ISO/FDIS 11551

Optics and photonics — Lasers and laser-related equipment — Test method for absorptance of optical laser components

This document specifies procedures and techniques for obtaining comparable values for the absorptance of optical laser components.

Optique et photonique — Lasers et équipements associés aux lasers — Méthode d'essai du facteur d'absorption des composants optiques pour lasers

Le présent document spécifie les modes opératoires et les techniques utilisés pour obtenir des valeurs comparables du facteur d'absorption des composants optiques pour lasers.

General Information

• Status: Not Published
• ICS: 31.260 - Optoelectronics. Laser equipment
• Technical Committee: ISO/TC 172/SC 9 - Laser and electro-optical systems
• Drafting Committee: ISO/TC 172/SC 9/WG 1 - Terminology and test methods for electro-optical systems
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 26-May-2026
• Completion Date: 26-May-2026

Relations

• Consolidates: FprEN ISO 11551 - Optics and photonics - Lasers and laser-related equipment - Test method for absorptance of optical laser components (ISO/FDIS 11551:2026)
• Effective Date: 12-Feb-2026
• Consolidates: ISO 23601:2020 - Safety identification — Escape and evacuation plan signs
• Effective Date: 01-Oct-2024
• Revises: ISO 11551:2019 - Optics and photonics — Lasers and laser-related equipment — Test method for absorptance of optical laser components
• Effective Date: 21-Sep-2024

Overview

ISO/FDIS 11551:2026, titled "Optics and photonics - Lasers and laser-related equipment - Test method for absorptance of optical laser components," establishes standardized procedures for measuring and comparing the absorptance values of optical components used in laser systems. Developed by ISO Technical Committee 172/SC 9, this standard aims to ensure reliable and consistent evaluation of the absorptance, which directly influences the efficiency and durability of laser optics across different applications and industries.

Absorptance, as defined by the standard, is the ratio of radiant flux absorbed by the optical component to the incident radiant flux. The procedures focus on calorimetric measurement methods, capturing the fraction of incident laser energy converted into heat, a critical parameter for maintaining the performance and safety of laser devices.

Key Topics

• Test Preparation and Environment

  • Adherence to cleanroom conditions (ISO 14644-1 Class 7 or better)
  • Control of environmental factors (humidity, dust, ambient temperature)
  • Proper mounting and handling to minimize measurement errors and sample damage

• Measurement Methodology

  • Detailed calibration of power and temperature sensors
  • Use of lasers matching operational parameters (wavelength, polarization, angle of incidence)
  • Separation of the measurement into drift, heating, and cooling intervals

• Evaluation Approaches

  • Exponential Method: Suitable for longer irradiation times, utilizing exponential fitting of temperature data to extract absorptance
  • Pulse Method: Optimal for short-duration tests, relying on extrapolation from temperature decay after laser exposure

• Reporting Requirements

  • Comprehensive documentation of test setup, environmental conditions, component details, and measurement results
  • Transparent disclosure of calibration steps, error budgets, and any deviations from standardized procedures

Applications

ISO/FDIS 11551 is highly relevant for industries where laser systems and optical components are critical, particularly in:

• Laser Manufacturing and Quality Control

  • Enables consistent testing of mirrors, lenses, windows, and coated optics for absorptance losses
  • Assists manufacturers in meeting product specifications and customer requirements

• Component Design and Material Research

  • Provides a standardized framework for benchmarking new materials or coatings under defined laser irradiation conditions
  • Supports the identification of factors influencing absorptance, such as wavelength dependence, polarization, and operating environment

• Laser System Maintenance and Reliability

  • Facilitates periodic inspection of installed laser optics for absorptance changes due to ageing, contamination, or damage
  • Contributes to improved predictive maintenance by identifying components at risk of degradation or failure

• Certification and Compliance

  • Used by testing laboratories and regulatory bodies to verify compliance with international standards on laser safety and performance

Related Standards

• ISO 11145: Vocabulary and symbols for lasers and laser-related equipment, referenced for terminology consistency
• ISO 14644-1: Cleanroom and controlled environment requirements, ensuring proper testing conditions
• ISO 80000-7: Quantities and units for light and radiation, ensuring correct measurement units and symbols throughout absorptance testing

Standardization documents like ISO/FDIS 11551 are crucial for global alignment on test methods, allowing manufacturers, laboratories, and users to achieve comparable and reliable measurement results for laser optical components. Meeting the requirements of this standard supports improved product reliability, operational safety, and consistent performance across diverse laser applications.

