ISO 13320:2020

Particle size analysis - Laser diffraction methods

This document provides guidance on instrument qualification and size distribution measurement of particles in many two-phase systems (e.g. powders, sprays, aerosols, suspensions, emulsions and gas bubbles in liquids) through the analysis of their light-scattering properties. It does not address the specific requirements of particle size measurement of specific materials. This document is applicable to particle sizes ranging from approximately 0,1 µm to 3 mm. With special instrumentation and conditions, the applicable size range can be extended above 3 mm and below 0,1 µm. For spherical and non-spherical particles, a size distribution is reported, where the predicted scattering pattern for the volumetric sum of spherical particles matches the measured scattering pattern. This is because the technique assumes a spherical particle shape in its optical model. For non-spherical particles the resulting particle size distribution is different from that obtained by methods based on other physical principles (e.g. sedimentation, sieving).

Analyse granulométrique — Méthodes par diffraction laser

General Information

• Status: Published
• Publication Date: 05-Jan-2020
• ICS: 19.120 - Particle size analysis. Sieving
• Technical Committee: ISO/TC 24/SC 4 - Particle characterization
• Drafting Committee: ISO/TC 24/SC 4/WG 6 - Laser diffraction methods
• Current Stage: 9093 - International Standard confirmed
• Start Date: 17-Jun-2025
• Completion Date: 13-Dec-2025

Relations

• Revises: ISO 13320:2009 - Particle size analysis - Laser diffraction methods
• Effective Date: 05-Nov-2015

Overview

ISO 13320:2020 - Particle size analysis - Laser diffraction methods - is the international guidance document for measuring particle size distributions (PSDs) using laser diffraction. It covers instrument qualification, measurement procedures, data conversion and reporting for many two‑phase systems (powders, sprays, aerosols, suspensions, emulsions and gas bubbles in liquids). The standard applies typically to particle sizes from approximately 0.1 µm to 3 mm, with extensions possible using special instrumentation and conditions.

Key technical topics and requirements

ISO 13320:2020 provides practical, technical guidance rather than prescriptive material‑specific limits. Major topics include:

• Principle and theory of laser diffraction and optical arrangements, including detector and scattering considerations.
• Instrument qualification: protocols for evaluating accuracy, repeatability and intermediate precision.
• Operational requirements: instrument location, sample inspection, preparation, dispersion (liquids and gases) and concentration control.
• Measurement procedures: blank measurements, data collection of scattering patterns, selection of optical models, and conversion of scattering data into PSDs.
• Measurement modes: off-line, in-line, online and at-line applications and their specific considerations.
• Reporting: standardized reporting of results, sample and dispersion details, and analyst identification.
• Annexed guidance: theoretical background (Annex A), dispersion liquids and methods (Annexes B–C), instrument preparation and error diagnosis (Annexes D–E), refractive index recommendations (Annex F), robustness/ruggedness (Annex G), and use of certified reference materials (Annex H).

Important technical note: the optical model assumes spherical particles, so for non‑spherical particles the reported PSD reflects the equivalent spherical volume distribution and can differ from results obtained by sedimentation or sieving.

Practical applications

Laser diffraction per ISO 13320:2020 is widely used for:

• Quality control and batch release in pharmaceuticals, chemicals, pigments and coatings
• R&D and formulation development in food, cosmetics and personal care products
• Environmental and aerosol monitoring (airborne particles)
• Process monitoring and optimization in powder processing, sprays and suspensions
• Materials science characterization where rapid, automated PSDs are required

The standard supports reliable comparison of results by defining instrument qualification, method repeatability and reporting expectations.

