ASTM D7708-23a

Standard Test Method for Microscopical Determination of the Reflectance of Vitrinite Dispersed in Sedimentary Rocks

SIGNIFICANCE AND USE
5.1 The mean reflectance of the vitrinite maceral in sedimentary rocks as determined by this test method is used as an indicator of thermal maturity, that is, the progressive geochemical alteration of dispersed organic material experienced during diagenesis, catagenesis, and metagenesis. In the case of hydrocarbon source rocks, three major categories of thermal maturity are defined by vitrinite reflectance: immature (Roran ≤ 0.5 %), mature (Roran ≈ 0.5 % to 1.35 %), and overmature (Roran ≥ 1.35 %) with respect to the generation of liquid hydrocarbons, although not all practitioners agree on these thermal boundaries (10). Thermal maturity as determined by the reflectance of vitrinite dispersed in sedimentary rocks is similar to the rank classification of coals as presented in Classification D388 and measured similarly to the reflectance of vitrinite in coal as presented in Test Method D2798. The mean reflectance of the vitrinite maceral in sedimentary rocks correlates with geochemically determined parameters of thermal maturity and can be used to characterize thermal maturation history, to calibrate burial history models, and to better understand the processes of hydrocarbon generation, migration, and accumulation in conventional and unconventional petroleum systems.
SCOPE
1.1 This test method covers the microscopical determination of the reflectance measured in immersion oil of polished surfaces of vitrinite dispersed in sedimentary rocks. This test method can also be used to determine the reflectance of macerals other than vitrinite dispersed in sedimentary rocks.  
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

• Status: Published
• Publication Date: 31-Aug-2023
• ICS: 91.100.15 - Mineral materials and products
• Technical Committee: D05 - Coal and Coke
• Drafting Committee: D05.28 - Petrographic Analysis of Coal and Coke

Relations

• Replaces: ASTM D7708-23 - Standard Test Method for Microscopical Determination of the Reflectance of Vitrinite Dispersed in Sedimentary Rocks
• Effective Date: 01-Sep-2023
• Refers: ASTM D121-15(2024) - Standard Terminology of Coal and Coke
• Effective Date: 01-Mar-2024
• Refers: ASTM D121-15 - Standard Terminology of Coal and Coke
• Effective Date: 01-Sep-2015

Overview

ASTM D7708-23a: Standard Test Method for Microscopical Determination of the Reflectance of Vitrinite Dispersed in Sedimentary Rocks is a globally recognized method developed by ASTM International. This standard provides a reliable approach to measuring the mean reflectance of vitrinite, an organic component found in sedimentary rocks, using reflected light microscopy in immersion oil. Vitrinite reflectance is a crucial parameter for assessing the thermal maturity of sedimentary rocks, which directly influences understanding the formation, generation, and accumulation of hydrocarbons in both conventional and unconventional petroleum systems.

Key Topics

• Vitrinite Reflectance as a Maturity Indicator:
  The method measures the mean reflectance of vitrinite macerals, serving as a direct indicator of the progressive geochemical alteration of organic material (‘thermal maturity’), including diagenesis, catagenesis, and metagenesis.

• Classification of Thermal Maturity:
  Based on vitrinite reflectance values:

  • Immature: Rₒran ≤ 0.5%
  • Mature: Rₒran ≈ 0.5% to 1.35%
  • Overmature: Rₒran ≥ 1.35% Note that specific boundaries may vary between practitioners.

• Correlation with Coal Rank:
  The determination of thermal maturity by this method is analogous to coal rank classification (see ASTM D388) and similar in procedure to the reflectance of vitrinite in coal (see ASTM D2798).

• Test Method Overview:

  • Uses a microscope-photometer system to measure light reflected from polished surfaces of vitrinite in sedimentary rocks.
  • Calibration is performed using standards of known reflectance and certified reference materials.
  • Measurements are done in immersion oil on prepared sample surfaces, ensuring accuracy and repeatability.

• Applicability Beyond Vitrinite:
  The method can also determine reflectance of macerals other than vitrinite, expanding its value for analysis of diverse organic components in geological samples.

Applications

• Hydrocarbon Exploration:
  Vitrinite reflectance data is fundamental in assessing the thermal maturation history of source rocks, crucial in petroleum system modeling, and in identifying regions with oil or gas generation potential.

• Burial History Modeling:
  The method is instrumental in calibrating geological models of sedimentary basin evolution and reconstructing burial and thermal histories.

• Petroleum System Analysis:
  Supports a better understanding of hydrocarbon generation, migration, and accumulation processes.

• Academic and Research Use:
  Used in academic studies for characterizing organic matter maturity in sedimentary basins, and in training and accreditation of organic petrographers.

• Quality Control in Drilling Projects:
  The test is used for characterizing core and cutting samples, supporting decisions on exploration and production.

Related Standards

• ASTM D121: Terminology of Coal and Coke
  Establishes key definitions relevant to organic petrography and maceral classification.

• ASTM D388: Classification of Coals by Rank
  Provides coal rank classifications correlated with vitrinite reflectance.

• ASTM D2797: Practice for Preparing Coal Samples for Microscopical Analysis
  Details sample preparation steps critical for accurate reflectance measurements.

• ASTM D2798: Microscopical Determination of the Vitrinite Reflectance of Coal
  Outlines a similar procedure for coal, complementing this standard for sedimentary rocks.

• ASTM E177 & E691: Guidance on Precision, Bias, and Interlaboratory Studies
  Offer best practices for ensuring measurement accuracy and statistical reliability.

Summary

ASTM D7708-23a is a vital method for geologists, petrographers, and energy industry professionals needing accurate assessment of sedimentary rock maturity via vitrinite reflectance microscopy. It enables effective resource evaluation, supports petroleum exploration and research, and aligns with international standards for reproducibility and reliability. For maximum benefit, this test method should be applied using proper safety and sample handling protocols, as outlined in the standard.

