Dual-phase LIA

The output from the detector preamplifier is also passed into a fourth LIA, D, referenced to the chopper, the output of which is the static absorption spectrum in other words, the conventional single-beam spectrum. Finally, the output of this LIA is input to another dual-phase LIA, E, referenced to the chopper firequency. The phase control of LIA E had been set prior to the experiment in the same way as LIA C. The outputs of LIA E are the in-phase and quadrature absorbance spectra. 

---