Who should use this standard

• Laboratory analysts and quality control teams performing PSD measurements
• Instrument manufacturers and service engineers developing or validating laser diffraction systems
• R&D scientists developing particulate formulations or processes
• Regulatory and compliance personnel seeking standardized measurement evidence

Related standards and keywords

Related ISO documents: ISO 9276‑1 and ISO 9276‑2 (representation and calculation of PSD results).
SEO keywords: ISO 13320:2020, particle size analysis, laser diffraction methods, particle size distribution (PSD), instrument qualification, laser diffraction standard, particle sizing, PSD measurement.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 13320
Second edition
2020-01
Particle size analysis — Laser
diffraction methods
Analyse granulométrique — Méthodes par diffraction laser
Reference number
©
ISO 2020
© ISO 2020
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: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols . 6
4 Principle . 8
4.1 General . 8
4.2 Theory . 8
4.3 Typical instrument and optical arrangement. 9
4.4 Measurement zone .11
4.5 Application and sample presentation .11
4.6 Off-line measurements .12
4.7 In-line measurements .12
4.8 Online measurements .12
4.9 At-line measurements .13
4.10 Scattering and detectors .13
5 Operational requirements and procedures .13
5.1 Instrument location .13
5.2 Dispersion gases .13
5.3 Dispersion liquids .14
5.4 Sample inspection, preparation, dispersion and concentration .14
5.4.1 Sample inspection .14
5.4.2 Preparation .14
5.4.3 Dispersion .14
5.4.4 Concentration .15
5.5 Measurement .15
5.5.1 Setting up instrument and blank measurement .15
5.5.2 Sample preparation .16
5.5.3 Data collection of the scattering pattern .16
5.5.4 Selection of an appropriate optical model .16
5.5.5 Conversion of scattering pattern into PSD .16
5.5.6 Robustness .17
5.6 Resolution and sensitivity .17
5.6.1 General.17
5.6.2 Resolution .17
5.6.3 Sensitivity and result variability .17
6 Accuracy repeatability and instrument qualification.18
6.1 General .18
6.2 Accuracy .19
6.2.1 Introduction .19
6.2.2 Accuracy test .19
6.3 Instrument repeatability .19
6.3.1 Introduction .19
6.3.2 Repeatability test .19
6.4 Method repeatability .20
6.4.1 Introduction .20
6.4.2 Method repeatability test .20
6.5 Accuracy under intermediate precision conditions .20
6.5.1 General.20
6.5.2 Intermediate precision conditions (general test) .21
7 Reporting of results .21
7.1 General .21
7.2 Sample .21
7.3 Dispersion .22
7.4 Laser diffraction measurement .22
7.5 Analyst identification: .22
Annex A (informative) Theoretical background of laser diffraction .24
Annex B (informative) Advice on dispersion liquids .41
Annex C (informative) Dispersion methods — Recommendations .42
Annex D (informative) Instrument preparation — Recommendations .44
Annex E (informative) Error sources and diagnosis .46
Annex F (informative) Refractive index — Recommendations .49
Annex G (informative) Laser diffraction robustness and ruggedness .51
Annex H (normative) Certified reference materials, reference materials and comparison
parameters .54
Bibliography .57
iv © ISO 2020 – 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 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 ISO/TC 24, Particle characterization including
sieving, Subcommittee SC 4, Particle characterization.
This second edition cancels and replaces the first edition (ISO 13320:2009), which has been technically
revised. The main changes compared to the previous edition are as follows:
a) protocols for evaluation of accuracy and qualification of instrument were newly developed;
b) new Annex H (normative) for usage of reference material has been added;
c) new descriptions for wider applications, such as off-line, online, in-line and at-line have been added;
d) some informative parts have been moved to new annexes;
e) minor revisions and updates have been made throughout the document.
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.
Introduction
The laser diffraction technique has evolved such that it is now a dominant method for determination
of particle size distributions (PSDs). The success of the technique is based on the fact that it can be
applied to a wide variety of particulate systems. The technique is fast and can be automated, and a
variety of commercial instruments is available. Nevertheless, the proper use of the instrument and the
interpretation of the results require the necessary caution.
Since ISO 13320-1:1999 was first published, the understanding of light scattering by different materials
and the design of instruments have advanced considerably. This is especially marked in the ability
to measure very fine particles. Therefore, it was replaced with the first edition of ISO 13320 in 2009,
and since then the method has been developed for a wider application. Additionally, demands raised
recently not only on establishment of accuracy of measurements but also on necessity of evaluation
of the accuracy and of qualification of instrument by users. Therefore, this document incorporates the
most recent advances in understanding.
vi © ISO 2020 – All rights reserved

