What is Infrared Radiation?
Technically speaking, infrared radiation is electromagnetic radiation whose wavelengths are greater than those of visible light but shorter than those of millimeter waves. In its most familiar form, it is radiated heat which can be sensed by our skin yet cannot be seen by our eyes.
All objects emit infrared radiation. The intensity of the emitted radiation depends on two factors, the temperature of the object and the ability to radiate. The latter is called emissivity or emittance.
There is a physical law that says that all matter that has a temperature above absolute zero, i.e. minus 273°C, radiates heat. Heat radiation is synonymous with infrared radiation. The hotter the object, the more intense the radiation.
However, for temperatures below about 500°C we cannot see the heat and
this is where thermography comes in. The camera can “see” the heat at much lower temperatures than the human eye. The camera converts the infrared radiation into a state that is visible to us. That is how we can see and measure the heat.
Normally we need to convert the measured heat into temperature as the infrared camera can only measure the heat intensity. To convert this value into temperature we require two things: a well known physical formula and knowledge about the radioactive properties of the object. This is called emissivity or emittance. It is essential that any use of infrared, has an understanding of the physical properties of the object.
How does an infrared camera create an infrared image?
Radiated heat does not require a medium for transfer of energy, and therefore no direct contact. It involves “waves of energy” leaving one substance and being received by another.
At the heart of every thermal imager is the infrared detector. The detector is a solid state component which converts infrared energy into an electrical signal. Through the electronic circuitry within the imager, this electrical signal is then converted into the visible image which appears on the imager’s display screen.
The most common detectors found in today’s modern cameras are know as the microbolometer. This microbolometer detector is constructed of two layers – a top plate and a substrate. Heat striking individual pixels creates electrical resistance between these layers. Resistance values of individual pixels are then converted by the imager’s electronics into thermal images and temperature values.