ASTM E2194-03

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Designation:E2194–03
Standard Practice for
Multiangle Color Measurement of Metal Flake Pigmented
Materials
This standard is issued under the fixed designation E 2194; 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
Surfaces that exhibit different colors depending on the angles of illumination or sensing are said to
be “gonioapparent.” Colorimetric values of reflecting gonioapparent materials are derived from
spectrophotometric (narrow band) or colorimetric (broad band) measurements of reflectance factor, at
variousanglesofilluminatingorsensing.Whenusingspectralvalues,tristimulusvaluesarecomputed
using the CIE Standard Observer and the spectrum of the illuminant, as described in Practice E 308.
This practice, E 2194, specifies the measurement of color observed at various aspecular angles.
1. Scope E 308 Practice for Computing the Colors of Objects by
Using the CIE System
1.1 This practice covers the instrumental requirements,
E 805 PracticeforIdentificationofInstrumentalMethodsof
standardization procedures, material standards, and parameters
Color or Color-Difference Measurement of Materials
needed to make precise instrumental measurements of the
E 1164 Practice for Obtaining Spectrophotometric Data for
colors of gonioapparent materials. This practice is designed to
Object Color Evaluation
encompass gonioapparent materials; such as, automotive coat-
E 1345 Practice for Reducing the Effect of Variability of
ings, paints, plastics, and inks.
Color Measurement by Use of Multiple Measurements
1.2 This practice addresses measurement of materials con-
E 1708 Practice for Electronic Interchange of Color and
taining metal flake and pigments. The optical characteristics of
Appearance Data
materials containing pearlescent and interference materials are
E 1767 Practice for Specifying the Geometry of Observa-
not covered by this practice. The measurement of materials
tions and Measurements to Characterize theAppearance of
containing metal flakes requires three angles of measurement
Materials
to characterize the colors of the specimen.
2.2 CIE Document:
NOTE 1—Datatakenbyutilizingthispracticeareforappearancequality
Publication No. 15.2 Colorimetry 2nd Edition
control purposes. This procedure may not necessarily supply appropriate
2.3 NIST (NBS) Publication:
data for pigment identification.
LC 1017 Standards for Checking the Calibration of Spec-
1.3 This standard does not purport to address all of the
trophotometers
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
3.1 Terms and definitions in Terminology E 284 are appli-
bility of regulatory limitations prior to use.
cabletothispractice.SeeSection“SpecializedTerminologyon
Gonioapparent Phenomena.”
2. Referenced Documents
3.2 Definitions—Usually the term metallic refers to a metal
2.1 ASTM Standards:
material. However, this standard employs the alternative defi-
D 2244 Practice for Calculation of Color Differences from
nition given in Terminology E 284 as:
Instrumentally Measured Color Coordinates
3.2.1 metallic, adj—pertaining to the appearance of a go-
E 284 Terminology of Appearance
nioapparent material containing metal flakes.
1 3
This practice is under the jurisdiction of ASTM Committee E12 on Color and Available from The U.S. National Committee of the CIE (International
AppearanceandisthedirectresponsibilityofSubcommitteeE12.12onMetallicand Commission on Illumination), C/o Thomas M. Lemons, TLA-Lighting Consultants,
Pearlescent Colors. Inc., 7 Pond St., Salem, MA 01970.
Current edition approved Feb. 10, 2003. Published May 2003. Available from National Institute of Standards and Technology (NIST), 100
Annual Book of ASTM Standards, Vol 06.01. Bureau Dr., Stop 3460, Gaithersburg, MD 20899-3460.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2194–03
4. Summary of Practice 7.1.1 A single bi-directional geometry is specified by illu-
mination and sensing angles with respect to the normal of the
4.1 This practice describes the procedures for the spectro-
plane of the specimen. Angles are measured relative to the
photometric and colorimetric measurement of metal flake
normal. Angles on the same side of the normal as the
pigmented materials. The results are reported in terms of CIE
illumination beam are written as positive angles; those on the
tristimulus values and other color coordinate systems. Stan-
other side are shown as negative, as shown in Fig. 1.
dardizationoftheinstrumentusedtomeasurethesematerialsis
defined. Guidelines are given for the selection of specimens
and a measurement protocol given. Characterization of these
materials requires measurement at a near-specular angle, a face
(mid-aspecular angle) and a flop (far-specular) angle. These
preferred aspecular angles are 15°, 45°, and 110°.
