ASTM E2758-22

Standard Guide for Selection and Use of Infrared Thermometers

SIGNIFICANCE AND USE
4.1 This guide provides guidelines and basic test methods for the use of infrared thermometers. The purpose of this guide is to provide a basis for users of IR thermometers to make more accurate measurements, to understand the error in measurements, and reduce the error in measurements.
SCOPE
1.1 This guide covers electronic instruments intended for measurement of temperature by detecting intensity of thermal radiation exchanged between the subject of measurement and the sensor.  
1.2 The devices covered by this guide are referred to as IR thermometers.  
1.3 The IR thermometers covered in this guide are instruments that are intended to measure temperatures below 2700 °C and measure a narrow to wide band of thermal radiation in the infrared region.  
1.4 This guide covers best practice in using IR thermometers. It addresses concerns that will help the user make better measurements. It also provides graphical tables to help determine the accuracy of measurements.  
1.5 Details on the design and construction of IR thermometers are not covered in this guide.  
1.6 This guide addresses general information on emissivity and how to deal with emissivity when making measurements with an IR thermometer.  
1.7 This guide contains basic information on the classification of different types of IR thermometers.  
1.8 The values of quantities stated in SI units are to be regarded as the standard. The values of quantities in parentheses are not in SI and are optional.  
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Sep-2022
• ICS: 17.200.20 - Temperature-measuring instruments
• Technical Committee: E20 - Temperature Measurement
• Drafting Committee: E20.02 - Radiation Thermometry

Relations

• Refers: ASTM E1256-15 - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-Jul-2015
• Refers: ASTM E1897-14 - Standard Practice for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging Radiometers
• Effective Date: 01-Dec-2014
• Refers: ASTM E1862-14 - Standard Practice for Measuring and Compensating for Reflected Temperature Using Infrared Imaging Radiometers
• Effective Date: 01-Dec-2014
• Refers: ASTM E1933-14 - Standard Practice for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers
• Effective Date: 01-Oct-2014
• Refers: ASTM E1256-11a - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-May-2011
• Refers: ASTM E1256-10 - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-Dec-2010
• Refers: ASTM E1933-99a(2010) - Standard Test Methods for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers
• Effective Date: 01-Jun-2010
• Refers: ASTM E1897-97(2010) - Standard Test Methods for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging Radiometers
• Effective Date: 01-May-2010
• Refers: ASTM E1862-97(2010) - Standard Test Methods for Measuring and Compensating for Reflected Temperature Using Infrared Imaging Radiometers
• Effective Date: 01-May-2010
• Refers: ASTM E1256-95(2007) - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 01-Nov-2007
• Refers: ASTM E1933-99a(2005)e1 - Standard Test Methods for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers
• Effective Date: 01-May-2005
• Refers: ASTM E1933-99a - Standard Test Methods for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers
• Effective Date: 10-Dec-1999
• Refers: ASTM E1897-97(2002)e1 - Standard Test Methods for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging Radiometers
• Effective Date: 10-Jul-1997
• Refers: ASTM E1862-97(2002)e1 - Standard Test Methods for Measuring and Compensating for Reflected Temperature Using Infrared Imaging Radiometers
• Effective Date: 10-Apr-1997
• Refers: ASTM E1256-95 - Standard Test Methods for Radiation Thermometers (Single Waveband Type)
• Effective Date: 10-Oct-1995

Overview

ASTM E2758-22: Standard Guide for Selection and Use of Infrared Thermometers provides comprehensive guidelines for the selection, setup, operation, and best practices in the use of infrared (IR) thermometers. Developed by ASTM, this standard addresses effective temperature measurement using IR devices by focusing on methods that optimize accuracy, account for measurement errors, and address key usage challenges. The guide is intended to help users make well-informed decisions to achieve reliable non-contact temperature measurements across a variety of surfaces and conditions.

Key Topics

• Scope of Application: Covers electronic infrared thermometers designed to detect thermal radiation for temperature measurement, specifically for temperatures below 2700 °C and across narrow to wide infrared spectral bands.
• Accuracy and Calibration: Provides foundational guidance on ensuring accurate temperature readings, including the importance of SI-unit traceable instrument calibration and the use of calibration adjustment or tables of offsets.
• Emissivity: Discusses the critical role of surface emissivity in infrared thermometry, how to set or compensate for emissivity, and how errors may be introduced when emissivity is not properly accounted for.
• Optical and Physical Considerations: Explains distance-to-spot ratio, field-of-view, lens cleanliness, and the impact of environmental factors such as thermal shock, atmospheric transmission, and lens contamination.
• Special Cases: Addresses measurement complexities on low-emissivity surfaces (e.g., metals), thin films, glass, hot gases, and surfaces below ambient temperature, offering practical tips to mitigate error.
• Measurement Practices: Outlines step-by-step procedures for proper instrument aiming, optimal scanning for hot/cold spot identification, and proper setup to ensure the measured area is appropriate relative to the spot size.

Applications

Infrared thermometers covered by ASTM E2758-22 are widely used in:

• Industrial Settings: Rapid, non-contact temperature checks for equipment, process lines, refractory materials, and moving or hard-to-reach components.
• Building Diagnostics: Surface temperature mapping, insulation inspection, leak detection, and HVAC system verification.
• Food and Agriculture: Surface temperature validation for food safety checks, storage monitoring, and process control.
• Automotive and Electronics: Thermal scanning of engines, electrical panels, and circuit boards for maintenance and troubleshooting.
• Materials Research: Studying thermal properties of materials, especially when traditional contact methods may be impractical or could alter the object’s condition.

Best Use Practices per the standard include:

• Ensuring the target’s size is larger than the instrument’s field-of-view spot size.
• Adjusting or compensating for surface emissivity for different materials-especially crucial for metals and non-homogeneous surfaces.
• Accounting for environmental influences like ambient conditions, reflected radiation from surroundings, and atmospheric absorption for distance measurements.
• Following manufacturer guidelines for cleaning and maintaining sensor lenses.

Related Standards

ASTM E2758-22 references and aligns with several key standards to ensure interoperability and technical accuracy:

• ASTM E1256: Test Methods for Radiation Thermometers (Single Waveband Type)
• ASTM E1862: Practice for Measuring and Compensating for Reflected Temperature Using Infrared Imaging Radiometers
• ASTM E1897: Practice for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging Radiometers
• ASTM E1933: Practice for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers
• IEC 62492-1: Industrial Process Control Devices-Radiation Thermometers-Part 1: Technical Data for Radiation Thermometers
• JCGM 200:2012 (VIM): International Vocabulary of Metrology-Basic and General Concepts and Associated Terms

