ASTM E2175-01

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E 2175 – 01
Standard Practice for
Specifying the Geometry of Multiangle Spectrophotometers
This standard is issued under the fixed designation E 2175; 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
The appearance of metallic coatings and plastics usually depends on the directions of illumination
and viewing, a phenomenon called “gonioappearance.” This phenomenon is also observed with other
materials, such as lustrous textiles and materials containing pearlescent or interference pigments. The
characteristic appearance of most such materials is accentuated by directional illumination, such as
that provided by the sun on a clear day or a small lamp at night. The variation in color, as a function
of geometry, is usually measured by spectrophotometry with several specified sets of geometric
conditions. Measurement of this kind, at a few selected angles, is called “multiangle spectrophotom-
etry,” as distinguished from measurement over a broad range of angles, which is called “goniospec-
trophotometry.” Spectrophotometric aspects of these measurements, including spectral resolution and
linearity of photometric scales, are treated in other standards, including Practice E 308 and Practice
E 1164. Practice E 1767 provides practice for specifying the geometry of measurements. Retroreflec-
tors exhibit a special kind of gonioappearance, which is treated in other ASTM documents. The
present document provides standard practice for specifying influx and efflux angles, angular
selectivity, spatial distributions of illuminators and receivers, and angular aspects of standardizing the
photometric scale, that are peculiar to multiangle spectrophotometry. Directional illumination
emphasizes the gonioappearance of most materials, but when interference pigments are used, such as
those used in ink to mark paper currency, the effect is observed with diffuse illumination and varying
angles of viewing, so these materials are also measured with diffuse illumination.
1. Scope 2. Referenced Documents
1.1 This practice provides a way of specifying the angular 2.1 ASTM Standards:
and spatial conditions of measurement and angular selectivity E 284 Terminology of Appearance
of a method of measuring the spectral reflectance factors of E 308 Practice for Computing the Colors of Objects by
opaque gonioapparent materials, for a small number of sets of Using the CIE System
geometric conditions. E 1164 Practice for Obtaining Spectrophotometric Data for
1.2 Measurements to characterize the appearance of retrore- Object-Color Evaluation
flective materials are of such a special nature that they are E 1767 Practice for Specifying the Geometry of Observa-
treated in other ASTM documents and are not included in the tions and Measurements to Characterize theAppearance of
scope of this standard. Materials
1.3 This standard does not purport to address all of the
3. Terminology
safety concerns, if any, associated with its use. It is the
3.1 Fordefinitionsofappearancetermsusedinthispractice,
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- refer to Terminology E 284.
bility of regulatory limitations prior to use.
This practice is under the jurisdiction of ASTM Committee E12 on Color and
Appearance and is the direct responsibility of Subcommittee E12.03 on Geometry.
Current edition approved Dec. 10, 2001. Published February 2002. Annual Book of ASTM Standards, Vol 06.01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 2175
4. Significance and Use direction of the beam from a directional illuminator after
specular reflection by an ideal plane mirror at the sampling
4.1 Thispracticeisfortheuseofmanufacturersandusersof
aperture.Anglessubtendedattheoriginandmeasuredfromthe
instruments to measure the appearance of gonioapparent ma-
specular direction are called “aspecular angles” and are posi-
terials, those writing standard specifications for such instru-
tive in sign when measured in the direction toward the normal.
ments, and others who wish to specify precisely the geometric
The normal and the axis of a directional illuminator define a
conditions of multiangle spectrophotometry. A prominent ex-
plane, known as the “plane of incidence.” The specular
ample of industrial usage is the routine application of such
direction necessarily lies in that plane.
measurements by material suppliers and automobile manufac-
6.1.1 To facilitate simple and precise geometric specifica-
turers to measure the colors of metallic paints and plastics.
tion of the sampling aperture, it shall be either circular or
5. Components of Apparatus
rectangular.
