ASTM E1791-96(2008)e1
(Practice)Standard Practice for Transfer Standards for Reflectance Factor for Near-Infrared Instruments Using Hemispherical Geometry
Standard Practice for Transfer Standards for Reflectance Factor for Near-Infrared Instruments Using Hemispherical Geometry
SIGNIFICANCE AND USE
Most commercial reflectometers and spectrophotometers with reflectance capability measure relative reflectance. The instrument reading is the ratio of the measured radiation reflected from the reference specimen to the measured radiation reflected by the 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 (5). These measurements are sufficiently difficult and of prohibitive cost 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 of the optical properties, especially those properties related to the geometric distribution of reflected radiation. Thus, reflectance factor standards that are sufficiently rugged or cleanable to use as permanent transfer standards, with the exception of the sintered PTFE standards, depart considerably from the perfect diffuser in the geometric distribution of reflected radiation.
The geometric distribution of reflected radiance from such standards is sufficiently diffuse that such a standard can provide a dependable calibration of a directional-hemispherical or certain directional-directional reflectometers. Although pressed powder standards are subject to contamination and breakage, the reflectance factor of pressed powder can be sufficiently reproducible from specimen to specimen from a given lot of powder to allow the assignment of absolute reflectance factor values to all of the powder in a lot.
Sintered PTFE materials exhibit sufficient reproducibility from within the same specimen after resurfacing or cleaning the specimen to allow the assign...
SCOPE
1.1 This practice covers procedures for the preparation and use of acceptable transfer standards for NIR spectrophotometers. Procedures for calibrating the reflectance factor of materials on an absolute basis are contained in CIE Publication No. 44 (9). Both the pressed powder samples and the sintered PTFE materials are used as transfer standards for such calibrations because they have very stable reflectance factors that are nearly constant with wavelength and because the distribution of flux resembles closely 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:E1791 −96(Reapproved2008)
Standard Practice for
Transfer Standards for Reflectance Factor for Near-Infrared
Instruments Using Hemispherical Geometry
This standard is issued under the fixed designation E1791; 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.
´ NOTE—Trademark names removed editorially in April 2008.
INTRODUCTION
The internationally accepted standard of reflectance is the perfect reflecting diffuser. This ideal
reflectingsurfacereflects100%oftheradiantpowerincidentonit,suchthattheradianceisthesame
foralldirectionswithinthehemisphereofsolidangles.Nophysicalrealizationofthisstandardexists.
Optical properties of standards prepared from pressed plaques of barium sulfate (BaSO)or
polytetrafluoroethylene (PTFE), as well as commercially available samples of sintered PTFE (1-4),
canapproximatethoseofawhitematerial.Forfurtherinformation,seeCommissionInternationalede
L’Eclairage (CIE) Publication No. 46 (5).Additional transfer standards are required that have a very
stable reflectance factor that is constant with wavelength and that have a range of values from near
zero to close to that of the perfect reflecting diffuser. Such materials as carbon-black doped sintered
PTFE (6-8) fulfill this requirement. The principle uses of a reflectance factor standard are for
transferring an absolute scale of reflectance to a more durable material or for calibrating near-infrared
(NIR) spectrophotometers for linearity of reflectance scale. In theory, this transfer, conducted from
first principles, should be quite easy. In practice, values are likely to be required for parameters that
are unknown, proprietary, or require a highly sophisticated level of skill. Some, but not all, of these
parameters are discussed in this practice.
1. Scope 1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This practice covers procedures for the preparation and
responsibility of the user of this standard to establish appro-
use of acceptable transfer standards for NIR spectrophotom-
priate safety and health practices and determine the applica-
eters. Procedures for calibrating the reflectance factor of
bility of regulatory limitations prior to use.
materialsonanabsolutebasisarecontainedinCIEPublication
No. 44 (9). Both the pressed powder samples and the sintered
2. Referenced Documents
PTFE materials are used as transfer standards for such calibra-
2.1 ASTM Standards:
tions because they have very stable reflectance factors that are
E131Terminology Relating to Molecular Spectroscopy
nearly constant with wavelength and because the distribution
E259Practice for Preparation of Pressed Powder White
of flux resembles closely that from the perfect reflecting
Reflectance Factor Transfer Standards for Hemispherical
diffuser.
and Bi-Directional Geometries
1.2 The values stated in SI units are to be regarded as the
E284Terminology of Appearance
standard. The values given in parentheses are for information
only.
3. Terminology
3.1 Definitions—Terms and definitions in Terminology
E284 are applicable to this practice.