By following ASTM E2758-22, users can maximize the reliability of infrared temperature measurements, reduce error, and ensure their practices are aligned with internationally recognized standards for non-contact thermometry.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E2758 − 22
Standard Guide for
Selection and Use of Infrared Thermometers
This standard is issued under the fixed designation E2758; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
1.1 This guide covers electronic instruments intended for
measurement of temperature by detecting intensity of thermal
2. Referenced Documents
radiation exchanged between the subject of measurement and
2.1 ASTM Standards:
the sensor.
E1256Test Methods for Radiation Thermometers (Single
1.2 The devices covered by this guide are referred to as IR
Waveband Type)
thermometers.
E1862Practice for Measuring and Compensating for Re-
1.3 The IR thermometers covered in this guide are instru-
flected Temperature Using Infrared Imaging Radiometers
ments that are intended to measure temperatures below E1897Practice for Measuring and Compensating for Trans-
2700°C and measure a narrow to wide band of thermal mittance of anAttenuating Medium Using Infrared Imag-
radiation in the infrared region. ing Radiometers
E1933Practice for Measuring and Compensating for Emis-
1.4 This guide covers best practice in using IR thermom-
sivity Using Infrared Imaging Radiometers
eters. It addresses concerns that will help the user make better
2.2 IEC Standards:
measurements. It also provides graphical tables to help deter-
IEC 62492-1 TS Industrial Process Control Devices—
mine the accuracy of measurements.
Radiation Thermometers—Part 1: Technical Data for
1.5 Details on the design and construction of IR thermom-
Radiation Thermometers
eters are not covered in this guide.
2.3 BIPM Standards:
1.6 This guide addresses general information on emissivity JCGM 200:2012International Vocabulary of Metrology—
and how to deal with emissivity when making measurements Basic and General Concepts andAssociatedTerms (VIM)
with an IR thermometer.
3. Terminology
1.7 This guide contains basic information on the classifica-
3.1 Definitions:
tion of different types of IR thermometers.
3.1.1 absolute zero, n—a temperature of 0 K (–273.15 °C).
1.8 The values of quantities stated in SI units are to be
3.1.2 atmospheric attenuation, n—a ratio showing how
regarded as the standard. The values of quantities in parenthe-
muchthermalradiationinagivenspectralrangeisabsorbedor
ses are not in SI and are optional.
scattered in air over a given distance.
1.9 This standard does not purport to address all of the
3.1.3 atmospheric transmission, n—a ratio showing how
safety concerns, if any, associated with its use. It is the
well thermal radiation in a given spectral range at a given
responsibility of the user of this standard to establish appro-
distance travels through a certain distance of air.
priate safety, health, and environmental practices and deter-
3.1.4 attenuating medium, n—a semi-transparent solid, liq-
mine the applicability of regulatory limitations prior to use.
uid or gas, such as a window, filter, external optics, or an
1.10 This international standard was developed in accor-
atmosphere that reduces thermal radiation, or combinations
dance with internationally recognized principles on standard-
thereof.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
3.1.5 background radiation—see reflected radiation.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This guide is under the jurisdiction ofASTM Committee E20 on Temperature contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Measurement and is the direct responsibility of Subcommittee E20.02 on Radiation Standards volume information, refer to the standard’s Document Summary page on
Thermometry. the ASTM website.
Current edition approved Oct. 1, 2022. Published November 2022. Originally Available from International Electrotechnical Commission (IEC), 3, rue de
approved in 1910. Last previous edition approved in 2021 as E2758–15A(2021). Varembé, 1st floor, P.O. Box 131, CH-1211, Geneva 20, Switzerland, https://
DOI:10.1520/E2758-22. www.iec.ch.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2758 − 22
3.1.6 blackbody, n—the perfect or ideal source of thermal 3.1.22 infrared reflector, n—a material with a reflectance in
radiant power having a spectral distribution described by the infrared region as close as possible to unity.
Planck’s Law.
3.1.23 infrared sensing device, n—one of a wide class of
3.1.7 blackbody simulator, n—a device with an emissivity instruments used to display or record (or both) information
related to the thermal radiation received from any object
close to unity that can be heated or cooled to a stable
temperature. surfaces viewed by the instrument.
3.1.8 calibration adjustment, n—the correction to an IR 3.1.24 infrared (IR) thermometer, n—optoelectronic instru-
thermometer based on its calibration. ment adapted for noncontact measurement of temperature of a
subject by utilizing thermal radiation exchange between the
3.1.9 celestial radiation, n—flux coming from the sky.
subject and the sensor.
3.1.10 center wavelength, n—the simple average of the
3.1.24.1 Discussion—IRthermometersareasubsetofradia-
lower and upper spectral range limits.
tion thermometers. Most manufacturers use the term IR ther-
3.1.11 contact thermometer, n—an instrument that is
mometer for handheld radiation thermometers. In general,
adapted for measuring temperature by means of thermal
these devices are wideband and use a thermopile detector.
conductance by determining the temperature at the moment
3.1.25 IR thermometry, n—the use of IR thermometers to
when negligible thermal energy flows between the thermom-
determine temperature by measuring thermal radiation.
eter and the object of measurement.
3.1.26 irradiance (E), n—the radiant flux (power) per unit
3.1.12 dew point, n—the temperature at which water vapor 2
area incident on a given surface in units of W/m .
condenses into liquid water.
3.1.27 limit of error, n—the extreme value of measurement
3.1.13 diffuse reflector, n—a surface that produces a diffuse
error of an infrared thermometer reading, relative to reference
image of a reflected source.
temperature standards, as permitted by a specification.
3.1.14 distance ratio, n—theratioofthemeasuringdistance
3.1.27.1 Discussion—Manufacturers sometimes use the
to the diameter of the field-of-view, when the target is in
term accuracy in their specifications to represent limit of error.
focus.
3.1.27.2 Discussion—A manufacturer’s accuracy specifica-
tion may apply only to well defined conditions.
3.1.15 electromagnetic radiation, n—physically occurring
radiant flux classified according to wavelength or frequency.
3.1.28 low-temperature, adj—for radiation and IR
thermometry, referring to any temperature below 660 °C.
3.1.16 emissivity (ε), n—the emissivity of a surface is the
ratio between the radiation emitted from this surface and the
3.1.29 measurement uncertainty (accuracy), n—non-
radiation from a blackbody at the same temperature.
negative parameter, characterizing the dispersion of the values
3.1.16.1 Discussion—The emissivity describes a thermo-
that could reasonably be attributed to the measurement of the
physical material characteristic, which in addition to the
quantity values being attributed to a measurand, based on the
4,5
chemical composition of the material may also be dependent
information used.
onthesurfacestructure(rough,smooth),theemissiondirection
3.1.30 measuring distance, n—distance or distance range
as well as on the observed wavelength and the temperature of
between the radiation thermometer and the target (measured
the measured object.
object) for which the radiation thermometer is designed.
3.1.17 emissivity setting, n—an adjustment on an IR ther-
3.1.31 measuring temperature range, n—temperature range
mometer to compensate for an emissivity of non-unity. 4
