ISO 18251-1:2017

Non-destructive testing - Infrared thermography - Part 1: Characteristics of system and equipment

ISO 18251-1:2017 describes the main components, and their characteristics, constituting an infrared (IR) imaging system and related equipment used in non-destructive testing (NDT). It also aims to assist the user in the selection of an appropriate system for a particular measurement task. The following items are specified: - objective lens; - detector; - image processor; - display; - thermal stimulation source; - accessories.

Essais non destructifs — Thermographie infrarouge — Partie 1: Caractéristiques du système et des équipements

General Information

• Status: Published
• Publication Date: 12-Jan-2017
• ICS: 19.100 - Non-destructive testing
• Technical Committee: ISO/TC 135/SC 8 - Thermographic testing
• Drafting Committee: ISO/TC 135/SC 8/WG 3 - NDT-IR-System and equipment
• Current Stage: 9060 - Close of review
• Completion Date: 02-Sep-2027

Relations

• Consolidated By: ISO 178:2019 - Plastics - Determination of flexural properties
• Effective Date: 06-Jun-2022

Overview

ISO 18251-1:2017 - "Non‑destructive testing - Infrared thermography - Part 1: Characteristics of system and equipment" defines the main components and performance characteristics of infrared (IR) imaging systems and related equipment used in industrial non‑destructive testing (NDT). The standard helps users and manufacturers select appropriate IR systems for specific measurement tasks by specifying requirements for lenses, detectors, image processing, displays, thermal stimulation sources and accessories.

Key topics and requirements

ISO 18251-1 addresses component-level and integrated system characteristics, including:

• Objective lens: spectral response, focal length, aperture (f‑number), interchangeable lens considerations.
• Detector: detector types and arrays, working wavelength range, number of pixels, bad/dead pixel handling, operability, thermal time constant, integration time and temperature range.
• Image processor & display: image acquisition (timing, triggers, freeze), display requirements, image analysis and recording.
• Image processing functions: bad/dead pixel replacement, non‑uniformity correction (NUC), image enhancement, filtering, time‑correlated processing, visible–infrared image fusion.
• Thermal stimulation sources: optical radiation, convective excitation, electromagnetic induction, mechanical excitation, and their advantages/drawbacks for active thermography.
• Integrated performance parameters: noise equivalent temperature difference (NETD), minimum resolvable temperature difference (MRTD), minimum detectable temperature difference (MDTD), field of view (FOV), instantaneous field of view (IFOV), minimum working distance, temperature measurement range and uniformity, operating temperature range.
• Interfaces & accessories: digital I/O, data transfer and video output interfaces; infrared mirrors, attenuation and spectral filters, tripods, reference blocks.

(ISO 10878 - vocabulary - is cited as a normative reference for terminology.)

Applications and who uses it

ISO 18251-1 is relevant for:

• NDT technicians and inspection engineers specifying IR cameras and systems for defect detection, condition monitoring, and quality control.
• Manufacturers and suppliers developing or certifying IR imaging equipment for industrial thermography.
• Procurement and R&D teams evaluating camera performance (NETD, MRTD, pixel operability, spectral response) for application fit.
• Training providers and consultants establishing best practices for IR system setup, thermal stimulation selection and image processing workflows.

Practical uses include selecting the right detector and lens for required spatial/temperature resolution, choosing suitable active excitation sources for subsurface inspection, and ensuring processed images meet detection and measurement needs.

Related standards

• ISO 10878 (Non‑destructive testing - Infrared thermography - Vocabulary)
• See the ISO 18251 series for other parts addressing test methods and procedures.

