“What is thermal Camera” could be a quite tricky question! A general perception is, a Thermal Camera is that camera which can “see in the dark” as there is temperature in the dark, though no light, while normal visual camera has capability to capture image when there is “light”. Most probably, this definition of explanation should be good enough for ‘normal people’ who do not want to be overwhelmed by jargon of a geek!
Personally, I would define a Thermal Camera like this : A thermal camera is that camera which can produce an image visible/comprehensible to human eye using temperature of objects instead of light as normally human eye or visible cameras do.
If one can understand how visible camera works, then s/he can understand how Thermal Camera works. Like any camera, thermal camera has a sensor, lens, some electronics and image processing software. Only difference is, a sensor of the thermal camera receives infrared rays emitted by objects ( within a fixed wavelength based on the temperature) while the sensor of visible camera ( Mobile Phone camera, DSLR Canon, Nikon, Sony etc) receives reflected light emitted by objects. Then the electronics and software parts of the camera creates an image to be displayed visible to human eye.
For technical minds, both light and infrared are just ‘rays’ of different wavelengths. IR ( Infrared) as in Wikipedia explains a lot where ‘light’ as we call is nothing but visible portion ( visible to human eye) of Electromagnetic Radiation with wavelength 400 nm–700 nm while that of IR is 700nm to 1mm. In other words, a sensor of Thermal Camera would responsive to wavelengths 700nm to 1mm while sensor of a normal camera ( lets call them visible camera) would responsive to wavelengths 400nm to 700nm.
Now just the wavelengths from 400nm to 700nm produces as many colors of the objects as human eye can see, similarly one can think of 700nm to 1 mm wavelengths will give out temperature values of the objects it senses. The computation of temperature values form that sensor data is secret of any Thermal Imaging Company!
I have tried to explain in simplest way as I could think of about Thermal Camera, if I miss anything, I hope to update this post in future. But Making Thermal Camera may sound easy, but its not so in real world! We hope to keep experiencing that as we change calendar!