A device for conducting a transmission measurement includes a system for
generating a light wave consisting of a first and a second wavelength
fraction, in addition to an optical channel guiding the light wave to a
sample tube with a sample, wherein the light wave permeates the sample
tube with a sample. The device also has a motion unit for producing a
relative movement of the light wave along the sample tube. A receiver for
receiving and separating the first and second wavelength fragments after
the light wave has permeated the sample tube with a sample is also
provided. A converter converts the first wavelength fraction into a first
digital measuring value describing the degree of transmission in the
first wavelength and converts the second wavelength fraction into a
second digital measuring value describing the degree of transmission in
the second wavelength. Quotients are formed from the first digital
measuring values and the second digital measuring values by an evaluation
system and the first derivative of the quotients is calculated.