The Thermo Scientific Model 450i is an analyzer that operates on the principle that H2S can be converted to SO2. As SO2 molecules absorb UV light and are excited at a certain wavelength, the molecules move to a lower energy state emitting UV light at a different wavelength. Specifically:
H2S -> SO2
SO2 + hv1 -> SO2* -> SO2 + hv2
The pulses of the UV lamp serve to increase the optical intensity, which causes higher UV energy to impinge on the sample to achieve lower SO2 concentrations.
The 450i analyzer uses bandpass reflector filters, which, compared to commonly used transmission filters, suffer less photochemical degradation and are more selective in terms of the bandpass of interest in SO2 measurement.