for which the radiation thermometer is designed.
3.1.17.1 Discussion—In most measuring situations a radia-
3.1.32 noise equivalent temperature difference (NETD),
tion thermometer is used on a surface with an emissivity
n—parameter which indicates the contribution of the measure-
significantly lower than one. For this purpose most thermom-
ment uncertainty in °C, which is due to instrument noise.
eters have the possibility of adjusting the emissivity setting.
3.1.33 opaque, adj—referring to the property of a material
The temperature reading is then automatically corrected.
whose transmittance is zero for a given spectral range.
3.1.18 emissivity tables, n—a list of objects and their mea-
3.1.34 operating temperature range and air humidity range,
sured emissivity for a particular IR thermometer.
n—the permissible temperature range and humidity range
3.1.19 field-of-view (FOV), n—a usually circular, flat sur-
within which the radiation thermometer may be operated. For
face of a measured object from which the radiation thermom-
this temperature range and humidity range the specifications
eter receives radiation.
are valid.
3.1.20 frost point, n—the temperature at which water vapor
3.1.34.1 Discussion—This is the range of ambient tempera-
condenses into solid water or ice.
ture and humidity the instrument may operate within and be
3.1.21 infrared (IR), adj—referring to electromagnetic ra- expected to meet its specification. It may be thought of as the
diation with a wavelength from approximately 0.7µm to ambientoperatingtemperaturerangeandtheambientoperating
30µm. humidity range.
4 5
See IEC62492-1. See BIPM JCGM 200:2012.
E2758 − 22
3.1.35 radiance (L), n—the flux per unit projected area per 3.1.47 table of offsets, n—a list of calibration points and
unit solid angle leaving a source or, in general, any reference calibration adjustments to be used when no internal calibration
surface. adjustment is available.
3.1.35.1 Discussion—If ∂ Φ is the flux emitted into a solid
3.1.48 thermal radiation, n—electromagnetic radiation
angle ∂ω by a source element of projected area ∂Acos(θ), the
which is caused by an object’s temperature and is predicted by
radiance is defined as:
Planck’s Law.
] Φ
3.1.49 thermal shock, n—subjecting an IR thermometer to a
L 5
]ω]Acos θ
~ !
rapid temperature change.
where:
3.1.50 thermopile detector, n—a thermopile detector’s out-
put is voltage. Incident radiation heats the disk. When the disk
θ = theanglebetweentheoutwardsurfacenormalofthearea
element ∂ A and the direction of observation (unit = is heated, its temperature rises above the sensor’s reference
temperature (ambient temperature) producing a temperature
W/sr•m ).
difference(∆T).Thepotentialofthethermopileisrelatedtothe
3.1.36 radiant power density (M), n—the radiant flux per
temperature difference based on the Seebeck Effect.
unit area leaving a surface that is,
3.1.51 transmittance (t), n—the ratio of the radiant flux
]Φ
M 5
transmitted through a body to that incident upon it.
]A
3.1.52 true temperature, n—temperature attributed to a
where:
particular site of a subject or object of measurement and
∂Φ = flux leaving a surface element ∂A (unit = W/m ).
accepted as having a specified uncertainty.
3.1.37 reflectance, n—the ratio of the radiant flux reflected
3.1.53 wideband, adj—referring to the situation where the
from a surface to that incident upon it. 1
spectral range of an instrument is at least ⁄10 of its center
3.1.38 reflected radiation, n—the thermal radiation incident wavelength.
upon and reflected from the measurement surface of the
specimen. 4. Significance and Use
3.1.39 reflected temperature, n—the temperature of the
4.1 This guide provides guidelines and basic test methods
radiant flux incident upon and reflected from the measurement
fortheuseofinfraredthermometers.Thepurposeofthisguide
surface of a specimen.
istoprovideabasisforusersofIRthermometerstomakemore
accurate measurements, to understand the error in
3.1.40 response time, n—time interval between the instant
measurements, and reduce the error in measurements.
of an abrupt change in the value of the input parameter (object
temperature or object radiation) and the instant from which the
5. Basic Use of IR Thermometry
measured value of the radiation thermometer (output param-
eter) remains within specified limits of its final value.
5.1 General Considerations:
3.1.41 sensor, n—devicedesignedtorespondtoIRradiation 5.1.1 An IR thermometer can be used in a number of
and convert that response into electrical signals. applications. Although they are generally not as accurate as
contact thermometers, their quickness of measurement and
3.1.42 size-of-source effect, n—the difference in the
their ability to measure the temperature of an opaque surface
radiance- or temperature reading of the radiation thermometer
without contacting it make them desirable instruments for
when changing the size of the radiating area of the observed
4 some temperature measurements.
source.
5.1.2 Most handheld IR thermometers are equipped with a
3.1.43 spectral range, n—parameter which gives the lower
trigger to start and stop the measurements.
and upper limits of the wavelength range over which the
5.1.3 As objects vary in temperature, they emit a varying
radiation thermometer operates.
amount of thermal radiation. This amount of thermal radiation
3.1.43.1 Discussion—Spectral range is sometimes referred
ispredictablebasedontheobject’stemperature,emissivityand
to as bandwidth.
reflected temperature.
3.1.43.2 Discussion—These limits are generally defined as
5.1.4 Handheld IR thermometers measure thermal radiation
the wavelengths where the power or signal is attenuated by a
in a given spectral range and determine the relationship
defined amount.
between the measured thermal radiation and temperature. The
3.1.44 spectral response, n—the numerical quantity of a
sensor mainly used in these instruments is a thermopile.
given phenomenon at a specific wavelength in the electromag-
5.2 Basic IR Measurement:
netic spectrum.
5.2.1 Before making a measurement, the emissivity setting
3.1.45 standard atmosphere, n—a model of how electro-
of the IR thermometer should be set to the object’s effective
magneticradiationistransmittedthroughtheatmospherebased
emissivity in the instrument’s spectral range. Some IR ther-
on variations in pressure, temperature and humidity.
mometersdonotallowtheusertoadjusttheemissivitybecause
3.1.46 surface-modifying material, n—any material that is their emissivity is fixed. In these cases there are mathematical
used to change the emissivity of the specimen surface. compensations that can be made.
E2758 − 22
FIG. 1 Basic IR Thermometer Measurement
5.2.2 To make a measurement, the IR thermometer’s lens 5.3.2 There are many other considerations in making accu-
should be pointed at the object being measured. The measure- rate measurements with IR thermometry. These are discussed
ment should be initiated. If the IR thermometer has a trigger, in the following sections.
thisisdonebypullingthetrigger.Thetriggershouldbeheldat
6. Wideband Instruments
least as long as the IR thermometer’s specified response time.
Themeasuredtemperatureisusuallyfrozenonthedisplayafter 6.1 Most handheld low-temperature IR thermometers are
the trigger is released. wideband instruments. As a result, their measurements can
5.2.3 Fig. 1 shows a diagram of how much of a surface an vary if emissivity varies over their spectral range. The most
IR thermometer measures. ‘S’ is the size or diameter that is commonspectralrangefortheseinstrumentsis8µmto14µm.
measured by the infrared thermometer’s field-of-view. ‘D’ is However,someinstrumentshaveaspectralrangeofupto5µm
themeasuringdistance.Subsection11.1discussesspotsizeand to 20µm. In any case, the IR thermometer should have a
distance ratio. specified spectral range. The end user of an IR thermometer
5.2.4 Fig. 2 shows how much surface area is needed for most likely will not have instrumentation to test the spectral
