ASTM D2798-11a(2019)
(Test Method)Standard Test Method for Microscopical Determination of the Vitrinite Reflectance of Coal
Standard Test Method for Microscopical Determination of the Vitrinite Reflectance of Coal
SIGNIFICANCE AND USE
5.1 The mean maximum reflectance of the vitrinite component in coal as determined by this test method is often used as an indicator of rank as presented in Classification D388, independent of petrographic composition, and in the characterization of coal as feedstock for carbonization, gasification, liquefaction, and combustion processes.
5.2 This test method is for use in scientific and industrial research.
SCOPE
1.1 This test method covers the microscopical determination of both the mean maximum and mean random reflectances measured in oil of polished surfaces of vitrinite and other macerals present in coals ranging in rank from lignite to anthracite. This test method can be used to determine the reflectance of other macerals.
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.
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Designation:D2798 −11a (Reapproved 2019)
Standard Test Method for
Microscopical Determination of the Vitrinite Reflectance of
Coal
This standard is issued under the fixed designation D2798; 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.
1. Scope 3.2 Abbreviations:
3.2.1 R max—mean maximum reflectance measured in oil.
o
1.1 Thistestmethodcoversthemicroscopicaldetermination
3.2.2 R ran—mean random reflectance measured in oil.
of both the mean maximum and mean random reflectances
o
3.2.2.1 Discussion—Other organizations may use other ab-
measured in oil of polished surfaces of vitrinite and other
breviations for mean maximum and mean random reflectance.
macerals present in coals ranging in rank from lignite to
anthracite. This test method can be used to determine the
4. Summary of Test Method
reflectance of other macerals.
4.1 Thereflectanceofthemaceralvitriniteorothermacerals
1.2 The values stated in SI units are to be regarded as
is determined in this test method by illuminating a polished
standard. No other units of measurement are included in this
surface of a section of coal in immersion oil using a micro-
standard.
scopic system that photometrically measures the amount of
1.3 This standard does not purport to address all of the
light reflected from the surface. The reflected light is recorded
safety concerns, if any, associated with its use. It is the
in percent reflectance after calibration of photometric equip-
responsibility of the user of this standard to establish appro-
ment by measuring the reflected light from standards of
priate safety, health, and environmental practices and deter-
reflectance as calculated from their refractive indices.
mine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accor-
5. Significance and Use
dance with internationally recognized principles on standard-
5.1 The mean maximum reflectance of the vitrinite compo-
ization established in the Decision on Principles for the
nent in coal as determined by this test method is often used as
Development of International Standards, Guides and Recom-
an indicator of rank as presented in Classification D388,
mendations issued by the World Trade Organization Technical
independent of petrographic composition, and in the charac-
Barriers to Trade (TBT) Committee.
terization of coal as feedstock for carbonization, gasification,
liquefaction, and combustion processes.
2. Referenced Documents
5.2 This test method is for use in scientific and industrial
2.1 ASTM Standards:
research.
D121Terminology of Coal and Coke
D388Classification of Coals by Rank
6. Apparatus
D2797Practice for Preparing Coal Samples for Microscopi-
6.1 Microscope—Any microscope equipped for reflected
cal Analysis by Reflected Light
light microscopy (such as a metallurgical or opaque-ore mi-
3. Terminology croscope) can be used, provided the lens combination of
objective and eyepieces permits examination of the specimen
3.1 Definitions—For definitions of terms, refer to Terminol-
at a magnification between 400 and×750, such that particles
ogy D121.
of1µmcanberesolved.Theobjectivesshallbeconstructedso
that samples can be examined in oil with plane-polarized light
This test method is under the jurisdiction of ASTM Committee D05 on Coal
and have the highest quality of antireflection coatings. The
and Coke and is the direct responsibility of Subcommittee D05.28 on Petrographic
microscope shall be able to project an image at similar
Analysis of Coal and Coke.
magnification to a photomultiplier tube and to support the
Current edition approved Nov. 1, 2019. Published November 2019. Originally
photomultiplier tube housing. Means shall be provided to
approved in 1969. Last previous edition approved in 2011 as D2798–11a. DOI:
10.1520/D2798-11AR19.