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: D7708 − 23a
Standard Test Method for
Microscopical Determination of the Reflectance of Vitrinite
Dispersed in Sedimentary Rocks
This standard is issued under the fixed designation D7708; 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 Determine the Precision of a Test Method
1.1 This test method covers the microscopical determination
3. Terminology
of the reflectance measured in immersion oil of polished
surfaces of vitrinite dispersed in sedimentary rocks. This test
3.1 Definitions—For definitions of terms, refer to Terminol-
method can also be used to determine the reflectance of
ogy D121.
macerals other than vitrinite dispersed in sedimentary rocks.
3.2 Abbreviations:
1.2 The values stated in SI units are to be regarded as
3.2.1 R ran—mean random reflectance measured in oil
o
standard. No other units of measurement are included in this
using a fixed microscope stage. Other organizations may use
standard.
other abbreviations for mean random reflectance.
1.3 This standard does not purport to address all of the
3.2.2 R max—mean apparent maximum reflectance mea-
o
safety concerns, if any, associated with its use. It is the
sured in oil using a fixed microscope stage and a rotating
responsibility of the user of this standard to establish appro-
polarizer in the incident light path. Other organizations may
priate safety, health, and environmental practices and deter-
use other abbreviations for mean apparent maximum reflec-
mine the applicability of regulatory limitations prior to use.
tance.
1.4 This international standard was developed in accor-
3.3 Definitions of Terms Specific to This Standard:
dance with internationally recognized principles on standard-
3.3.1 alginite, n—a primary liptinite maceral occurring in
ization established in the Decision on Principles for the
structured morphologies, telalginite, and unstructured
Development of International Standards, Guides and Recom-
morphologies, lamalginite.
mendations issued by the World Trade Organization Technical
3.3.2 bituminite, n—an amorphous primary liptinite maceral
Barriers to Trade (TBT) Committee.
with low reflectance, occasionally characterized by colored
internal reflections in reflected white light and weak orange- to
2. Referenced Documents
2 brown fluorescence, derived from bacterial biomass and the
2.1 ASTM Standards:
bacterial decomposition of algal material and faunal plankton
D121 Terminology of Coal and Coke
(1). Bituminite is equivalent to the amorphous organic matter
D388 Classification of Coals by Rank
recognized in strew slides of concentrated kerogen (2).
D2797 Practice for Preparing Coal Samples for Microscopi-
3.3.2.1 Discussion—Bituminite may be distinguished from
cal Analysis by Reflected Light
vitrinite by lower reflectance, as well as higher fluorescence
D2798 Test Method for Microscopical Determination of the
intensity if fluorescence is present in vitrinite. Bituminite has
Vitrinite Reflectance of Coal
poorly-defined wispy boundaries and may be speckled or
E177 Practice for Use of the Terms Precision and Bias in
unevenly colored from dark brown, dark gray, to almost black
ASTM Test Methods
in reflected white light, whereas vitrinite has distinct boundar-
E691 Practice for Conducting an Interlaboratory Study to
ies and is blockier and evenly colored. The occurrence of
bituminite in association with lamalginite and micrinite is
common. Bituminite may be expected to occur in lacustrine or
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
marine settings. It is less commonly present in fluvial or similar
Analysis of Coal and Coke.
proximal depositional environments, where vitrinite may be
Current edition approved Sept. 1, 2023. Published October 2023. Originally
expected to occur in greater abundance.
approved in 2011. Last previous edition approved in 2023 as D7708 – 23. DOI:
10.1520/D7708-23A.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The boldface numbers in parentheses refer to a list of references at the end of
the ASTM website. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7708 − 23a
3.3.3 chitinozoan, n—a group of flask-shaped, sometimes 3.3.12 liptinite, n—maceral group with macerals that exhibit
ornamented marine microfossils of presumed metazoan origin lower reflectance than other organic components in the same
which are composed of ‘pseudochitin’ proteinic material and sample of sedimentary rocks and coal, appearing black to dark
which occur individually or in chains. Chitinozoan cell walls gray in reflected white light and that fluoresce under blue to
are thin, opaque to translucent, and range from dark gray to ultraviolet light in coals ranked medium volatile bituminous
white in reflected white light similar to vitrinite. Chitinozoans and lower. Liptinite maceral fluorescence can be used as a
are common in Ordovician to Devonian marine shales. qualitative thermal maturity indicator as fluorescence changes
from green to yellow to orange before becoming extinguished
3.3.4 conodont, n—the phosphatic, tooth-like remains of
at advanced thermal maturity.
marine vertebrate worm-like animals present from the Cam-
3.3.12.1 Discussion—Liptinite macerals are observed only
brian through Triassic, composed predominantly of apatite
in coals of coal rank or degree of coalification up to approxi-
with subordinate amounts of organic matter. Conodont mor-
mately the high volatile bituminous to medium volatile bitu-
phology is variable, but often well-defined denticles and blades
minous transition, and in sedimentary rocks of equivalent
are preserved. In reflected white light examination conodonts
thermal maturity up to the late stages of oil generation and
range from pale yellow to light brown to dark brown and to
early stages of gas generation. Liptinite macerals undergo
black.
chemical changes during maturation which render their optical
3.3.5 fusinite, n—an inertinite maceral distinguished princi-
distinction from vitrinite and inertinite macerals difficult at
pally by the preservation of some feature(s) of the plant cell
rank higher than medium volatile bituminous.
wall structure, high relief, and reflectance substantially higher
3.3.13 maceral, n—an organic component occurring in sedi-
than first cycle vitrinite in the same sample. When less than
mentary rocks that is distinguished on the basis of its optical
50-μm in size this maceral is assigned to inertodetrinite. Other
microscopic properties, primarily reflectance and morphology.
organizations may define macerals using different technical