temperature measurement when considering the IR thermom- range.
eter’s spot size. The part of the figure labeled ‘poor’ shows a
6.2 Atmospheric transmission is dependent on spectral
situation where the object being measured is smaller than the
range. Any measurement made over a long distance should
spotsize.Suchsituationsareundesirable.Thepartofthefigure
consultastandardatmospheremodeltodetermineatmospheric
labeled ‘OK’ shows a situation where the object being mea-
transmission. Guidance on accounting for atmospheric trans-
sured is slightly larger than the spot size. Such situations
mission is given in subsection 12.5.
should produce acceptable temperature measurements. The
part of the figure labeled ‘better’ shows a situation where the 7. Spectral Emissivity
object being measured is significantly larger than the spot size.
7.1 Spectral Emissivity in General:
Thissituationwillproducethebesttemperaturemeasurements.
7.1.1 An IR thermometer measures the thermal radiation
5.3 Accuracy:
coming off of an object. If an object is opaque, this radiation
5.3.1 To make accurate measurements, many factors must energyisacombinationoftheobject’semittedradiationandits
be considered. The first is that the IR thermometer in use reflected radiation. The ability to emit energy is known as
should be calibrated with traceability to the International emissivity. A perfect blackbody has an emissivity of unity,
System of Units (SI) through a national metrological institute ε=1. All actual surfaces emit less thermal radiation than a
(NMI). A list of NMIs can be found by visiting the BIPM perfect blackbody and have an emissivity less than unity. Fig.
website: http://www.bipm.org/en/cipm-mra/participation/ 3 shows the relationship between the radiation emitted by a
signatories.html. Calibration results can be implemented in perfectblackbody,E(T),andtheradiationemittedbyasurface,
subsequent measurements in two ways. If the IR thermometer εE(T). In reality, a perfect blackbody is not achievable. One
has an internal calibration adjustment, the user can use the possible approximation to a perfect blackbody is a cavity
reading on the readout. Some IR thermometer calibrations will radiator.
provide a table of offsets. In such cases, the user must make a 7.1.2 The higher the emissivity an object has, the better the
manual calculation to determine the true temperature. temperature can be determined from its thermal radiation.
E2758 − 22
FIG. 2 Filling the IR Thermometer’s Spot
FIG. 3 Blackbody and Surface Emissivity
Non-metals tend to have much higher emissivity values than 8.2.1 Fourier Transform Infrared (FTIR) testing collects
metals. Non-oxidized metals tend to have lower emissivity data through a reflective method. It is normally done in a
than oxidized metals. Rough surfaces will have higher emis- laboratoryandmostlikelywillnotbeavailablefortheenduser
sivity values than polished surfaces of the same material. of an IR thermometer.
7.1.3 Wideband infrared thermometers are excellent tools
8.2.2 FTIR data provides spectral emissivity values at
for measuring the surface temperatures of materials with high
various wavelengths in the electromagnetic spectrum. The
emissivity values. Materials such as wood, brick, painted
spectral emissivity values are derived from the reflectivity
surfaces, plants and foods generally have emissivity values of
results obtained in the tests. Fig. 4 shows an example of FTIR
0.85 or higher.
test results.
7.2 Determining and Compensating for Emissivity:
8.3 Compensating for Emissivity:
7.2.1 A number of methods to determine and compensate
8.3.1 The preferred way to compensate for unknown emis-
for emissivity are included in Section 16.
sivity is to use the emissivity setting on the IR thermometer.
8.3.2 If the IR thermometer does not have an adjustable
8. Methods of Determining Emissivity
emissivitysetting,usethemathematicsdescribedinsubsection
8.1 Emissivity Tables:
X2.3todeterminehowmuchdifferencethereisbetweentheIR
8.1.1 Manymanufacturerswillprovideatableofemissivity
thermometer reading and the true temperature of the surface.
values for specific materials. These tables are instrument-
This calculation has a degree of uncertainty.
specific. They also contain a certain amount of uncertainty.
8.3.3 If the emissivity of an object is given as a range, it is
8.2 Fourier Transform Infrared Testing: best to measure temperature with the emissivity set to at least
E2758 − 22
Courtesy of Surface Optics Corporation, San Diego, California.
FIG. 4 Example of FTIR Test Results
the lower and upper end of the range, plus the middle of the 10. Measurements of Surfaces Below Ambient
range. For instance, if a materials emissivity range is given as Temperature
0.80 to 0.86, temperature measurements should be taken at
10.1 Reflected Radiation:
ε=0.80, ε=0.83, and ε=0.86. This will indicate a median
10.1.1 The effects of reflected temperature are much greater
temperature (at ε=0.83) along with a low temperature (at
at temperatures below ambient temperatures. These effects are
ε=0.80) and a high temperature (at ε = 0.86).
shown in subsection X2.4.
10.2 Dew Point or Frost Point:
9. Reflected Radiation
10.2.1 If the surface being measured is below the dew point
9.1 General Considerations:
or frost point, there are two additional problems which need to
9.1.1 The thermal radiation detected by an IR thermometer
be considered.
measuring an opaque object is a combination of the thermal
10.2.2 Inthissituation,theemissivityofthesurfaceislikely
radiation emitted by the object and the reflected radiation,
to change. If the surface is completely covered with dew, then
which is radiation originating from other sources and reflected
the surface will have the emissivity of the liquid water formed
by the object. IR thermometers will compensate for the
on the surface. If the surface is completely covered with frost,
reflected radiation in some manner.This compensation may be
then the emissivity will be that of the frost.
byareflectedtemperatureorareflectedtemperaturesetting.In
10.2.3 Another effect is that frost or liquid water forms an
some cases reflected radiation compensation is done inside the
insulating layer between the object being measured and the
IRthermometer.Fig.5showstherelationshipbetweenemitted
surroundingair.Thesurfacetemperatureoftheinsulatinglayer
radiation and reflected radiation.
may be closer to ambient, depending on how deep the
9.1.2 Reflected radiation is minimized by measuring a flat
insulative layer is.
or convex object that has surroundings at a temperature much
less than the measured object (see Section 10). Reflected
11. Optical Considerations
radiation can be very high if the measured object is at a
relativelylowtemperatureorhassurroundingsatatemperature 11.1 Distance Ratio:
equal or greater to the measured object (for example, the 11.1.1 Most IR thermometers come with a distance-to-size
measured object is inside an operating furnace). Reflected diagram or specification. This specification may be referred to
radiation may also be very high when the temperature of as D:S, distance ratio, distance-to-size ratio, field of view, or
reflective surfaces such as metals are being measured. Low- size of source.
temperature measurements are covered in Section 10. Mea- 11.1.2 This distance ratio specification shows that at a
surementofthetemperatureofmetalsiscoveredinSection13. distance D, a certain percentage of the thermal radiation
In such cases it is important that the reflected temperature is measured by the IR thermometer is within a diameter S. Care
known and well controlled. should be taken to ensure that the object being measured is
9.1.3 When making measurements outdoors, it is important larger than this diameter, or it is within the IR thermometer’s
to shield the measured object from reflected celestial radiation. field of view.
Celestialradiationcanhaveatemperatureanywherefromclose 11.1.3 Fig. 6 shows two examples of D:S diagrams that
