Basics
Measurement of Infrared Temperature
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3 Basics
3.1 Measurement of Infrared Temperature
All surfaces emit infrared radiation. The intensity of this infrared radiation changes according to the temperature of
the object. Depending on the material and surface properties, the emitted radiation lies in a wavelength spectrum
of approximately 1 to 20 µm. The intensity of the infrared radiation (heat radiation) is dependent on the material.
For many substances, this material-dependent constant is known. This constant is referred to as the
emissivity
value
.
Infrared thermometers are optical-electronic sensors. These sensors are sensitive to the emitted radiation. Infrared
thermometers are made up of a lens, a spectral filter, a sensor, and an electronic signal processing unit. The task
of the spectral filter is to select the wavelength spectrum of interest. The sensor converts the infrared radiation into
an electrical signal. The signal processing electronics analyze the electrical signal and convert it into a temperature
measurement. As the intensity of the emitted infrared radiation is dependent on the material, the required emissivity
can be selected on the sensor.
The biggest advantage of the infrared thermometer is its ability to measure temperature without touching an object.
Consequently, surface temperatures of moving or hard to reach objects can easily be measured.
3.2 Emissivity for 1-Color Measurements
Emissivity is a calculated ratio of infrared energy emitted by an object to the energy emitted by a blackbody at the
same temperature (a perfect radiator has an emissivity of 1.00). For information on determining an unknown
emissivity and for sample emissivities, see section 15.4
If emissivity is low, measured results could be falsified by interfering infrared radiation from background objects
(such as heating systems, flames, fireclay bricks, etc. located close beside or behind the target object). This type
of problem can occur when measuring reflective surfaces and very thin materials, such as plastic film and glass.
This measurement error can be reduced to a minimum, if care is taken during installation and the sensing head is
shielded from these reflecting radiation sources.