specifications. 3.3.14 maceral classification, n—the systematic division of
the organic components (macerals) in sedimentary rocks and
3.3.6 graptolite, n—colonial, chitinous animal which occurs
coal based on their appearance in the optical microscope under
as thin, elongate bodies sometimes showing complex skeletal
reflected white light and epi-fluorescence.
morphology and with reflective dark gray to white color in
reflected white light similar to vitrinite (3). Graptolites occur 3.3.15 micrinite, n—an inertinite maceral, generally
from the Cambrian through Carboniferous.
nonangular, exhibiting no relict plant cell wall structure,
smaller than 10 μm and most commonly occurring as granular
3.3.7 huminite, n—maceral group present in lignite, some
particles around 1 μm to 5 μm diameter. Other organizations
subbituminous coal, and thermally immature sedimentary
may define macerals using different technical specifications.
rocks with reflectances intermediate to those of associated
3.3.15.1 Discussion—Micrinite is a secondary maceral
darker liptinites and brighter inertinites in reflected white light
formed from liptinite macerals during maturation. Other ori-
(4). The huminite maceral group is equivalent to the vitrinite
gins may be possible, including bacterial degradation of
maceral group that occurs in some subbituminous and higher
liptinite macerals, or diffuse scattering of reflected light from
rank coals with measured reflectance values greater than 0.5 %
inorganic constituents.
R ran(5).
o
3.3.16 mineral matter, n—in sedimentary rocks, the non-
3.3.8 inertinite, n—maceral group with macerals that exhibit
organic fraction composed of physically discrete particles of
higher reflectance than other organic components in the same
minerals such as clays, pyrite, quartz, carbonates, etc., and all
sample: for example, semifusinite, fusinite, and inertodetrinite.
elements other than carbon, hydrogen, oxygen, nitrogen and
Inertinite macerals generally lack fluorescence and usually
sulfur in the organic fraction.
retain preserved plant cell wall structure (6). Individual mac-
erals of the inertinite group generally are not distinguished in
3.3.17 recycled vitrinite, n—vitrinite that has undergone at
practice when dispersed in sedimentary rocks (7). least one additional cycle of burial, exhumation, and erosion in
contrast to first cycle vitrinite which has undergone only one
3.3.9 inertodetrinite, n—an inertinite maceral occurring as
burial cycle. The additional cycle may result in exposure to
individual, angular, clastic fragments incorporated within the
thermal maturation, chemical or thermal oxidative processes,
matrix of other macerals (commonly vitrinite) or minerals, and
or both, and mechanical abrasion (sometimes resulting in
in the size range from 2 μm to 50 μm. Other organizations may
increased particle rounding) that is not experienced by first
define macerals using different technical specifications.
cycle vitrinite contained in the same sample.
3.3.9.1 Discussion—Inertodetrinite is derived through the
3.3.17.1 Discussion—Recycled vitrinite has higher reflec-
disintegration of other inertinite macerals, that is, fusinite and
tance than co-occurring first cycle vitrinite, and sometimes is
semifusinite, by mechanical abrasion during transport.
less angular, due to the rounding of grain boundaries experi-
3.3.10 kerogen, n—fraction of dispersed or concentrated
enced during transportation. Recycled vitrinite may have bright
organic matter, or both, occurring in sediments and sedimen-
or dark halos, representing thermal oxidation and weathering
tary rocks that is insoluble in non-polar organic solvents.
processes, respectively, which are not present in the co-
3.3.11 lamalginite, n—an unstructured liptinite maceral occurring first cycle vitrinite. Recycled vitrinite has a higher
with low reflectance distinguished primarily by the presence of variance of reflectance values, representative of the many
fluorescence and lamellar character. possible sources and processes occurring during transportation,
D7708 − 23a
and may show greater relief than first cycle vitrinite in the same insertion of a post-sample analyzer into a polarized light path
sample. Recycling of vitrinite may be inferred from the may help to distinguish mosaic bitumens. Solid bitumens may
geologic context; for example, a higher proportion of recycled be deposited in voids and fractures with orientations normal to
vitrinite may be observed in a catchment collecting sediments parallel to the sedimentary bedding. Solid bitumens may occur
derived from an active orogenic belt. as droplets and may be translucent (recognized by reflections
from pyrite inclusions) and contain pyrite crystals at edges.
3.3.18 scolecodont, n—the chitinous, variably mineralized
Rock type, thermal maturity, and geologic occurrence can be
fossil remains of the jaw elements of polychaete annelid
used to interpret the potential presence of solid bitumens; for
worms, which occur as lamellar to tooth-like structures with
example, bitumens may be present if the sample is or occurs in
spongy, laminated, or granular texture, and with reflective dark
proximity to a thermally mature hydrocarbon source rock or if
gray to white color similar to vitrinite. Scolecodonts occur
the sample is from an exhumed oil reservoir. Solid bitumens
from the Ordovician to recent.
can be physically associated with bituminite or other liptinite
3.3.19 semifusinite, n—an inertinite maceral with morphol-
macerals from which they are derived. Some solid bitumens
ogy like fusinite sometimes with less distinct evidence of
are soluble in non-polar organic solvents and may be distin-
cellular structure, and with reflectance ranging from slightly
guished from vitrinite in low maturity source rocks by low
greater than that of the associated vitrinite to that of the least
magnification observation of fluorescence streaming after pi-
reflective fusinite. Semifusinite may show irregular mosaic
petted solvation of the examination surface.
texture or satin anisotropy when viewed under polarized
3.3.21 telalginite, n—a liptinite maceral characterized by
reflected white light.
strong fluorescence and structured morphologies at lower
3.3.19.1 Discussion—Low-reflecting semifusinite may be
thermal maturity conditions. Common botanical varieties in-
distinguished from vitrinite by higher reflectance and relief,
clude Botryococcus, a freshwater to brackish water indicator,
and the presence of more arcuate boundaries. The most reliable
and Tasmanites, a marine indicator. Fluorescence intensity