position the tube housing laterally to obtain maximum re-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
sponse. The microscope shall have a circular stage that is
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
capable of rotating a specimen through 360°. The mechanical
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. stage attached to the microscope stage shall enable the analyst
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2798−11a (2019)
to move the specimen accurately (within 0.1 mm) to a given 6.9 Calibration Standards—Prisms constructed of high-
field location. A combination of objective and circular stage index glasses or synthetic minerals shall be used as standards
shall permit centering. The viewing eyepiece shall be supplied tocalibratethephotometerforreflectancemeasurement.These
with a crosshair or grid to be used as a reference to locate standardsmustbedurable,isotropic,resistanttocorrosion,free
from internal flaws or fractures, and have negligible light
precisely the area sampled by the phototube. During
measurement, no light shall be permitted to enter the observ- absorption.Aprismwithsidesthatforma30-60-90°triangleis
the most effective shape, with the side between the 30 and 90°
er’s end of the viewing eyepiece.
angles highly polished and used as the reflectance-measuring
6.2 Polarizer and Illuminator—The light incident on the
surface. The prisms shall be enclosed, except for the polished
vertical illuminator of the microscope shall be plane-polarized
surface, in a durable, lightabsorbent, water- and oil-resistant
by a prism or sheet polarizer. The vertical illuminator can
mount; polyester or epoxy resin, made light absorbent with a
contain a Berek prism, a Smith illuminator, or high-quality
dyeorfiller,servesadequately.Itisdesirabletohaveanumber
glass plate.The polarizer shall be oriented at 45° when using a
of different standards with reflectances near those of the
Berek prism or at 0° when using a Smith illuminator or glass
vitrinite studied; these also serve to check the linear response
plate.
of the photometer. The reflectance of each standard shall be
calculated to the nearest 0.001% by means of the following
6.3 Photomultiplier Tube—In combination with the micro-
equation:
scope optical system, light source, and filter used, the photo-
2 2
multiplier photometer shall be capable of detecting the mini-
R 5 100 n 2 1.5180 / n 11.5180 (1)
~ ! ~ !
s g g
mumlightreflectedfromthelimitedportionofthecoalsample
where:
(see6.8).Thehighvoltagesuppliedtothephotomultipliertube
R = standard reflectance in oil of the glass, % and
must be within the prescribed range to obtain linearity of s
n = refractive index of the glass at 546-nm wavelength, to
g
response. This is usually from 300 to 1100 V for side-window
the nearest 0.0001 index value.
tubes and from 1000 to 1500 V for end-window tubes.
NOTE 1—Most coal laboratories in North America use the following
Photodiode arrays, channeltrons, or other light-measuring de-
Bausch and Lomb Co. or Schott Co. optical glasses (the reported
vices are acceptable alternatives providing that sufficient gray
refractive index at 546 nm and the calculated standard reflectance in oil
levels obtainable will enable reliable differentiation of signal
are given in parentheses):
equivalentto0.01%reflectanceandthatthesystemislinearin
Bausch and Lomb Schott
the range of the reflectance measured.
689 309 (1.6935; 0.299 %) SF8-689-312 (1.6945; 0.303 %)
6.4 Photometer Amplifier—The signal from the photomulti-
751 278 (1.7566; 0.532 %) SF13-714-276 (1.7477; 0.496 %)
827 250 (1.8351; 0.895 %) LaF12-836-423 (1.8400; 0.921 %)
plier tube shall be amplified and displayed by a galvanometer,
850 324 (1.8543; 0.996 %) LaSF9-850-322 (1.8567; 1.009 %)
digital meter, or recorder. When adjusted for operation, the
915 213 (1.9235; 1.390 %) LaSF18-913-325 (1.9273; 1.413 %)
amplifier and meter shall be capable of reliably distinguishing 980 222 (1.9907; 1.817 %) LaSF6-961-249 (1.9670; 1.662 %)
differences in signal equivalent to 0.01% reflectance and shall Other standards available that can be used include the following:
be linear in the range of reflectance measured. Leucosapphire (1.77; 0.59 %)
Yttrium aluminum garnet, YAG (1.84; 0.92 %)
6.5 Recorder or Meter—The recorder or meter used shall Gadolinium gallium garnet, 3G (1.98; 1.73 %)
Silicon carbide (2.663; 7.52 %)
have a response time at full scale of no more than1sto detect
the maximum reflectance level during rotation of the micro-
6.10 Immersion Oil—The oil shall be a nondrying, noncor-
scope stage. rosive type that will not react with coal, does not contain
carcinogens, and has a refractive index within the range from
6.6 Light Source—The light source shall have a regulated
1.515to1.519at546nmand25°C.Withinthespecifiedrange,
power supply to provide for stable output. Some photometers
the refractive index of the oil is not critical provided the
and recorders require supplemental voltage-stabilizing trans-
specified value of 1.5180 is used in calculating reflectance of
formers if the line voltage fluctuates.