Standards Content (Sample)

FINAL DRAFT
International
Standard
ISO/TC 172/SC 9
Optics and photonics — Lasers and
Secretariat: DIN
laser-related equipment — Test
Voting begins on:
method for absorptance of optical
2026-05-26
laser components
Voting terminates on:
2026-07-21
Optique et photonique — Lasers et équipements associés aux
lasers — Méthode d'essai du facteur d'absorption des composants
optiques pour lasers
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/CEN PARALLEL PROCESSING LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 172/SC 9
Optics and photonics — Lasers and
Secretariat: DIN
laser-related equipment — Test
Voting begins on:
method for absorptance of optical
laser components
Voting terminates on:
Optique et photonique — Lasers et équipements associés aux
lasers — Méthode d'essai du facteur d'absorption des composants
optiques pour lasers
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/CEN PARALLEL PROCESSING
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units of measure . 1
5 Preparation of test sample and measuring arrangement . 2
6 Characteristic features of the laser radiation . 4
7 Test procedure . 5
7.1 General .5
7.2 Calibration .5
7.2.1 Calibration of the radiant power signal .5
7.2.2 Calibration of the temperature signal .5
7.2.3 Calibration of the thermal response .5
7.2.4 Measurement of the background signal .6
7.3 Determining the absorptance .6
8 Evaluation . 6
8.1 General .6
8.2 Elimination of drift .7
8.3 Exponential method .7
8.4 Pulse method .8
9 Test report . 9
Annex A (informative) Effects changing absorptance .11
Annex B (informative) Influence of signal distortions . 14
Annex C (informative) Algorithm for parameterizing the temperature data . 17
Bibliography .18

iii
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, 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 www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee 172, Optics and Photonics, Subcommittee SC 9, Laser
and electro-optical systems, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 123, Lasers and Photonics, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
This fourth edition cancels and replaces the third edition (ISO 11551:2019), which has been technically
revised.
The main changes are as follows:
— harmonization of terms and environmental conditions to current laser measurement standards;
— minor adjustments of formulae and figures;
— modified text and additional figures in A.1 and A.3.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
To characterize an optical component, it is important to know its absorptance. When radiation impinges
upon a component, a part of that radiation is absorbed, increasing the temperature of the component. In this
document only the part of the absorbed radiant power/energy, that is converted into heat, is measured. If
enough radiant energy is absorbed, the optical properties of the component can change, and the component
can even be destroyed. Absorptance is the ratio of the radiant flux absorbed to the radiant flux of the incident
radiation.
In the procedures described in this document, the absorptance is determined calorimetrically as the ratio
of radiant power or radiant energy absorbed by the component to the total radiant power or radiant energy,
respectively, impinging upon the component. The assumption is made that the absorptance of the test sample
is constant within the temperature fluctuations experienced by the component during the measurement.

