A quality test of fluids of biological origin can be performed optically
with applying a suitable evaluation method. In case of two components to
be determined in a such fluid, an extinction spectrum is approximated in a
first wavelength range by a combination of a merely theoretical curve and
the spectrum of the pure first substance in a first wavelength range, and
this evaluation is repeated in a second wavelength range this time by
approximating the measured spectrum (62) by a combination of a
hypothetical curve, the spectrum (64) of the first component with the
already determined concentration, and the spectrum (65) of the pure second
component. Furthermore, it is feasible to subtract the first and second
spectrum and analyze the so obtained differential spectrum (66) in view of
anomalies. The hypothetical curves are preferably straight lines which are
defined by slope and ordinate section. In the praxis of the quality test
of blood, bilirubin and hemoglobin may be quantitatively be determined,
whilst the background together with the lipid component can be
qualitatively examined by means of the differential spectrum.