standards as specified in 6.9. Periodic checking of the refrac-
6.7 Filters—The light shall be made approximately mono- tive index of the oil is discretionary.
chromatic green by passage through an interference filter or
6.11 Sample-Leveling Press—Aconventional manual level-
combination of filters with peak transmittance of 546 65nm
ing device can be used to level sample briquettes and glass
and a half-peak transmittance bandwidth of less than 20 nm.
standards when they are mounted on microscope slides with
Insert the filter into the light path after the sample and before
modeling clay.
the photomultiplier tube.
6.8 Limiting Aperture—A limiting aperture made of nonre- 7. Test Specimen
flecting and opaque material shall be placed approximately in
7.1 PreparethesamplebriquetteinaccordancewithPractice
thefocalplaneoftheeyepieceatitscentralaxistorestrictlight
D2797.
tothephotomultipliertubewindowsothatonlyasmallareaof
the reflectance standard or sample is sensed. The diameter of
8. Setting Up and Calibrating the Apparatus
the aperture shall be selected to provide an effective field of
measurement (sensed spot) of about 5 µm diameter or about 8.1 Turn on the photometer and light source and allow
20-µm area. equipment to warm up for at least ⁄2 h.
D2798−11a (2019)
8.2 Mount the glass standards and a polished briquette glass reflectance standard. If values change during rotation
containing the sample on slides using modeling clay and a morethan0.03%,thenthesystemalignmentshallbechecked.
leveling press or use a leveling briquette holder.
8.12 Measure the same areas of the glass reflectance stan-
dards each time the calibration is made.
8.3 Place the mounted briquette on the stage, apply immer-
sionoil,andverifylevelingofthemountandstagebychecking
8.13 Glass standards should be cleaned at least once a
that there is no systematic focus change when the briquette is
month to avoid oxidation and changes in reflectances.
moved laterally on the stage. Use Köhler illumination. To
minimize glare, restrict the illuminated field by means of the
9. Procedure for Measuring Maximum Reflectance of a
field diaphragm so that the diameter is about one third or less
Sample
thanthesizeofthefullfield.Adjustanyotherprovisionsofthe
9.1 Immediately after calibrating the system, place a pol-
illuminator to reduce scattered light in the system.
ished briquette of the sample on the microscope stage and
8.4 Verify the position of the limiting aperture of the
apply immersion oil.
photometer with respect to the field of view. This can be done
9.2 Adopt a systematic scheme of transection of the bri-
by moving a small bright object of the sample across the
quette for selection of areas to be measured. Transect intervals
position of the crosshair or reticle that marks the photometer-
shall be such that the entire surface of the briquette or
sensed spot, ascertaining that readings are highest when the
briquettes will be sampled for the component being measured.
bright object is within the sensed area or by using back-lit
The transect spacing shall be suitable for a total of 100
illumination of the measuring aperture if so equipped.
measurements.
8.5 Using a small, distinctive feature of the sample as a
9.3 Using the procedure specified in 9.2, select the location
guide, adjust the microscope so that the axis of rotation of the
tobemeasured.Slightadjustmentstothemaceralpositionmay
stage is coincident with the photometer-sensed spot. This is
be made to obtain a scratch-free area of uniform appearance.
accomplished by adjusting the centering screws of the objec-
Rotate the circular stage slowly (approximately 4 r/min)
tive or stage. The purpose is to eliminate movement of the
through 360°. Reflectance will vary progressively from a
object grain or area from the sensed spot when the stage is
maximum value to a minimum value as the stage is rotated.
rotated.
