Infrared thermometry is a convenient and reliable method of measuring the temperature of objects such as hot steel ingots that cannot be handled or silicon wafers undergoing processing in protective atmospheres and food products that should not be handled because of contamination.
Heated objects emit visible and infrared radiation and the amounts emitted are dependent on the temperature of the object. Engineers frequently assess the temperature of a steel component by its colour and brightness during tempering and hardening as the human eye makes an accurate analogy of an Infrared thermometer. With suitable calibration, usually by long experience, skilled workers estimate temperatures in the range of 700oC (dull cherry red) to 1000oC (yellow red] with astonishing accuracy and repeatability.
The amount of infrared radiation from a heated body is much greater than visible light. A billet of steel heated to 650 degrees C emits about 10 times as much infrared as visible light. Emitted energy in the visible light region below 650 degrees C is so small that it is of little use for measurement purposes. However, there is significant radiation at longer wavelengths, which can be utilised for temperature measurement purposes.
DETECTING BANDS OF RADIANCE
The longer wavelength of infrared radiation cannot be seen by the eye, because it is not sensitive to emission at these wavelengths. Since all radiance is fundamentally the same, differing only in its wavelength and the amount of energy it contains, it is possible to utilise different types of detectors for different bands of radiance
The radiated emissions can be considered as elementary packets of energy called photons, which travel in straight lines but can be reflected and refracted by mirrors or lenses. As the radiance is dependent on the temperature of the object, it is possible to measure the temperature of the body.
The detectors used for infrared thermometry fall into two classes. A thermal detector absorbs the incident flux falling onto it, thus raising its temperature. Changes in the detector’s physical characteristics, for example electrical resistance, can be measured and the object’s temperature computed. This type is sensitive to all wavelengths but the response time tends to be long compared to the quantum type.
The quantum detector operates in fundamental manner. An incident photon impinging on the detector interacts with a bound electron within the crystal lattice. Given that it contains enough energy, it will free that electron from its bound state allowing the electron to move throughout the crystal structure. After its energy is given up, producing an electrical signal that can be measured, the electron will fall back to its original state. If there are sufficient incident photons, then a continuous voltage is produced from which the temperature of the body can be computed. Different semiconductor materials may be used, each differing in its sensitivity to different wavelengths.
Steve Green, Ircon (c/o Fairey Group plc) Tel: 01784 485323