating SR current by dividing a measured light intensity above the cut-off wavelength (sample LI) by a measured light intensity below the cut-off wavelength (base LI) for a current sample, and calculating SR OPPM by dividing sample LI by base LI for the blank sample, and thereafter calculating absorption by the following: Absorbance = Lo g 10 SR OPPM SR current.