ASTM E259-06(2011)
(Practice)Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical and Bi-Directional Geometries
Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical and Bi-Directional Geometries
SIGNIFICANCE AND USE
All commercial reflectometers measure relative reflectance. The instrument reading is the reflectance factor, the ratio of the light reflected by a reference specimen to that reflected by a test specimen. That ratio is dependent on specific instrument parameters.
National standardizing laboratories and some research laboratories measure reflectance on instruments calibrated from basic principles, thereby establishing a scale of absolute reflectance as described in CIE Publication No. 44 (2). These measurements are sufficiently difficult that they are usually left to laboratories that specialize in them.
A standard that has been measured on an absolute scale could be used to transfer that scale to a reflectometer. While such procedures exist, the constraints placed on the mechanical properties restrict the suitability of some optical properties, especially those properties related to the geometric distribution of the reflected light. Thus, reflectance factor standards which are sufficiently rugged and able to be cleaned, depart considerably from the perfect diffuser in the geometric distribution of reflected radiance.
The geometric distribution of reflected radiance from a pressed powder plaque is sufficiently diffuse to provide a dependable calibration of a directional-hemispherical reflectometer. Although pressed powder standards are subject to contamination and breakage, the directional-hemispherical reflectance factor of pressed powder standards can be sufficiently reproducible from specimen to specimen made from a given lot of powder, so as to allow one to assign absolute reflectance factor values to all the powder in a lot.
This practice describes how to prepare white reflectance factor standards from a powder in a manner that allows a standardizing laboratory to assign the absolute scale of reflectance to the plaque.
Note—The collar and receptacle should be securely held in place before pressing the powder. FIG. 1 Example Powder Press
SCOPE
1.1 This practice covers procedures for preparing pressed powder transfer standards. These standards can be used in the near-ultraviolet, visible and near-infrared region of the electromagnetic spectrum. Procedures for calibrating the reflectance factor of materials on an absolute basis are contained in CIE Publication No. 44 (2). Pressed powder standards are used as transfer standards for such calibrations because they have a high reflectance factor that is nearly constant with wavelength, and because the geometric distribution of reflected flux resembles that from the perfect reflecting diffuser.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E259 −06(Reapproved 2011)
Standard Practice for
Preparation of Pressed Powder White Reflectance Factor
Transfer Standards for Hemispherical and Bi-Directional
Geometries
This standard is issued under the fixed designation E259; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The internationally accepted standard of reflectance is the perfect reflecting diffuser. This ideal
reflecting surface reflects 100% of the incident radiant power such that the radiance is the same for
all directions within the hemisphere above the surface. No physical realization of this standard exists.
Optical properties of standards prepared from pressed plaques of magnesium carbonate (MgCO ),
barium sulfate (BaSO ), or polytetrafluoroethylene (PTFE) can approximate an ideal standard. For
further information see CIE Publication No. 46 (1). The principal use of a white reflectance factor
standard is to transfer an absolute scale of reflectance to a more durable material or from one
instrument to another. In theory, it should be easy to do this transfer from first principles. In practice,
one is likely to need values for parameters that are unknown, proprietary, or require a high level of
skill. Some, but not all, of those parameters are discussed in this practice.
1. Scope 2. Referenced Documents
1.1 This practice covers procedures for preparing pressed 2.1 ASTM Standards:
powder transfer standards. These standards can be used in the E284Terminology of Appearance
near-ultraviolet, visible and near-infrared region of the electro-
3. Terminology
magnetic spectrum. Procedures for calibrating the reflectance
factor of materials on an absolute basis are contained in CIE
3.1 Terms and definitions in Terminology E284 are appli-
Publication No. 44 (2). Pressed powder standards are used as
cable to this practice.
transfer standards for such calibrations because they have a
3.2 Definitions:
high reflectance factor that is nearly constant with wavelength,
3.2.1 The following definitions are particularly important to
and because the geometric distribution of reflected flux re-
this practice:
sembles that from the perfect reflecting diffuser.
3.2.2 perfect reflecting diffuser, n—ideal reflecting surface
1.2 The values stated in SI units are to be regarded as
that neither absorbs nor transmits light, but reflects diffusely,
standard. No other units of measurement are included in this
withtheradianceofthereflectingsurfacebeingthesameforall
standard.
reflecting angles, regardless of the angular distribution of the
incident light. (1990)
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.2.3 reflectance, ρ,n—ratio of the reflected radiant or
responsibility of the user of this standard to establish appro-
luminous flux to the incident flux in the given conditions.
A
priate safety and health practices and determine the applica-
[CIE]
bility of regulatory limitations prior to use.
3.2.4 Theterm reflectanceisoftenusedinageneralsenseor
as an abbreviation for reflectance factor. Such usage may be
assumed unless the above definition is specifically required by
This practice is under the jurisdiction ofASTM Committee E12 on Color and
the context. (1989b)
Appearance and is the direct responsibility of Subcommittee E12.02 on Spectro-
photometry and Colorimetry.