to absolute zero to the temperature of the sun. commonly come with IR thermometers. An example is given
9.1.4 The effect of miscalculation of background tempera- for an open focus IR thermometer and a closed focus IR
ture is shown in subsection X2.4. thermometer.
E2758 − 22
FIG. 5 Emission and Reflection
FIG. 6 Distance-to-size Diagrams
E2758 − 22
11.1.4 It should be noted that in most cases, a significant 12.3 Steam and Dust:
amountofenergycomesfromoutsideofthisdiameterD.Ifthis 12.3.1 IR thermometry measurements should not be made
energy is not accounted for, it can cause inaccuracy in IR when there is a significant amount of particulates between the
thermometry measurements. Section 8 of Test Methods E1256 surfacebeingmeasuredandtheIRthermometer.Insuchcases,
can be used to determine this effect. asignificantamountofthethermalradiationincidentontheIR
thermometercomesfromtheparticulatesandnotthesurfaceof
11.2 Laser Pointers:
interest.
11.2.1 An IR thermometer is often equipped with a laser
pointer. The purpose of the laser pointer is to give the user a 12.4 Surface Measurement of Temperature:
12.4.1 When measuring the temperature of a surface, it is
rough idea of where the IR thermometer is pointed. The point
where the laser pointer strikes the object may not be where the important to keep in mind that the temperature measured is the
surface temperature. It may not be an accurate representation
exact center of the spot is located.
11.2.2 Atest similar to that described in subsection 8.3.3 of of the temperature below the surface. This is especially true
when thermally insulative materials are being measured or
Test Methods E1256 can be used to determine how close the
when the object being measured is thick.
laser pointer is to the center of the spot for a given distance.
12.5 Atmospheric Transmission:
11.3 Lens Cleanliness:
12.5.1 An IR thermometer measures thermal radiation
11.3.1 Since these measurements are based on optics, it is
through a certain distance of atmosphere.Acertain amount of
important that the lens of the IR thermometer be kept free of
attenuation that takes place over this distance. At distances
foreign objects, such as dust and grease, and free of scratches.
under 2 m, this attenuation is small. At greater distances,
11.3.2 The IR thermometer manufacturer’s instructions
attenuation may become significant.
should be consulted for acceptable cleaning and maintenance
12.5.2 For distances under 1 m, the worst case attenuation
practices.
factor is 0.0003. The calculation of the effect of this attenua-
tion on temperature measurement is discussed in subsection
12. Physical Considerations
X2.5.
12.1 Thermal Shock:
12.5.3 There are three areas in the IR region of the electro-
12.1.1 Thermal shock is caused by quickly changing the
magnetic spectrum that are usually considered windows be-
temperature of the IR thermometer housing. This can happen
cause there is little atmospheric attenuation in these regions.
when transporting the IR thermometer from one environment
These regions are 2.0µm to 2.5 µm, 3.5µm to 4.2µm, and
toanother,especiallywhentheenvironmentshavesignificantly
8µmto14µm.The8µmto14µmwindowisthemostsuitable
different temperatures.
for low-temperature IR measurements.
12.1.2 When changing the ambient temperature of an IR
13. Measuring Metal Surfaces
thermometer, it is best to allow the IR thermometer to reach
thermal equilibrium before taking measurements. This will
13.1 General Considerations:
vary from IR thermometer to IR thermometer, depending on
13.1.1 Wideband generally refers to IR thermometers mea-
the IR thermometer’s thermal mass.
suring radiation in the 8-14 µm region. Given the scope of this
12.1.3 If the IR thermometer manufacturer recommends a
standard, it must be understood by the user of this document
time to allow for an IR thermometer to reach thermal
thatbaremetalsurfacesinherentlycannotbemeasuredreliably
equilibrium, this time should be used. Otherwise, some experi-
with wideband low-temperature infrared thermometers.
mentation should be done to determine this time.
However, in some cases, an inaccurate but highly repeatable
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2758 − 15a (Reapproved 2021) E2758 − 22
Standard Guide for
Selection and Use of Wideband, Low Temperature Infrared
Thermometers
This standard is issued under the fixed designation E2758; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide covers electronic instruments intended for measurement of temperature by detecting intensity of thermal radiation
exchanged between the subject of measurement and the sensor.
1.2 The devices covered by this guide are referred to as IR thermometers.
1.3 The IR thermometers covered in this guide are instruments that are intended to measure temperatures below 1000 °C 2700 °C
and measure a narrow to wide band of thermal radiation in the infrared region.
1.4 This guide covers best practice in using IR thermometers. It addresses concerns that will help the user make better
measurements. It also provides graphical tables to help determine the accuracy of measurements.
1.5 Details on the design and construction of IR thermometers are not covered in this guide.
1.6 This guide does not cover medium- and high-temperature IR thermometry (above 1000 °C). It does not address the use of
narrowband IR thermometers.addresses general information on emissivity and how to deal with emissivity when making
measurements with an IR thermometer.
1.7 This guide contains basic information on the classification of different types of IR thermometers.
1.8 The values of quantities stated in SI units are to be regarded as the standard. The values of quantities in parentheses are not
in SI and are optional.
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.10 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
This guide is under the jurisdiction of ASTM Committee E20 on Temperature Measurement and is the direct responsibility of Subcommittee E20.02 on Radiation
Thermometry.
Current edition approved May 1, 2021Oct. 1, 2022. Published June 2021November 2022. Originally approved in 1910. Last previous edition approved in 20152021 as
E2758 – 15A. DOI:10.1520A (2021). DOI:10.1520/E2758-22.⁄E2758-15AR21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2758 − 22
2. Referenced Documents
2.1 ASTM Standards:
E1256 Test Methods for Radiation Thermometers (Single Waveband Type)
E1862 Practice for Measuring and Compensating for Reflected Temperature Using Infrared Imaging Radiometers
E1897 Practice for Measuring and Compensating for Transmittance of an Attenuating Medium Using Infrared Imaging
Radiometers
E1933 Practice for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers
2.2 IEC Standards:
IEC 62492-1 TS Industrial Process Control Devices—Radiation Thermometers—Part 1: Technical Data for Radiation Ther-
mometers
2.3 BIPM Standards:
JCGM 200:2012 International Vocabulary of Metrology—Basic and General Concepts and Associated Terms (VIM)
3. Terminology
3.1 Definitions:
3.1.1 absolute zero, n—a temperature of 0 K (–273.15 °C).
3.1.2 atmospheric attenuation, n—a ratio showing how much thermal radiation in a given spectral range is absorbed or scattered
in air over a given distance.
3.1.3 atmospheric transmission, n—a ratio showing how well thermal radiation in a given spectral range at a given distance travels
through a certain distance of air.
3.1.4 attenuating medium, n—a semi-transparent solid, liquid or gas, such as a window, filter, external optics, or an atmosphere
that reduces thermal radiation, or combinations thereof.
3.1.5 background radiation—see reflected radiation.
3.1.6 blackbody, n—the perfect or ideal source of thermal radiant power having a spectral distribution described by Planck’s Law.
3.1.7 blackbody simulator, n—a device with an emissivity close to unity that can be heated or cooled to a stable temperature.
3.1.8 calibration adjustment, n—the correction to an IR thermometer based on its calibration.
3.1.9 celestial radiation, n—flux coming from the sky.
