Infrared photography is often confused with thermal imaging. The difference is basically one of wavelength of electromagnetic radiation since the infrared light with which my photographs are taken lies just beyond visible red light: this is why it is called near infrared. Near infrared radiation is not related to the temperature of the object being photographed - unless the object is extremely hot - since the object is not actually the source of the radiation. It is just reflecting it.
Hand print left behind on a table cloth
FLiR One thermal-visible hybrid camera
If you make an object hot enough it will start to radiate heat and light. An electric fire is usually hot enough for us to see reddish light as well as the infrared (and heat) it radiates. An electric iron (or flat-iron for ironing clothes) is not hot enough to radiate visible light but it does radiate near infrared light and can, in theory anyway, be used as a source of illumination. In his book Photography by Infrared, Walter Clark explains that infrared photography can see hot bodies at temperatures from about 250 C up to the point where they begin to glow visibly at about 500-600 C, depending on how dark it is and how much your eyes have adjusted to the darkness.
So here is the source of the confusion: you can photograph in infrared by the 'light' of a hot object and yet the infrared illumination is not, in itself, heat anymore than red light is heat.
Imaging things using the heat radiated by the objects themselves is known as thermal imaging. It needs specialist electronic equipment to produce usable images, and such devices are expensive: the cheapest being several hundred dollars and having a resolution in the thousands of pixels. Standard TV resolution is regarded as high resolution for thermal imaging. (There are inexpensive devices that can detect a change in temperature and these are used for intruder alarms. It's known as Passive IR or PIR and has been used for low-resolution thermal imaging.)
A human body, at around 37 C, radiates at a wavelength of about 8 to 9 thousand nanometres (8-9 micrometres), so around eight times the wavelength that can be recorded on film. These wavelengths behave differently to visible light in many significant ways. Glass is opaque but a balloon or bin-bag is transparent. Fog and distant haze are also transparent while infrared photography can only penetrate a light haze. Hot gas, such as from an engine exhaust,becomes visible as if it were a solid object. A hand print will be visible on a surface for a time after the hand is removed. Contrary to many TV cop shows, thermal imaging will not see through a wall to tell you where people are, unless their body heat affects the outside wall enough to show a slight 'glow'.
As an example, here is an image taken of a Boeing 747 taking off, that was shot at LA airport. You can see that the tyres are bright (hot), there is a plume of heat firing back from the engines and the leading edge of the wing is black (cold).
This image was supplied (in 2001) by Mark McKelvey, who at the time was a member of the Infrared Focal Plane Array Technology Group at NASA's Jet Propulsion Laboratory in Pasadena. The image is slightly cropped at the bottom.
Back in the mid-1990s, the Infrared Focal Plane Array Technology Group at JPL produced a thermal imaging camera that super-cools the sensor and still looked rather like an ordinary camcorder of the time. It used a technology called a Quantum Well Infrared Photodetector (QWIP for short) and at the time their thermal imaging camera was the world's smallest.