METROLOGY 101

Infrared Thermometer Calibration

By Frank Liebmann
Training Objective: The objective of this article is to give laboratory personnel a basis to set up a calibration program
for infrared thermometers. While this information is not a complete set of instructions, it contains a number of factors that
commonly result in errors to people who are calibrating these devices.

What Is an Infrared Field-of-View

Thermometer Measuring? Handheld IR thermometers usually come equipped with
a laser pointer. This serves as a guide to show where the
An infrared thermometer is a non-contact thermometer, infrared thermometer is pointed. However, these pointers
since it doesn’t touch the surface being measured. It can be misleading in two respects. First, the laser provides
a finite point. In fact, the infrared thermometer is typically
measures thermal radiation in the infrared region of the
measuring a non-finite area or spot which will be discussed
electromagnetic spectrum beyond where the eye can see.
shortly. Second, typically the laser center does not represent
A common spectral band for measuring temperatures from
the center of the spot.
below ambient up to 500 °C or 1000 °C is the 8 – 14 µm
Using IEC terminology, the measure for the size of
band. This is partly because at room temperature, the peak
this spot is field-of-view [3]. In a nutshell, field-of-view
energy occurs just below 10 µm.
specifies that the infrared thermometer will measure a
As temperatures get higher, this peak wavelength
certain percentage of energy within a specified diameter
becomes shorter. Most people have seen a “red hot” piece
at a given distance. What about the energy measured
of metal. This is because the human eye can see this thermal outside of this diameter? This is called scatter, and the
radiation. The metal is red hot because the radiation has infrared thermometer is measuring it as well. Most infrared
a significant enough amount of energy in the shorter thermometers come with a diagram, or a specification of
wavelengths where the human eye can see, between 0.3 distance to size ratio (D:S). The diameter specified by this
and 0.7 µm. This occurs at some point above 600 °C. The ratio only contains a certain percentage of the radiation
Sun’s surface temperature is at a temperature between 5000 received by the infrared thermometer. For a measurement,
and 5500 K. The peak wavelength for these temperatures it is best to have at least two times this ratio in diameter [1]
is roughly 0.5µm, right in the middle of the range visible as is shown in Figure 2. For calibration, the diameter of the
to the human eye. source should be at least three times this diameter [4]. For
this reason, a flat-plate is often used as a thermal radiation
Pitfalls in Infrared source instead of a cavity. At a minimum, the measuring
Thermometer Measurement distance and diameter of the source should be stated on
the calibration certificate.
There are a number of factors which can increase Effects of Emissivity Change (Tamb = 23°C, ε = 0.95)

uncertainty and cause errors when using infrared 8

Change in Measured Temperature (0.01 change in emissivity) (°C)

thermometers. An adequate uncertainty budget should 7

help point these out. There are two which cause people 6
more problems than others, emissivity and size-of-source.
5

Emissivity 4

Emissivity is a material’s ability to radiate compared to 3

a perfect blackbody. It can have a value from anywhere 2

from 0.0 to 1.0. Bare metal tends to have a low emissivity;
oxidized metal tends to have a moderate emissivity; non 1

metals tend to have high emissivity [1]. Typically, it is 0

difficult to control a surface’s emissivity to within ±0.01 -1

[2]. In the 8 – 14 µm band, an uncertainty in emissivity of
0.01 translates to a uncertainty of 0.6 K at 100 °C and 3.4 K -2
0 200 400 600 800 1000
at 500 °C. This is illustrated in Figure 1. Source Temperature (°C)

Figure 1. Effect of emissivity error.

Cal Lab: The International Journal of Metrology 20 Jul • Aug • Sep 2011
 METROLOGY 101

Figure 2. Proper measurement size-of-source.

Equipment Needed for Calibration Calibration of the Thermal Radiation Source

There are two methods to calibrate the radiation source.
Table 1 provides a list of equipment for infrared One is using a contact transfer and the other a radiometric
thermometer calibration [5]. transfer. The contact calibration has the advantage in
that it is not wavelength dependant. The contact transfer
Mandatory Equipment
does not account for the ‘heat exchange’ error [6]. When
Two Types: a flat-plate source is used, it also may result in a large
• Cavity (preferred)
Thermal Radiation Source uncertainty for emissivity [5]. The radiometric transfer
• Flat-plate (large size-of-
source) has the advantage in that it accounts for the errors caused
Two Types:
by heat exchange (between the reference probe and
Transfer Standard • Contact thermometer the radiation source’s surface) and for not well defined
• Radiation thermometer emissivity. The radiometric transfer standard must be of the
Ambient Temperature Monitors laboratory same wavelength as the infrared thermometer’s calibrated
Thermometer temperature. using the thermal radiation source [5].
Tripod, fixture, or technician’s
Mounting Device
hand.
Basic Infrared Thermometer Calibration
Distance Measuring Can be a by ruler, tape
Device measure, or fixturing.
Procedure
Optional Equipment
Before calibrating an infrared thermometer, the
Needed only if requested infrared thermometer should be allowed to reach room
Aperture by user or required by temperature. This is especially important when moving an
manufacturer.
infrared thermometer from one environment to another.
• Cold Temperatures: Typically, 30 minutes is sufficient.
Prevents ice or dew
build-up
The basic infrared thermometer procedure for a
Purge Device • High Temperatures: calibration point should include the following steps [5]:
Prevents oxidation 1. If a purge device is used, set up the purge.
• May also improve
temperature gradients 2. Allow the thermal radiation source to stabilize at
its set-point.
Table 1. Mandatory and optional calibration equipment.