distinguishing feature of low-reflecting semifusinite is the
diminishes and fluorescence color shifts toward red wave-
frequent presence of well-preserved cellular structure or open
lengths with increasing thermal maturity in the oil window.
cell lumens, or both. However, it is not unusual for cell lumens
to also remain open in vitrinite when deposited in clay-rich 3.3.22 thermal maturity, n—the degree of thermal matura-
sediments. Semifusinite usually has more distinct particle
tion of the dispersed organic matter contained in sedimentary
boundaries, which distinguishes it from vitrinite which has a rocks, synonymous with coal rank, and commonly related to oil
more porous and textured surface. Geologic context is impor-
generation. Thermal maturity of organic matter in sedimentary
tant; a greater proportion of semifusinite can be expected in rocks commonly is well-defined by vitrinite reflectance. In
sediments or coals associated with more arid locations, addition, spectral fluorescence, X-ray diffraction
climates, and time periods. crystallography, or organic geochemical parameters can be
applied to derive the level of thermal maturity.
3.3.20 solid bitumen, n—a secondary maceral associated
with hydrocarbon generation from sedimentary organic matter. 3.3.23 vitrinite, n—vitrinite dispersed in Upper Silurian (9)
Solid bitumen is distinguished primarily by its conformation to and stratigraphically younger age sedimentary rocks is the
pores, voids and fractures in the rock matrix, microtextural remains of coalified humic material from vascular land plants.
embayment by authigenic mineral grains, and the absence of Vitrinite dispersed in sedimentary rocks may be representative
features such as cellular structure indicating derivation from of a large variety of precursor plant materials with differing
precursor plant material. Solid bitumens may show homog- original chemistries and structures. Vitrinite typically occurs as
enous or granular textures; irregular anisotropic mosaic tex- finely comminuted dark gray to white particles (in reflected
tures also are common, particularly at advanced stages of white light) of sizes less than 100 μm, and usually only 5 μm to
thermal maturity (8). Solid bitumens may exhibit fluorescence 10 μm, dispersed throughout the mineral matrix although
and zonation at low thermal maturity. particles of larger size can also be present. Vitrinite dispersed
3.3.20.1 Discussion—For the purpose of reflectance mea- in sedimentary rocks may infrequently occur as fragments of
surement it is important to distinguish solid bitumen from coal, which includes other macerals, such as inertinite and
vitrinite since both macerals appear gray under reflected white liptinite.
light and the reflectance of both advances with increasing 3.3.23.1 Discussion—The identification of the primary vi-
maturity. Several populations of solid bitumen with distinct trinite (first cycle vitrinite) population is essential for deter-
reflectance ranges can be present in a single whole-rock mining the thermal maturity experienced by organic matter in
sample. Solid bitumens are characterized by their pore-filling a sedimentary rock. This can be complicated by: the chemical
or anastamosing forms. Boundaries of solid bitumen can be and structural heterogeneity of dispersed vitrinite reflecting
well-defined by micro-textural embayment by authigenic min- multiple sources; the presence of similar organic components
erals such as calcite and dolomite that commonly form resembling vitrinite, including solid bitumen, bituminite, re-
contemporaneously with solid bitumen formation. However, cycled vitrinite, low-reflecting semifusinite, and zooclasts;
vitrinite can be replaced by authigenic minerals and therefore vitrinite reflectance retardation or suppression, or both; altera-
micro-textures indicative of embayment or mineral inclusion tion by oxidation or weathering from sample handling or by
are not always diagnostic of solid bitumen. Solid bitumen may exposure to the atmosphere at outcrop; and the potential for
exhibit mosaic anisotropic domains at higher thermal maturity, contamination such as cavings and drilling mud additives in the
whereas vitrinite does not. Use of cross-polarized light by case of drill cuttings. The term vitrinite is currently used as
D7708 − 23a
both a maceral and maceral group. Individual macerals of the 4.1.2 The scholarly organization International Committee
vitrinite group generally are not distinguished in practice when for Coal and Organic Petrology hosts a biennial accreditation
dispersed in sedimentary rocks (7). program for organic petrographers using D7708 to measure
reflectance of vitrinite dispersed in sedimentary rocks.
3.3.24 vitrinite reflectance retardation, n—a reduction in
vitrinite reflectance values below thermal maturity levels
5. Significance and Use
determined by geochemical or other petrographic parameters.
5.1 The mean reflectance of the vitrinite maceral in sedi-
Vitrinite reflectance retardation occurs due to decreased reac-
mentary rocks as determined by this test method is used as an
tion rate and inhibition of the rearrangement of vitrinite
indicator of thermal maturity, that is, the progressive geo-
molecular structure principally as a result of overpressure.
chemical alteration of dispersed organic material experienced
3.3.24.1 Discussion—The presence of vitrinite reflectance
during diagenesis, catagenesis, and metagenesis. In the case of
retardation can only be assessed if other thermal maturity
hydrocarbon source rocks, three major categories of thermal
parameters are available for the same sample or if vitrinite
maturity are defined by vitrinite reflectance: immature (R ran ≤
o
reflectance data from different depths or locations in an area are
0.5 %), mature (R ran ≈ 0.5 % to 1.35 %), and overmature
o
available for comparison. Vitrinite reflectance retardation can-
(R ran ≥ 1.35 %) with respect to the generation of liquid
o
not be assessed from the reflectance result of a single sample or
hydrocarbons, although not all practitioners agree on these
the appearance of a single vitrinite particle.
thermal boundaries (10). Thermal maturity as determined by
3.3.25 vitrinite reflectance suppression, n—a reduction in
the reflectance of vitrinite dispersed in sedimentary rocks is
vitrinite reflectance values below thermal maturity levels
similar to the rank classification of coals as presented in
determined by geochemical or other petrographic parameters.
Classification D388 and measured similarly to the reflectance