INTERNATIONAL STANDARD ISO 13320:2020(E)
Particle size analysis — Laser diffraction methods
1 Scope
This document provides guidance on instrument qualification and size distribution measurement of
particles in many two-phase systems (e.g. powders, sprays, aerosols, suspensions, emulsions and gas
bubbles in liquids) through the analysis of their light-scattering properties. It does not address the
specific requirements of particle size measurement of specific materials.
This document is applicable to particle sizes ranging from approximately 0,1 µm to 3 mm. With special
instrumentation and conditions, the applicable size range can be extended above 3 mm and below 0,1 µm.
For spherical and non-spherical particles, a size distribution is reported, where the predicted scattering
pattern for the volumetric sum of spherical particles matches the measured scattering pattern. This
is because the technique assumes a spherical particle shape in its optical model. For non-spherical
particles the resulting particle size distribution is different from that obtained by methods based on
other physical principles (e.g. sedimentation, sieving).
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 9276-1, Representation of results of particle size analysis — Part 1: Graphical representation
ISO 9276-2, Representation of results of particle size analysis — Part 2: Calculation of average particle
sizes/diameters and moments from particle size distributions
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1.1
absorption
reduction of intensity of a light beam not due to scattering
3.1.2
accuracy
closeness of agreement between a test result or measurement result and the true value
Note 1 to entry: In practice, the accepted reference value is substituted for the true value.
Note 2 to entry: The term “accuracy”, when applied to a set of test or measurement results, involves a combination
of random components and a common systematic error or bias component.
Note 3 to entry: Accuracy refers to a combination of trueness and precision.
[SOURCE: ISO 3534-2:2006, 3.3.1]
3.1.3
aspect ratio
ratio of the minimum to the maximum Feret diameter
Note 1 to entry: For not very elongated particles.
[SOURCE: ISO 26824:2013, 4.5]
3.1.4
certified reference material
CRM
reference material (3.1.16) characterised by a metrologically valid procedure for one or more specified
properties, accompanied by an RM certificate that provides the value of the specified property, its
associated uncertainty, and a statement of metrological traceability
Note 1 to entry: The concept of value includes a nominal property or a qualitative attribute such as identity or
sequence. Uncertainties for such attributes may be expressed as probabilities or levels of confidence.
Note 2 to entry: Metrologically valid procedures for the production and certification of RMs are given in, among
others, ISO 17034 and ISO Guide 35.
Note 3 to entry: ISO Guide 31 gives guidance on the contents of RM certificates.
Note 4 to entry: ISO/IEC Guide 99:2007, 5.14 has an analogous definition.
[SOURCE: ISO Guide 35:2017, 3.2]
3.1.5
complex refractive index
n
p
refractive index of a particle, consisting of a real and an imaginary (absorption) part
Note 1 to entry: The complex refractive index of a particle can be expressed mathematically as
n = n − ik
p p p
where
i is the square root of −1;
k is the positive imaginary (absorption) part of the refractive index of a particle;
p
n is the positive real part of the refractive index of a particle.
p
Note 2 to entry: In contrast to ISO 80000-7, this document follows the convention of adding a minus sign to the
imaginary part of the refractive index.
3.1.6
deconvolution
mathematical procedure whereby the size distribution of an ensemble of particles is
inferred from measurements of their scattering pattern
3.1.7
diffraction
scattering of light around the contour of a particle, observed at a substantial
distance (in the ‘far field’)
2 © ISO 2020 – All rights reserved