3.1.10 center wavelength, n—the simple average of the lower and upper spectral range limits.
3.1.11 contact thermometer, n—an instrument that is adapted for measuring temperature by means of thermal conductance by
determining the temperature at the moment when negligible thermal energy flows between the thermometer and the object of
measurement.
3.1.12 dew point, n—the temperature at which water vapor condenses into liquid water.
3.1.13 diffuse reflector, n—a surface that produces a diffuse image of a reflected source.
3.1.14 distance ratio, n—the ratio of the measuring distance to the diameter of the field-of-view, when the target is in focus.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from International Electrotechnical Commission (IEC), 3, rue de Varembé, 1st floor, P.O. Box 131, CH-1211, Geneva 20, Switzerland, https://www.iec.ch.
See IEC 62492-1.
E2758 − 22
3.1.15 electromagnetic radiation, n—physically occurring radiant flux classified according to wavelength or frequency.
3.1.16 emissivity (ε), n—the emissivity of a surface is the ratio between the radiation emitted from this surface and the radiation
from a blackbody at the same temperature.
3.1.16.1 Discussion—
The emissivity describes a thermo-physical material characteristic, which in addition to the chemical composition of the material
may also be dependent on the surface structure (rough, smooth), the emission direction as well as on the observed wavelength and
the temperature of the measured object.
3.1.17 emissivity setting, n—an adjustment on an IR thermometer to compensate for an emissivity of non-unity.
3.1.17.1 Discussion—
In most measuring situations a radiation thermometer is used on a surface with an emissivity significantly lower than one. For this
purpose most thermometers have the possibility of adjusting the emissivity setting. The temperature reading is then automatically
corrected.
3.1.18 emissivity tables, n—a list of objects and their measured emissivity for a particular IR thermometer.
3.1.19 field-of-view (FOV), n—a usually circular, flat surface of a measured object from which the radiation thermometer receives
radiation.
3.1.20 frost point, n—the temperature at which water vapor condenses into solid water or ice.
3.1.21 infrared (IR), adj—referring to electromagnetic radiation with a wavelength from approximately 0.70.7 μm to 30
μm.30 μm.
3.1.22 infrared reflector, n—a material with a reflectance in the infrared region as close as possible to unity.
3.1.23 infrared sensing device, n—one of a wide class of instruments used to display or record (or both) information related to
the thermal radiation received from any object surfaces viewed by the instrument.
3.1.24 infrared (IR) thermometer, n—optoelectronic instrument adapted for noncontact measurement of temperature of a subject
by utilizing thermal radiation exchange between the subject and the sensor.
3.1.24.1 Discussion—
IR thermometers are a subset of radiation thermometers. Most manufacturers use the term IR thermometer for handheld radiation
thermometers. In general, these devices are wideband and use a thermopile detector.
3.1.25 IR thermometry, n—the use of IR thermometers to determine temperature by measuring thermal radiation.
3.1.26 irradiance (E), n—the radiant flux (power) per unit area incident on a given surface in units of W/m .
3.1.27 limit of error, n—the extreme value of measurement error of an infrared thermometer reading, relative to reference
temperature standards, as permitted by a specification.
3.1.27.1 Discussion—
Manufacturers sometimes use the term accuracy in their specifications to represent limit of error.
3.1.27.2 Discussion—
A manufacturer’s accuracy specification may apply only to well defined conditions.
3.1.28 low-temperature, adj—for radiation and IR thermometry, referring to any temperature below 660 °C.
E2758 − 22
3.1.29 measurement uncertainty (accuracy), n—non-negative parameter, characterizing the dispersion of the values that could
reasonably be attributed to the measurement of the quantity values being attributed to a measurand, based on the information
4,5
used.
3.1.30 measuring distance, n—distance or distance range between the radiation thermometer and the target (measured object) for
which the radiation thermometer is designed.
3.1.31 measuring temperature range, n—temperature range for which the radiation thermometer is designed.
3.1.32 noise equivalent temperature difference (NETD), n—parameter which indicates the contribution of the measurement
uncertainty in °C, which is due to instrument noise.
3.1.33 opaque, adj—referring to the property of a material whose transmittance is zero for a given spectral range.
3.1.34 operating temperature range and air humidity range, n—the permissible temperature range and humidity range within
which the radiation thermometer may be operated. For this temperature range and humidity range the specifications are valid.
3.1.34.1 Discussion—
This is the range of ambient temperature and humidity the instrument may operate within and be expected to meet its specification.
It may be thought of as the ambient operating temperature range and the ambient operating humidity range.
3.1.35 radiance (L), n—the flux per unit projected area per unit solid angle leaving a source or, in general, any reference surface.
3.1.35.1 Discussion—
If ∂ Φ is the flux emitted into a solid angle ∂ω by a source element of projected area ∂Acos(θ), the radiance is defined as:
] Φ
L 5
]ω]Acos~θ!
where:
θ = the angle between the outward surface normal of the area element ∂ A and the direction of observation (unit = W/sr•m ).
3.1.36 radiant power density (M), n—the radiant flux per unit area leaving a surface that is,
]Φ
M 5
]A
where:
∂Φ = flux leaving a surface element ∂A (unit = W/m ).
3.1.37 reflectance, n—the ratio of the radiant flux reflected from a surface to that incident upon it.
3.1.38 reflected radiation, n—the thermal radiation incident upon and reflected from the measurement surface of the specimen.
3.1.39 reflected temperature, n—the temperature of the radiant flux incident upon and reflected from the measurement surface of
a specimen.
3.1.40 response time, n—time interval between the instant of an abrupt change in the value of the input parameter (object
temperature or object radiation) and the instant from which the measured value of the radiation thermometer (output parameter)
remains within specified limits of its final value.
3.1.41 sensor, n—device designed to respond to IR radiation and convert that response into electrical signals.
See BIPM JCGM 200:2012.
E2758 − 22
3.1.42 size-of-source effect, n—the difference in the radiance- or temperature reading of the radiation thermometer when changing
the size of the radiating area of the observed source.
3.1.43 spectral range, n—parameter which gives the lower and upper limits of the wavelength range over which the radiation
thermometer operates.
3.1.43.1 Discussion—
Spectral range is sometimes referred to as bandwidth.
3.1.43.2 Discussion—
These limits are generally defined as the wavelengths where the power or signal is attenuated by a defined amount.
3.1.44 spectral response, n—the numerical quantity of a given phenomenon at a specific wavelength in the electromagnetic
spectrum.
3.1.45 standard atmosphere, n—a model of how electromagnetic radiation is transmitted through the atmosphere based on
variations in pressure, temperature and humidity.
3.1.46 surface-modifying material, n—any material that is used to change the emissivity of the specimen surface.
3.1.47 table of offsets, n—a list of calibration points and calibration adjustments to be used when no internal calibration adjustment
is available.
3.1.48 thermal radiation, n—electromagnetic radiation which is caused by an object’s temperature and is predicted by Planck’s
Law.
3.1.49 thermal shock, n—subjecting an IR thermometer to a rapid temperature change.
3.1.50 thermopile detector, n—a thermopile detector’s output is voltage. Incident radiation heats the disk. When the disk is heated,