Jul • Aug • Sep 2011 21 Cal Lab: The International Journal of Metrology
 METROLOGY 101

3. If available, set the infrared thermometer’s reflected • BIPM CCT-WG5: http://www.bipm.org/wg/CCT/CCT-

temperature setting to the reflected temperature. WG5/Allowed/Miscellaneous/Low_T_Uncertainty_
4. Set the infrared thermometer’s emissivity to the Paper_Version_1.71.pdf
emissivity of the thermal radiation source. • MSL TG22: http://www.msl.irl.cri.nz/sites/all/files/
5. Set the measuring distance of the infrared training-manuals/tg22-july-2009v2.pdf
thermometer.
6. Align the infrared thermometer so that it is centered References
on the thermal radiation source.
7. Perform the measurement. [1] ASTM E2758 - 10 Standard Guide for Selection and Use of
8. Repeat these steps for repeatability if needed.
Wideband, Low Temperature Infrared Thermometers, ASTM,
There are a few notes and exceptions to consider when West Conshohocken, PA, 2010.
performing these steps. First, most infrared thermometers
do not have a reflected temperature setting. Instead, the [2] F. Liebmann, “Quality Control for Emissive Surfaces,”
reflected temperature is detected within the instrument.
Proceedings of the National Conference of Standards
Second, some infrared thermometers do not have an
adjustable emissivity setting. In these cases, if the emissivity Laboratories International, 2009.
setting of the infrared thermometer does not match the
emissivity of the thermal radiation source, mathematical [3] IEC/TS 62492-1 Ed. 1.0 Industrial Process Control Devices
corrections may be made. - Radiation Thermometers - Part 1: Technical data for radiation
When using a handheld infrared thermometer, it is
thermometers, IEC, Geneva, 2008.
typical to initiate a measurement by pulling a trigger. The
trigger should be held a significant amount of time longer
than the infrared thermometer’s specified response time. [4] F. Liebmann, “Determining Size of Source for Hand-
Finally, the number of set points measured should be held Infrared Thermometers – Theory and Practice,” Pro-
driven by the customer. If the infrared thermometer is only ceedings of the Measurement Science Conference, 2008.
used over a narrow temperature range, one or two points
may be sufficient. If the infrared thermometer is used over
a wide range, three or more points may be necessary. This [5] ASTM WK27665 - New Procedure for Accuracy Verifica-
should be driven by the customer. However, the calibration tion of Wideband Infrared Thermometers, ASTM, West Con-
laboratory should be ready to offer advice. shohocken, PA, not yet published.

Where to go for more information? [6] J. Fischer, P. Saunders, M. Sadli, M. Battuello, C. W.

Park, Z. Yuan, H. Yoon, W. Li, E. van der Ham, F. Sakuma,
Reading
Y. Yamada, M. Ballico, G. Machin, N. Fox, J. Hollandt, S.
• Radiometric Temperature Measurements, Vol. 1:
Fundamentals, eds. Z. Zhang, B. Tsai, G. Machin (2009, Ugur, M. Matveyev and P. Bloembergen, “Uncertainty
Academic Press) budgets for calibration of radiation thermometers below
• Theory and Practice of Radiation Thermometry by D.P. the silver point”, CCT-WG5 working document CCT-
DeWitt and Gene D. Nutter (John Wiley & Sons) WG508-03, Sèvres, France, May 2008.
• Radiation Thermometry: Fundamentals and Application in
the Petrochemical Industry by Peter Saunders (SPIE)

Courses
• Radiation Thermometry Short Course (NIST) – Held Frank Liebmann is a Senior Design Engineer for
once a year in Gaithersburg, MD Fluke Calibration.
• Snell Thermography Courses – Held at various locations
in the US and Canada
• Fluke Infrared Thermometry Metrology Seminar – Held
once a year in American Fork, UT

Standards Organizations
• ASTM: http://www.astm.org/; http://irthermometry.
blogspot.com/

Cal Lab: The International Journal of Metrology 22 Jul • Aug • Sep 2011