A brief introduction to infrared: wavelength and colour
On the electromagnetic spectrum, infrared radiation (or infrared light) lies in the range between the end of the visible-light range and the start of the microwave range. The infrared range can be divided into three regions:
Near infrared with wavelengths 0.78 to 2.5 micrometres approximately
Middle infrared with wavelengths 2.5 to 50 micrometers
Far infrared with wavelengths 50 to 1,000 micrometers
Since the infrared radiation lies out of the visible light range, this also means that infrared light cannot be seen by the human eye. Humans can only “see” infrared light through the use of infrared sensor (IR sensor) in infrared cameras/thermal cameras. This is known as infrared thermography. Those colours on the thermogram should not be mistaken as colours of infrared light. Infrared light is invisible - it does not have a colour. The colours seen on the thermogram are a visual aid to help someone better understand the temperature gradient in the image.
Detecting infrared radiation
Let’s delve into the detection of infrared radiation using IR sensor. Different bands of infrared will require different detectors for the purpose. In the near infrared (NIR) spectrum, relatively low cost Si or GaAs or InP based detectors will be adequate. This of course also depends on the wavelength and speed of radiation that you are measuring. As for mid wave infrared detection, long wave detection (LWIR detection) or far infrared detection, these will require different materials for the detectors. A compromise between cost, sensitivity and intended application will have to be made.
These infrared sensors do work in the dark. The function of an IR sensor is to sense the invisible infrared waves, so the darkness of its surroundings will not hinder the IR sensor from doing its job. However, it is important for the user to verify the setup and ensure that it allows the IR sensor to perform the work it is intended for. The user should also check if there are other sources of infrared radiation that could be picked up by the infrared sensor as that will affect the accuracy of the measurement. Once the sensing system is set up correctly, the user can then turn off the light of the room, or put the system in a dark enclosure, and the sensor will do its work.
Without the use of IR cameras and IR sensors, humans can detect infrared radiation as heat energy - a feeling of warmth on the skin. This brings us to a common question: Is infrared radiation the same as thermal radiation? No, they are not the same. Thermal radiation refers to all the electromagnetic waves given off by an object (including humans - read "Common Questions about Infrared Thermometers" to find out how infrared thermometers work) because of its temperature. In other words, all objects emit infrared radiation when they are above absolute zero temperature (0 K). Infrared radiation is simply one of the electromagnetic waves. In other words, infrared radiation is part of thermal radiation.
Does infrared light hurt your eyes?
Technically all infrared light, visible light and ultraviolet electromagnetic radiation can cause injury to your eyes if in sufficient concentrations. Infrared light needs to be extremely intense, such that your eyes absorb too much infrared light, to cause eye damage. However, such occurrences are quite rare.