v
FINAL DRAFT International Standard ISO/FDIS 11551:2026(en)
Optics and photonics — Lasers and laser-related equipment
— Test method for absorptance of optical laser components
WARNING — Laser calorimetric measurements may involve high power lasers, the use of which
may come with significant risks, which may include, but are not limited to; eye injury to people;
laser burns to people or equipment; ignition of materials; generating debris of toxic materials in
the substrate or coating; electrical hazards. It is the responsibility of the user to comply with local
guidelines and regulations for their particular set-up.
1 Scope
This document specifies procedures and techniques for obtaining comparable values for the absorptance of
optical laser components.
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 11145, Optics and photonics — Lasers and laser-related equipment — Vocabulary and symbols
ISO 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by
particle concentration
ISO 80000-7, Quantities and units — Part 7: Light and radiation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11145 and ISO 80000-7 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
absorptance
a
ratio of the radiant flux absorbed to the radiant flux of the incident radiation
Note 1 to entry: The definition of absorptance used for this document is limited to absorptance processes which convert
the absorbed radiant energy to heat. For certain types of optics and radiation, additional non-thermal processes can
result in absorption losses which will not be detected by the test procedure described here (see Annex A).
4 Symbols and units of measure
The symbols and units of measurement used are the following:

Table 1 — Symbols and units of measure
Symbol Term Unit
Thermal capacity of test sample, holder, etc. J/K
Ceff
c Specific heat capacity of test sample J/(kg·K)
p
d , d Beam width on test sample mm
σx σy
m Mass of test sample, holder, etc. kg
i
P cw radiant power W
P Average laser power for continuous pulse mode operation W
av
Typical peak radiant power for repetitive pulse mode oper-
P W
pk
ation
t Duration of irradiation s
B
t Duration of cooling s
C
t Duration of drift s
D
Δt Time interval s
T Ambient temperature K
amb
ΔT Temperature difference K
a Absorptance 1
β Angle of incidence Rad
γ Thermal loss coefficient 1/s
λ Wavelength nm
κ Heat conductivity W/(m·K)
η Mass density kg/m
σ Error sum —
min
Q Heat source W/m
5 Preparation of test sample and measuring arrangement
Storage, cleaning and the preparation of the test samples are carried out in accordance with the
manufacturer’s instructions for normal use.
The environment of the testing place shall be adapted to the application and test wavelength. It should consist
of dust-free filtered air with relative humidity between 40 % and 60 %. The residual dust shall be reduced in
accordance with cleanroom class 7 as defined in ISO 14644-1. However, some specific spectral ranges might
could require nitrogen purged environments (deep UV) or zero humidity (several IR wavelengths). Nitrogen
quality for the deep UV range should be at 99,999 % or higher. If these conditions cannot be supplied,
absorption within the surrounding atmosphere will be included in the test result. An environment free from
draughts is very important in order to keep thermal disturbances and heat loss by convection as small as
possible. Measurements in ambient atmosphere and vacuum can have different influences on the measured
absorptance.
A laser shall be used as the radiation source. To keep errors as low as possible, the laser power chosen
for measurements is as high as possible but without causing any deterioration to the component. At high
irradiance, it shall be ensured that the sample is not damaged. This shall be ensured by the fact that the
measurement shall be reproducible within the specified error limits.
Wavelength, angle of incidence and state of polarization of the laser radiation used for the measurement
shall correspond to the values specified by the manufacturer for the use of the test sample. If also ranges
are accepted for these three quantities, any combination of wavelength, angle of incidence and state of
polarization may be chosen from those ranges. The absorption of an optical component can depend on
further parameters, e.g. irradiance or irradiation dose. In such cases, the measurement sequence should be
chosen individually. For more details, refer to Annex A.