Observeandrecordthemaximumvalue.Iftheeffectivefieldof
8.6 Adjust the polarizer to a 45° position when using a measurement does not remain on the component being mea-
sured when the stage is rotated, then recenter the objective or
Berek prism or 0° when using a Smith illuminator or glass
plate. Place a glass standard covered with clean immersion oil stage as described in 8.5.Avoid taking measurements of areas
that are near highly reflecting grains such as pyrite. Because
on the microscope stage and focus on the polished surface.
some relief and nonplanarity may develop during polishing,
8.7 With no light reflected from the standard to the
avoid edges of particles and particles near the edge of the
phototube, adjust the photometer zero setting or dark current.
briquette.
8.8 Place on the microscope stage a briquette of opaque
9.4 Move the stage to the next area to be measured and
resin that has a hole 5 mm in diameter and 5 mm deep which
repeat 9.3. Continue the location selecting and measuring
isfilledwithimmersionoil.Measurethereflectanceofthehole
procedure. After approximately ⁄2 h of operation, remove the
to ensure that a reflectance of 0.00 6 0.03% is obtained
briquette and recheck the calibration of the glass standards. If
thereby ensuring that parasitic reflectances of the objective are
this value indicates a drift equivalent to more than 0.01%
minimal.Ifthereflectanceoftheholeexceedsthestatedlimits,
reflectance of the initial standard reflectance value, discard the
then another objective having a higher quality anti-reflection
set of readings on the coal sample
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D2798 − 11a D2798 − 11a (Reapproved 2019)
Standard Test Method for
Microscopical Determination of the Vitrinite Reflectance of
Coal
This standard is issued under the fixed designation D2798; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the microscopical determination of both the mean maximum and mean random reflectances
measured in oil of polished surfaces of vitrinite and other macerals present in coals ranging in rank from lignite to anthracite. This
test method can be used to determine the reflectance of other macerals.
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 safety, health, and healthenvironmental 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.
2. Referenced Documents
2.1 ASTM Standards:
D121 Terminology of Coal and Coke
D388 Classification of Coals by Rank
D2797 Practice for Preparing Coal Samples for Microscopical Analysis by Reflected Light
3. Terminology
3.1 Definitions—For definitions of terms, refer to Terminology D121.
3.2 Abbreviations:
3.2.1 R max—mean maximum reflectance measured in oil.
o
3.2.2 R ran—mean random reflectance measured in oil.
o
3.2.2.1 Discussion—
Other organizations may use other abbreviations for mean maximum and mean random reflectance.
4. Summary of Test Method
4.1 The reflectance of the maceral vitrinite or other macerals is determined in this test method by illuminating a polished surface
of a section of coal in immersion oil using a microscopic system that photometrically measures the amount of light reflected from
the surface. The reflected light is recorded in percent reflectance after calibration of photometric equipment by measuring the
reflected light from standards of reflectance as calculated from their refractive indices.
This test method is under the jurisdiction of ASTM Committee D05 on Coal and Coke and is the direct responsibility of Subcommittee D05.28 on Petrographic Analysis
of Coal and Coke.
Current edition approved June 1, 2011Nov. 1, 2019. Published July 2011November 2019. Originally approved in 1969. Last previous edition approved in 2011 as
D2798 – 11.D2798 – 11a. DOI: 10.1520/D2798-11A.10.1520/D2798-11AR19.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2798 − 11a (2019)
5. Significance and Use
5.1 The mean maximum reflectance of the vitrinite component in coal as determined by this test method is often used as an
indicator of rank as presented in Classification D388, independent of petrographic composition, and in the characterization of coal
as feedstock for carbonization, gasification, liquefaction, and combustion processes.
5.2 This test method is for use in scientific and industrial research.
6. Apparatus
6.1 Microscope—Any microscope equipped for reflected light microscopy (such as a metallurgical or opaque-ore microscope)
can be used, provided the lens combination of objective and eyepieces permits examination of the specimen at a magnification
between 400 and × 750, such that particles of 1 μm can be resolved. The objectives shall be constructed so that samples can be
examined in oil with plane-polarized light and have the highest quality of antireflection coatings. The microscope shall be able to
project an image at similar magnification to a photomultiplier tube and to support the photomultiplier tube housing. Means shall
be provided to position the tube housing laterally to obtain maximum response. The microscope shall have a circular stage that
is capable of rotating a specimen through 360°. The mechanical stage attached to the microscope stage shall enable the analyst to
move the specimen accurately (within 0.1 mm) to a given field location. A combination of objective and circular stage shall permit
centering. The viewing eyepiece shall be supplied with a crosshair or grid to be used as a reference to locate precisely the area
sampled by the phototube. During measurement, no light shall be permitted to enter the observer’s end of the viewing eyepiece.
