Keller 1110968, Operating Instructions Manual, Page: 18 / 30

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Operating instructions CellaTemp PR xx
12 Theory of optical temperature measurements
All materials radiate thermal energy in all states of aggregation above absolute
zero� This radiation is mainly caused by atomic or molecular oscillations�
This temperature radiation is only a limited sector within the total electromagnetic
radiation spectrum� It extends from the visible range starting at wavelengths
of approx. 0.5 μm to the infrared range with wavelenghs of more than 40 μm.
Radiation pyrometers detect infrared radiation for optical temperature measure-
ment�
12.1 Advantages of optical temperature measurement
• Non-contact temperature detection means cost-effective temperature
measurement because this technique only requires a single in-vestment in an
instrument without any follow-up costs for consumables such as thermocou-
ples�
• This method enables temperature detection of moving objects - quick temper-
ature measurements within milliseconds - for example at automatic welding
processes�
• Small objects with medium and high temperatures can also be easily and
accurately measured�
• When measuring materials with low specific heat, a non-contact method does
not induce heat loss which would distort the temperature reading (as is the
case with contact temperature probes)� Non-contact temperature detection
is ideal with corrosive molten materials for which the use of thermocouples is
hardly feasible�
• Voltage-carrying objects can also be measured�
12.2 Measurements at Black Bodies (Cavity Radiators)
A black body or a black radiator is used to calibrate radiation pyrome-ters� This
black body is designed in a way that its radiation does not
depend on material characteristics, but only on its temperature� A black body emits
at any wavelength the maximum energy possible for the specific temperature� Real
bodies do not have this ability� In other words, a black body completely absorbs
the radiation without reflection or transmission losses� The spectral emissivity
coefficient e(*) of a black body is equal to 1 or 100 %� The emissivity coefficient