ASTM E1791-96(2000)

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Designation: E 1791 – 96 (Reapproved 2000)
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 (e) 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,seeCIEPublicationNo.46(5)and
Specification D1457. 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
oftheperfectreflectingdiffuser.Suchmaterialsascarbon-blackdopedsinteredPTFE (6-8)fulfillthis
requirement. The principle uses of a reflectance factor standard are for transferring an absolute scale
ofreflectancetoamoredurablematerialorforcalibratingnear-infrared(NIR)spectrophotometersfor
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
requireahighlysophisticatedlevelofskill.Some,butnotall,oftheseparametersarediscussedinthis
practice.
1. Scope priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
1.1 This practice covers procedures for the preparation and
use of acceptable transfer standards for NIR spectrophotom-
2. Referenced Documents
eters. Procedures for calibrating the reflectance factor of
2.1 ASTM Standards:
materialsonanabsolutebasisarecontainedinCIEPublication
D1457 Specification for Polytetrafluoroethylene (PTFE)
No. 44 (9). Both the pressed powder samples and the sintered
Molding and Extrusion Materials
PTFE materials are used as transfer standards for such calibra-
E131 Terminology Relating to Molecular Spectroscopy
tions because they have very stable reflectance factors that are
E259 Practice for Preparation of Pressed Powder White
nearly constant with wavelength and because the distribution
Reflectance Factor Transfer Standards for Hemispherical
of flux resembles closely that from the perfect reflecting
Geometry
diffuser.
E284 Terminology of Appearance
1.2 The values stated in SI units are to be regarded as the
standard. The values given in parentheses are for information
3. Terminology
only.
3.1 Definitions—Terms and definitions in Terminology
1.3 This standard does not purport to address all of the
E284 are applicable to this practice.
safety concerns, if any, associated with its use. It is the
3.2 Descriptions of Terms Specific to This Standard—The
responsibility of the user of this standard to establish appro-
followingdefinitionsareparticularlyimportanttothispractice.
3.2.1 linearity—the ability of a photometric system to yield
This practice is under the jurisdiction ofASTM Committee E-13 on Molecular a linear relationship between the radiant power incident on its
Spectroscopy and is the direct responsibility of Subcommittee E13.11 on Chemo-
metrics.
Current edition approved March 10, 1996. Published May 1996. Annual Book of ASTM Standards, Vol 08.01.
2 4
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Annual Book of ASTM Standards, Vol 03.06.
this practice. 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 1791 – 96 (2000)
TABLE 1 6°/Typical Diffuse Reflectance for Sintered (Labsphere TABLE 2 6°/Typical Diffuse Reflectance for Three Sintered
A
SpectralonY SRS-99) Carbon-Black Doped PTFE (Labsphere SpectralonY SRS-80,
A
SRS-10, and SRS-02)
Wavelength, nm Reflectance Factor
Wavelength, nm SRS-80 SRS-10 SRS-02
250 0.940
300 0.977
250 0.774 0.106 0.015
400 0.991
300 0.793 0.099 0.016
500 0.991
400 0.795 0.097 0.017
600 0.991
500 0.796 0.099 0.017
700 0.990
600 0.797 0.101 0.017
800 0.991
700 0.799 0.103 0.017
900 0.991
800 0.802 0.105 0.018
1000 0.990
900 0.803 0.105 0.017
1100 0.990
1000 0.805 0.106 0.018
1200 0.989
1100 0.806 0.108 0.017
1300 0.988 1200 0.807 0.109 0.018
1400 0.986
1300 0.808 0.111 0.018
1500 0.988
1400 0.808 0.112 0.018
1600 0.987
1500 0.810 0.113 0.020
1700 0.984
1600 0.811 0.114 0.021
1800 0.984
1700 0.812 0.115 0.023
1900 0.978
1800 0.813 0.116 0.024
2000 0.970
1900 0.811 0.118 0.026
2100 0.950
2000 0.814 0.117 0.027
2200 0.963 2100 0.809 0.114 0.030
2300 0.955
2200 0.812 0.110 0.032
2400 0.944
2300 0.813 0.110 0.035
2500 0.940
2400 0.809 0.103 0.034
Density = 1500 kg/m ; thickness$ 7 mm.
2500 0.809 0.101 0.038
A
Available from Labsphere, Inc., P.O. Box 70, North Sutton, NH 03260-0070;
thickness#7 mm thickness#5 mm thickness#3mm
uncertainty of measurement 60.002.
A
Available from Labsphere, Inc., P.O. Box 70, North Sutton, NH 03260-0070.
Materials are available in nominal reflectance factor values at 600 nm from
between 0.02 and 0.99.
detectorandsomemeasurablequantityprovidedbythesystem.
(E 131)
4.2 Sintered carbon-black doped PTFE samples are also
3.2.2 near-infrared, adj—the region of the electromagnetic
commercially available and are described in Table 2. These
spectrum for radiation of wavelengths between 780 and 2500
materials provide close approximation to the optical properties
nm (0.78 and 2.50 µm).
of a perfect reflecting diffuser with spectrally neutral absor-
3.2.3 perfect reflecting diffuser—idealreflectingsurfacethat
bance features and may be used to transfer a scale of linearity
neither absorbs nor transmits light, but reflects diffusely,
in reflectance factor to another material or instrument.
withtheradianceofthereflectingsurfacebeingthesameforall
reflecting angles, regardless of the angular distribution of the
5. Significance and Use
incident light.
5.1 Most commercial reflectometers and spectrophotom-
3.2.4 reflectance, r, n—ratio of the reflected radiant or
eters with reflectance capability measure relative reflectance.
luminous flux to the incident flux in the given conditions (1).
The instrument reading is the ratio of the measured radiation
3.2.4.1 The term reflectance is often used in a general sense
reflected from the reference specimen to the measured radia-
orasanabbreviationforreflectancefactor.Suchusagemaybe
tion reflected by the test specimen. That ratio is dependent on
assumed unless the definition is specifically required by the
specific instrument parameters.
context.
5.2 National standardizing laboratories and some research
3.2.5 reflectance factor, R, n—ratio of the flux reflected
laboratories measure reflectance on instruments calibrated
from the specimen to the flux reflected from the perfect
from basic principles, thereby establishing a scale of absolute
reflecting diffuser under the same geometric and spectral
reflectance as described in CIE Publication No. 44 (5). These
conditions of measurement (2).
measurements are sufficiently difficult and of prohibitive cost
thattheyareusuallylefttolaboratoriesthatspecializeinthem.
4. Summary of Practice
5.3 Astandard that has been measured on an absolute scale
4.1 Procedures for the preparation of packed powder
could be used to transfer that scale to a reflectometer. While
samples of barium sulfate and PTFE can be found in Practice
suchproceduresexist,theconstraintsplacedonthemechanical
E259. Sintered PTFE samples are commercially available.
properties restrict the suitability of some of the optical prop-
Reflectance data for this material are given in Table 1. These
erties, especially those properties related to the geometric
materials provide close approximation to the optical properties
distribution of reflected radiation. Thus, reflectance factor
of the perfect reflecting diffuser and may be used to transfer a
standards that are sufficiently rugged or cleanable to use as
scale of reflectance factor to another material or instrument.
permanenttransferstandards,withtheexceptionofthesintered
6 7
Such material is available under the trade name Spectralon SRS-99 from Such material is available under the trade name Spectralon SRS-XX from
Labsphere, Inc., P.O. Box 70, North Su
...