ASTM E2808-19
(Guide)Standard Guide for Microspectrophotometry in Forensic Paint Analysis
Standard Guide for Microspectrophotometry in Forensic Paint Analysis
SIGNIFICANCE AND USE
5.1 This guide is designed to assist an analyst in the selection of appropriate sample preparation methods and instrumental parameters for the analysis and comparison of paint pigments and colors. When used for comparison purposes, the goal is to determine whether any meaningful differences exist between the samples.
5.2 Paint sample spectra can be measured by reflectance or transmittance spectroscopy for comparison purposes. Transmittance measurements are generally preferred and are especially necessary for the analysis of UV absorbers in clear coats and the detailed analysis of effect pigments that are not opaque. Emission comparison via fluorescence is also measurable.
5.3 It is not the intention of this guide to present comprehensive theories and methods of MSP. It is necessary that the analyst have an understanding of UV-Vis-NIR MSP and general concepts of specimen preparation before using this guide. This information is available from manufacturers’ reference materials, training courses, and references such as Eyring (1),3 Stoecklein (2), and Purcell (3).
SCOPE
1.1 This guide is intended to assist forensic analysts who conduct UV, visible, NIR, or fluorescence emission spectral analyses on small fragments of paint or use Guide E1610, as this guide is to be used in conjunction with a broader analytical scheme.
1.2 This guide deals primarily with color measurements within the visible spectral range but will also include some details concerning measurements in the UV and NIR spectral ranges. The particular method(s) employed by each analyst depends upon available equipment, examiner training, sample suitability, and sample size.
1.3 This guide provides basic recommendations and information about microspectrophotometers.
1.4 This guide does not address other areas of color evaluation such as colorimetric values, paint surface texture or pigment particle size, shape, or dispersion within a paint film that are evaluated by other forms of microscopy.
1.5 This guide is directed at the color analysis of commercially prepared paints and coatings. It does not address the analysis or determination of provenance of artistic, historical, or restorative paints, but it may be useful in those fields.
1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.7 This standard cannot replace knowledge, skill, or abilities acquired through education, training, and experience and is to be used in conjunction with professional judgment by individuals with such discipline-specific knowledge, skills, and abilities.
1.8 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.
1.9 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: E2808 − 19
Standard Guide for
1
Microspectrophotometry in Forensic Paint Analysis
This standard is issued under the fixed designation E2808; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Color is one of the most important comparative characteristics of paints. The comparison of color
isoneofthefirststepstakeninaforensicpaintcomparison.Subjectivetermssuchas“blue,”“violet,”
or “purple” are descriptors of color but are inadequate for clear communication of color as terms may
suggest different colors to different people. It is essential to note that this guide does not propose the
use of instrumental color comparison for objects that are distinguishable to the eye. Since the 1940s,
analytical instruments have been able to discriminate colors that the average human eye cannot
distinguish. Microspectrophotometers (MSPs), in particular, allow for an objective measurement of
thecolorofsmall,millimetreorsubmillimetresamplesandaregenerallymoresensitivethanthemore
subjective results of visual microscopical color comparisons.
Suitableinstrumentswithappropriateoptics,sensitivity,resolution,anddynamicrangecanmeasure
spectral curves from small samples as that light is transmitted, absorbed, reflected, or emitted (via
fluorescence)bythesample.Thespectrallimitsofdifferentinstrumentscanvaryandmayextendfrom
theultraviolet(UV)(~190to380nm)throughthevisiblespectralregion(~380to780nm)tothenear
infraredregion(NIR)(~780to2500nm).MSPsshouldnotbeconfusedwithbroadbandorabsorption
filter-based tristimulus systems.
1. Scope 1.4 This guide does not address other areas of color evalu-
ation such as colorimetric values, paint surface texture or
1.1 This guide is intended to assist forensic analysts who
pigment particle size, shape, or dispersion within a paint film
conduct UV, visible, NIR, or fluorescence emission spectral
that are evaluated by other forms of microscopy.
analyses on small fragments of paint or use Guide E1610,as
1.5 This guide is directed at the color analysis of commer-
thisguideistobeusedinconjunctionwithabroaderanalytical
cially prepared paints and coatings. It does not address the
scheme.
analysis or determination of provenance of artistic, historical,
1.2 This guide deals primarily with color measurements
or restorative paints, but it may be useful in those fields.
within the visible spectral range but will also include some
1.6 The values stated in SI units are to be regarded as
details concerning measurements in the UV and NIR spectral
standard. No other units of measurement are included in this
ranges. The particular method(s) employed by each analyst
standard.
depends upon available equipment, examiner training, sample
1.7 This standard cannot replace knowledge, skill, or abili-
suitability, and sample size.
ties acquired through education, training, and experience and
1.3 This guide provides basic recommendations and infor-
is to be used in conjunction with professional judgment by
mation about microspectrophotometers.
individuals with such discipline-specific knowledge, skills, and
abilities.