The test sample is mounted in a suitable holder. It is preferable to mount the sample in a manner that
minimizes any thermal contact between the sample and the holder. In this arrangement, the thermal sensor
is attached directly to the sample surface. Reproducible thermal contact between the thermal sensor and the
sample surface is important. Also, care should be taken to maintain constant thermal impedance between
the sample and the holder. Accurate calibration is critically dependent on the location of the thermal sensor,
on the material the sample is made of, and on the sample geometry. Refer to Annex B for a detailed discussion
of these considerations.
It can be difficult to attach the thermal sensor to a small test sample or a sample having an irregular shape.
Such a sample is mounted to the holder in a manner that maximizes thermal contact between the sample
and the holder, while the thermal sensor is attached to or integrated into the holder. Reproducible thermal
contact between the thermal sensor and the holder is important. Also, care should be taken to maintain
constant thermal conductance between the sample and the holder.
In order to increase the precision of the measurements, the sample should be mounted inside a chamber
designed for thermal shielding, with apertures for the laser beam. Special attention shall be given to ensure
that the temperature measurement itself does not cause a change of the sample temperature.
Suitable diaphragms should be placed in the beam path in front of and behind the test sample to ensure
that only the test sample is irradiated by the measuring beam and that reflected or stray radiation will
not strike the holder or the chamber walls. The number of transmissive optics employed for beam guiding
should be minimized in order to reduce possible distortions by multi-reflections or scattered radiation. The
transmitted and reflected partial beams shall be directed on to beam dumps with minimized back scatter.
Figure 1 shows a schematic measuring arrangement. The curved folding mirror M1 is recommended for
imaging the laser output window on to the sample in order to avoid diffracted radiation influencing the
measurement.
Key
1 laser
2 mirror M1
3 optical axis
4 mirror M2
5 test chamber
6 sample holder
7 test sample
8 personal computer
9 beam stop
10 thermal sensor
11 control unit
12 radiant power detector
Figure 1 — Typical arrangement for measurement of the absorptance
6 Characteristic features of the laser radiation
The following physical quantities are needed for characterizing the laser radiation used for the test:
— wavelength, λ;
— angle of incidence, β;
— state and degree of polarization;
— beam widths on the test sample, d , d ;
σx σy
— average radiant power, P , for cw or continuously pulsed lasers;
av
— typical peak radiant power, P , and pulse energy, Q, in the case of pulsed lasers;
pk
— duration of irradiation, t .
B
7 Test procedure
7.1 General
The following auxiliary tests shall be performed on a regular basis and whenever the measuring arrangement
has been altered.
7.2 Calibration
7.2.1 Calibration of the radiant power signal
Calibrate the radiant power signal by placing a calibrated laser power detector at the location of the test
components and, in order to obtain correct calibration, compare the measured laser power to the signal of
the power monitor used during absorptance tests.
7.2.2 Calibration of the temperature signal
Calibrate the temperature signal by fixing a test sample, to which a calibrated thermal sensor is attached, to
the sample holder. Compare the temperature signals of this calibrated sensor and the sensors used during
absorptance tests while varying the ambient temperature slowly over the linearity range of the temperature
detectors at the typical test temperature.
7.2.3 Calibration of the thermal response
For certain types of sample materials and geometries, the temperature rise induced by the absorbed heat
can differ from the theoretical response expected for ideal materials having infinite thermal conductivity.
In these cases, a correction factor, f , shall be determined, which compensates for the influence of such
c
phenomena on the absorptance test result. f is unity if the influence of limited thermal conductivity can
c
be neglected. In order to derive a correct value for f , the heating scheme of the calibration routine needs to
c
be consistent with the heating characteristic of the samples to be tested. Surface absorbers shall be related
to a correction factor derived from a calibration based on surface heating. And a bulk absorber shall be
corrected with a bulk heated calibration sample.
For calibration, a reference sample of known absorptance, which is identical to the samples under
investigation with respect to substrate geometry and thermal diffusivity, is tested for absorptance as in
Annex B. The irradiation time and evaluation method used for calibration shall be the same as for the sample
under test.
Depending on the evaluation method used for the absorptance test, the correction coefficient can be
calculated by substituting the value of the known calibration sample absorptance for a in Formula (2) (see
8.3) or Formula (5) (see 8.4), and solving for f .
c
A known absorptance can be achieved by applying a thin,
...

ISO/TC 172/SC 09 9
Secretariat: DIN
Date: 2026-04-08xx
Optics and photonics — Lasers and laser-related equipment — Test
method for absorptance of optical laser components
Optique et photonique — Lasers et équipements associés aux lasers — Méthode d'essai du facteur d'absorption
des composants optiques pour lasers
FDIS stage
TThhiis drs draafftt i is s susubbmmiitttteed d ttoo aa ppaarraallellel l vvoottee i inn IISSOO,, CCEEN.N.