Vitrinite reflectance suppression is related to factors causing
of vitrinite in coal as presented in Test Method D2798. The
decreased reaction rate and inhibition of the rearrangement of
mean reflectance of the vitrinite maceral in sedimentary rocks
vitrinite molecular structure. Suppression factors may include
correlates with geochemically determined parameters of ther-
an atypical hydrogen-rich vitrinite chemistry inherited from the
mal maturity and can be used to characterize thermal matura-
precursor plant material or introduced into the vitrinite by the
tion history, to calibrate burial history models, and to better
chemical microenviroments of deposition, diagenesis, and
understand the processes of hydrocarbon generation,
catagenesis, among other causes.
migration, and accumulation in conventional and unconven-
3.3.25.1 Discussion—The presence of vitrinite reflectance tional petroleum systems.
suppression can only be assessed if other thermal maturity
6. Apparatus
parameters are available for the same sample or if vitrinite
6.1 Microscope—Any microscope equipped for reflected
reflectance data from different depths or locations in an area are
light microscopy (such as an upright research metallurgical or
available for comparison. Vitrinite reflectance suppression
opaque-ore microscope) can be used. The microscope shall be
cannot be assessed from the reflectance result of a single
able to project an image to a photomultiplier tube or other light
sample or the appearance of a single vitrinite particle.
detection system and to support the photomultiplier tube/light
3.3.26 zooclast, n—faunal relics such as chitinozoans,
detection system housing.
graptolites, scolecodonts, and conodonts which may show
6.2 Light Sources—The white light source used for measur-
similar optical properties to dispersed vitrinite (reflective dark
ing reflectance shall have a regulated power supply to provide
gray to white color) in reflected white light and which increase
for stable output. White light delivered from a 12 V 100 W
in reflectance with increasing thermal maturity. The reflectance
tungsten halogen bulb is routinely employed; other illumina-
of zooclasts may be measured and used for thermal maturity
tion devices such as LEDs are acceptable provided they have
information of sedimentary rocks of pre-Upper Silurian age
similar emission spectra to that from tungsten halogen. Some
which do not contain vitrinite, or in addition to vitrinite
lamps require supplemental voltage-stabilizing transformers if
reflectance in Upper Silurian and younger rocks.
the line voltage fluctuates. The microscope may also be
equipped with low wavelength fluorescence illumination from
4. Summary of Test Method
mercury or xenon gas discharge lamps, LEDs, or other devices
4.1 In this test method, light reflected from a polished
with similar emission spectra. A beam-splitting mirror is used
surface of vitrinite or other organic matter type is measured by
to switch illumination sources.
a microscope-photometer or other detection system. The re-
6.3 Vertical Illuminator and Polarizer—The vertical illumi-
flected light is recorded in percent reflectance after calibration
nator can contain a Berek prism, a Smith illuminator, or a
of the detection equipment. The calibration of the detection
high-quality glass plate. Some fixed-stage microscope systems
equipment is accomplished by measuring light reflected from
employ a computer-controlled, rotating, gearcoupled polarizer
standards of known reflectance as calculated from their refrac-
in the incident light path for apparent maximum reflectance
tive indices (see 6.13, Calibration Standards), and measured
measurements, with data acquisition and data processing con-
against reference standards. All measurements are made on
trolled by a computer or microcontroller.
surfaces covered by an immersion oil.
4.1.1 Color photomicrographs of vitrinite and other organic 6.4 Fluorescence Filter Set—For fluorescence microscopy,
materials dispersed in sedimentary rocks are available from the microscope can be equipped with appropriate filter sets
various publications and websites. designed to observe the fluorescence emission spectra of the
D7708 − 23a
sample. Typically, the sets contain a bandpass excitation filter, 6.11 Photometer Amplifier—The signal from the photomul-
a long pass beam splitter, which serves as the vertical tiplier tube or detector shall be amplified and displayed by a
illuminator, and a long pass emission filter. galvanometer, digital meter, or recorder. When adjusted for
operation, the amplifier and meter shall be capable of reliably
6.5 Field Diaphragm—The light incident on the vertical
distinguishing differences in signal equivalent to 0.01 % reflec-
illuminator of the microscope shall be limited by an adjustable
tance and shall be linear in the range of reflectance measured.
or fixed diaphragm field stop that should close to approxi-
6.12 Ocular—The viewing ocular shall be supplied with a
mately ⁄3 of the field or smaller as projected on the image. An
crosshair or grid to be used as a reference to locate precisely
adjustable field stop shall be limited by means of a set screw or
the area sampled by the phototube. During measurement, no
similar mechanism so as to close to precisely the same
light shall be permitted to enter the observer’s end of the
diameter each time it is employed.
viewing ocular.
6.6 Objective—The oil immersion microscope objective
6.13 Calibration Standards—Prisms constructed of high-
shall be constructed of high quality lenses with anti-reflection
index glasses or synthetic minerals shall be used as standards
coatings such that a minimum of stray light enters the light
to calibrate the detection system for reflectance measurement.
path. The combined magnification of objective and oculars
These standards must be durable, isotropic, resistant to corro-
shall permit examination of the specimen at a magnification
sion and tarnish, free from internal flaws or fractures, and have
between 400× and 750×, such that particles of 1 μm can be
negligible light absorption. A prism with sides that form a
resolved. Objectives of 40× or 50× magnification are routinely
30°-60°-90° triangle is the most effective shape, with the side
employed with oculars of 10× magnification.
between the 30° and 90° angles highly polished and used as the
6.7 Stage—The microscope stage can be capable of rotating
reflectance-measuring surface. The prisms shall be enclosed,
through 360° or can be fixed. The mechanical stage a
...

