Black body radiation is the emission of electromagnetic energy by an object which is in a thermodynamic equilibrium. The blackbody emits an amount of energy depends on its temperature, with ideal blackbody absorbing and re-emitting all the incident radiations it receives at any wavelength.
The blackbody radiation is a common phenomenon, observed when the temperature of an object increases. The electromagnetic radiation occupies a wide spectrum, both in the visible and invisible regions depending on the temperature of the object and amount of radiation.
Black body radiation sources
All objects are black body radiators, the amount of radiation and position in the spectrum depends on the object temperature as well its emissivity. Some examples of blackbody radiators that emit visible light or whose radiation is used for other processes include the electric heaters, incandescent light bulbs, stoves, the sun, the stars, night vision equipment, burglar alarms, warm-blooded animals, etc.
Typical blackbody radiations:
- A filament bulb converts the electrical energy into light energy, at switch on or switch off, when the filament is not fully heated or cooling down, it radiates energy in the infrared regions, initially, and goes to red, then yellow until it reaches an almost white light when fully energized.
- As the welding temperature of a piece of metal increases, it initially glows red, then orange, and the color continues changing until it becomes a bright, bluish cast. The glow at very high temperatures is very intense and painful to look at using the naked eyes. Due to this, welders usually use dark goggles to prevent damage to their eyes.
An object that is not very hot still emits radiation, but in the infrared region. This is utilized in night vision equipment which is used to detect the infrared radiation and convert this into a visible image. This allows the detection of warm-blooded animals and people at night.
The black body radiation from animals is usually in the infrared radiation, and cannot be seen with the naked eye; however, a thermal camera can be used to see the thermal radiation from an animal. The image appears as a glowing object due to the black body radiation, unlike during the day when the person reflects the light falling on them.
The hot and warmer objects emit more energetic radiation and cool faster than the cooler objects. In the absence of a heat source, the objects will eventually reach the same temperature as the surroundings and are said to be in thermal equilibrium.
Selecting a black body radiation source
The blackbody radiators for commercial applications almost approach an ideal blackbody and are used for a variety of applications. The choice of the blackbody radiation sources depends on the temperature, type of the application and environment. The major factors considered include;
- Temperature
- Emissivity
- Size of Emissive area
- Warm-up time
- Cooling time
- Regulation stability
The temperature depends on the object under test. For example, a low-temperature blackbody is suitable for applications such as calibrating IR sensor that looks at buildings, vehicles, or human bodies.
The blackbody radiation sources are available in three main categories, based on the temperature range.
- Low-temperature blackbody with a range of between -40 °C to +150 °C
- High temperature extended area blackbody - from ambient temperature up to +600 °C
- High-temperature cavity blackbody - from ambient temperature up to 1200 °C
Applications
The blackbodies are used for lighting, heating, security, thermal imaging, as well as testing and measurement applications.
Since the intensity of the energy at any temperature and wavelength and can be determined using the Planck Law of radiation. A blackbody radiation source with a known temperature, or, whose temperature can be measured, is usually used for calibrating and testing the radiation thermometers.