3.1.8
equivalent spherical diameter
particle size reported from a distribution of spherical particles that creates a
scattering pattern that matches the light scattering distribution observed from the measurement
Note 1 to entry: The scattering pattern of the spherical particles is calculated according to an optical model.
3.1.9
extinction
attenuation of a light beam traversing a medium through absorption and
scattering
3.1.10
intermediate precision
accuracy and precision under intermediate precision conditions (3.1.11)
[SOURCE: ISO 3534-2:2006, 3.3.15, modified — field of application has been added.]
3.1.11
intermediate precision conditions
conditions where test results or measurement results are obtained on different
laser diffraction instruments and with different operators using the same prescribed method
Note 1 to entry: There are four elements to the operating condition: time, calibration, operator and equipment.
3.1.12
multiple scattering
consecutive scattering of light by more than one particle, causing a scattering pattern that is no longer
the sum of the patterns from all individual particles
3.1.13
obscuration
fraction of incident light that is attenuated due to extinction (scattering and/or absorption) by particles
Note 1 to entry: Obscuration can be expressed as a percentage.
Note 2 to entry: When expressed as fractions, obscuration plus transmission (3.1.29) equal unity.
[SOURCE: ISO 8130-13:2019, 3.1, modified — words “percentage” and “during a laser diffraction
measurement” have been omitted because of context.]
3.1.14
optical model
theoretical model used for computing the model matrix for optically homogeneous and isotropic
spheres with, if necessary, a specified complex refractive index
EXAMPLE Fraunhofer diffraction model, Mie scattering model.
3.1.15
precision
closeness of agreement between independent test/measurement results obtained under stipulated
conditions
Note 1 to entry: Precision depends only on the distribution of random errors and does not relate to the true value
or the specified value.
Note 2 to entry: The measure of precision is usually expressed in terms of imprecision and computed as a
standard deviation of the test results or measurement results. Less precision is reflected by a larger standard
deviation.
Note 3 to entry: Quantitative measures of precision depend critically on the stipulated conditions. Repeatability
conditions and reproducibility conditions are particular sets of extreme stipulated conditions.
[SOURCE: ISO 3534-2:2006, 3.3.4]
3.1.16
reference material
RM
material, sufficiently homogeneous and stable with respect to one or more specified properties, which
has been established to be fit for its intended use in a measurement process
Note 1 to entry: RM is a generic term.
Note 2 to entry: Properties can be quantitative or qualitative, e.g. identity of substances or species.
Note 3 to entry: Uses may include the calibration of a measurement system, assessment of a measurement
procedure, assigning values to other materials, and quality control.
Note 4 to entry: ISO/IEC Guide 99:2007 has an analogous definition but restricts the term “measurement” to
apply to quantitative values. However, ISO/IEC Guide 99:2007, 5.13, Note 3 (VIM), specifically includes qualitative
properties, called “nominal properties”.
[SOURCE: ISO Guide 35:2017, 3.1]
3.1.17
reflection
change of direction of a light wave at a surface without a change in wavelength
or frequency
3.1.18
refraction
process by which the direction of a radiation is changed as a result of changes in its velocity of
propagation in passing through an optically non-homogeneous medium, or in crossing a surface
separating different media
Note 1 to entry: The process occurs in accordance with Snell's law:
n sinθ = n sinθ
m m p p
See 3.2 for symbol definitions.
3.1.19
relative refractive index
m
rel
ratio of the complex refractive index of a particle to the real part of the dispersion medium
[SOURCE: ISO 24235:2007, 3.3, modified — “absolute refractive index” has been replaced by “complex
refractive index” and “the sample” has been replaced by “a particle”.]
Note 1 to entry: In many applications, the medium is transparent and, thus, its refractive index has a negligible
imaginary part.
Note 2 to entry: The relative refractive index can be expressed mathematically as
m = n /n
rel p m
where
n is the real part of the refractive index of the medium;
m
n is the complex refractive index of a particle.
p
See single scattering (3.1.26).
4 © ISO 2020 – All rights reserved

