The Principle of an Optical Pyrometer Temperature Sensor

A heated object emits electromagnetic radiation. At temperatures below approximately 400 °C this radiation can be felt as heat. As the temperature rises, the object starts to emit visible radiation passing from red through yellow to white. Intuitively we can use this radiation to qualitatively measure temperature as illustrated in the table below:

Pyrometers employ the same effect to provide non-contact temperature measurement.

The object whose temperature is to be measured is viewed through a fixed aperture by temperature measuring device. A portion of the radiation emitted by the object falls on the temperature sensor causing its temperature to rise. The object’s temperature is then inferred from the rise in temperature seen by the sensor. The sensor size must be very small, normally of the order of 1 mm diameter. Often a circular ring of thermocouples connected in series (called thermopile) is employed. Alternatively a small resistance thermometer (called a bolometer) may be used. Some pyrometers measure the radiation directly using photo-electric detectors.

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A key advantage of pyrometers is the temperature measurement is independent of distance from the object provided the field of view is full.

Although all operators operate on radiated energy there are various ways in which temperature can be deduced from the received radiation. The simplest method measures the total energy received from the object (which is proportional to T⁴ where T is the temperature in kelvin). This technique is susceptible to errors from lack of knowledge of the emissivity of the object’s surface. This error can be reduced by using filters to restrict the measuring range to frequencies where the object’s emissivity approaches unity.

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An alternative technique takes two measurements at two different frequencies, that is, two different colours and compares the relative intensities to give an indication of temperature. This method significantly reduces emissivity errors. Pyrometers have a few key restrictions. The foremost problem is, they measure surface temperature and only surface temperature. Lack of knowledge of the emissivity is a also a major source of error.

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