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: D7708 − 23 D7708 − 23a
Standard Test Method for
Microscopical Determination of the Reflectance of Vitrinite
Dispersed in Sedimentary Rocks
This standard is issued under the fixed designation D7708; 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 the reflectance measured in immersion oil of polished surfaces of
vitrinite dispersed in sedimentary rocks. This test method can also be used to determine the reflectance of macerals other than
vitrinite dispersed in sedimentary rocks.
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.
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
D2798 Test Method for Microscopical Determination of the Vitrinite Reflectance of Coal
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions—For definitions of terms, refer to Terminology D121.
3.2 Abbreviations:
3.2.1 R ran—mean random reflectance measured in oil using a fixed microscope stage. Other organizations may use other
o
abbreviations for mean random reflectance.
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 Jan. 1, 2023Sept. 1, 2023. Published February 2023October 2023. Originally approved in 2011. Last previous edition approved in 20142023 as
D7708 – 14. DOI: 10.1520/D7708-23.23. DOI: 10.1520/D7708-23A.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. 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
D7708 − 23a
3.2.2 R max—mean apparent maximum reflectance measured in oil using a fixed microscope stage and a rotating polarizer in the
o
incident light path. Other organizations may use other abbreviations for mean apparent maximum reflectance.
3.3 Definitions of Terms Specific to This Standard:
3.3.1 alginite, n—a primary liptinite maceral occurring in structured morphologies, telalginite, and unstructured morphologies,
lamalginite.
3.3.2 bituminite, n—an amorphous primary liptinite maceral with low reflectance, occasionally characterized by colored internal
reflections in reflected white light and weak orange- to brown fluorescence, derived from bacterial biomass and the bacterial
decomposition of algal material and faunal plankton (1). Bituminite is equivalent to the amorphous organic matter recognized in
strew slides of concentrated kerogen (2).
3.3.2.1 Discussion—
Bituminite may be distinguished from vitrinite by lower reflectance, as well as higher fluorescence intensity if fluorescence is
present in vitrinite. Bituminite has poorly-defined wispy boundaries and may be speckled or unevenly colored from dark brown,
dark gray, to almost black in reflected white light, whereas vitrinite has distinct boundaries and is blockier and evenly colored. The
occurrence of bituminite in association with lamalginite and micrinite is common. Bituminite may be expected to occur in
lacustrine or marine settings. It is less commonly present in fluvial or similar proximal depositional environments, where vitrinite
may be expected to occur in greater abundance.
3.3.3 chitinozoan, n—a group of flask-shaped, sometimes ornamented marine microfossils of presumed metazoan origin which are
composed of ‘pseudochitin’ proteinic material and which occur individually or in chains. Chitinozoan cell walls are thin, opaque
to translucent, and range from dark gray to white in reflected white light similar to vitrinite. Chitinozoans are common in
Ordovician to Devonian marine shales.
3.3.4 conodont, n—the phosphatic, tooth-like remains of marine vertebrate worm-like animals present from the Cambrian through
Triassic, composed predominantly of apatite with subordinate amounts of organic matter. Conodont morphology is variable, but
often well-defined denticles and blades are preserved. In reflected white light examination conodonts range from pale yellow to
light brown to dark brown and to black.
3.3.5 fusinite, n—an inertinite maceral distinguished principally by the preservation of some feature(s) of the plant cell wall
structure, high relief, and reflectance substantially higher than first cycle vitrinite in the same sample. When less than 50-μm in
size this maceral is assigned to inertodetrinite. Other organizations may define macerals using different technical specifications.
3.3.6 graptolite, n—colonial, chitinous animal which occurs as thin, elongate bodies sometimes showing complex skeletal
morphology and with reflective dark gray to white color in reflected white light similar to vitrinite (3). Graptolites occur from the
Cambrian through Carboniferous.
3.3.7 huminite, n—maceral group present in lignite, some subbituminous coal, and thermally immature sedimentary rocks with
reflectances intermediate to those of associated darker liptinites and brighter inertinites in reflected white light (4). The huminite
maceral group is equivalent to the vitrinite maceral group that occurs in some subbituminous and higher rank coals with measured
reflectance values greater than 0.5 % R ran(5).
o
3.3.8 inertinite, n—maceral group with macerals that exhibit higher reflectance than other organic components in the same sample:
for example, semifusinite, fusinite, and inertodetrinite. Inertinite macerals generally lack fluorescence and usually retain preserved
plant cell wall structure (6). Individual macerals of the inertinite group generally are not distinguished in practice when dispersed
in sedimentary rocks (7).
3.3.9 inertodetrinite, n—an inertinite maceral occurring as individual, angular, clastic fragments incorporated within the matrix of
other macerals (commonly vitrinite) or minerals, and in the size range from 2 μm to 50 μm. Other organizations may define
macerals using different technical specifications.
3.3.9.1 Discussion—
Inertodetrinite is derived through the disintegration of other inertinite macerals, that is, fusinite and semifusinite, by mechanical
abrasion during transport.
The boldface numbers in parentheses refer to a list of references at the end of this standard.
D7708 − 23a
3.3.10 kerogen, n—fraction of dispersed or concentrated organic matter, or both, occurring in sediments and sedimentary rocks
that is insoluble in non-polar organic solvents.
3.3.11 lamalginite, n—an unstructured liptinite maceral with low reflectance distinguished primarily by the presence of
fluorescence and lamellar character.
3.3.12 liptinite, n—maceral group with macerals that exhibit lower reflectance than other organic components in the same sample
of sedimentary rocks and coal, appearing black to dark gray in reflected white light and that fluoresce under blue to ultraviolet light
in coals ranked medium volatile bituminous and lower. Liptinite maceral fluorescence can be used as a qualitative thermal maturity
indicator as fluorescence changes from green to yellow to orange before becoming extinguished at advanced thermal maturity.
3.3.12.1 Discussion—
Liptinite macerals are observed only in coals of coal rank or degree of coalification up to approximately the high volatile
bituminous to medium volatile bituminous transition, and in sedimentary rocks of equivalent thermal maturity up to the late stages
of oil generation and early stages of gas generation. Liptinite macerals undergo chemical changes during maturation which render
their optical distinction from vitrinite and inertinite macerals difficult at rank higher than medium volatile bituminous.
3.3.13 maceral, n—an organic component occurring in sedimentary rocks that is distinguished on the basis of its optical
microscopic properties, primarily reflectance and morphology.
3.3.14 maceral classification, n—the systematic division of the organic components (macerals) in sedimentary rocks and coal
based on their appearance in the optical microscope under reflected white light and epi-fluorescence.
3.3.15 micrinite, n—an inertinite maceral, generally nonangular, exhibiting no relict plant cell wall structure, smaller than 10 μm
and most commonly occurring as granular particles around 1 μm to 5 μm diameter. Other organizations may define macerals using
different technical specifications.
3.3.15.1 Discussion—
Micrinite is a secondary maceral formed from liptinite macerals during maturation. Other origins may be possible, including
bacterial degradation of liptinite macerals, or diffuse scattering of reflected light from inorganic constituents.