its temperature rises above the sensor’s reference temperature (ambient temperature) producing a temperature difference (ΔT). The
potential of the thermopile is related to the temperature difference based on the Seebeck Effect.
3.1.51 transmittance (t), n—the ratio of the radiant flux transmitted through a body to that incident upon it.
3.1.52 true temperature, n—temperature attributed to a particular site of a subject or object of measurement and accepted as having
a specified uncertainty.
3.1.53 wideband, adj—referring to the situation where the spectral range of an instrument is at least ⁄10 of its center wavelength.
4. Significance and Use
4.1 This guide provides guidelines and basic test methods for the use of infrared thermometers. The purpose of this guide is to
provide a basis for users of IR thermometers to make more accurate measurements, to understand the error in measurements, and
reduce the error in measurements.
5. Basic Use of IR Thermometry
5.1 General Considerations:
5.1.1 An IR thermometer can be used in a number of applications. Although they are generally not as accurate as contact
thermometers, their quickness of measurement and their ability to measure the temperature of an opaque surface without contacting
it make them desirable instruments for some temperature measurements.
5.1.2 Most handheld IR thermometers are equipped with a trigger to start and stop the measurements.
E2758 − 22
FIG. 1 Basic IR Thermometer Measurement
5.1.3 As objects vary in temperature, they emit a varying amount of thermal radiation. This amount of thermal radiation is
predictable based on the object’s temperature, emissivity and reflected temperature.
5.1.4 Handheld IR thermometers measure thermal radiation in a given spectral range and determine the relationship between the
measured thermal radiation and temperature. The sensor mainly used in these instruments is a thermopile.
5.2 Basic IR Measurement:
5.2.1 Before making a measurement, the emissivity setting of the IR thermometer should be set to the object’s effective emissivity
in the instrument’s spectral range. Some IR thermometers do not allow the user to adjust the emissivity because their emissivity
is fixed. In these cases there are mathematical compensations that can be made.
5.2.2 To make a measurement, the IR thermometer’s lens should be pointed at the object being measured. The measurement should
be initiated. If the IR thermometer has a trigger, this is done by pulling the trigger. The trigger should be held at least as long as
the IR thermometer’s specified response time. The measured temperature is usually frozen on the display after the trigger is
released.
5.2.3 Fig. 1 shows a diagram of how much of a surface an IR thermometer measures. ‘S’ is the size or diameter that is measured
by the infrared thermometer’s field-of-view. ‘D’ is the measuring distance. Subsection 11.1 discusses spot size and distance ratio.
5.2.4 Fig. 2 shows how much surface area is needed for temperature measurement when considering the IR thermometer’s spot
size. The part of the figure labeled ‘poor’ shows a situation where the object being measured is smaller than the spot size. Such
situations are undesirable. The part of the figure labeled ‘OK’ shows a situation where the object being measured is slightly larger
than the spot size. Such situations should produce acceptable temperature measurements. The part of the figure labeled ‘better’
shows a situation where the object being measured is significantly larger than the spot size. This situation will produce the best
temperature measurements.
5.3 Accuracy:
5.3.1 To make accurate measurements, many factors must be considered. The first is that the IR thermometer in use should be
calibrated with traceability to the International System of Units (SI) through a national metrological institute (NMI). A list of NMIs
can be found by visiting the BIPM website: http://www.bipm.org/en/cipm-mra/participation/signatories.html. Calibration results
can be implemented in subsequent measurements in two ways. If the IR thermometer has an internal calibration adjustment, the
user can use the reading on the readout. Some IR thermometer calibrations will provide a table of offsets. In such cases, the user
must make a manual calculation to determine the true temperature.
E2758 − 22
FIG. 2 Filling the IR Thermometer’s Spot
5.3.2 There are many other considerations in making accurate measurements with IR thermometry. These are discussed in the
following sections.
6. Wideband Instruments
6.1 Most handheld low-temperature IR thermometers are wideband instruments. As a result, their measurements can vary if
emissivity varies over their spectral range. The most common spectral range for these instruments is 88 μm to 14 μm. 14 μm.
However, some instruments have a spectral range of up to 55 μm to 20 μm. 20 μm. In any case, the IR thermometer should have
a specified spectral range. The end user of an IR thermometer most likely will not have instrumentation to test the spectral range.
6.2 Atmospheric transmission is dependent on spectral range. Any measurement made over a long distance should consult a
standard atmosphere model to determine atmospheric transmission. Guidance on accounting for atmospheric transmission is given
in subsection 12.5.
7. Spectral Emissivity
7.1 Spectral Emissivity in General:
7.1.1 An IR thermometer measures the thermal radiation coming off of an object. If an object is opaque, this radiation energy is
a combination of the object’s emitted radiation and its reflected radiation. The ability to emit energy is known as emissivity. A
perfect blackbody has an emissivity of unity, ε = 1. All actual surfaces emit less thermal radiation than a perfect blackbody and
have an emissivity less than unity. Fig. 3 shows the relationship between the radiation emitted by a perfect blackbody, E(T), and
the radiation emitted by a surface, εE(T). In reality, a perfect blackbody is not achievable. One possible approximation to a perfect
blackbody is a cavity radiator.
7.1.2 The higher the emissivity an object has, the better the temperature can be determined from its thermal radiation. Non-metals
tend to have much higher emissivity values than metals. Non-oxidized metals tend to have lower emissivity than oxidized metals.
Rough surfaces will have higher emissivity values than polished surfaces of the same material.
7.1.3 Wideband infrared thermometers are excellent tools for measuring the surface temperatures of materials with high emissivity
values. Materials such as wood, brick, painted surfaces, plants and foods generally have emissivity values of 0.85 or higher.
7.2 Determining and Compensating for Emissivity:
7.2.1 A number of methods to determine and compensate for emissivity are included in Section 16.
8. Methods of Determining Emissivity
8.1 Emissivity Tables:
8.1.1 Many manufacturers will provide a table of emissivity values for specific materials. These tables are instrument-specific.
They also contain a certain amount of uncertainty.
8.2 Fourier Transform Infrared Testing:
E2758 − 22
FIG. 3 Blackbody and Surface Emissivity
8.2.1 Fourier Transform Infrared (FTIR) testing collects data through a reflective method. It is normally done in a laboratory and
most likely will not be available for the end user of an IR thermometer.
8.2.2 FTIR data provides spectral emissivity values at various wavelengths in the electromagnetic spectrum. The spectral
emissivity values are derived from the reflectivity results obtained in the tests. Fig. 4 shows an example of FTIR test results.
8.3 Compensating for Emissivity:
8.3.1 The preferred way to compensate for unknown emissivity is to use the emissivity setting on the IR thermometer.
8.3.2 If the IR thermometer does not have an adjustable emissivity setting, use the mathematics described in subsection X2.3 to
determine how much difference there is between the IR thermometer reading and the true temperature of the surface. This
