APPLICATIONS

The fact that every object emits radiation can be exploited for a number of uses. For example, the human body at a temperature of about 312 K emits a large amount of infrared radiation centered about a wavelength of 10 μm. This is used for security and convenience lighting purposes by passive infrared (PIR) motion detectors such as that in Figure 1.1. Lenses focus radiation from the area under surveillance onto a sensitive pyro electric detector that detects changes in the incident radiation. Pyro electric detectors are made from ferroelectric crystals and work by allowing incident radiation to change the temperature of the crystal, which affects the charge on the electrodes. They have a wide response range of 1 to 100 μm and are quite rapid at detecting change. As a human (or any other warm object) walks across the area in front of the detector, the amount of radiation received changes suddenly, triggering a light or alarm. It may be worth mentioning that clothing and other items normally opaque to visible light are often quite transparent to far-infrared light. To this end a commercially viable security camera is available which, quite literally, has the ability to see through clothing. In this age of tight airport security, this would make it quite impossible to conceal a weapon of any type, including those that do not affect metal detectors. Of course, the privacy ramifications have not yet been dealt with fully. This technology has already been used by border patrols to identify illegal aliens hiding in concealed compartments of trucks. The IR radiation emitted from the humans inside is easily detected by an IR camera, which clearly

Figure 1.1. Passive infrared detector.

shows the outlines of the (warm) figures concealed inside. More mundane applications of blackbody radiation include noncontact thermometers used worldwide by the medical community. These in-the-ear thermometers measure the IR radiation emitted from the eardrum (which is inside the body and hence accurately reflects the body’s core temperature) and calculate body temperature from this.