3.1.20
repeatability
precision under repeatability conditions (3.1.21)
Note 1 to entry: Repeatability can be expressed quantitatively in terms of the dispersion characteristics of the
results.
[SOURCE: ISO 3534-2:2006, 3.3.5]
3.1.21
repeatability conditions
observation conditions where independent test/measurement results are obtained with the same
method on identical test/measurement items in the same test or measuring facility by the same
operator using the same equipment within short intervals of time
Note 1 to entry: Repeatability conditions include:
— the same measurement procedure or test procedure;
— the same operator;
— the same measuring or test equipment used under
— the same conditions;
— the same location;
— repetition over a short period of time.
[SOURCE: ISO 3534-2:2006, 3.3.6]
3.1.22
method repeatability
closeness of agreement between multiple measurement results of a given property in different aliquots
of a sample, executed by the same operator using the same instrument under identical conditions
within a short period of time
Note 1 to entry: The variability includes the variabilities of sub sampling technique, of the sampled material
together and of the instrument.
3.1.23
scattering
change in propagation of light at the interface of two media having different optical properties
3.1.24
scattering angle
angle between the principal axis of the incident light beam and the scattered light
3.1.25
scattering pattern
angular pattern of light intensity, I(θ), or spatial pattern of light intensity, I(r), originating from
scattering, or the related energy values taking into account the sensitivity and the geometry of the
detector elements
3.1.26
single scattering
scattering whereby the contribution of a single member of a particle population to the total scattering
pattern remains independent of the other members of the population
3.1.27
single shot
for an analysis, for which the entire content of a test sample container is used
3.1.28
test sample
sample that is entirely used for a property characterization
[SOURCE: ISO 14488:2007, 3.12]
3.1.29
transmission
fraction of incident light that remains un-attenuated by the particles
Note 1 to entry: Transmission can be expressed as a percentage.
Note 2 to entry: When expressed as fractions, obscuration (3.1.13) plus transmission equal unity.
3.1.30
true value
quantity or quantitative characteristic supposed to be “true” as the target value of the measurement
according to the definition of the measurement
Note 1 to entry: The true value is a theoretical concept and, in general, cannot be known exactly.
Note 2 to entry: For an explanation of the term “quantity”, refer to ISO 3534-2:2006.
3.1.31
trueness
closeness of agreement between the expectation of a test result or a measurement result and a true value
Note 1 to entry: The measure of trueness is usually expressed in terms of bias
Note 2 to entry: Trueness is sometimes referred to as “accuracy of the mean”. This usage is not recommended.
Note 3 to entry: In practice, the accepted reference value is substituted for the true value.
[SOURCE: ISO 3534-2:2006, 3.3.3]
3.2 Symbols
A extinction efficiency of size class i
i
C particulate concentration, volume fraction
CF coverage factor
D particle diameter (x may also be used)
D particle diameter corresponding to the 10th percentile of the cumulative undersize
10,3
distribution (here by volume)
D median particle diameter corresponding to the 50th percentile of the cumulative
50,3
undersize distribution (here by volume)
D particle diameter corresponding to the 90th percentile of the cumulative undersize
90,3
distribution (here by volume)
I(θ) angular intensity distribution of light scattered by particles (scattering pattern)
I intensity of horizontally polarized light at a given angle
h
I(r) spatial intensity distribution of light scattered by particles on the detector
elements (measured scattering pattern by detector)
6 © ISO 2020 – All rights reserved

