Appendix C Spectroscopy Background
Double modulation
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the blue trace is prevented from going to zero intensity by the contribution of the red
trace.
Much effort has been put in the design of the new MB3000 series as well as the
original FTLA legacy series of FTIRs to avoid reflecting back radiation once it is
modulated. For example the source elements are placed at an angle so that reflection
is sent beside the interferometer. The detector optics is also designed to send
reflected light beside the interferometer.
When measuring a sample that might have reflective surfaces that are placed
perpendicular to the sample beam, there is a chance to reflect modulated light of the
sample (such as a liquid cell window) back to the interferometer and have some of it
return as double modulated light. This is particularly the case for a liquid cell with
Figure C- 5.
