Place two objects in a void of empty space, one object hot, the other cold. The two objects will exchange energy and eventually come to the same temperature. It is the mechanism of this exchange that is the basis of blackbody theory.
By this theory, the temperature of the hot object is a measure of the kinetic energy of the random vibrational motion of its atoms. As the atoms vibrate they drag their electrons with them. The moving electrons cause a disturbance in the electromagnetic field, draining energy from them. The emitted energy from the disturbed field is called blackbody radiation and is described by “Planks law”. Because the electrons are bound to matter, not all vibrational frequencies are generated in equal amounts, so only a certain quantity of energy at each wavelength is radiated. This quantity is precisely determined by the temperature, and is shown in Figure 1, the blackbody curve. This is why a blackbody can be used as a precision radiation source for instrument calibration.
As an object’s temperature increases, the total amount of power it radiates rises as well. Since higher energy photons travel at higher frequencies, the warmer an object is, the shorter the wavelengths of light it emits. Because Plank’s Law allows for the precise calculation of wavelength from the temperature of an object, a blackbody makes an excellent standard source for instrument calibration.
To see this theory put into action on your design problem, download the blackbody calculator (.xls) here.
Minimum Resolvable Temperature Difference (MRTD)
Minimum Resolvable Temperature Difference (MRTD) is a measure of an infrared camera’s ability to resolve a target. Conceptually this measurement is similar to that done with a resolution test chart in a visible system. A typical test chart such as the 1951 U.S. Air Force resolution test chart, presents white bars against a black background at various sizes (frequencies).
A simple measurement determines the smallest feature (highest line frequency) that can be resolved.
In the Infrared regime, black and white bars are created not by variation in reflectivity, but by variation in temperature. For a high temperature difference (∆T=5°C) bars can be resolved to a high frequency. As the temperature difference is reduced the bar contrast will fade-out. The highest frequencies will fade out first, until the whole image is lost in a grey background.
The temperature contrast at which the bars disappear is recorded. The temperature contrast is then inverted, cold bars on a hot background, and the measurements are repeated and averaged together. Figure 3 depicts a typical MRTD curve.