Our eyes only see the tiny fraction of energy emitted by the sun in the form of visible light. However, if we could see the infrared rays emitted by all bodies - organic and inorganic - we could effectively see in the dark. Though invisible to the human eye, infrared radiation can be detected as a feeling of warmth on the skin, and even objects that are colder than ambient temperature radiate infrared energy.
Some animals such as rattlesnakes, have small infrared temperature sensors located under each eye which can sense the amount of heat being given off by a body. These sensors help them to locate prey and protect themselves from predators.
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Non-contact temperature sensors use the concept of infrared radiant energy to measure the temperature of objects from a distance. After determining the wavelength of the energy being emitted by an object, the sensor can use integrated equations that take into account the body's material and surface qualities to determine its temperature. In this chapter, we will focus on the history of radiation thermometry and the development of non-contact temperature sensors.
IR Through the Ages
Although not apparent, radiation thermometry has been practiced for thousands of years. The first practical infrared thermometer was the human eye. The human eye contains a lens which focuses emitted radiation onto the retina. The retina is stimulated by the radiation and sends a signal to the brain, which serves as the indicator of the radiation. If properly calibrated based on experience, the brain can convert this signal to a measure of temperature.
People have been using infrared heat to practical advantage for thousands of years. There is proof from clay tablets and pottery dating back thousands of years that the sun was used to increase the temperature of materials in order to produce molds for construction. Pyramids were built from approximately 2700-2200 B.C. of sun-dried bricks. The Egyptians also made metal tools such as saws, cutting tools, and wedges, which were crafted by the experienced craftsmen of their time. The craftsmen had to know how hot to make the metal before they could form it. This was most likely performed based on experience of the color of the iron.
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