6.2 Polarizer and Illuminator—The light incident on the vertical illuminator of the microscope shall be plane-polarized by a
prism or sheet polarizer. The vertical illuminator can contain a Berek prism, a Smith illuminator, or high-quality glass plate. The
polarizer shall be oriented at 45° when using a Berek prism or at 0° when using a Smith illuminator or glass plate.
6.3 Photomultiplier Tube—In combination with the microscope optical system, light source, and filter used, the photomultiplier
photometer shall be capable of detecting the minimum light reflected from the limited portion of the coal sample (see 6.8). The
high voltage supplied to the photomultiplier tube must be within the prescribed range to obtain linearity of response. This is usually
from 300 to 1100 V for side-window tubes and from 1000 to 1500 V for end-window tubes. Photodiode arrays, channeltrons, or
other light-measuring devices are acceptable alternatives providing that sufficient gray levels obtainable will enable reliable
differentiation of signal equivalent to 0.01 % reflectance and that the system is linear in the range of the reflectance measured.
6.4 Photometer Amplifier—The signal from the photomultiplier tube shall be amplified and displayed by a galvanometer, digital
meter, or recorder. When adjusted for operation, the amplifier and meter shall be capable of reliably distinguishing differences in
signal equivalent to 0.01 % reflectance and shall be linear in the range of reflectance measured.
6.5 Recorder or Meter—The recorder or meter used shall have a response time at full scale of no more than 1 s to detect the
maximum reflectance level during rotation of the microscope stage.
6.6 Light Source—The light source shall have a regulated power supply to provide for stable output. Some photometers and
recorders require supplemental voltage-stabilizing transformers if the line voltage fluctuates.
6.7 Filters—The light shall be made approximately monochromatic green by passage through an interference filter or
combination of filters with peak transmittance of 546 6 5 nm and a half-peak transmittance bandwidth of less than 20 nm. Insert
the filter into the light path after the sample and before the photomultiplier tube.
6.8 Limiting Aperture—A limiting aperture made of nonreflecting and opaque material shall be placed approximately in the
focal plane of the eyepiece at its central axis to restrict light to the photomultiplier tube window so that only a small area of the
reflectance standard or sample is sensed. The diameter of the aperture shall be selected to provide an effective field of measurement
(sensed spot) of about 5 μm diameter or about 20-μm area.
6.9 Calibration Standards—Prisms constructed of high-index glasses or synthetic minerals shall be used as standards to
calibrate the photometer for reflectance measurement. These standards must be durable, isotropic, resistant to corrosion, free from
internal flaws or fractures, and have negligible light absorption. A prism with sides that form a 30-60-90° triangle is the most
effective shape, with the side between the 30 and 90° angles highly polished and used as the reflectance-measuring surface. The
prisms shall be enclosed, except for the polished surface, in a durable, lightabsorbent, water- and oil-resistant mount; polyester or
epoxy resin, made light absorbent with a dye or filler, serves adequately. It is desirable to have a number of different standards with
reflectances near those of the vitrinite studied; these also serve to check the linear response of the photometer. The reflectance of
each standard shall be calculated to the nearest 0.001 % by means of the following equation:
2 2
R 5 100~n 2 1.5180! /~n 11.5180! (1)
s g g
where:
R = standard reflectance in oil of the glass, % and
s
n = refractive index of the glass at 546-nm wavelength, to the nearest 0.0001 index value.
