Ok, so I am just an undergraduate intern this summer. We have light source shining through an optical chopper, reflecting of a lit display, and going to a detector. So, we have a signal on top of a DC signal coming from the fact that the display is also emitting light. We want to measure both the DC signal and the chopped signal and compare them. The signal from the optical chopper is our reference for measuring the chopped signal. My advisor is convinced that we should be able use a second lock-in amplifier to measure the DC component of our signal by somehow using the same chopping frequency as a reference signal. He won't really listen to me, but I am convinced otherwise (that we need more than just another lock-in). (Essentially, he believes that we should just be able to look 180 degs phase shifted from the reference and be able to see just the DC signal. I've tried telling him that that is not how a lock-in works). He wants me to call tech support for Stanford Research (the makers of our lock-ins) to ask them for advice. It seems silly. So far, I've thought of a few other ways to accomplish the problem: -split the signal from the detector, send half to the lock-in and the other half through a low-pass filter -split the signal, send half to the lock-in and mix the other half with a signal generated by a frequency generator, then send that to another lock-in (using the generated frequency as the reference) He has pretty much dismissed both of these ideas, thinking that his solution is more elegant. What should I do? Thanks!