Keywords: ISO 18251-1, infrared thermography, NDT, IR imaging system, detector, objective lens, NETD, MRTD, thermal stimulation.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 18251-1
First edition
2017-02
Non-destructive testing — Infrared
thermography —
Part 1:
Characteristics of system and
equipment
Essais non destructifs — Thermographie infrarouge —
Partie 1: Caractéristiques du système et des équipements
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 IR system setup . 1
5 Objective lens . 2
5.1 General . 2
5.2 Spectral response. 2
5.3 Focal length (mm) . 2
5.4 Aperture f-number . 2
5.5 Interchangeable object lenses . 3
6 Detector . 3
6.1 General . 3
6.2 Detector types . 3
6.3 Detector arrays . 3
6.4 Scanning systems . 3
6.5 Working wavelength range . 3
6.6 Number of pixels . 4
6.7 Bad/dead pixel . 4
6.8 Detector operability . 4
6.9 Thermal time constant . 4
6.10 Integration time . 4
6.11 Temperature range . 4
7 Image processor . 4
7.1 General . 4
7.2 Image acquisition . 4
7.2.1 Timing acquisition . 4
7.2.2 Trigger acquisition . 5
7.2.3 Image freeze . 5
7.3 Image display . 5
7.4 Image analysis . 5
7.5 Image processing . 5
7.5.1 General. 5
7.5.2 Bad/dead pixel replacement . 5
7.5.3 Non-uniformity correction . 5
7.5.4 Image enhancement . 5
7.5.5 Filtering . 6
7.5.6 Time correlated processing method . 6
7.5.7 Visible-infrared image fusion. 6
7.6 Image recording . 6
8 Thermal stimulation source . 6
8.1 General . 6
8.2 Optical radiation devices . 6
8.3 Convective excitation devices . 6
8.4 Electromagnetic induction devices . 7
8.5 Mechanical excitation devices . 7
8.6 Advantages and drawbacks of thermal stimulation sources . 7
9 Integrated characteristics and functions of infrared systems and equipment.7
9.1 Integrated performance parameters. 7
9.1.1 Noise equivalent temperature difference (NETD) . 7
9.1.2 Minimum resolvable temperature difference (MRTD) . 8
9.1.3 Minimum detectable temperature difference (MDTD) . 8
9.1.4 Field of view (FOV) . 8
9.1.5 Instantaneous field of view (IFOV) . 8
9.1.6 Minimum working distance . 8
9.1.7 Maximum temperature measurement range . 8
9.1.8 Temperature measurement uniformity . 8
9.1.9 Operating temperature range . 9
9.2 Integrated functions . 9
9.2.1 Digital input/output interface . 9
9.2.2 Data transfer interface . 9
9.2.3 Video output interface . 9
10 Accessories . 9
10.1 Infrared mirror . 9
10.2 Attenuation filter . 9
10.3 Spectral filters . 9
10.4 Tripod . 9
10.5 Reference blocks . 9
Bibliography .10
iv © ISO 2017 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the
Technical Barriers to Trade (TBT) see the following URL: www . i so .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 135, Non-destructive testing,
Subcommittee SC 8, Thermographic testing.
A list of all parts in the ISO 18251 series can be found on the ISO website.
Introduction
The industrial applications of infrared thermographic testing in non-destructive testing (NDT) are
growing, along with a remarkable improvement in thermographic technologies. The effectiveness
of any application of infrared thermographic testing depends upon proper and correct usage of the
system and equipment. The purpose of this document is to provide the characterization description of
system and equipment for infrared thermography in the field of industrial NDT. The development of this
document resolves the lack of International Standards on infrared equipment and systems. The main
interested parties who will benefit from this document are manufacturers and users of such equipment
and systems.
vi © ISO 2017 – All rights reserved

INTERNATIONAL STANDARD ISO 18251-1:2017(E)
Non-destructive testing — Infrared thermography —
Part 1:
Characteristics of system and equipment
1 Scope
This document describes the main components, and their characteristics, constituting an infrared (IR)
imaging system and related equipment used in non-destructive testing (NDT). It also aims to assist the
user in the selection of an appropriate system for a particular measurement task.
The following items are specified:
— objective lens;
— detector;
— image processor;
— display;
— thermal stimulation source;
— accessories.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 10878, Non-destructive testing — Infrared thermography — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10878 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
4 IR system setup
Figure 1 represents an imaging arrangement including the IR system. The lens focuses an image of
the object on the detector. The array of pixels in the detector produces electrical signals dependent on
infrared radiation intensity. The electrical signals are processed to produce an image that is shown on a
display and available for storage or further processing.
Figure 1 — IR system setup
5 Objective lens
5.1 General
The objective lens of an optical system is the element, or combination of elements, that focuses radiant
energy from the object and forms the primary image.
Interchangeable lenses are used to reach a desired spatial resolution of the investigated object, or
object detail.
5.2 Spectral response
IR-cameras are adapted to the transmission properties of the atmosphere for infrared radiation
(atmospheric windows):
— Short Wave, SW: wavelength between approx. 0,8 µm and 2 µm;
— Mid Wave, MW: wavelength between approx. 3 µm and 5 µm;
— Long Wave, LW: wavelength between approx. 8 µm and 14 µm.
The spectral response of the IR-camera depends on the used detector. The transmission of the objective
lens system should be adapted to the spectral response of the detector. The detector is selected
according to the test problem.
5.3 Focal length (mm)
The focal length is the distance between optical centre of the lens and the focal plane point of the
detector. The image of an observed object differs according to the focal length of the lens. A long focal
length results in a smaller field of view and a larger image on the focal plane; this can be useful for
increasing the working distance or visualizing fine details of an object.
5.4 Aperture f-number
The aperture defines the opening through which the rays come to a focus on the focal plane array. The
effective size of the lens aperture affects the amount of radiant energy that passes through a lens. It
is usually specified as an f-number, the ratio of the focal length to the effective aperture diameter. It
strongly influences the sensitivity of the infrared detector. A larger aperture, a lower f-number, allows
more radiant energy to reach the detector, increasing sensitivity of the system. These are “fast”
apertures like f/1.1 or f/2. A smaller aperture, a higher f-number, allows less light that gets in. These are
“slow” apertures like f/3 or f/4.5.
2 © ISO 2017 – All rights reserved

The lens aperture diameter needs to be greater than the detector diagonal to ensure th
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