ISO 20351:2024

International
Standard
ISO 20351
Second edition
Fine ceramics (advanced ceramics,
2024-10
advanced technical ceramics) —
Test method for optical properties
of ceramic phosphors for white
light-emitting diodes using an
integrating sphere
Reference number
© ISO 2024
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Published in Switzerland
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measurement apparatus . 2
4.1 Apparatus configuration .2
4.2 Light source unit .4
4.3 Sample unit .4
4.3.1 Cell .4
4.3.2 White diffuser or reference cell .4
4.3.3 Integrating sphere .4
4.4 Detection unit .5
4.4.1 Directing optical system .5
4.4.2 Spectrometer and detector .5
4.4.3 Amplifier .5
4.5 Signal and data processing unit .5
5 Calibration, inspection and maintenance of measurement apparatus . 5
5.1 General .5
5.2 Wavelength calibration of light source unit .5
5.3 Cells and cover glasses .5
5.4 Integrating sphere walls and white diffusers .5
5.5 Wavelength calibration of detection unit .5
5.6 Spectral responsivity correction .6
6 Samples . 6
6.1 Reference material .6
6.2 Storage and pre-processing .6
6.3 Filling cells with samples .6
7 Procedure, calculation and report for absolute measurement . 6
7.1 Measurement method .6
7.1.1 Measurement environment .6
7.1.2 Light spectrum without phosphor sample .7
7.1.3 Light spectrum with phosphor sample .7
7.2 Calculations .7
7.2.1 Conversion to photon-number-based spectra .7
7.2.2 Fluorescence spectrum .8
7.2.3 Internal quantum efficiency .8
7.3 Test report .8
8 Procedure, calculation and report for substitution measurement . 9
8.1 Measurement procedures .9
8.1.1 Measurement environment .9
8.1.2 Spectrometer setup for substitution measurement .9
8.1.3 Measurement for reference material .10
8.1.4 Measurement for phosphor material under test .10
8.2 Calculation .10
8.2.1 Spectral responsivity correction .10
8.2.2 Conversion to photon number-based spectral distribution .10
8.2.3 Calculation of scattered light and fluorescence photon numbers .11
8.2.4 External quantum efficiency . 12
8.2.5 Absorptance . 12
8.3 Test report . 12

iii
Bibliography . 14

iv
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
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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 document 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
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This document was prepared by Technical Committee ISO/TC 206, Fine ceramics.
This second edition cancels and replaces the first edition (ISO 20351:2017), which has been technically
revised.
The main changes are as follows:
— scope of the document is expanded by additionally implementing the reference material-based
substitution measurement method to measure external quantum efficiency and absorptance.
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.

v
Introduction
White light-emitting diode (LED) based solid-state lighting (SSL) has been widely used for a variety of
applications as alternatives for incandescent and fluorescent lamps. Initially, white LEDs (comprising blue
LEDs and yellow phosphors) became popular as backlight sources for small-size liquid-crystal displays
(LCDs) used in mobile phones and digital cameras. These were followed by white LEDs (consisting of
blue LEDs combined with green and red phosphors) applied to backlight sources for large-area LCDs.
Subsequently, LED lamps were commercialised for general lighting, replacing conventional luminaires and
capitalising on their advantages, such as compactness, high luminous efficiency, high brightness below 0 °C
or higher ambient temperatures, long life, and controllability of light intensity and colour temperature.
The previous edition of this document (ISO 20351:2017) was developed based on the need for standardizing
a test method for internal quantum efficiency of phosphors using an integrating sphere. This standard test
method has the advantage of a short measurement time and being available to those with no expertise in
precise optical measurement. The external quantum efficiency and absorptance were out of the scope of
ISO 20351:2017 due to an insufficient understanding of the source of variation in these measurement values.
ISO 23946 was then developed to provide alternative absolute measurement methods for the external
quantum efficiency, internal quantum efficiency and absorptance using a gonio-spectrofluorometer.
The application of ISO 23946 is assumed to be limited to those who intend to determine the optical
properties of phosphor materials to be utilized as reference materials due to its complicated equipment
and time-consuming procedure. ISO 13915 was developed following ISO 23946 and provides substitution
measurement methods comparing with reference materials with values evaluated in accordance with
ISO 23946 to provide external quantum efficiency, internal quantum efficiency and absorptance by using a
commercially available fluorescence spectrophotometer.
It was found that the external quantum efficiency and absorptance of a phosphor material under test can
also be appropriately obtained using and integrating sphere-based spectrometer with the substitution
measurement method as described in ISO 13915, where the measurement using and integrating sphere-
based equipment is excluded.
Therefore, the revised second edition of this document is intended to expand the scope by additionally
implementing the substitution measurement method for obtaining external quantum efficiency and
absorptance using an integrating sphere to the existing absolute method to obtain internal quantum
efficiency. This expanded scope will benefit those who routinely use integrating sphere-based equipment.
In this document, measurement conditions and procedures that can affect the measurement values are
described in detail, helping those who address high-performance phosphors for competitive SSL products to
obtain appropriate information on their competitiveness.
This document can also be adopted for phosphors used in non-white LEDs, e.g. green, orange, pink and purple.

