Abstract: A heterodyning lock-in amplifier in which the reference signal is shifted up in frequency (as in prior art designs), but then shifted back down in frequency prior to mixing with the input signal.

Abstract: The invention relates to an infrared optical system which is capable of providing an effective focal length of about 240 mm, but fitting into a spherical package whose diameter is approximately 150 mm. To achieve such a degree of compactness, the invention employs an optical path including three reflections, which in combination define a question-mark-shaped optical path. Between the first and second reflections, the transmitted beam is passed through a focal plane and then is recollimated, and the collimated beam is then passed through the succeeding two reflections, after which it is directed telecentrically onto a detector. The invention further relates to a specific combination of lenses.