Just addressing that if you want to turn out the lights while monitoring the prisoner then a FLIR will do a good job, so yes to a degree I’m ignoring cost. There was a discussion up thread about the effect that continuous lighting has on well being.
However regarding cost, a microbolomoter FLIR is cheap but has low resolution (good enough for firefighters to find people in burning buildings though though). A more pricey InSb sensor with a cooled FPA will give plenty of resolution. Definitely good enough to allow you to tell if a person is moving and breathing. On the super cheap end are light amplifiers, the type used on the silly ghost-hunter shows. Those only require a brightness of about 3 millilux to be effective.
