Yes, we have considered using thermal cameras. In fact, we have tested three different thermal cameras, but they are generally very expensive and are subject to the many of the same issues as a non-3D optical camera as well as a some unique ones. We have seen issues like "heat shadows", blasts of wind when it is either very warm or cold outside, people bundled in winter clothing, hot food, or cold drinks throwing off thermal cameras.
I guess what I'm really asking is if anyone is (or knows someone who is) an optics expert and would be able to take a look at the camera to see what they could do to make this work as an IR cam (i.e. putting in a filter that permits IR light to pass and calibrate both lenses).
I appreciate all the feedback from everyone on the forum though!
Have you considered using a thermal camera? At least the image would be consistent day and night making it a lot easier to calibrate for people counting. I've never tried this but it's just a thought.