6.1.2 To facilitate simple and precise geometric specifica-
5.1 The apparatus shall consist of one or more illuminators
tion of directional influx or efflux distributions, they shall be
and one or more spectrometric receivers at fixed or adjustable
either conical or pyramidal. For purposes of describing geom-
angleswithrespecttoareferenceplane,ameansofpositioning
etry by functional notation, a diffuse distribution may be
specimens in a reference plane, a means of indicating the area
considered a conical distribution centered on the normal and
on the specimen to be measured, shielding to avoid stray light,
having a half angle of 90 degrees.
and a means of displaying spectral or colorimetric data and/or
6.1.3 In a uniplanar configuration, a directional illuminator
communicating such data to a data-recorder or computer. (The
is used, the axes of the receivers lie in the plane of incidence,
terms “light,” “illuminator,” “illumination,” and “illuminance”
and their positions are specified by aspecular angles. A
are used here for simplicity, though the corresponding terms
uniplanar configuration is illustrated in Fig. 1. To simplify the
“radiant power,” “irradiator,” “irradiation,” and “irradiance”
figure, only one receiver is shown.
would be more accurate when the incident flux includes
6.1.3.1 For a conical influx distribution, the flux incident on
ultraviolet flux, as is necessary if the appearance of a fluores-
the origin comes from an area of a directional illuminator
cent material is measured.)
uniformly filling a circle on a plane normal to the beam. For a
6. Geometric Types of Apparatus
conical efflux distribution, flux from the origin is uniformly
6.1 The geometric configuration of the instrument may be collected and evaluated over an area of the receiver that is a
circle on a plane normal to the beam.Auniplanar configuration
uniplanar, annular, circumferential, or diffuse. In all cases, the
specimen is taken to be a flat surface lying in a plane called the with conical influx and efflux distributions is illustrated in Fig.
2. To simplify the figure, only one receiver is shown.
“reference plane,” which is designated the x, y plane. When
there is a single directional illuminator, the x direction is the 6.1.3.2 For a pyramidal influx distribution, flux incident on
direction of the projection of the axis of the incident beam on the origin comes from an area of a directional illuminator
the reference plane. If there are several directional illuminators uniformlyfillingarectangleonaplanenormaltothebeam.For
or a single diffuse illuminator, the direction of the x-axis must a pyramidal efflux distribution, flux from the origin is uni-
be selected and specified. The area of the reference plane on formly collected and evaluated over an area of the receiver that
which measurements are made is called the “sampling aper- is a rectangle on a plane normal to the beam. A pyramidal
ture” and the center of that area is designated the origin, o, of configuration can be used to subtend a small angle in the plane
the geometric space used to specify the configuration. The of incidence, to enhance angular selectivity, but have a large
normal to the sampling aperture, at the origin, is the -z-axis. enough solid angle to provide adequate flux for reliable
Angles subtended at the origin and measured from that normal measurements.Auniplanarconfigurationwithpyramidalinflux
are called “anormal angles.” The specular direction is the and efflux distributions is illustrated in Fig. 3. To simplify the
FIG. 1 Uniplanar Configuration
E 2175
FIG. 2 Uniplanar Configuration with Conical Influx and Efflux Distributions
FIG. 3 Uniplanar Configuration with Pyramidal Influx and Efflux Distributions
figure, only one receiver is shown and the angles d and e are 6.1.5.1 The discrete illuminators shall all have the same
shown for the receiver, but not for the illuminator. nominal angle of incidence, for a given measurement.
6.1.4 In an annular configuration, the incident beam uni- 6.1.6 In a diffuse configuration, the incident flux is diffuse.
formly fills the space between two right-circular cones, with Ideally, the illuminator illuminates the sampling aperture at all
their axes on the normal and apices at the origin. An annular angles within the hemisphere on the -z side of the reference
configuration can be used to provide a flux distribution with a plane, except those directions occupied by receivers. The use
small range of anormal angles, to enhance anormal angular of an integrating sphere to produce uniform diffuse illumina-
selectivity, but of large enough solid angle to provide adequate tion requires non-selective diffusing baffles to obscure the
flux for reliable measurements. The nominal angle of an entrance port and the area on the sphere wall at which the flux
annular distribution is the average of the half-angles of the two entering the sphere is first reflected. When diffuse illuminati
...