vi
International Standard ISO 20351:2024(en)
Fine ceramics (advanced ceramics, advanced technical
ceramics) — Test method for optical properties of ceramic
phosphors for white light-emitting diodes using an
integrating sphere
1 Scope
This document specifies test methods for the use of an integrating sphere to measure the optical properties
of ceramic phosphor powders, which are used in white light-emitting diodes (LEDs) and emit visible
light when excited by UV or blue light. This document specifies an absolute method to measure internal
quantum efficiency and a substitution method to measure external quantum efficiency and absorptance.
The substitution method uses reference materials whose external quantum efficiency and absorptance have
been obtained in accordance with ISO 23946.
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 23946, Fine ceramics (advanced ceramics, advanced technical ceramics) — Test methods for optical
properties of ceramic phosphors for white light-emitting diodes using a gonio-spectrofluorometer
CIE S 017/E:2020, ILV: International Lighting Vocabulary, 2nd edition
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 23946 and CIE S 017/E 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
internal quantum efficiency
ratio of the number of photons emitted in free space from the phosphor to the number of excitation light
photons absorbed by the phosphor
3.2
external quantum efficiency
ratio of the number of photons emitted in free space from the phosphor to the number of excitation light
photons incident on the phosphor
3.3
absorptance
ratio of the number of excitation light photons absorbed by the phosphor to the number of excitation light
photons incident on the phosphor

3.4
cell
container filled with a sample or a white material such as barium sulfate
Note 1 to entry: A cell is typically a flat plate sample holder with a cylindrical hollow, a petri dish or a rectangular cell
used in a spectrophotometer.
3.5
reference cell
cell filled with a white powder which has a high spectral diffuse reflectance over the whole visible spectrum
(such as barium sulfate or alumina), used when measuring the excitation light spectrum
Note 1 to entry: This term is only for use in the absolute method.
3.6
white diffuser
white plate which has a high spectral diffuse reflectance over the whole visible spectrum (such as barium
sulfate or polytetrafluoroethylene (PTFE)), used when measuring the excitation light spectrum
Note 1 to entry: This term is only for use in the absolute method.
3.7
secondary absorption
absorption of indirect incident light from every direction of the sphere wall by the phosphor sample
Note 1 to entry: The excitation light illuminating the sample is not entirely absorbed by the sample but is partially
scattered or reflected and then repeatedly reflected on the sphere wall. Some of the scattered/reflected light can
illuminate the sample again and be absorbed.
3.8
self absorption
absorption of photoluminescent photons emitted by the sample itself
4 Measurement apparatus
4.1 Apparatus configuration
The apparatus includes a light source unit, a sample unit, a detection unit and a signal and data processing
unit. Figure 1 and Figure 2 illustrate the typical configurations of a measurement apparatus.
The light source unit generates monochromatic excitation light and comprises a white light source, a power
supply for the light source, a focusing optical system, a wavelength selection unit (monochromator for the
white light source) and an optical system for irradiation. A collimated laser beam can also be used as the
monochromatic light source.
The sample unit comprises a cell and an integrating sphere.
The detecting unit comprises directing optics for collecting light, a spectrometer, a detector and an amplifier.
A fluorescence spectrophotometer equipped with a sample unit (including an integrating sphere), and
equipment combining a light source unit and an array spectrometer together with the sample unit, are
typical examples.
Key
A light source unit 1 light source
B sample unit 2 excitation monochromator
C detecting unit 3 integrating sphere
4 cell (sample)
5 emission monochromator
6 detector
Figure 1 — Example configuration of measuring equipment (fluorescence spectrophotometer type)
Key
A light source unit 1 light source
B sample unit 2 monochromator
C detecting unit 3 optical fibre
4 integrating sphere
5 cell (sample)
6 optical fibre
7 array spectrometer
Figure 2 — Example configuration of measuring equipment (array spectrometer type)

4.2 Light source unit
The spectral width of the excitation light is limited by the monochromator. The half-width of the excitation
light spectrum should be 15 nm or less.
The generated excitation light is introduced into the integrating sphere via the excitation light
...