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Pyrometry is the process of measuring the temperature of an object by gauging its infrared radiation and is usually assumed to refer to operations under high-temperature conditions. An instrument called a pyrometer, which is a type of thermometer, is used for this process, and several types exist. Most modern pyrometers require no physical contact with the object being measured, making them suitable for measuring the temperature of extremely hot objects.
Early pyrometers used a technique called disappearing filament, or brightness pyrometry, to measure temperature. The pyrometer used a filament of known composition and chemical properties to compare the object to be measured with the filament, which glowed at a certain brightness upon reaching a known temperature. These devices only worked when measuring objects with a marked luminosity, or brightness. They gave inaccurate results under certain conditions, as the luminosity of an object at a specific temperature can vary according to a number of factors, such as surface texture or shape.
As this instrument proved to be less than satisfactory for some applications, a device known as a two color, or ratio pyrometer, was developed. This device was essentially two brightness pyrometers combined into one device. The new pyrometer relied on the principle of physics known as Planck's law to compare two readings of one object to determine its temperature. While more accurate than earlier pyrometers, these devices were still subject to some of the same problems.
Modern pyrometers have advanced the science and technology of pyrometry even further. They are often described as point-and-shoot pyrometers and provide accurate measurements of surface temperatures for almost any object. They require no contact with the object being measured and give results quickly and reliably.
There are many applications for pyrometry, found both in the everyday life and industrial processes. Pyrometers are frequently used in foundries and other factories to measure temperatures of various materials, including molten metals, gases, and salt baths. In 2011, inexpensive hand-held pyrometers, also known as infrared thermometers, are becoming more and more common in health care settings as a means of taking a patient's temperature easily and almost instantly. These types of pyrometers are especially popular for use on small children and infants, as it can be difficult to use more traditional thermometers if the child or infant is uncooperative.