The invention relates to a device and a method for optically measuring the concentrations
of a substance contained in a fluid medium. With the solution according to the
invention the accuracy of measurement is to be increased in accordance with the
object over a greater period without any additional calibration measurements, and
ageing of a fluorescent substance within a layer is to be taken into consideration,
in particular, and the long-time drift of such a measurement system is to be reduced.
To achieve this, a layer containing such a known fluorescent substance or layer
system are employed in which fluorescence is excited. The fluorescence changes
depending on a substance concentration or a pH-value, and the intensity varying
correspondingly in time or phase shift of the fluorescent light is allowed to be
measured with at least one optical detector with the known intensity or phase of
the fluorescence exciting light. On that occasion, the exciting light is directed
upon at least two areas being optically separated from each other of a layer containing
a fluorescent substance or two equal layers each having a different excitation
energy during the useful life, and measured values of fluorescence intensity, fluorescence
decay times or phase shifts being locally and/or timely separated of both areas
or layers are fed to an electronic comparative and correction value determination unit.
