ASTM E1458-92

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Designation: E 1458 – 92 An American National Standard
AMERICAN SOCIETY FOR TESTING AND MATERIALS
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Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Calibration Verification of Laser Diffraction Particle Sizing
Instruments Using Photomask Reticles
This standard is issued under the fixed designation E 1458; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
There exists a large variety of techniques and instruments for the sizing of particles and droplets in
fluid suspension. These instruments are based on a number of different physical phenomena and
interlaboratory comparisons of data on, for example, reference liquid sprays have shown significant
variability. This test method evolved in conjunction with efforts to explain the observed variability.
The effectiveness of this test method can be traced to the fact it circumvents difficulties associated with
producing, replicating, and maintaining a standard sample of liquid particles in a spray. This test
method uses a photomask reticle to provide a simulation of some of the optical properties of a
reference population of spherical particles. This test method is only applicable to optical particle sizing
instruments that are based on measurement and analysis of light scattered in the forward direction by
particles illuminated by a light beam. Since modern optical instruments generally use a laser to
produce a light beam, and since the light scattered in the forward direction by particles can often be
accurately described using diffraction theory approximations, the class of instruments for which this
test method applies have become generally known as laser diffraction particle sizing instruments.
2 3
Because it is specifically Fraunhofer diffraction theory , that is used in the approximation, these
instruments are also known as Fraunhofer diffraction particle sizing instruments.
The diffraction approximation to the general problem of electromagnetic wave scattering by
particles is strictly valid only if three conditions are satisfied. The conditions are: particle sizes must
be significantly larger than the optical wavelength, particle refractive indices must be significantly
different than the surrounding medium, and only very small (near-forward) scattering angles are
considered. For the case of spherical particles with sizes on the order of the wavelength or for large
2 3
scattering angles, the complete Lorenz-Mie scattering theory , rather than the Fraunhofer diffraction
approximation must be used. If the size and angle constraints are satisfied but the particle refractive
index is very close to that of the medium, the anomalous diffraction approximation may be used.
A complication is introduced by the fact that the optical systems of most laser diffraction particle
sizing instruments can be used, with only minor modifications such as changing a lens or translating
the sample, for measurement configurations outside the particle size or scattering angle range for
which the diffraction approximation is valid. In this situation the scattering inversion software in the
instrument would generally incorporate a scattering model other than Fraunhofer diffraction theory, in
which case the term “laser diffraction instrument” might be considered a misnomer. However, such an
instrument is still in essence a laser diffraction instrument, modified to decrease the lower particle size
limit. A calibration verification procedure as described by this test method would be applicable to all
instrument configurations (or operational modes) where the photomask reticle accurately simulates the
relevant optical properties of the particles.
The ideal calibration test samples for laser diffraction particle sizing instruments would be
comprised of the actual particle or droplet material of interest in the actual environment of interest
with size distributions closely approximating those encountered in practice. However, the use of such
calibration test samples is not currently feasible because multi-phase mixtures may undergo changes
during a test and because actual samples (for example, a spray) are not easily collected and stabilized
for long periods of time. The subject of this test method is an alternative calibration test sample
comprised of a two-dimensional array of thin, opaque circular discs (particle artifacts) deposited on
a transparent substrate (the photographic negative, that is, clear apertures in an opaque substrate, may
be used as well). Each disc or particle artifact represents the orthogonal projection of the cross-section
E 1458
of one member of a population of spherical particles comprising the reference population. The
collection of particle artifacts on a reticle represents an orthogonal projection of all the particles in the
reference population for one particular three-dimensional arrangement of the population where the
member particles are positioned within a finite reference volume. The reference volume is generally
defined such that the area covered by particle artifacts on the reticle is roughly equivalent to the
cross-section of the instrument light beam. The reference population would generally contain a large
number of particles, with a size distribution that approximates distributions of practical interest,
randomly distributed over the reference volume. Large numbers and random positions minimize
complications that can arise from optical coherence effects (interference).
Of importance here is the fact that the near-forward scattering characteristics of the orthogonal
projections of the particle cross-sections onto the reticle plane accurately simulate, in regimes where
the diffraction approximation is valid, the near-forward scattering characteristics of the reference
population (independent of the chemical composition of the particles in the reference population). In
other words the photomask reticle, when illuminated with a laser beam of known properties, generates
a reference scattered light signature which can be predicted analytically from a knowledge of the size
distribution of the reference population. The properties of the reference population can be inferred
from a characterization (using optical microscopy) of the sizes of the particle artifacts on the reticle.
As the instrument is operated away from the diffraction regime, the scattering properties of the
photomask reticle diverge from that which would be produced by the reference population and
interpretation of the measurements becomes more problematic.
The most complete test result for this test method would be a discrete size distribution reported for
a very large number of size class intervals, but intercomparisons of such distributions are difficult. For
that reason statistical parameters (for example, representative diameters and measures of the
dispersion) of the particle size distribution are used. Two examples of statistical parameters are the
volume median diameter D and the relative span (D − D )/D as defined in Practice
V0.5 V0.9 V0.1 V0.5
E 799 (recall that volume parameters such as D for a photomask reticle are defined in the sense that
Vf
two-dimensional particle artifacts scatter light like spherical particles of the same diameter). Estimates
of the true values of these statistical parameters for a photomask reticle (or more precisely the true
values for the reference population simulated by the reticle) can be established using optical or
electron microscope measurements of the diameters of the particle artifacts on the reticle. The values
so established are termed image-analysis reference values and will be used herein as the accepted
reference values. It is the stability of D , the relative span, and all other statistical parameters
V0.5
representative of the particle artifact size distribution for a reticle and the ability to produce nearly
identical replicate copies of the reticles that make this test method useful. A comparison of the
accepted reference value of D , the relative span, or any other parameter of a reticle with a
V0.5
corresponding test result from the instrument under evaluation can be used to assess the acceptability
of the instrument and of the data routinely obtained with the instrument.