calculation has a degree of uncertainty.
8.3.3 If the emissivity of an object is given as a range, it is best to measure temperature with the emissivity set to at least the lower
and upper end of the range, plus the middle of the range. For instance, if a materials emissivity range is given as 0.80 to 0.86,
temperature measurements should be taken at ε = 0.80, ε = 0.83, and ε = 0.86. This will indicate a median temperature (at ε = 0.83)
along with a low temperature (at ε = 0.80) and a high temperature (at ε = 0.86).
9. Reflected Radiation
9.1 General Considerations:
9.1.1 The thermal radiation detected by an IR thermometer measuring an opaque object is a combination of the thermal radiation
emitted by the object and the reflected radiation, which is radiation originating from other sources and reflected by the object. IR
thermometers will compensate for the reflected radiation in some manner. This compensation may be by a reflected temperature
or a reflected temperature setting. In some cases reflected radiation compensation is done inside the IR thermometer. Fig. 5 shows
the relationship between emitted radiation and reflected radiation.
9.1.2 Reflected radiation is minimized by measuring a flat or convex object that has surroundings at a temperature much less than
the measured object (see Section 10). Reflected radiation can be very high if the measured object is at a relatively low temperature
E2758 − 22
Courtesy of Surface Optics Corporation, San Diego, California.
FIG. 4 Example of FTIR Test Results
FIG. 5 Emission and Reflection
or has surroundings at a temperature equal or greater to the measured object (for example, the measured object is inside an
operating furnace). Reflected radiation may also be very high when the temperature of reflective surfaces such as metals are being
measured. Low-temperature measurements are covered in Section 10. Measurement of the temperature of metals is covered in
Section 13. In such cases it is important that the reflected temperature is known and well controlled.
9.1.3 When making measurements outdoors, it is important to shield the measured object from reflected celestial radiation.
Celestial radiation can have a temperature anywhere from close to absolute zero to the temperature of the sun.
9.1.4 The effect of miscalculation of background temperature is shown in subsection X2.4.
E2758 − 22
10. Measurements of Surfaces Below Ambient Temperature
10.1 Reflected Radiation:
10.1.1 The effects of reflected temperature are much greater at temperatures below ambient temperatures. These effects are shown
in subsection X2.4.
10.2 Dew Point or Frost Point:
10.2.1 If the surface being measured is below the dew point or frost point, there are two additional problems which need to be
considered.
10.2.2 In this situation, the emissivity of the surface is likely to change. If the surface is completely covered with dew, then the
surface will have the emissivity of the liquid water formed on the surface. If the surface is completely covered with frost, then the
emissivity will be that of the frost.
10.2.3 Another effect is that frost or liquid water forms an insulating layer between the object being measured and the surrounding
air. The surface temperature of the insulating layer may be closer to ambient, depending on how deep the insulative layer is.
11. Optical Considerations
11.1 Distance Ratio:
11.1.1 Most IR thermometers come with a distance-to-size diagram or specification. This specification may be referred to as D:S,
distance ratio, distance-to-size ratio, field of view, or size of source.
11.1.2 This distance ratio specification shows that at a distance D, a certain percentage of the thermal radiation measured by the
IR thermometer is within a diameter S. Care should be taken to ensure that the object being measured is larger than this diameter,
or it is within the IR thermometer’s field of view.
11.1.3 Fig. 6 shows two examples of D:S diagrams that commonly come with IR thermometers. An example is given for an open
focus IR thermometer and a closed focus IR thermometer.
11.1.4 It should be noted that in most cases, a significant amount of energy comes from outside of this diameter D. If this energy
is not accounted for, it can cause inaccuracy in IR thermometry measurements. Section 8 of Test Methods E1256 can be used to
determine this effect.
E2758 − 22
FIG. 6 Distance-to-size Diagrams
11.2 Laser Pointers:
11.2.1 An IR thermometer is often equipped with a laser pointer. The purpose of the laser pointer is to give the user a rough idea
of where the IR thermometer is pointed. The point where the laser pointer strikes the object may not be where the exact center
of the spot is located.
11.2.2 A test similar to that described in subsection 8.3.3 of Test Methods E1256 can be used to determine how close the laser
pointer is to the center of the spot for a given distance.
11.3 Lens Cleanliness:
11.3.1 Since these measurements are based on optics, it is important that the lens of the IR thermometer be kept free of foreign
objects, such as dust and grease, and free of scratches.
11.3.2 The IR thermometer manufacturer’s instructions should be consulted for acceptable cleaning and maintenance practices.
12. Physical Considerations
12.1 Thermal Shock:
12.1.1 Thermal shock is caused by quickly changing the temperature of the IR thermometer housing. This can happen when
transporting the IR thermometer from one environment to another, especially when the environments have significantly different
temperatures.
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12.1.2 When changing the ambient temperature of an IR thermometer, it is best to allow the IR thermometer to reach thermal
equilibrium before taking measurements. This will vary from IR thermometer to IR thermometer, depending on the IR
thermometer’s thermal mass.
12.1.3 If the IR thermometer manufacturer recommends a time to allow for an IR thermometer to reach thermal equilibrium, this
time should be used. Otherwise, some experimentation should be done to determine this time.
12.1.4 If the thermal shock results in the IR thermometer temperature being lower than the dew or frost point temperature of the
ambient environment, moisture may condense on the IR thermometer optics. Measurements should wait until the optics show no
signs of moisture condensation.
12.2 Measurements at Short Distance:
12.2.1 Being too close to the surface being measured presents several problems. This is because the IR thermometer is absorbing
a certain amount of convected heat from the surface being measured.
12.2.2 Having the IR thermometer too close to the surface being measured can cause thermal shock which is discussed in
subsection 12.1.
12.2.3 Having the IR thermometer too close to the surface being measured can cause damage to the IR thermometer due to heating
from the convective source. This damage may be to just the housing. However, it may also cause damage to the lens and even the
sensor if held too close to the surface being measured.
12.3 Steam and Dust:
12.3.1 IR thermometry measurements should not be made when there is a significant amount of particulates between the surface
being measured and the IR thermometer. In such cases, a significant amount of the thermal radiation incident on the IR
thermometer comes from the particulates and not the surface of interest.
12.4 Surface Measurement of Temperature:
12.4.1 When measuring the temperature of a surface, it is important to keep in mind that the temperature measured is the surface
temperature. It may not be an accurate representation of the temperature below the surface. This is especially true when thermally
insulative materials are being measured or when the object being measured is thick.
12.5 Atmospheric Transmission:
12.5.1 An IR thermometer measures thermal radiation through a certain distance of atmosphere. A certain amount of attenuation
that t
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