It strikes me that a chopper amplifier (not chopper stabilized but proper, old school chopper amplifier) is either similar in concept to or one specific implementation of a lock-in amplifier.
Both have the concept of synchronous modulation and demodulation with amplification in between, followed by a low-pass filter (and perhaps more amplification).
It seems the chopper amplifier only has the X output of a more full-featured lock-in amplifier, but otherwise the concepts are quite similar. Am I missing some distinction here?