I shall keep this brief as I really do not have the information or pictures needed to make a detailed assessment of your system.......
1. The camera head contains the Cooler, sensor array and Optics. It may also contain a buffer amplifier for the 'video' output.
2. The camera head receives all the required power, bias and timing signals from the separate controller/PC required to create the thermal scene video signal.
3. The Controller/PC creates the timing for the system so it can interpret the 'RAW' video signal coming out of the camera head in the time domain. This enables construction of a viewable video image frame.
4. The controller/PC not only generates the required power supplies and timing signals, it also converts the RAW video signal into a digital format that it can process. This will be the ADC video card that you appear to be missing. Once the RAW video is in the digital domain, it has the required video processing and calibration corrections applied. These corrections are used to remove dead pixels from the image and create a uniformly 'flat' image output from the thermal sensor array. Without this correction the image will contain many dead pixels and possibly appear to have light and dark pixels all over it. Basically, pretty much unuseable RAW imagery.
4. The controller/PC is a 'System' and is likely 'closed loop'. That is to say, when it boots, it tests itself and checks for the presence of all cards and expected inter component communications. This can include a true 'BITE' sub system that will test and diagnose faults within the equipment. If any part of the system is faulty or missing, it is highly likely that the operating system will spot this as an error and will not provide the camera head with any signals to drive the sensor. Power supplies to the head are likely completely separate to the image control functions so they are present to cool the sensor but no bias voltages or timing signals will be present at the sensor. The camera head is basically like. Zombie .... the lights are on, but no one is at home
Sadly you have an incomplete thermal imaging system. It is unlikely to boot up correctly and drive the camera head unless you so,be the missing video card problem. You cannot just tap the video output from the camera head as it is unlikely that the sensor is actually doing anything and is still 'asleep'.
The fact that the camera head is not 'on-line' means the controller/PC is not putting the head into operation mode so it is, as already stated, cooled but asleep.
Sorry it is not better news.
I have worked on different manufacturers products that appear to be constructed in the same way as your unit. They are complete closed loop systems that are very fault intolerant and shut down the camera head whenevervan issue is detected. The running of the Cooler is not proof of life in the imaging head sensor array. It is purely a simple power control of a motor driving a pump. Pretty dumb electronics that regulate sensor temperature to -196C
Some pictures of the system would have helped me identify any similar/re-badged versions that may exist.
As the old saying goes.... a picture is worth a thousand words.
Fraser