Abstract
Determining the size of source effect of a radiation thermometer is not an
easy task and manufacturers of these thermometers usually do not indicate
the deviation to the measured temperature due to this effect. It is one of
the main uncertainty components when measuring with a radiation
thermometer and it may lead to erroneous estimation of the actual
temperature of the measured target. We present an empiric model to
estimate the magnitude of deviation of the measured temperature with a
long-wavelength infrared radiation thermometer due to the size of source
effect. The deviation is calculated as a function of the field of view of
the thermometer and the diameter of the radiating source. For thermometers
whose field of view size at 90% power is approximately equal to the
diameter of the radiating source, it was found that this effect may lead
to deviations of the measured temperature of up to 6% at 200ºC and up to
14% at 500ºC. Calculations of the temperature deviation with the proposed
model are performed as a function of temperature and as a function of the
first order component of electrical signal.
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