Current edition approved Nov. 1, 2011. Published November 2011. Originally
approved in 1965. Last previous edition approved in 2006 as E259–06. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/E0259-06R11. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Standards volume information, refer to the standard’s Document Summary page on
this practice. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E259−06 (2011)
3.2.5 reflectance factor, R, n—ratioofthefluxreflectedfrom
the specimen to the flux reflected from the perfect reflecting
diffuser under the same geometric and spectral conditions of
B
measurement. [CIE] (1988)
4. Summary of Practice
4.1 Procedures are given for the preparation of white
reference standards of diffuse reflectance factor and diffuse
radiancefactor.Therecommendedmaterialsarewhitepowders
that are pressed into plaques. These plaques provide close
approximations to the optical properties of the perfect reflect-
ing diffuser, and may be used to transfer a scale of absolute
reflectance to another material or to an instrument.
NOTE 1—The collar and receptacle should be securely held in place
before pressing the powder.
5. Significance and Use
FIG. 1Example Powder Press
5.1 All commercial reflectometers measure relative reflec-
tance.Theinstrumentreadingisthereflectancefactor,theratio
to provide a matte finish on the pressed plaque. Powder
of the light reflected by a reference specimen to that reflected
receptaclesshouldbeatleast5mmdeepforBaSO andatleast
by a test specimen. That ratio is dependent on specific
10 mm deep for PTFE.
instrument parameters.
5.2 National standardizing laboratories and some research 7. Reagents and Materials
laboratories measure reflectance on instruments calibrated
7.1 Barium Sulfate—The barium sulfate should be of the
from basic principles, thereby establishing a scale of absolute
highest purity. It should be specially refined for optical and
reflectance as described in CIE Publication No. 44 (2). These
spectroscopic use (3).
measurementsaresufficientlydifficultthattheyareusuallyleft
7.2 Polytetrafluoroethylene—The PTFE (4) should also be
to laboratories that specialize in them.
specially refined for optical and spectroscopic use but some
5.3 Astandard that has been measured on an absolute scale
commercial grades have been found to be acceptable substi-
could be used to transfer that scale to a reflectometer. While
tutes. (5) Large quantities (drums) of commercial grade PTFE
suchproceduresexist,theconstraintsplacedonthemechanical
can be obtained from manufacturers.
properties restrict the suitability of some optical properties,
especiallythosepropertiesrelatedtothegeometricdistribution
8. Procedure
of the reflected light. Thus, reflectance factor standards which
8.1 Store all powdered reflectance standards in tightly
are sufficiently rugged and able to be cleaned, depart consid-
capped glass containers. If the powder is purchased in plastic
erablyfromtheperfectdiffuserinthegeometricdistributionof
containers, transfer it to a glass container as soon as possible.
reflected radiance.
Before using the powder, place it in a glass blender equipped
5.4 The geometric distribution of reflected radiance from a
with stainless-steel or PTFE-coated blades and pulverize to a
pressed powder plaque is sufficiently diffuse to provide a
uniform consistency. Transfer the quantity of powder to be
dependable calibration of a directional-hemispherical reflecto-
used with stainless steel or PTFE-coated spoons. Perform the
meter. Although pressed powder standards are subject to
whole operation in a draft-free location, away from sources of
contamination and breakage, the directional-hemispherical re-
small particulate contamination, filters, sweaters, windows,
flectancefactorofpressedpowderstandardscanbesufficiently
ovens, etc. Perform all measurements (weight, height, width,
reproduciblefromspecimentospecimenmadefromagivenlot
depth, volume, area, etc.) with adequate precision to ensure
of powder, so as to allow one to assign absolute reflectance
that the final density is within 5% of the specified value. The
factor values to all the powder in a lot.
most reproducible standards are made by pressing the powder
toaspecificdensity.Thus,determinethemassofthepowderto
5.5 Thispracticedescribeshowtopreparewhitereflectance
be used from the volume of the receptacle.
factor standards from a powder in a manner that allows a
standardizing laboratory to assign the absolute scale of reflec-
8.2 Barium Sulfate—Press BaSO to a density of 2000
3 3
tance to the plaque.
kg/m (2.0 gm/cm ) and a thickness of at least 5 mm. Press
several specimens in succession. Select matched pairs to be
6. Apparatus
representativeofthecontentsofthebottleofpowder.Keepthe
6.1 The basic apparatus for producing a pressed powder pressed plaques in a covered desiccator when not in use. Some
standard includes a powder press, powder containers and a suppliers of BaSO provide calibration values with each bottle
balance. There are currently two commercial suppliers of ofpowder,othersuppliersprovideonly3or4qualityreference
powder presses.The press and receptacles can also be made in checks and a
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