All rights reserved. Unless otherwise specified, or required in the context of its implementation, 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.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
EmailE-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units of measure . 1
5 Preparation of test sample and measuring arrangement . 2
6 Characteristic features of the laser radiation . 4
7 Test procedure . 5
7.1 General . 5
7.2 Calibration . 5
7.3 Determining the absorptance . 6
8 Evaluation . 6
8.1 General . 6
8.2 Elimination of drift . 7
8.3 Exponential method . 7
8.4 Pulse method . 8
9 Test report . 9
Annex A (informative) Effects changing absorptance . 11
Annex B (informative) Influence of signal distortions . 14
Annex C (informative) Algorithm for parameterizing the temperature data . 17
Bibliography . 18

iii
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, 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 www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee 172, Optics and Photonics, Subcommittee SC 09 9, Laser
and electro-optical systems, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 123, Lasers and Photonics, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
This fourth edition cancels and replaces the third edition (ISO 11551:2019), which has been technically
revised.
The main changes are as follows:
— — harmonization of terms and environmental conditions to current laser measurement standards;
— — minor adjustments of formulae and figures;
— — modified text and additional figures in A.1A.1 and A.3A.3.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
To characterize an optical component, it is important to know its absorptance. When radiation impinges upon
a component, a part of that radiation is absorbed, increasing the temperature of the component. In this
document only the part of the absorbed radiant power/energy, that is converted into heat, is measured. If
enough radiant energy is absorbed, the optical properties of the component can change, and the component
can even be destroyed. Absorptance is the ratio of the radiant flux absorbed to the radiant flux of the incident
radiation.
In the procedures described in this document, the absorptance is determined calorimetrically as the ratio of
radiant power or radiant energy absorbed by the component to the total radiant power or radiant energy,
respectively, impinging upon the component. The assumption is made that the absorptance of the test sample
is constant within the temperature fluctuations experienced by the component during the measurement.
v
Optics and photonics — Lasers and laser-related equipment — Test
method for absorptance of optical laser components
WARNING — Laser calorimetric measurements may involve high power lasers, the use of which may
come with significant risks, which may include, but are not limited to; eye injury to people; laser burns
to people or equipment; ignition of materials; generating debris of toxic materials in the substrate or
coating; electrical hazards. It is the responsibility of the user to comply with local guidelines and
regulations for their particular set-up.
1 Scope
This document specifies procedures and techniques for obtaining comparable values for the absorptance of
optical laser components.
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 11145, Optics and photonics — Lasers and laser-related equipment — Vocabulary and symbols
ISO 14644--1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by
particle concentration
ISO 80000--7, Quantities and units — Part 7: Light and radiation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11145 and ISO 80000-7 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
3.1 3.1
absorptance
a
ratio of the radiant flux absorbed to the radiant flux of the incident radiation
Note 1 to entry: The definition of absorptance used for this document is limited to absorptance processes which convert
the absorbed radiant energy to heat. For certain types of optics and radiation, additional non-thermal processes can
result in absorption losses which will not be detected by the test procedure described here (see Annex AAnnex A).).
4 Symbols and units of measure
The symbols and units of measurement used are the following:
Table 1 — Symbols and units of measure
Symbol Term Unit
Ceff Thermal capacity of test sample, holder, etc. J/K
cp Specific heat capacity of test sample J/(kg·K)
d , d Beam width on test sample mm
σx σy
mi Mass of test sample, holder, etc. kg
P cw radiant power W
Pav Average laser power for continuous pulse mode operation W
Typical peak radiant power for repetitive pulse mode
P W
pk
operation
tB Duration of irradiation s
tC Duration of cooling s
t Duration of drift s
D
Δt Time interval s
T Ambient temperature K
amb
ΔT Temperature difference K