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
3.2 Descriptions of Terms Specific to This Standard—The
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
Current edition approved March 15, 2008. Published April 2008. Originally
approved in 1996. Last previous edition approved in 2000 as E1791–96(2000). For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/E1791-96R08E01. 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
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E1791−96(2008)
TABLE 2 6°/Typical Diffuse Reflectance for Three Sintered
followingdefinitionsareparticularlyimportanttothispractice.
Carbon-Black Doped PTFE
3.2.1 linearity—the ability of a photometric system to yield
Wavelength, nm 80 % Standard 10 % Standard 2 % Standard
a linear relationship between the radiant power incident on its
250 0.774 0.106 0.015
detectorandsomemeasurablequantityprovidedbythesystem.
300 0.793 0.099 0.016
(E131)
400 0.795 0.097 0.017
500 0.796 0.099 0.017
3.2.2 near-infrared, adj—the region of the electromagnetic
600 0.797 0.101 0.017
spectrum for radiation of wavelengths between 780 and 2500
700 0.799 0.103 0.017
nm (0.78 and 2.50 µm). 800 0.802 0.105 0.018
900 0.803 0.105 0.017
3.2.3 perfect reflecting diffuser—idealreflectingsurfacethat
1000 0.805 0.106 0.018
neither absorbs nor transmits light, but reflects diffusely, with 1100 0.806 0.108 0.017
1200 0.807 0.109 0.018
the radiance of the reflecting surface being the same for all
1300 0.808 0.111 0.018
reflecting angles, regardless of the angular distribution of the
1400 0.808 0.112 0.018
incident light. 1500 0.810 0.113 0.020
1600 0.811 0.114 0.021
3.2.4 reflectance, r, n—ratio of the reflected radiant or
1700 0.812 0.115 0.023
1800 0.813 0.116 0.024
luminous flux to the incident flux in the given conditions (1).
1900 0.811 0.118 0.026
3.2.4.1 The term reflectance is often used in a general sense
2000 0.814 0.117 0.027
or as an abbreviation for reflectance factor. Such usage may be
2100 0.809 0.114 0.030
2200 0.812 0.110 0.032
assumed unless the definition is specifically required by the
2300 0.813 0.110 0.035
context.
2400 0.809 0.103 0.034
3.2.5 reflectance factor, R, n—ratioofthefluxreflectedfrom
2500 0.809 0.101 0.038
thickness#7 mm thickness#5 mm thickness#3mm
the specimen to the flux reflected from the perfect reflecting
diffuser under the same geometric and spectral conditions of
measurement (2).
materials provide close approximation to the optical properties
4. Summary of Practice
of a perfect reflecting diffuser with spectrally neutral absor-
bance features and may be used to transfer a scale of linearity
4.1 Procedures for the preparation of packed powder
in reflectance factor to another material or instrument.
samples of barium sulfate and PTFE can be found in Practice
E259. Sintered PTFE samples are commercially available.
5. Significance and Use
Reflectance data for this material are given in Table 1. These
5.1 Most commercial reflectometers and spectrophotom-
materials provide close approximation to the optical properties
eters with reflectance capability measure relative reflectance.
of the perfect reflecting diffuser and may be used to transfer a
The instrument reading is the ratio of the measured radiation
scale of reflectance factor to another material or instrument.
reflected from the reference specimen to the measured radia-
4.2 Sintered carbon-black doped PTFE samples are also
tion reflected by the test specimen. That ratio is dependent on
commercially available and are described in Table 2. These
specific instrument parameters.
5.2 National standardizing laboratories and some research
TABLE 1 6°/Typical Diffuse Reflectance for Sintered PTFE
laboratories measure reflectance on instruments calibrated
Wavelength, nm Reflectance Factor from basic principles, thereby establishing a scale of absolute
250 0.940 reflectance as described in CIE Publication No. 44 (5). These
300 0.977
measurements are sufficiently difficult and of prohibitive cost
400 0.991
thattheyareusuallylefttolaboratoriesthatspecializeinthem.
500 0.991
600 0.991
5.3 Astandard that has been measured on an absolute scale
700 0.990
could be used to transfer that scale to a reflectometer. While
800 0.991
900 0.991
suchproceduresexist,theconstraintsplacedonthemechanical
1000 0.990
properties restrict the suitability of some of the optical
1100 0.990
properties, especially those properties related to the geometric
1200 0.989
1300 0.988
distribution of reflected radiation. Thus, reflectance factor
1400 0.986
standards that are sufficiently rugged or cleanable to use as
1500 0.988
permanenttransferstandards,withtheexceptio
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