Thermal imaging or infrared (IR) cameras
The camera sees light that is within the heat spectrum that exists just beyond the spectrum that can be seen with the naked eye. Differing heat signatures are displayed in the camera’s viewfinder as a gradient color scheme, with hotter areas displayed as brighter colors, and cooler areas as darker colors.

What We see (Missing Insulation) Vs What You See
Detecting moisture intrusion problems is an excellent application for thermal imaging or infrared (IR) cameras. Moisture intrusion may be identified through basic visual inspection if it has lead to obvious defects, such as staining or mold growth. By the time visible evidence has presented itself, however, significant damage may have already been done. In many cases, moisture or water intrusion may have been developing for a while before obvious signs become apparent. By employing thermal imaging and a moisture meter, inspectors can locate moisture issues before they become large problems and lead to serious damage, as well as gather details in cases where moisture intrusion has already become obvious.

What We see (Water Leak) Vs What You See