Originally Posted by Digerati
Okay - I see what you are doing now. Thanks for clarifying.
You might have to come up with something that will get a bit hotter as ambient temperatures in some facilities without air conditioning on a hot summer day may approach 90°F. Or the beating sun on an outside wall may obscure the image.
Note that component heating is incremental, not absolute, so higher ambient temperature means that the heating component will end up at equally higher end temperature.
But... If the device is moved from cooler place (say, from a car with air conditioning, or carried outside for a while during a cold day) and put to immediate use in a warmer place, then the "ambient" inside the device (what the heating resistor sees as ambient) is still cool for a while, until the device gets warmed up. This can be solved quite easily with enough ventilation holes in the device, for example on both top and bottom, which lets the air in the environment flush through relatively quickly.
However, even if the device was cooler, a good thermal imager might still show the "hot spot". The imager should show the device as cooler than environment, with a "less cool spot" than the rest of the device. I guess this depends on the imager's capabilities.