I intensity of vertically polarized light at a given angle
v
J first order Bessel Function of the first kind
k wave number in medium: 2πn /λ
m
ik imaginary (absorption) part of the refractive index of a particle
p
l distance from scattering object to detector
a
l illuminated path length containing particles
b
L vector of photocurrents (i , i , . i )
n 1 2 n
m relative, complex refractive index of particle to medium
rel
M model matrix, containing calculated detector signals per unit volume of particles in all
size classes
n real part of refractive index of medium
m
n real part of refractive index of particle
p
n complex refractive index of particle
p
O obscuration (1 − transmission);
r radial distance from focal point in focal plane
u standard uncertainty for the parameter and value specified
p
u standard uncertainty of the certified value
crm
u standard uncertainty of in-house reference material value
house
U expanded uncertainty of the certified value
crm
U expanded uncertainty of in-house reference material value
house
U expanded tolerance limit defined by calculation
lim
V volume content of size class i
i
v velocity of particles in dry disperser
x particle diameter (D may also be used)
x geometric mean particle size of size class i
i
x particle diameter corresponding to 10th percentile of the cumulative undersize distribu-
10,3
tion (here by volume)
x median particle diameter corresponding to the 50th percentile of the cumulative under-
50,3
size distribution (here by volume)
x particle diameter corresponding to 90th percentile of the cumulative undersize distribu-
90,3
tion (here by volume)
x volume-weighted mean diameter
13,
α dimensionless size parameter: πxn /λ
m
ΔQ volume fraction within size class i
3,i
θ scattering angle with respect to forward direction
θ angle with respect to perpendicular at boundary for a light beam in medium
m
(see definition 3.1.18)
θ angle with respect to perpendicular at boundary for a light beam in particle
p
(see definition 3.1.18)
λ wavelength of illuminating light source in vacuum
σ standard deviation
ω angular velocity
4 Principle
4.1 General
1)
The laser diffraction or scattering technique for the determination of particle size distributions,
PSDs, is based upon the phenomenon that the angular distribution of the intensity of scattered light
by a particle (scattering pattern) is dependent on the particle size. When the scattering is from a cloud
or ensemble of particles the intensity of scattering for any given size class is related to the number of
[5][20]
particles and their optical properties, present in that size class .
A test sample, dispersed at an adequate concentration in a suitable liquid or gas, is passed through the
beam of a monochromatic light source, usually a laser. The light scattered by the particles, at various
angles, is measured by an array of photo detectors. The numerical values from each detector are
recorded for subsequent analysis. Within certain limits, such as of particle concentration in measuring
zone, the scattering pattern of an ensemble of particles is identical to the sum of the individual
scattering patterns of all particles. The theoretical scattering patterns of unit volumes of particles in
selected size classes are used to build a matrix and together with a mathematical procedure are used to
solve the inverse problem, providing a volumetric particle size distribution (PSD), iterated to provide a
[18]
best fit to the measured scattering pattern .
4.2 Theory
The theoretical scattering pattern of a single spherical homogeneous particle is given by Mie-theory
[4]
in general . If the particle size is relatively large (in terms of size parameter, α = π × n /λ > 10) and
m
[4][5]
is opaque, Fraunhofer diffraction theory is available only for small angle forward scattering . The
Fraunhofer approximation is an analytical method that does not require the optical properties of the
material.
Some other theoretical approximations are available for numerical realization of the Mie-theory, and
these are called optical models in general. Choosing a relevant optical model for the inverse problem to
yield a proper PSD is important.
Laser diffraction records the scattering pattern from the particles presented. This composite pattern is
converted to a size distribution of spherical particles that would provide the same composite scattering
pattern using an appropriate optical model and data inversion routine. It therefore provides a size
distribution of laser diffraction equivalent spheres. If the test sample is not spherical, the same basic
procedure is used and the resulting size distribution is formed. Thus, PSD’s for non-spherical particles
1)  Early instruments had very limited computer capacity and were restricted to using a laser with Fraunhofer
Diffraction. Often a model form of particle size distribution was iterated to fit the scattering data. The term Laser
Diffraction rapidly became the dominant descriptor. This has continued despite the technique having advanced to
use different light sources and more sophisticated optical theories and data analysis.
8 © ISO 2020 – All rights reserved

are likely to be different from other particle sizing techniques measuring the same material. The details
of the theory are given in Annex A.
4.3 Typical instrument and optical arrangement
The system consists of a monochromatic light source, sample feeder, optical system, light detectors,
and control-calculation device. To extend the applicable range of particle size and its analysis, multiple
light sources, additional light detecting systems and related optical systems can be used.
The light source is typically a laser or other narrow-wavelength source to generate a monochromatic
beam. This is followed by a beam-processing unit producing an extended and nearly ideal, Gaussian
distributed beam to illuminate the dispersed particles. The illuminating light beam passes through the
measuring zone of the optical system.
A computer is used to control the measurement, to store and to process the data, and to solve the
inversion problem from the data of the detected signals to the particle size distribution. It may provide
automated instrument operation.
Typical diagrams of the set-up of laser diffraction/scattering instruments are given in Figures 1 to 4.
Key
1 light source assembly [with one or more light 3 forward scattering multi-element detector
source(s)] including beam expansion and (with obscuration/transmission detector)
collimation 4 wide angle scattering detector(s)
2 measurement zone (for details, see Figure 2) 5 back scattering detector
6 Fourier lens
Figure 1 — Fourier optical arrangement
Key
1 forward scattering multi-element detector 5 focal distance
(including obscuration/transmission detector) 6 incident light beam
2 Fourier lens 7 scattering light
3 ensemble of dispersed particles r radius of multi-element detector
4 working distance within measurement zone θ scattering angle
Figure 2 — Fourier optical arrangement — Scattering angle
Key
1 light source assembly [with one or more light 3 forward scattering multi-element detector
source(s)] including beam expansion and (with obscuration/transmission detector)
collimation 4 wide angle scattering detector(s)
2 measurement zone (for details, see Figure 4) 5 back scattering detector
6 reverse Fourier lens
Figure 3 — Reverse Fourier optical arrangement
10 © ISO 2020 – All rights reserved