a Absorptance 1
β Angle of incidence Rad
γ Thermal loss coefficient 1/s
λ Wavelength nm
κ Heat conductivity W/(m·K)
η Mass density kg/m
σ Error sum -—
min
Q Heat source W/m
5 Preparation of test sample and measuring arrangement
Storage, cleaning and the preparation of the test samples are carried out in accordance with the
manufacturer’s instructions for normal use.
The environment of the testing place shall be adapted to the application and test wavelength. It should consist
of dust-free filtered air with relative humidity between 40 % and 60 %. The residual dust shall be reduced in
accordance with cleanroom class 7 as defined in ISO 14644-1. However, some specific spectral ranges might
could require nitrogen purged environments (deep UV) or zero humidity (several IR wavelengths). Nitrogen
quality for the deep UV range should be at 99,999 % or higher. If these conditions cannot be supplied,
absorption within the surrounding atmosphere will be included in the test result. An environment free from
draughts is very important in order to keep thermal disturbances and heat loss by convection as small as
possible. Measurements in ambient atmosphere and vacuum can have different influences on the measured
absorptance.
A laser shall be used as the radiation source. To keep errors as low as possible, the laser power chosen for
measurements is as high as possible but without causing any deterioration to the component. At high
irradiance, it shall be ensured that the sample is not damaged. This shall be ensured by the fact that the
measurement shall be reproducible within the specified error limits.
Wavelength, angle of incidence and state of polarization of the laser radiation used for the measurement shall
correspond to the values specified by the manufacturer for the use of the test sample. If also ranges are
accepted for these three quantities, any combination of wavelength, angle of incidence and state of
polarization may be chosen from those ranges. The absorption of an optical component can depend on further
parameters, e.g. irradiance or irradiation dose. In such cases, the measurement sequence should be chosen
individually. For more details, refer to Annex AAnnex A.
The test sample is mounted in a suitable holder. It is preferable to mount the sample in a manner that
minimizes any thermal contact between the sample and the holder. In this arrangement, the thermal sensor is
attached directly to the sample surface. Reproducible thermal contact between the thermal sensor and the
sample surface is important. Also, care should be taken to maintain constant thermal impedance between the
sample and the holder. Accurate calibration is critically dependent on the location of the thermal sensor, on
the material the sample is made of, and on the sample geometry. Refer to Annex BAnnex B for a detailed
discussion of these considerations.
It can be difficult to attach the thermal sensor to a small test sample or a sample having an irregular shape.
Such a sample is mounted to the holder in a manner that maximizes thermal contact between the sample and
the holder, while the thermal sensor is attached to or integrated into the holder. Reproducible thermal contact
between the thermal sensor and the holder is important. Also, care should be taken to maintain constant
thermal conductance between the sample and the holder.
In order to increase the precision of the measurements, the sample should be mounted inside a chamber
designed for thermal shielding, with apertures for the laser beam. Special attention shall be given to ensure
that the temperature measurement itself does not cause a change of the sample temperature.
Suitable diaphragms should be placed in the beam path in front of and behind the test sample to ensure that
only the test sample is irradiated by the measuring beam and that reflected or stray radiation will not strike
the holder or the chamber walls. The number of transmissive optics employed for beam guiding should be
minimized in order to reduce possible distortions by multi-reflections or scattered radiation. The transmitted
and reflected partial beams shall be directed on to beam dumps with minimized back scatter.
Figure 1Figure 1 shows a schematic measuring arrangement. The curved folding mirror M1 is recommended
for imaging the laser output window on to the sample in order to avoid diffracted radiation influencing the
measurement.
11551_ed4fig1.EPS
Key
1 laser
2 mirror M1
3 optical axis
4 mirror M2
5 test chamber
6 sample holder
7 test sample
8 personal computer
9 beam stop
10 thermal sensor
11 control unit
12 radiant power detector
Figure 1 — Typical arrangement for measurement of the absorptance
6 Characteristic features of the laser radiation
The following physical quantities are needed for characterizing the laser radiation used for the test:
