The electromagnetic spectrum is the collective term for all known frequencies and their linked wavelengths of the known photons ( electromagnetic radiation ). The "electromagnetic spectrum" of an object has a different meaning, and is instead the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object. If this sounds confusing, watch this video by FLIR:
What is Infrared?
Thermography and Infrared Light
Normally, our vision is limited to a very small portion of the electromagnetic spectrum. Thermal energy has a much longer wavelength than visible light. So long, in fact, that the human eye can't even see it, just like we can't see radio waves.
With thermal imaging, the portion of the spectrum we perceive is dramatically expanded, helping us "see" and "measure" thermal energy emitted from an object. Unlike visible light, in the infrared world, everything with a temperature above absolute zero emits heat. Even very cold objects, like ice cubes, emit infrared. And visible light doesn't affect the thermal world, so you can see equally well in highly lit and totally dark environments.
How Does an IR (thermal) Camera Work?
Infrared Energy Detection
An infrared camera is a non-contact device that detects infrared energy (heat) and converts it into an electronic signal, which is then processed to produce a thermal image or video, on which you can perform temperature calculations. Heat sensed by an infrared camera can be very precisely quantified, or measured, allowing you to not only monitor thermal performance, but also identify and evaluate the relative severity of heat-related problems.
References: FLIR and https://en.wikipedia.org/wiki/Electromagnetic_spectrum