How Do Infrared Cameras Work?
Infrared cameras are able to detect infrared energy (or heat) without being in close contact with the subjects. Analog infrared cameras use films that are sensitive to stray light, which means special IR-sensitive film (like the Kodak HIE) is all you really need to produce IR images with them.
Digital infrared cameras, on the other hand, are conveniently built with infrared sensors that are sensitive to thermal radiation. But despite them being able to detect infrared energy, they don’t normally produce infrared images because most manufacturers sell them with hot mirrors or infrared cut-off filters that block IR light from reaching the sensor and producing weird colors as a result of mixed IR and visible light.
For digital cameras to detect near-infrared spectrum at higher sensitivity, you have two options: You can use an infrared-passing filter (or cold mirrors) that will allow IR light to enter your camera and block visible light or have the camera converted to permanently lose the installed blocking filter and allow more flexibility with a wide range of filters (also known as full spectrum camera conversion).
An infrared film camera will directly record radiated infrared energy onto the loaded IR film, whereas a digital infrared camera will convert detected infrared energy into electronic signals and process these heat signals to produce a corresponding and precise thermal image on the monitor.
If you decide to add enhancements with an infrared-passing filter—like the Hoya R72 ($67.69)—with your DSLR lens without removing the internal infrared blocking filter, your camera will need you to do long exposures even in broad daylight to allow enough time for reflected IR light to pass through.
Infrared vs Thermal Imagery
Thermal cameras also use infrared radiation to detect humans (among many others) and form heat zones on the camera viewfinder or monitor to provide a visualization of varying levels of heat within the scene—even in pitch darkness. However, near-infrared and thermal cameras have very important distinctions, particularly in heat sensitivity.
Infrared film and digital camera sensors typically have a much smaller IR-sensitivity range of 250 to 500°C (482 to 932 °F). Meanwhile, thermographic IR film and specialized sensors in dedicated digital thermal cameras are developed to be extra sensitive to a wider range of infrared radiation, specifically between -50 to 2,000°C (−58 to 3,632 °F).
High-quality digital thermal cameras offered by established brands like FLIR are designed to detect infrared radiation that is way hotter than 250 °C (482 °F). Therefore, you can’t possibly use regular IR film or even an IR-modified DSLR to produce thermographic results needed for demanding commercial, industrial, enterprise, and military applications.
How to Get the Best Thermal Images
For quality, high resolution thermal images, it helps to understand that both environmental conditions and the quality of your thermal imaging camera will drastically affect the look of your photographs.
If you’re converting a standard digital camera into a thermographic camera, it helps to start with a high resolution camera body capable of taking detailed images. Pixel count matters if you intend to create large prints from your images. Many photographers use their spare camera bodies and are happy with the results. Consider converting a camera with live-view and touchscreen functionality for easy setting adjustments and real-time previews of how your images will look.
Several environmental factors will affect the quality of your infrared images as well. Make sure the field of view is filled with a variety of subject matter. Natural scenes featuring water, clouds, sky, and foliage often provide enough infrared variation to create an interesting IR image. Remember that it may take some tweaking in post-production to get the exact coloration you’re looking for.