422 · BIOMEDICAL SENSORS implementation methods required in order to discriminate correctly between passengers or personnel with and without clinical fever. 9.3. INFRARED RADIATION 9.3.1. THE IR SPECTRUM In objects at a temperature above absolute zero, -273.16°C (or 0 K), every atom and every molecule vibrates kinetically. According to the laws of electrodynamics, an oscillating electric charge is associated with a variable electric field and an alternating magnetic field. These vibrations produce electromagnetic waves that radiate from the object at the speed of light. The transfer of radiant energy by electromagnetic waves in the thermal range occurs between 0.1 µm and 100 µm, as shown in Figure 9.5. The optimum wavelength of an IR radiometer is determined by the wavelength distribution of the emitted radiation and the type of detector. Another consideration is the transparency of the atmosphere for the transmission of IR radiation between the object and the radiometer. At particular wavelengths there is a "lack of radiative transparency" within segments of the IR spectrum. There are, for example, high levels of IR transparency between 3.5 µm and 4.1 µm (with poor transmission at 4.2 µm due to carbon dioxide absorption) and between 8 µm and 14 µm (although there is a marked reduction above 12.5 µm), as seen in Figure 9.6. The IR segment of the electromagnetic spectrum is at wavelengths just beyond the visible spec- trum, as shown in Figure 9.7, and can be divided into three more segments by wavelength. These, measured in micrometers, are