5. Significance and Use
5.1 Instrumental Measurement Angles—This practice is de-
signed to provide color data at specific measurement angles
that can be utilized for quality control, color matching, and
formulating in the characterization of metal flake pigmented
materials.
5.2 Materials—This practice provides meaningful color
information for metal flake pigmented materials, but has not
been evaluated for use with pearlescent materials or other
gonioapparent materials. This practice has been tested and
NOTE 1—Anormal illumination angle = 45° and anormal sensing
verified on paint and coatings, and the same principles should angle = 65°; therefore, aspecular angle = 45 + 65 = 110°.
FIG. 1 Example of Illuminating and Sensing Geometry
apply to plastics containing metallic flake.
5.3 Utilization—This practice is appropriate for measure-
ment and characterization of metal flake pigmented materials.
7.2 Multiangle Uniplanar Measurement—The color of me-
These data may be used for quality control, incoming inspec-
tallic materials specimens varies with the angle of view. Thus
tion, or color correction purposes.
measurements must be taken at more than one angle to
5.4 Specimen Requirements—Even though a pair of speci-
characterize the change of color with angle. The measurement
mens have the same color values at three angles, if there are
geometry for multiangle measurements is specified by aspecu-
differences in gloss, orange peel, texture, or flake orientation,
lar angles. The aspecular angle is the sensing angle measured
they may not be a visual match.
from the specular direction, in the illuminator plane. The angle
is considered positive when measured from the specular
NOTE 2—Information presented in this practice is based upon data
taken on metallic materials coatings.Applicability of this practice to other
directiontowardsthenormaldirection.Thus,ifthespecimenis
materials should be confirmed by the user.
illuminated at 45° to the normal the specular reflection will be
at -45° (See Fig. 1). Sensing at 65° from the normal, and on the
6. Apparatus
same side of normal as the illumination, is sensing 110° away
6.1 Instrument—This practice requires measurement at
from the specular direction; that is an aspecular angle of 110°.
multiple angles of illumination and sensing, usually accom-
Thus, the aspecular angle is the sum of the materials colors, a
plished by the use of a multiangle spectrophotometer as
specific aspecular angle gives the same measurement regard-
specified in this practice to characterize metal flake pigmented 5
less of angle of illumination.
materials. Measurement with a single geometry cannot char-
7.3 Annular and Circumferential Geometry—Annular illu-
acterize the gonioappearance of these materials.
mination provides incident light to a sample at all azimuthal
6.2 Standardization—A standardization plaque with as-
angles. This type of illumination minimizes the contribution
signedspectralreflectancefactorortristimulusvaluestraceable
from directional effects such as the venetian blind effect and
to a national standardizing laboratory for each specified as-
surface irregularities. Circumferential illumination is an ap-
pecular angle is required to standardize the instrument. The
proximation to annular illumination, incident light being pro-
instrument manufacturer typically assigns the values to this
vided from a discrete number of representative azimuthal
plaque.
angles. A large number or an odd number of illumination
sources more closely approximates annular illumination. An-
7. Geometric Conditions
nular or circumferential illumination minimizes directional
7.1 Conventional Color Measurement—In general purpose effects. Therefore, measurements with annular or circumferen-
colorimetry, the common geometry involves illuminating at tial illumination may or may not correlate with how that
45° and sensing at 0°.This geometry is designated 45:0 (45/0). specimen appears under directional illumination. For example,
Reverse geometry has the illumination at 0° and the sensing at this system averaging may cause the measured color values of
45°. That is, the illuminator and sensing geometries are
interchanged. This reciprocal geometry is designated 0:45
(0/45). Either geometry is used. Rodrigues, Die Farbe 37, pp. 65-78 (1990).
E2194–03
two specimens to be the same or similar, even though these
same two specimens would not match visually due to the fact
that one specimen exhibits the venetian blind effect.
7.4 Recommended Geometry—The instrument shall con-
formtothefollowinggeometricrequirementsformeasurement
of reflectance factor unless otherwise agreed upon between the
buyer and the seller. The preferred aspecular angles for
measurement are 15°, 45°, and 110°.
