9 Reference Data
105
9.3
Measurement Principle
The rotary cell length modulation method, which combines high performance and reliability, is
adopted in this unit.
Two cylinder cells are placed, arranged with the light source and detector in a position where
they are decentered from the center of cylinder as shown in the figure below. By rotating these
two cylinder cells with the centers of the light source and detector as the rotation center, the
distance between the 2 cylinders (cell length) is modulated continuously and periodically. The
signal input to the detector from the light source is the signal which changes the difference in
light intensity between the maximum cell length (
= 180 degrees) and the minimum cell
length (
= 0 degree) into the amplitude. Wide range measurement becomes possible by
selecting the optimum cell length and calculating according to the change of cell length.
Optical compensation
Optical compensation is attained by the rotary cell length modulation method. Information
required for optical compensation is the change of light intensity due to the light source,
detector, contamination of the measuring cell etc. This change in light intensity information can
be obtained by monitoring the light intensity when at the minimum cell length (
= 0 degree).
Turbidity compensation
This unit can measure ultraviolet (UV) and visible light (VIS) absorbances independently. UV-
VIS display is possible by setting the turbidity correction factor (
), thus a more reliable
turbidity compensation can be performed.
Cell length
Output signal
Rotation center of decentered cell
Center of cylinder cell
Center of light source
Rotation center of decentered cell
Center of detector