1. Scope the near-forward light scattering properties are accurately
modeled by the instrument data processing and data reduction
1.1 This test method describes a procedure necessary to
software. The precision and bias limits presented herein are,
permit a user to easily verify that a laser diffraction particle
therefore, estimates of the instrument performance under ideal
sizing instrument is operating within tolerance limit specifica-
conditions. Nonideal factors that could be present in actual
tions, for example, such that the instrument accuracy is as
applications and that could significantly increase the bias errors
stated by the manufacturer. The recommended calibration
of laser diffraction instruments include vignetting (that is,
verification method provides a decisive indication of the
where light scattered at large angles by particles far away from
overall performance of the instrument at the calibration point
the receiving lens does not pass through the receiving lens and
or points, but it is specifically not to be inferred that all factors
therefore does not reach the detector plane), the presence of
in instrument performance are verified. In effect, use of this test
nonspherical particles, the presence of particles of unknown
method will verify the instrument performance for applications
refractive index, and multiple scattering.
involving spherical particles of known refractive index where
1.2 This test method shall be used as a significant test of the
instrument performance. While the procedure is not designed
for extensive calibration adjustment of an instrument, it shall
This test method is under the jurisdiction of ASTM Committee E-29 on Particle
Size Measurement and is the direct responsibility of Subcommittee E29.04 on be used to verify quantitative performance on an ongoing basis,
Liquid Particle Measurement.
Current edition approved May 20, 1992. Published July 1992.
Bohren, C. F. and Huffman, D. R. Absorption and Scattering of Light by Small
Particles, John Wiley and Sons, New York, 1983. Hirleman, E. D., Oechsle, V., and Chigier, N. A., “Response Characteristics of
van de Hulst, H. C. Light Scattering by Small Particles, Dover Publications Laser Diffraction Particle Sizing Systems: Optical Sample Volume and Lens
Inc., New York, 1981. Effects,” Optical Engineering, Vol 23, 1984, pp. 610–619.
E 1458
to compare one instrument performance with that of another, ISO Guide 2A General Terms and Their Definitions Con-
and to provide error limits for instruments tested. cerning Standardization Certification, and Testing Lab.
1.3 This test method provides an indirect measurement of Accreditation
some of the important parameters controlling the results in
3. Terminology
particle sizing by laser diffraction. A determination of all
3.1 Current ASTM Standard Definitions—Definitions of the
parameters affecting instrument performance would come
terms listed below, as used in this test method are from the
under a calibration adjustment procedure.
Compilation of ASTM Standard Definitions, :
1.4 This test method shall be performed on a periodic and
3.1.1 accuracy—see Terminology D 123, (Committee
regular basis, the frequency of which depends on the physical
D-13).
environment in which the instrumentation is used. Thus, units
3.1.2 assignable cause—see Terminology E 456, (Commit-
handled roughly or used under adverse conditions (for ex-
tee E-11).
ample, exposed to dust, chemical vapors, vibration, or combi-
3.1.3 bias—see Terminology D 123, (Committee D-13).
nations thereof) shall undergo a calibration verification more
3.1.4 calibration—see Terminology E 1187, (Committee
frequently than those not exposed to such conditions. This
E-36).
procedure shall be performed after any significant repairs are
3.1.5 Discussion—This and many other commonly used
made on an instrument, such as those involving the optics,
definitions for calibration are very broad in the sense that they
detector, or electronics.
could encompass a wide range of tasks. (See for example
1.5 The values stated in SI units are to be regarded as the
MIL-STD-45662, NBS (NIST) SP 676I, and ANSI ASQC Z-1
standard.
Draft Standard for Calibration Systems). For example, in some
1.6 This standard does not purport to address all of the
cases calibration is only the determination of whether or not an
safety problems, if any, associated with its use. It is the
instrument is operating within accuracy specifications (toler-
responsibility of the user of this standard to establish appro-
ance testing in NBS SP 676I). In other cases calibration
priate safety and health practices and determine the applica-
includes reporting of differences between the instrument re-
bility of regulatory limitations prior to use.
sponse and the accepted value of the standard, for example, to
2. Referenced Documents
produce a 88Table of Corrections” to be used with the instru-
ment. Finally, calibration can also include any repairs or
2.1 ASTM Standards:
adjustments required to make the instrument response consis-
A 340 Terminology of Symbols and Definitions Relating to
tent with the standard within the stated accuracy specifications.
Magnetic Testing
To clarify the situation it is proposed that the more specific
D 123 Terminology Relating to Textile Materials
terms calibration verification and calibration adjustment (see
D 3244 Practice for Utilization of Test Data to Determine
3.4) both of which would fall under these broad definitions of
Conformance with Specifications
calibration.
E 131 Terminology Relating to Molecular Spectroscopy
3.1.6 coeffıcient of variation—see Terminology D 123,
E 135 Terminology Relating to Emission Spectroscopy
(Committee D-13). Also known as the relative standard
E 284 Terminology of Appearance
deviation (see Terminology E 135, Committee E-1).
E 456 Terminology Relating to Quality and Statistics
3.1.7 reference material—see Terminology E 1187, (Com-
E 799 Practice for Determining Data Criteria and Process-
mittee E-36) (see ISO Guide 2, A).
ing for Liquid Drop Size Analysis
3.1.8 scattering—see Terminology E 284, (Committee
E 1187 Terminology Relating to Laboratory Accreditation
E
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