g
NOTE 1—Most coal laboratories in North America use the following Bausch and Lomb Co. or Schott Co. optical glasses (the reported refractive index
at 546 nm and the calculated standard reflectance in oil are given in parentheses):
D2798 − 11a (2019)
Bausch and Lomb Schott
689 309 (1.6935; 0.299 %) SF8-689-312 (1.6945; 0.303 %)
751 278 (1.7566; 0.532 %) SF13-714-276 (1.7477; 0.496 %)
827 250 (1.8351; 0.895 %) LaF12-836-423 (1.8400; 0.921 %)
850 324 (1.8543; 0.996 %) LaSF9-850-322 (1.8567; 1.009 %)
915 213 (1.9235; 1.390 %) LaSF18-913-325 (1.9273; 1.413 %)
980 222 (1.9907; 1.817 %) LaSF6-961-249 (1.9670; 1.662 %)
Other standards available that can be used include the following:
Leucosapphire (1.77; 0.59 %)
Yttrium aluminum garnet, YAG (1.84; 0.92 %)
Gadolinium gallium garnet, 3G (1.98; 1.73 %)
Silicon carbide (2.663; 7.52 %)
6.10 Immersion Oil—The oil shall be a nondrying, noncorrosive type that will not react with coal, does not contain carcinogens,
and has a refractive index within the range from 1.515 to 1.519 at 546 nm and 25°C.25 °C. Within the specified range, the refractive
index of the oil is not critical provided the specified value of 1.5180 is used in calculating reflectance of standards as specified in
6.9. Periodic checking of the refractive index of the oil is discretionary.
6.11 Sample-Leveling Press—A conventional manual leveling device can be used to level sample briquettes and glass standards
when they are mounted on microscope slides with modeling clay.
7. Test Specimen
7.1 Prepare the sample briquette in accordance with Practice D2797.
8. Setting Up and Calibrating the Apparatus
8.1 Turn on the photometer and light source and allow equipment to warm up for at least ⁄2 h.
8.2 Mount the glass standards and a polished briquette containing the sample on slides using modeling clay and a leveling press
or use a leveling briquette holder.
8.3 Place the mounted briquette on the stage, apply immersion oil, and verify leveling of the mount and stage by checking that
there is no systematic focus change when the briquette is moved laterally on the stage. Use Köhler illumination. To minimize glare,
restrict the illuminated field by means of the field diaphragm so that the diameter is about one third or less than the size of the
full field. Adjust any other provisions of the illuminator to reduce scattered light in the system.
8.4 Verify the position of the limiting aperture of the photometer with respect to the field of view. This can be done by moving
a small bright object of the sample across the position of the crosshair or reticle that marks the photometer-sensed spot, ascertaining
that readings are highest when the bright object is within the sensed area or by using back-lit illumination of the measuring aperture
if so equipped.
8.5 Using a small, distinctive feature of the sample as a guide, adjust the microscope so that the axis of rotation of the stage
is coincident with the photometer-sensed spot. This is accomplished by adjusting the centering screws of the objective or stage.
The purpose is to eliminate movement of the object grain or area from the sensed spot when the stage is rotated.
8.6 Adjust the polarizer to a 45° position when using a Berek prism or 0° when using a Smith illuminator or glass plate. Place
a glass standard covered with clean immersion oil on the microscope stage and focus on the polished surface.
8.7 With no light reflected from the standard to the phototube, adjust the photometer zero setting or dark current.
8.8 Place on the microscope stage a briquette of opaque resin that has a hole 5 mm in diameter and 5 mm deep which is filled
with immersion oil. Measure the reflectance of the hole to ensure that a reflectance of 0.00 6 0.03 % is obtained thereby ensuring
that parasitic reflectances of the objective are minimal. If the reflectance of the hole exceeds the stated limits, then another objective
having a higher quality anti-reflection coating shall be used.
8.9 Then allow the reflected light to impinge on the tube. Adjust the photometer amplifier or the illumination to obtain a meter
or recorder scale setting that conveniently represents the calculated reflectance of the glass standard (see Appendix X1).
8.10 Without changing the settings, measure the reflectance of one or more additional standards to check that the photometer
system measures correctly in the range to be studied.
8.10.1 Because the photometric system cannot give a linear response to a wide range of light flux, standards with reflectance
values close to that of the coal being measured should be used. At least two standards having reflectances that span the range of
the coal being measured should be used.
8.11 Make all standardization measurements under the same conditions used in measuring vitrinite reflectance. When measuring
mean maximum reflectance of vitrinite, rotate the stage through 360° and note the maximum reflectance value of the glass
reflectance standard. If values change during rotation more than 0.03 %, then the system alignment shall be checked.
8.12 Measure the same areas of the glass reflectance standards each time the calibration is made.
D2798 − 11a (2019)
8.13 Glass standards should be cleaned at least once a month to avoid oxidation and changes in reflectances.
9. Procedure for Measuring Maximum Reflectance of a Sample
9.1 Immediately after calibrating the system, place a polishe
...
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