Thermal imaging lenses uses cameras that “see” heat instead of light. Sometimes referred to as FLIR cameras, they produce an image that portrays objects using their radiated heat instead of their visible properties.
So how does thermal imaging lenses see heat? All objects warmer than absolute zero (-273°C/-459°F) emit infrared radiation in the MWIR and LWIR wavelengths (3µm–14µm) in an amount proportional to the temperature of the object. Thermal imaging lenses focuses and detects this radiation, then translates the temperature variations into a greyscale image, using brighter and darker shades of grey to represent hotter and cooler temperatures, which gives a visual representation to the heat profile of the scene.
Many thermal imaging lenses can also apply color profiles to these images, showing hotter objects as yellow and cooler objects as blue for example, to make it easier to compare temperatures in the image.