1.8 This standard does not purport to address all of the
1
This guide is under the jurisdiction of ASTM Committee E30 on Forensic
safety concerns, if any, associated with its use. It is the
Sciences and is the direct responsibility of Subcommittee E30.01 on Criminalistics.
responsibility of the user of this standard to establish appro-
Current edition approved Oct. 15, 2019. Published October 2019. Originally
priate safety, health, and environmental practices and deter-
approved in 2011. Last previous edition approved in 2011 as E2808 – 11. DOI:
10.1520/E2808-19. mine the applicability of regulatory limitations prior to use.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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E2808 − 19
1.9 This international standard was developed in accor- 3.2.6 metameric samples, n—two or more samples that
dance with internationally recognized principles on standard- appear to have the same color under one type of illumination
ization established in the Decision on Principles for the but may appear dissimilar under different lighting conditions,
Development of International Standards, Guides and Recom- or two or more samples that appear to be the same color under
mendations issued by the World Trade Organization Technical alllightingconditions,yettheirreflectance/transmittancespec-
Barriers to Trade (TBT) Committee. tral curves are different.
3.2.7 microspectrophotometer (MSP), n—a special
...
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: E2808 − 11 E2808 − 19
Standard Guide for
Microspectrophotometry and Color Measurement in
1
Forensic Paint Analysis
This standard is issued under the fixed designation E2808; 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.
INTRODUCTION
Color is one of the most important comparative characteristics of paints. The comparison of color
is one of the first steps taken in a forensic paint comparison and it comparison. Subjective terms such
as “blue,” “violet,” or “purple” are descriptors of color but are inadequate for clear communication of
color as terms may suggest different colors to different people. It is essential to note that this guide
does not propose the use of instrumental color comparison for objects that are distinguishable to the
eye. Since the 1940s, analytical instruments have been able to discriminate colors that the average
human eye cannot distinguish. Microspectrophotometers (MSPs), in particular, allow for an objective
measurement of the color of small, millimetre or submillimetre samples and are more precise or
quantitative compared to generally more sensitive than the more subjective results of visual
microscopical color comparisons.
Suitable instruments with appropriate optics, sensitivity, resolution, and dynamic range can measure
and produce spectral curves of light energy from small samples as that light is transmitted, absorbed,
or reflected reflected, or emitted (via fluorescence) by the sample. These spectra are collected over
small measurement steps or increments of one-half to a few nanometers each. MSPs typically operate
in The spectral limits of different instruments can vary and may extend from the ultraviolet (UV)
(~190 to 380 nm) through the visible spectral region (~380 to ~800 nm) and also in the ultraviolet
region (~190 to ~380 nm). They 780 nm) to the near infrared region (NIR) (~780 to 2500 nm). MSPs
should not be confused with broadband or absorption filter-based, tristimulus systems or low-
resolution, large measurement step (5 nm or more) spectral analyzers.filter-based tristimulus systems.
The spectral limits of different instruments can vary in all of the above noted spectral regions and
may also include the near infrared region from about 780 to 2100 nm. The usefulness of this last
spectral region in the analysis and comparison of paint fragments is currently indeterminate and will
not be covered in this guide.
Subjective terms such as “blue, “violet,” or “purple” are inadequate descriptors of actual color. MSP
instruments can be used to describe or numerically “name” an item’s color by calculating the item’s
colorimetric values. These values, or chromaticity coordinates, can be expressed in any one of several
coordinate systems and are useful in the development and maintenance of color comparison databases.
Colorimetric values are of limited use in actual color comparison of evidence samples because
differing spectral curves can yield identical colorimetric values. This is commonly found in industrial
or commercial paint-matching protocols in which the only requirement is to paint an item so it appears
to be the same as others. The eye’s perception and the colorimetric values of two items may indicate
that they are the same color, but the spectral curves of those items may still be distinguished. This
leads to the use of the MSP in the comparison of visually indistinguishable colored items.
1. Scope
1.1 This guide is intended to assist individuals and laboratories that conduct forensic visible and ultraviolet (UV) forensic
analysts who conduct UV, visible, NIR, or fluorescence emission spectral analyses on small fragments of paint using or use Guide
E1610. , as this guide is to be used in conjunction with a broader analytical scheme.
1
This guide is under the jurisdiction of ASTM Committee E30 on Forensic Sciences and is the direct responsibility of Subcommittee E30.01 on Criminalistics.
Current edition approved March 1, 2011Oct. 15, 2019. Published May 2011October 2019. Originally approved in 2011. Last previous edition approved in 2011 as E2808
– 11. DOI: 10.1520/E2808-11.10.1520/E2808-19.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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E2808 − 19
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