NOTE 3—Given a geometric configuration, the reverse geometry is
consideredequivalent,ifallothercomponentsoftheinstrumentdesignare
equivalent; for example, in the example shown in Fig. 1, the same result
wouldbeobtainedwiththeilluminationangleat65°andthesensingangle
NOTE 1—Solid lines indicate preferred angles.
at 45°. The aspecular angle would still be 110°.
FIG. 2 Diagram of Aspecular Angles
NOTE 4—Measurement angles below are stated in terms of aspecular
angles. It has been established that for metallic materials colors, a specific
aspecular angle gives the same measurement regardless of angle of
angles down to 70° give acceptable results. (Warning—Visual
illumination. For pearlescent materials, it is known that color is also a
function of angle of illumination. The importance of this phenomenon in
assessments of gonioapparent matches typically cover a wide
measurement of pearlescent and interference materials for color difference
range of aspecular angles, from very near specular, all the way
for quality control or color correction purposes has not been established.
to flop angles of 110° or even higher. Therefore, instrumental
NOTE 5—Uniplanar instruments can measure the venetian blind effect.
measurementatflopanglesbelow110°mayoccasionallyresult
Circumferential and annular illumination will not quantify this gonioap-
in measurements not agreeing with typical visual assessments.
parent effect.
This will occur when specimens are an acceptable visual and
NOTE 6—There are instruments commercially available with uniplanar,
multiangle geometries that give results that characterize gonioapparent instrumental match at angles such as 75° but unacceptable at
materials.These instruments will detect the venetian blind effect and other
110°.)
anomalies. Table 1 delineates the preferred angles. Note that circumfer-
7.4.4 Illuminating and Sensing BeamApertureAngles—The
ential geometry is limited to <90° aspecular angle. With the variety of
illuminating beam aperture angle and the sensing beam aper-
instrumentation in common usage, it is incumbent upon the user to
ture angle must be less than 8°.
determine if an instrument with angles other than the preferred angles is
7.4.5 Tolerances on Measurement Geometries—
appropriate in their application. Fig. 2
Instrumental measurement of specimens entails illumination of
7.4.1 Near Specular Angle—The near specular angle used
a sample and detection of light reflected at an aspecular angle.
should be as close to the specular direction as possible, without
Illumination and detection may be collimated or non-
detecting specular light. Surface imperfections can cause light
collimated. The specimen may be under-illuminated or over-
to be reflected in a direction slightly away from the nominal
illuminated. The size of the illuminator, detector, and speci-
specular direction. Measurement at 15° from the specular
men, the distance between them, and uniformity of
minimizes the effects of surface imperfections encountered in
illumination or detection, will all provide different effective
most practical industrial specimens. Differences in surface
aspecular angles. The following ray tracing-procedure must be
texture may result from spray application differences which
used to determine the effective aspecular angle and the distri-
can cause flake orientation differences. Measurement at 20° or
bution of the aspecular angles to ensure that the instrument
25° from specular may be chosen when less sensitivity to
design meets the specifications in Table 2. This ensures
application differences between standard and batch is desired.
equivalent color readings between instruments differing in
In critical color matching applications, batches should be
optical design. Fig. 3 schematically shows a procedure for ray
resampled and resprayed to eliminate surface differences and
tracing in 2-dimensional space. In actuality, we are dealing
measurements shall be performed at 15°.
with 3-dimensional space and all angles must be calculated in
7.4.2 Face Angle—The face color measurement shall be at
3-dimensional space relative to the surface normal of the
an aspecular angle of 45° conforming to the geometrical
specimen.
specifications of CIE 15.2. The geometrical requirements are
7.4.5.1 All rays from the illuminator (I to I ) to all points
1 n
specified as follows:
on the sample area (S to S ) result in mirrored rays in the
1 n
7.4.3 Flop Angle—Visual observation of color differences
specular direction (Sp to Sp ). For each of these specular rays
1 n
in a few cases detects sidetone scattering better at angles
and for each point on the detector area (D to D ) the aspecular
1 n
further away from specular; hence, 110° is the preferred
Fig. 3 the aspecular angles
angles have to be calc
...