3.3.16 mineral matter, n—in sedimentary rocks, the non-organic fraction composed of physically discrete particles of minerals
such as clays, pyrite, quartz, carbonates, etc., and all elements other than carbon, hydrogen, oxygen, nitrogen and sulfur in the
organic fraction.
3.3.17 recycled vitrinite, n—vitrinite that has undergone at least one additional cycle of burial, exhumation, and erosion in contrast
to first cycle vitrinite which has undergone only one burial cycle. The additional cycle may result in exposure to thermal
maturation, chemical or thermal oxidative processes, or both, and mechanical abrasion (sometimes resulting in increased particle
rounding) that is not experienced by first cycle vitrinite contained in the same sample.
3.3.17.1 Discussion—
Recycled vitrinite has higher reflectance than co-occurring first cycle vitrinite, and sometimes is less angular, due to the rounding
of grain boundaries experienced during transportation. Recycled vitrinite may have bright or dark halos, representing thermal
oxidation and weathering processes, respectively, which are not present in the co-occurring first cycle vitrinite. Recycled vitrinite
has a higher variance of reflectance values, representative of the many possible sources and processes occurring during
transportation, and may show greater relief than first cycle vitrinite in the same sample. Recycling of vitrinite may be inferred from
the geologic context; for example, a higher proportion of recycled vitrinite may be observed in a catchment collecting sediments
derived from an active orogenic belt.
3.3.18 scolecodont, n—the chitinous, variably mineralized fossil remains of the jaw elements of polychaete annelid worms, which
occur as lamellar to tooth-like structures with spongy, laminated, or granular texture, and with reflective dark gray to white color
similar to vitrinite. Scolecodonts occur from the Ordovician to recent.
3.3.19 semifusinite, n—an inertinite maceral with morphology like fusinite sometimes with less distinct evidence of cellular
structure, and with reflectance ranging from slightly greater than that of the associated vitrinite to that of the least reflective fusinite.
Semifusinite may show irregular mosaic texture or satin anisotropy when viewed under polarized reflected white light.
3.3.19.1 Discussion—
Low-reflecting semifusinite may be distinguished from vitrinite by higher reflectance and relief, and the presence of more arcuate
D7708 − 23a
boundaries. The most reliable distinguishing feature of low-reflecting semifusinite is the frequent presence of well-preserved
cellular structure or open cell lumens, or both. However, it is not unusual for cell lumens to also remain open in vitrinite when
deposited in clay-rich sediments. Semifusinite usually has more distinct particle boundaries, which distinguishes it from vitrinite
which has a more porous and textured surface. Geologic context is important; a greater proportion of semifusinite can be expected
in sediments or coals associated with more arid locations, climates, and time periods.
3.3.20 solid bitumen, n—a secondary maceral associated with hydrocarbon generation from sedimentary organic matter. Solid
bitumen is distinguished primarily by its conformation to pores, voids and fractures in the rock matrix, microtextural embayment
by authigenic mineral grains, and the absence of features such as cellular structure indicating derivation from precursor plant
material. Solid bitumens may show homogenous or granular textures; irregular anisotropic mosaic textures also are common,
particularly at advanced stages of thermal maturity (8). Solid bitumens may exhibit fluorescence and zonation at low thermal
maturity.
3.3.20.1 Discussion—
For the purpose of reflectance measurement it is important to distinguish solid bitumen from vitrinite since both macerals appear
gray under reflected white light and the reflectance of both advances with increasing maturity. Several populations of solid bitumen
with distinct reflectance ranges can be present in a single whole-rock sample. Solid bitumens are characterized by their pore-filling
or anastamosing forms. Boundaries of solid bitumen can be well-defined by micro-textural embayment by authigenic minerals such
as calcite and dolomite that commonly form contemporaneously with solid bitumen formation. However, vitrinite can be replaced
by authigenic minerals and therefore micro-textures indicative of embayment or mineral inclusion are not always diagnostic of
solid bitumen. Solid bitumen may exhibit mosaic anisotropic domains at higher thermal maturity, whereas vitrinite does not. Use
of cross-polarized light by insertion of a post-sample analyzer into a polarized light path may help to distinguish mosaic bitumens.
Solid bitumens may be deposited in voids and fractures with orientations normal to parallel to the sedimentary bedding. Solid
bitumens may occur as droplets and may be translucent (recognized by reflections from pyrite inclusions) and contain pyrite
crystals at edges. Rock type, thermal maturity, and geologic occurrence can be used to interpret the potential presence of solid
bitumens; for example, bitumens may be present if the sample is or occurs in proximity to a thermally mature hydrocarbon source
rock or if the sample is from an exhumed oil reservoir. Solid bitumens can be physically associated with bituminite or other liptinite
macerals from which they are derived. Some solid bitumens are soluble in non-polar organic solvents and may be distinguished
from vitrinite in low maturity source rocks by low magnification observation of fluorescence streaming after pipetted solvation of
the examination surface.
3.3.21 telalginite, n—a liptinite maceral characterized by strong fluorescence and structured morphologies at lower thermal
maturity conditions. Common botanical varieties include Botryococcus, a freshwater to brackish water indicator, and Tasmanites,
a marine indicator. Fluorescence intensity diminishes and fluorescence color shifts toward red wavelengths with increasing thermal
maturity in the oil window.
3.3.22 thermal maturity, n—the degree of thermal maturation of the dispersed organic matter contained in sedimentary rocks,
synonymous with coal rank, and commonly related to oil generation. Thermal maturity of organic matter in sedimentary rocks
commonly is well-defined by vitrinite reflectance. In addition, spectral fluorescence, X-ray diffraction crystallography, or organic
geochemical parameters can be applied to derive the level of thermal maturity.
3.3.23 vitrinite, n—vitrinite dispersed in Upper Silurian (9) and stratigraphically younger age sedimentary rocks is the remains of
coalified humic material from vascular land plants. Vitrinite dispersed in sedimentary rocks may be representative of a large variety
of precursor plant materials with differing original chemistries and structures. Vitrinite typically occurs as finely comminuted dark
gray to white particles (in reflected white light) of sizes less than 100 μm, and usually only 5 μm to 10 μm, dispersed throughout
the mineral matrix although particles of larger size can also be present. Vitrinite dispersed in sedimentary rocks may infrequently
occur as fragments of coal, which includes other macerals, such as inertinite and liptinite.
3.3.23.1 Discussion—
The identification of the primary vitrinite (first cycle vitrinite) population is essential for determining the thermal maturity
experienced by organic matter in a sedimentary rock. This can be complicated by: the chemical and structural heterogeneity of
dispersed vitrinite reflecting multiple sources; the presence of similar organic components resembling vitrinite, including solid
bitumen, bituminite, recycled vitrinite, low-reflecting semifusinite, and zooclasts; vitrinite reflectance retardation or suppression,
or both; alteration by oxidation or weathering from sample handling or by exposure to the atmosphere at outcrop; and the potential
for contamination such as cavings and drilling mud additives in the case of drill cuttings. The term vitrinite is currently used as
both a maceral and maceral group. Individual macerals of the vitrinite group generally are not distinguished in practice when
dispersed in sedimentary rocks (7).
3.3.24 vitrinite reflectance retardation, n—a reduction in vitrinite reflectance values below thermal maturity levels determined by