— — wavelength, λ;
— — angle of incidence, β;
— — state and degree of polarization;
— — beam widths on the test sample, d , d ;
σx σy
— — average radiant power, P , for cw or continuously pulsed lasers;
av
— — typical peak radiant power, P , and pulse energy, Q, in the case of pulsed lasers;
pk
— — duration of irradiation, t .
B
7 Test procedure
7.1 General
The following auxiliary tests shall be performed on a regular basis and whenever the measuring arrangement
has been altered.
7.2 Calibration
7.2.1 Calibration of the radiant power signal
Calibrate the radiant power signal by placing a calibrated laser power detector at the location of the test
components and, in order to obtain correct calibration, compare the measured laser power to the signal of the
power monitor used during absorptance tests.
7.2.2 Calibration of the temperature signal
Calibrate the temperature signal by fixing a test sample, to which a calibrated thermal sensor is attached, to
the sample holder. Compare the temperature signals of this calibrated sensor and the sensors used during
absorptance tests while varying the ambient temperature slowly over the linearity range of the temperature
detectors at the typical test temperature.
7.2.3 Calibration of the thermal response
For certain types of sample materials and geometries, the temperature rise induced by the absorbed heat can
differ from the theoretical response expected for ideal materials having infinite thermal conductivity. In these
cases, a correction factor, f , shall be determined, which compensates for the influence of such phenomena on
c
the absorptance test result. f is unity if the influence of limited thermal conductivity can be neglected. In order
c
to derive a correct value for f , the heating scheme of the calibration routine needs to be consistent with the
c
heating characteristic of the samples to be tested. Surface absorbers shall be related to a correction factor
derived from a calibration based on surface heating. And a bulk absorber shall be corrected with a bulk heated
calibration sample.
For calibration, a reference sample of known absorptance, which is identical to the samples under
investigation with respect to substrate geometry and thermal diffusivity, is tested for absorptance as in
Annex BAnnex B. The irradiation time and evaluation method used for calibration shall be the same as for the
sample under test.
Depending on the evaluation method used for the absorptance test, the correction coefficient can be calculated
by substituting the value of the known calibration sample absorptance for a in Formula (2)Formula (2) (see
8.38.3)) or Formula (5)Formula (5) (see 8.48.4),), and solving for f .
c
A known absorptance can be achieved by applying a thin, high-absorbing coating to the sample surface area
that is exposed to irradiation. High absorptance values can be determined with sufficient accuracy, e.g. by
measuring the fraction of transmitted, reflected and scattered radiation. For absorptance testing of samples
with high absorptance values, the laser power should be suitably attenuated in order to avoid damage to the
samples and to ensure that the resulting temperature rise is in the same order of magnitude as the
temperature which is achieved for typical measurements. This procedure applies only for samples of high
surface absorption, where bulk absorption can be neglected.
As an alternative to irradiating a calibration sample of known absorptance with the laser beam, the thermal
energy may be deposited electrically in the test sample by attaching an electric resistor to the tested surface.
The absorbed electrical power is given by RI , where R is the electrical resistance and I is the electric current
during “irradiation”. Care should be taken to ensure good thermal contact between resistor and sample.
Furthermore, especially in the case of samples with low thermal conductivity, the area of the resistor should
match the area irradiated by the laser beam under normal test conditions. This procedure can in principle be
applied to both, surface and bulk absorbing samples. Care should be taken to ensure that the heating scheme
of the calibration sample is close or identical to the expected heating scheme of the test samples.
7.2.4 Measurement of the background signal
For maximum accuracy and suppression of possible signal distortions, the imaging and alignment of the laser
beam shall be optimized. A measurement with an empty holder or with an absorptance-free component can
be used to verify that the measuring arrangement is not influenced by reflected or stray radiation. The
amplitude of the temperature fluctuations during the test interval shall be at least one order of magnitude
below t
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