ASTM E1791-96(2021)
(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
5.1 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.
5.2 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.
5.3 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.
5.4 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.
5.5 Sintered PTFE materials exhibit sufficient reproducibility from within the same specimen after resurfacing or cleaning the spec...
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 standard. No other units of measurement are included in this standard.
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation:E1791 −96 (Reapproved 2021)
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.
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 responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
1.1 This practice covers procedures for the preparation and
mine the applicability of regulatory limitations prior to use.
use of acceptable transfer standards for NIR spectrophotom-
1.4 This international standard was developed in accor-
eters. Procedures for calibrating the reflectance factor of
dance with internationally recognized principles on standard-
materialsonanabsolutebasisarecontainedinCIEPublication
ization established in the Decision on Principles for the
No. 44 (9). Both the pressed powder samples and the sintered
Development of International Standards, Guides and Recom-
PTFE materials are used as transfer standards for such calibra-
mendations issued by the World Trade Organization Technical
tions because they have very stable reflectance factors that are
Barriers to Trade (TBT) Committee.
nearly constant with wavelength and because the distribution
of flux resembles closely that from the perfect reflecting
2. Referenced Documents
diffuser. 3
2.1 ASTM Standards:
1.2 The values stated in SI units are to be regarded as
E131Terminology Relating to Molecular Spectroscopy
standard. No other units of measurement are included in this
E259Practice for Preparation of Pressed Powder White
standard.
Reflectance Factor Transfer Standards for Hemispherical
and Bi-Directional Geometries
1.3 This standard does not purport to address all of the
E284Terminology of Appearance
safety concerns, if any, associated with its use. It is the
3. Terminology
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
3.1 Definitions—Terms and definitions in Terminology
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
E284 are applicable to this practice.
mittee E13.03 on Infrared and Near Infrared Spectroscopy.
Current edition approved Sept. 1, 2021. Published September 2021. Originally
approved in 1996. Last previous edition approved in 2014 as E1791–96(2014). For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/E1791-96R21. 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
E1791−96 (2021)
3.2 Descriptions of Terms Specific to This Standard—The of the perfect reflecting diffuser and may be used to transfer a
followingdefinitionsareparticularlyimportanttothispractice. scale of reflectance factor to another material or instrument.
3.2.1 linearity—the ability of a photometric system to yield
4.2 Sintered carbon-black doped PTFE samples are also
a linear relationship between the radiant power incident on its
commercially available and are described in Table 2. These
detectorandsomemeasurablequantityprovidedbythesystem.
materials provide close approximation to the optical properties
(E131)
of a perfect reflecting diffuser with spectrally neutral absor-
3.2.2 near-infrared, adj—the region of the electromagnetic bance features and may be used to transfer a scale of linearity
spectrum for radiation of wavelengths between 780 and 2500 in reflectance factor to another material or instrument.
nm (0.78 and 2.50 µm).
5. Significance and Use
3.2.3 perfect reflecting diffuser—idealreflectingsurfacethat
5.1 Most commercial reflectometers and spectrophotom-
neither absorbs nor transmits light, but reflects diffusely, with
eters with reflectance capability measure relative reflectance.
the radiance of the reflecting surface being the same for all
The instrument reading is the ratio of the measured radiation
reflecting angles, regardless of the angular distribution of the
reflected from the reference specimen to the measured radia-
incident light.
tion reflected by the test specimen. That ratio is dependent on
3.2.4 reflectance, r, n—ratio of the reflected radiant or
specific instrument parameters.
luminous flux to the incident flux in the given conditions (1).
5.2 National standardizing laboratories and some research
3.2.4.1 The term reflectance is often used in a general sense
laboratories measure reflectance on instruments calibrated
or as an abbreviation for reflectance factor. Such usage may be
from basic principles, thereby establishing a scale of absolute
assumed unless the definition is specifically required by the
reflectance as described in CIE Publication No. 44 (5). These
context.
measurements are sufficiently difficult and of prohibitive cost
3.2.5 reflectance factor, R, n—ratioofthefluxreflectedfrom
thattheyareusuallylefttolaboratoriesthatspecializeinthem.
the specimen to the flux reflected from the perfect reflecting
5.3 Astandard that has been measured on an absolute scale
diffuser under the same geometric and spectral conditions of
could be used to transfer that scale to a reflectometer. While
measurement (2).
suchproceduresexist,theconstraintsplacedonthemechanical
properties restrict the suitability of some of the optical
4. Summary of Practice
properties, especially those properties related to the geometric
4.1 Procedures for the preparation of packed powder
distribution of reflected radiation. Thus, reflectance factor
samples of barium sulfate and PTFE can be found in Practice
standards that are sufficiently rugged or cleanable to use as
E259. Sintered PTFE samples are commercially available.
permanenttransferstandards,withtheexceptionofthesintered
Reflectance data for this material are given in Table 1. These
PTFE standards, depart considerably from the perfect diffuser
materials provide close approximation to the optical properties
in the geometric distribution of reflected radiation.
TABLE 2 6°/Typical Diffuse Reflectance for Three Sintered
TABLE 1 6°/Typical Diffuse Reflectance for Sintered PTFE Carbon-Black Doped PTFE
Wavelength, nm Reflectance Factor Wavelength, nm 80 % Standard 10 % Standard 2 % Standard
250 0.940 250 0.774 0.106 0.015
300 0.977 300 0.793 0.099 0.016
400 0.991 400 0.795 0.097 0.017
500 0.991 500 0.796 0.099 0.017
600 0.991 600 0.797 0.101 0.017
700 0.990 700 0.799 0.103 0.017
800 0.991 800 0.802 0.105 0.018
900 0.991 900 0.803 0.105 0.017
1000 0.990 1000 0.805 0.106 0.018
1100 0.990 1100 0.806 0.108 0.017
1200 0.989 1200 0.807 0.109 0.018
1300 0.988 1300 0.808 0.111 0.018
1400 0.986 1400 0.808 0.112 0
...
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