8-1
8. Principle
8.1 CD Measurement Principle
Hg-Xe Lamp
Lens1
Polarizer
Grating
PEM
Lens 2
Flowcell
Detector
Fig.8.1
Optical diagram
The light emitted from the light source(Hg-Xe Lamp) is collimated by the Lens 1 and
directed to the polarizer. The collimated light(unpolarized) is converted to linearly
polarized light by the polarizer. The linearly polarized light is dispersed to each
wavelength and directed to the PEM(photo-elastic modulator) by the grating. Then the
light is converted to LCP(left circularly polarized light) and RCP(right circularly polarized
light) alternately (with 50kHz) by the PEM and pass through the flow cell. The light which
pass through the flow cell directed to the photodiode, the light signal energy is
transformed to electric signal.
If there are no CD active components in the flow cell, the intensities of LCP and RCP
electric signals are the same, resulting DC electric signal.
(See Fig.8.2)
If there are CD active components in the flow cell, LCP and RCP electric signals show
different intensity, resulting AC electric signal.(See Fig.8.3)
The electric signal is converted to digital signal by the A/D converter, calculated to
produce CD signal and UV signal by the CPU. These CD signal and UV signal are re-
converted to analogue signal by the D/A converter and outputted as a output signal.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com