ASTM E2808-11
(Guide)Standard Guide for Microspectrophotometry and Color Measurement in Forensic Paint Analysis
Standard Guide for Microspectrophotometry and Color Measurement in Forensic Paint Analysis
SIGNIFICANCE AND USE
Paint sample colors can be measured by reflectance (visible range) or transmission (UV-Vis) for comparison purposes. Transmission measurements are especially necessary for the analysis of UV absorbers in clear coats, the identification of pigments, and the detailed analysis of effect pigments that are not opaque.
This guide is designed to assist an analyst in the selection of appropriate sample preparation methods and instrumental parameters for the analysis, comparison, or identification of paint pigments and colors.
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 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 “Visible Microscopical Spectrophotometry in the Forensic Sciences” and “The Role of Colour and Microscopic Techniques for the Characterisation of Paint Fragments.”
SCOPE
1.1 This guide is intended to assist individuals and laboratories that conduct forensic visible and ultraviolet (UV) spectral analyses on small fragments of paint using Guide E1610.
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 range.
1.3 This guide does not address other areas of color evaluation such as paint surface texture or paint pigment particle size, shape, or dispersion within a paint film that are evaluated by other forms of microscopy. Other techniques such as spectral luminescence, fluorescence, and near infrared (NIR) are not included in this guide because of their limited use, lack of validation, or established efficacy in forensic paint analysis.
1.4 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 found useful in those fields.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E2808 − 11
Standard Guide for
Microspectrophotometry and Color Measurement in
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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
is one of the first steps taken in a forensic paint comparison and 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 the more subjective results of visual microscopical color comparisons.
Suitableinstrumentswithappropriateoptics,sensitivity,resolution,anddynamicrangecanmeasure
and produce spectral curves of light energy from small samples as that light is transmitted, absorbed,
or reflected 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 visible spectral region (~380 to
~800nm) and also in the ultraviolet region (~190 to ~380nm). They should not be confused with
broadband or absorption filter-based, tristimulus systems or low-resolution, large measurement step
(5nm or more) spectral analyzers.
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 2100nm. 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.
Subjectivetermssuchas“blue,“violet,”or“purple”areinadequatedescriptorsofactualcolor.MSP
instruments can be used to describe or numerically “name” an item’s color by calculating the item’s
colorimetricvalues.Thesevalues,orchromaticitycoordinates,canbeexpressedinanyoneofseveral
coordinatesystemsandareusefulinthedevelopmentandmaintenanceofcolorcomparisondatabases.
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
orcommercialpaint-matchingprotocolsinwhichtheonlyrequirementistopaintanitemsoitappears
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 1.3 This guide does not address other areas of color evalu-
ation such as paint surface texture or paint pigment particle
1.1 This guide is intended to assist individuals and labora-
size, shape, or dispersion within a paint film that are evaluated
tories that conduct forensic visible and ultraviolet (UV) spec-
by other forms of microscopy. Other techniques such as
tral analyses on small fragments of paint using Guide E1610.
spectral luminescence, fluorescence, and near infrared (NIR)
1.2 This guide deals primarily with color measurements
are not included in this guide because of their limited use, lack
within the visible spectral range but will also include some
of validation, or established efficacy in forensic paint analysis.
details concerning measurements in the UV range.
1.4 This guide is directed at the color analysis of commer-
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This guide is under the jurisdiction of ASTM Committee E30 on Forensic
cially prepared paints and coatings. It does not address the
Sciences and is the direct responsibility of Subcommittee E30.01 on Criminalistics.
analysis or determination of provenance of artistic, historical,
Current edition approved March 1, 2011. Published May 2011. DOI: 10.1520/
E2808-11. or restorative paints, but it may be found useful in those fields.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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E2808 − 11
1.5 The values stated in SI units are to be regarded as length interval over which radiant energy is greater than 50%
standard. No other units of measurement are included in this of the maximum intensity.
standard.
3.2.4 charge-coupled device (CCD), n—a silicon-based
1.6 This standard do
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