How does CRDS compare to FTIR, NDIR, and TDLAS?
CRDS differs from these other techniques in that it uses a time-based measurement to interrogate the absorption spectrum of the gas, rather than a traditional intensity-based absorption method. This approach has the advantage that it is independent of laser noise. Since absorbance-based instruments such as FT-IR and NDIR measure the ratio of the absorbed-to-incident light, sensitivity is limited by noise from the light source, mirrors, and detector. CRDS also has a significantly longer path length than the other techniques, resulting in higher sensitivity, precision and lower detection limits. The “optical path length” of the measurement cell is the primary theoretical determinant of sensitivity for all these techniques. CRDS uses a coherent laser light source while FTIR and NDIR instruments use an incoherent light source to generate photons. The properties of incoherent light limit the optical path length, and therefore sensitivity, of these instruments. Although TDLAS uses a coherent
