1. m
SPECIALUdakara (Assistant Director, Sri Lanka Standards Institution)
Ms Sujeewa
Mr. Ganbold Altangerel (Researcher, Mongolian Agency for Standardization and Metrology)
Basic Principles of
Radiation Thermometry
and Thermometer Calibrations
Introduction
This paper was written to share our knowledge which we gained during the training
on strengthening of Measurement Standards Institutes of Asia Pacific Countries from 16th
January to 24th February at National Measurement Institute, Thailand.
Temperature is the one of the most frequently measured physical quantity, second
only to time. It can be measured by contact thermometers or non contact thermometers.
Non contact measurements are taken by using radiation thermometers. This paper describes
10 the brief outline of radiation thermometry and calibration of radiation thermometers.
Blackbody Radiation
All forms of materials with temperature (T) above absolute zero emit thermal
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radiation. An ideal blackbody will absorb all incident electromagnetic radiation at all
wavelengths and all directions. A blackbody is simply a perfectly black surface, emitting
exactly what it absorbs when it is in equilibrium with its surrounding and it is an idealized
object.
The term emissivity is used to quantify the energy-emitting characteristics of
different objects. Emissivity of the material(є) is the relative ability of its surface to emit
energy by radiation. For a given temperature,
Emissivity of a material(є) = Energy radiated by a particular material
Energy radiated by the blackbody
Emissivity of blackbody depends on shape, material, surface and oxidation.
Bodies with emissivity less than 1 are called gray bodies. However, the bodies
with emissivity changes with wavelength are called non-gray bodies.
Plank’s radiation law describes the amount of energy emitted by blackbody in
radiation of a certain wavelength.
c1
Lλ (T ) =
Radiance flux ; λ5 (exp(c 2 λT ) − 1)
Where, λ – wavelength, T – absolute temperature, c1-0.595x10-8 W.m-2, c2=0.014388
m.K
The intensity of an object's emitted infrared energy is proportional to its temperature.
This energy, measured as the target's emissivity that indicates an object's temperature. By
understanding how this radiation depends on temperature we can make accurate
measurements of temperature over a wide range.

2. Radiation thermometers
Infrared radiation is part of the electromagnetic spectrum and the range of the
spectrum is from 0.7 micron to 1000 micron. However, only the 0.7-micron to 14-micron
band is used for Infrared temperature measurement, Noncontact Infrared thermometers
can focus on nearly any portion of the 0.7-micron to 14-micron band. It consists of advanced
optic systems and detectors.
Spectral Radiation
Thermometers
Most radiation thermometers are of the type known as spectral band thermometers.
They measure the radiance over a relatively narrow band of wavelengths somewhere within
the range 0.5 µm to 25µm. The choice of wavelength depends on the temperature range,
the environment, and the type of surface under investigation. The spectral band grouping
includes most industrial radiation thermometers and all primary and transfer standard
thermometers.
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Figure 1: Schematic diagram of a spectral band thermometer
In a radiation thermometer, the sensor collects the energy emitted from the object
base on its optics and location. The filter selects the range of wavelength to be measured.
Then, the detector and signal processing system measures the energy and converts into
an electrical signal.
The factors considered for designing of radiation thermometers are temperature
range, target size, wavelength range, response time, and distance between radiation
thermometer to target .
How do you select suitable distance for non contact
temperature measurement?
The optical resolution of a radiation thermometer is defined as the relationship
between the distance of the measuring device from the target, and the diameter of the spot
(D:S). The greater this value, the better the optical resolution of the measuring device, and
the smaller the target can be at a given distance. For accurate measurements, spot size
shall be less or equal to half of the diameter of object.

3. est Good Wrong
Sensor
Object is bigger Object is equal Object is smaller
than spot size spot size than spot size
Figure 2 : Suitable distance for accurate non contact temperature measurement
Sensitivity of detectors
Infrared detectors fall into two main groups: quantum detectors and thermal
detectors. Quantum detectors (photodiodes) interact directly with the impacting photons,
resulting in electron pairs and therefore an electrical signal. Thermal detectors are much
slower than quantum detectors due to the self-heating required.
A variety of detectors are used to maximize the sensitivity of sensors. Eg: PbS has
the greatest sensitivity, while the thermopile has the least sensitivity.
12 Advantages of radiation thermometers
i) used for high temperature measurement for long period
ii) measurements on moving object
iii) measurements with fast respond
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iv) avoid the disturbance and contamination of measured system
Calibration of radiation thermometer
Calibration of radiation thermometer can be done by using the following methods.
Uncertainty sources of radiation thermometer calibrations
i) Fixed point calibration: used to ii) Spectral Resposivity measurement
calibrate transfer standards : used to calibrate radiation
(temperature range:157oC to thermometer which is using for
1084oC) high temperature measurements.
(temperature range : above
1084oC)
Figure 3 : Blackbody furnaces Figure 4: Spectral resposivity
measurement system

4. iii) Comparison with standard iv) Comparison with Platinum
radiation thermometer : used to resistance thermometer or
calibrate standards radiation thermocouple : used to calibrate
thermometers/radiation Infrared thermometers
thermometers (temperature range: -30oC to
(temperature range:150oC to 500oC)
1000oC)
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Figure 5 : measurement setup Figure 6 : calibration setup
The uncertainties of radiation thermometer calibrations fall into three main groups.
Uncertainties related to i) the standard used ii) the blackbody source
iii) the radiation thermometer under calibration
Example : Calibration of radiation thermometer using standard radiation thermometer
as a standard
Uncertainty sources:
i) Size of source effect
ii) Distance effect
iii) Emissivity effect
iv) Wavelength difference between standard thermometer and calibration
thermometer
v) Temperature source : uniformity, stability, emissivity
Acknowledgement
Finally We wish to express our deep gratitude to the Japan International Cooperation
Agency(JICA) and Thailand International Development Cooperation Agency (TICA) for
sponsoring for us for TCPT training programme. Also our special thanks go to Dr. Narudom
Noulkhow and the staff of National Measurement Institute, Thailand (NIMT) for organizing
and providing a valuable technical training for us in the field of radiation thermometry.