Apparatus for measuring the proportion or quantity of a component in a radiation-transparent mixture

In an apparatus for measuring the proportion or quantity of a component of a radiation transparent mixture by discrete radiation absorption, including means for passing a beam through the mixture;

• beam splitting means disposed downstream of the mixture with respect to the travelling direction of the beam for splitting the beam into a measuring beam and a reference beam;
• a first narrow-band interference filter arranged in the path of the measuring beam;
• a second narrow-band interference filter arranged in the path of the reference beam;
• means disposed downstream of the filters for separately detecting the radiation intensities of the measuring beam and the reference beam;
• and means for forming the difference between the two radiation intensities, the difference between the two radiation intensities being a function of the proportion and quantity of the component;
• the improvement wherein said first filter has a transmitting wavelength slightly above the discrete absorption wavelength of the component for a perpendicular penetration of radiation and said second filter has a transmitting wavelength outside the absorption spectrum of the component;
• further comprising means for oscillating said filters in phase in a range of oscillation in which said first filter traverses the absorption maximum.