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.
  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 (Figure 1-1). 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.

Figure1-1: The first IR Thermometer

  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.
  Because fuel for firing was scarce, builders of Biblical times had to depend on the sun's infrared radiation to dry the bricks for their temples and pyramids. The Mesopotamian remains of the Tower of Babel indicate that it was made of sun-dried brick, faced with burnt brick and stone. In India, a sewer system dating back to 2500 B.C. carried wastewater through pottery pipes into covered brick drains along the street and discharged from these into brick culverts leading into a stream.
  In ancient Greece, as far back as 2100 B.C., Minoan artisans produced things such as vases, statues, textiles. By using sight, they could approximate when a piece of material could be shaped. Terra-cotta pipes were built by heating them to a certain temperature and casting them into a mold.
  In more recent years, special craftsmen have relied on their own senses to visualize when a material is the correct temperature for molding or cutting. Sight has been used for steel working, glass working, wax molding, and pottery. From experience, skilled craftsmen learned to estimate the degree of heat required in the kiln, smelter, or glass furnace by the color of the interior of the heating chamber. Just as a classical blacksmith, for example, might judge the malleability of a horseshoe by its cherry-red color.
  In countries around the world, the technique of sight is still being used. In Europe, glass molding craftsmen use sight to determine when glass is ready to be shaped (Figure 1-2). They put a large piece of glass in a heating furnace by use of a large metal rod. When the glass reaches the desired color and brightness, they pull it out of the oven and immediately form it into the shape they want. If the glass cools and loses the desired color or brightness, they put it back in the oven or dispose of it. The glass makers know when the glass is ready, by sight. If you have a chandelier made of glass, or hand-made glasses from Europe, most likely they were formed in this way.

Figure1-2:Glass Manufacturer Using Visual IR Temperature Measurement