Key
1 forward scattering multi-element detector 5 incident light
(including obscuration/transmission detector) 6 scattering light
2 flow through cuvette for dispersed particles r radius of multi-element detector
3 particle(s) θ scattering angle
4 focal distance
Figure 4 — Reverse Fourier optical arrangement — Scattering angle
4.4 Measurement zone
The locations of the two possible measurement zones, are illustrated in Figures 2 and 4. The Fourier
optical arrangement allows, within certain limits, the particles to traverse the beam in a wide range
of positions along the laser axis. By contrast, the presentation of particles within the reverse Fourier
optical arrangement shall be confined to a short distance along the laser axis if errors in sizing are to
be avoided. The measurement zone shall be located at a defined distance from the low angle detector.
A test sample of particles, dispersed as necessary, is introduced into the measuring zone at an adequate
[20]
concentration. Scattering theories show that the scattering from each particle can be combined if
the concentration is low enough, thus providing single scattering. It also requires that the particles
move freely relative to each other. It is also necessary that all particles traverse the laser beam at the
same velocity if the effects of velocity bias are to be avoided. Ideally, the selected concentration should
remain fairly constant during the measurement.
4.5 Application and sample presentation
The laser diffraction/scattering method is applicable to both wet and dry systems, and also to off-line,
online, at-line and in-line measurements. Sprays and gas bubbles in liquid can also be measured directly,
provided that their concentration is at an adequate level (see 5.4.4).
a) Off-line; (Laboratory) a sample is removed from the process which may or may not require reducing
to a test sample. The test sample is dispersed and manually introduced into the measurement zone,
forming a discontinuous measurement sequence whose operational parameters are adapted to the
product.
b) At-line; (Laboratory) a test sample is automatically taken, at or close to the process, dispersed and
introduced into the measurement zone forming a discontinuous measurement sequence whose
operational parameters are most likely dedicated to the product.
c) Online; (Process) a test sample is automatically removed from the process and introduced into
the measurement zone. A quasi-continuous real-time measurement, dedicated to close-to-process
conditions is made.
d) In-line; (Process) a test sample remains in the process line during the measurement, a quasi-
continuous real-time measurement is made under process conditions.
During the introduction of any of the above methods, material inspection and preliminary investigation
shall be performed to establish whether appropriate values for sample concentration, dispersion state,
resistance to optical fouling and other parameters are being achieved to ensure the desired particle
sizing result.
4.6 Off-line measurements
In the case of off-line measurement of wet systems, a test sample is dispersed in an appropriate
transparent liquid medium, and transported to an optical (sample) cell, in which the measuring zone
is formed. (See Annex B) In off-line measurement, dispersants (wetting agents; stabilizers) and/
or mechanical forces (agitation; sonication) are applied for de-agglomeration of the particles and for
the stabilization of the dispersion. A recirculation system is often used, consisting of an optical cell, a
sample bath (with agitation and/or sonication), a pump and tubing. A small volume cell with limited
stirring is also available for particles with very slow sedimentation velocity. Small volume cells should
be used with care as the very small test sample volume, required to provide single scattering, may fall
below a minimum test sample requirement.
In the case of off-line measurement of dry powder systems, particles are converted into aerosols by a
dry powder feeder and a disperser, before being introduced into the measuring zone. The aerosolised
particle stream is either blown or sucked through the measurement zone. It is preferred that the
concentration of aerosolised powder remains steady during the averaging of the scattered light.
Dispersed dry powders can also be measured online or in-line. Sprays are usually measured in-line.
A representative PSD analysis requires that the powder stream is not segregated. Alternatively, the
degree of segregation can be measured by analysis of different sections of the powder stream. At all
times, the particulate concentration should remain within the concentration limits for single scattering,
be fairly stable and all sizes of particles should pass through the measurement zone at the same speed
to avoid velocity bias.
4.7 In-line measurements
In the case of in-line measurement, the laser beam illuminates particles in a flowing stream within
t
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