This article will start by assuming that you are very familiar with your camera and how to use it in manual mode. You need to understand apertures, shutter times, how to use the cameras histogram to understand over- and under exposure.
If you are not familiar with these topics, then it would be better if you familiarized yourself with them before. There are a number of articles you can find that will take you through this of course.
What is infrared light?
Infrared light is light that has a frequency that is lower than the visible light in the spectrum. We can also say that the wavelength is longer than the visible light. Infrared light have various properties that are different from visible light and it reflects and refracts different from visible light through the camera lens.
Here is a diagram that shows how the infrared spectrum relates to the visible light spectrum. In this article we will only be talking about photographing in the infrared spectrum that is close to visible light, between 1200-730 nm roughly. Infrared photography in this aspect is not to be confused with heat signature cameras and systems such as FLIR (example below) which are used for example to detect body heat. There is also the ”far infrared spectrum” said to have medicinal use and influence for example the production of vitamin D in the skin of humans and a few other species.
Radation in the thermal heat range and the far IR range does not really affect the result of IR photography as we have limited our reception range to the near visible light IR spectrum. The sensors in a digital camera is also not very sensitive in ranges under 1000 nm so it makes little sense trying.
Looking at the picture above we can see that the visible light spectrum ranges from around 730 nm to about 370 nm. Above we have X-ray and gamma rays, nasty stuff to us humans but below is the infrared portion of the band.
Here to the left is a picture taken with a different technology (FLIR) that captures the heat radiated from a body.
We can here see the lighter areas correspond to hotter parts of the animal. This is a technique that is quite often used for tecnhical purposes such as trying to find leaks in insulations and heat pipes and similar.
This is known as thermal imaging but is the area around 100 µm and photographic sensors for normal cameras don’t work well in this range. This is however what most people think about first when subjected to the term ”infrared photography” and it is important to distinguish between the two.
So the infrared spectrum we are interested in lies between the lowest portion of the visible light spectrum around 730 nm and down to perhaps something like 1200 nm. This light comes normally from very ”hot” sources, such as the sun, or a light bulb perhaps.
Those of you who are physiscists may remember the black body radiation formula and put it to use here :)