D7708 − 23a
geochemical or other petrographic parameters. Vitrinite reflectance retardation occurs due to decreased reaction rate and inhibition
of the rearrangement of vitrinite molecular structure principally as a result of overpressure.
3.3.24.1 Discussion—
The presence of vitrinite reflectance retardation can only be assessed if other thermal maturity parameters are available for the same
sample or if vitrinite reflectance data from different depths or locations in an area are available for comparison. Vitrinite reflectance
retardation cannot be assessed from the reflectance result of a single sample or the appearance of a single vitrinite particle.
3.3.25 vitrinite reflectance suppression, n—a reduction in vitrinite reflectance values below thermal maturity levels determined by
geochemical or other petrographic parameters. Vitrinite reflectance suppression is related to factors causing decreased reaction rate
and inhibition of the rearrangement of vitrinite molecular structure. Suppression factors may include an atypical hydrogen-rich
vitrinite chemistry inherited from the precursor plant material or introduced into the vitrinite by the chemical microenviroments
of deposition, diagenesis, and catagenesis, among other causes.
3.3.25.1 Discussion—
The presence of vitrinite reflectance suppression can only be assessed if other thermal maturity parameters are available for the
same sample or if vitrinite reflectance data from different depths or locations in an area are available for comparison. Vitrinite
reflectance suppression cannot be assessed from the reflectance result of a single sample or the appearance of a single vitrinite
particle.
3.3.26 zooclast, n—faunal relics such as chitinozoans, graptolites, scolecodonts, and conodonts which may show similar optical
properties to dispersed vitrinite (reflective dark gray to white color) in reflected white light and which increase in reflectance with
increasing thermal maturity. The reflectance of zooclasts may be measured and used for thermal maturity information of
sedimentary rocks of pre-Upper Silurian age which do not contain vitrinite, or in addition to vitrinite reflectance in Upper Silurian
and younger rocks.
4. Summary of Test Method
4.1 In this test method, light reflected from a polished surface of vitrinite or other organic matter type is measured by a
microscope-photometer or other detection system. The reflected light is recorded in percent reflectance after calibration of the
detection equipment. The calibration of the detection equipment is accomplished by measuring light reflected from standards of
known reflectance as calculated from their refractive indices (see 6.13, Calibration Standards), and measured against reference
standards. All measurements are made on surfaces covered by an immersion oil.
4.1.1 Color photomicrographs of vitrinite and other organic materials dispersed in sedimentary rocks are available from various
publications and websites.
4.1.2 The scholarly organization International Committee for Coal and Organic Petrology hosts a biennial accreditation program
for organic petrographers using D7708 to measure reflectance of vitrinite dispersed in sedimentary rocks.
5. Significance and Use
5.1 The mean reflectance of the vitrinite maceral in sedimentary rocks as determined by this test method is used as an indicator
of thermal maturity, that is, the progressive geochemical alteration of dispersed organic material experienced during diagenesis,
catagenesis, and metagenesis. In the case of hydrocarbon source rocks, three major categories of thermal maturity are defined by
vitrinite reflectance: immature (R ran ≤ 0.5 %), mature (R ran ≈ 0.5 % to 1.35 %), and overmature (R ran ≥ 1.35 %) with respect
o o o
to the generation of liquid hydrocarbons, although not all practitioners agree on these thermal boundaries (10). Thermal maturity
as determined by the reflectance of vitrinite dispersed in sedimentary rocks is similar to the rank classification of coals as presented
in Classification D388 and measured similarly to the reflectance of vitrinite in coal as presented in Test Method D2798. The mean
reflectance of the vitrinite maceral in sedimentary rocks correlates with geochemically determined parameters of thermal maturity
and can be used to characterize thermal maturation history, to calibrate burial history models, and to better understand the processes
of hydrocarbon generation, migration, and accumulation in conventional and unconventional petroleum systems.
6. Apparatus
6.1 Microscope—Any microscope equipped for reflected light microscopy (such as an upright research metallurgical or
opaque-ore microscope) can be used. The microscope shall be able to project an image to a photomultiplier tube or other light
detection system and to support the photomultiplier tube/light detection system housing.
6.2 Light Sources—The white light source used for measuring reflectance shall have a regulated power supply to provide for stable
D7708 − 23a
output. White light delivered from a 12 V 100 W tungsten halogen bulb is routinely employed; other illumination devices such as
LEDs are acceptable provided they have similar emission spectra to that from tungsten halogen. Some lamps require supplemental
voltage-stabilizing transformers if the line voltage fluctuates. The microscope may also be equipped with low wavelength
fluorescence illumination from mercury or xenon gas discharge lamps, LEDs, or other devices with similar emission spectra. A
beam-splitting mirror is used to switch illumination sources.
6.3 Vertical Illuminator and Polarizer—The vertical illuminator can contain a Berek prism, a Smith illuminator, or a high-quality
glass plate. Some fixed-stage microscope systems employ a computer-controlled, rotating, gearcoupled polarizer in the incident
light path for apparent maximum reflectance measurements, with data acquisition and data processing controlled by a computer
or microcontroller.
6.4 Fluorescence Filter Set—For fluorescence microscopy, the microscope can be equipped with appropriate filter sets designed
to observe the fluorescence emission spectra of the sample. Typically, the sets contain a bandpass excitation filter, a long pass beam
splitter, which serves as the vertical illuminator, and a long pass emission filter.
6.5 Field Diaphragm—The light incident on the vertical illuminator of the microscope shall be limited by an adjustable or fixed
diaphragm field stop that should close to approximately ⁄3 of the field or smaller as projected on the image. An adjustable field
stop shall be limited by means of a set screw or similar mechanism so as to close to precisely the same diameter each time it is
employed.
6.6 Objective—The oil immersion microscope objective shall be constructed of high quality lenses with anti-reflection coatings
such that a minimum of stray light enters the light path. The combined magnification of objective and oculars shall permit
examination of the specimen at a magnification between 400× and 750×, such that particles of 1 μm can be resolved. Objectives
of 40× or 50× magnification are routinely employed with oculars of 10× magnification.
6.7 Stage—The microscope stage can be capable of rotating through 360° or can be fixed. 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 of the optical path.
6.8 Measuring Aperture—A measuring aperture made of non-reflecting, opaque material shall be placed approximately in the focal
plane of the ocular 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 an area of about 20 μm . Digital reflectance systems which employ a grayscale calibration to
measure reflected light by pixel intensity may use a broader measurement field.
6.9 Filters—The light reflected from the surface of the sample or standard shall be converted to monochromatic green by inserting
an interference filter, or combination of filters, into the light path. The filters shall have peak transmittance of 546 nm 6 5 nm and
a half-peak transmittance bandwidth of less than 20 nm. The filters shall be inserted into the light path immediately before the
photomultiplier tube or other detector type.
6.10 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 sample. The high voltage
supplied to the photomultiplier tube must be within the prescribed range to
...