1、Designation: E2808 11Standard Guide forMicrospectrophotometry and Color Measurement inForensic Paint Analysis1This standard is issued under the fixed designation E2808; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of la
2、st revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONColor is one of the most important comparative characteristics of paints. The comparison of coloris one of the first ste
3、ps taken in a forensic paint comparison and it is essential to note that this guidedoes not propose the use of instrumental color comparison for objects that are distinguishable to theeye. Since the 1940s, analytical instruments have been able to discriminate colors that the averagehuman eye cannot
4、distinguish. Microspectrophotometers (MSPs), in particular, allow for an objectivemeasurement of the color of small, millimetre or submillimetre samples and are more precise orquantitative compared to the more subjective results of visual microscopical color comparisons.Suitable instruments with app
5、ropriate optics, sensitivity, resolution, and dynamic range can measureand 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 ofone-half to a few nanomete
6、rs each. MSPs typically operate in the visible spectral region (380 to800 nm) and also in the ultraviolet region (190 to 380 nm). They should not be confused withbroadband or absorption filter-based, tristimulus systems or low-resolution, large measurement step(5 nm or more) spectral analyzers.The s
7、pectral limits of different instruments can vary in all of the above noted spectral regions andmay also include the near infrared region from about 780 to 2100 nm. The usefulness of this lastspectral region in the analysis and comparison of paint fragments is currently indeterminate and willnot be c
8、overed in this guide.Subjective terms such as “blue, “violet,” or “purple” are inadequate descriptors of actual color. MSPinstruments can be used to describe or numerically “name” an items color by calculating the itemscolorimetric values. These values, or chromaticity coordinates, can be expressed
9、in any one of severalcoordinate 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 becausediffering spectral curves can yield identical colorimetric values. This is commonly fou
10、nd in industrialor commercial paint-matching protocols in which the only requirement is to paint an item so it appearsto be the same as others. The eyes perception and the colorimetric values of two items may indicatethat they are the same color, but the spectral curves of those items may still be d
11、istinguished. Thisleads to the use of the MSP in the comparison of visually indistinguishable colored items.1. Scope1.1 This guide is intended to assist individuals and labora-tories that conduct forensic visible and ultraviolet (UV) spec-tral analyses on small fragments of paint using Guide E1610.1
12、.2 This guide deals primarily with color measurementswithin the visible spectral range but will also include somedetails concerning measurements in the UV range.1.3 This guide does not address other areas of color evalu-ation such as paint surface texture or paint pigment particlesize, shape, or dis
13、persion within a paint film that are evaluatedby other forms of microscopy. Other techniques such asspectral luminescence, fluorescence, and near infrared (NIR)are not included in this guide because of their limited use, lackof validation, or established efficacy in forensic paint analysis.1This gui
14、de is under the jurisdiction of ASTM Committee E30 on ForensicSciences and is the direct responsibility of Subcommittee E30.01 on Criminalistics.Current edition approved March 1, 2011. Published May 2011. DOI: 10.1520/E2808-11.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West C
15、onshohocken, PA 19428-2959, United States.1.4 This guide is directed at the color analysis of commer-cially prepared paints and coatings. It does not address theanalysis or determination of provenance of artistic, historical,or restorative paints, but it may be found useful in those fields.1.5 The v
16、alues stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate
17、safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D16 Terminology for Paint, Related Coatings, Materials,and ApplicationsD1535 Practice for Specifying Color by the Munsell SystemD2244 Practice for Calculatio
18、n of Color Tolerances andColor Differences from Instrumentally Measured ColorCoordinatesE179 Guide for Selection of Geometric Conditions forMeasurement of Reflection and Transmission Properties ofMaterialsE275 Practice for Describing and Measuring Performanceof Ultraviolet and Visible Spectrophotome
19、tersE284 Terminology of AppearanceE308 Practice for Computing the Colors of Objects byUsing the CIE SystemE805 Practice for Identification of Instrumental Methods ofColor or Color-Difference Measurement of MaterialsE1610 Guide for Forensic Paint Analysis and Comparison3. Terminology3.1 DefinitionsFo
20、r definitions of paint-associated termi-nology used in this guide, see Terminologies D16, E284, andE1610.3.2 Definitions of Terms Specific to This Standard:3.2.1 absorbance, nlogarithm to the base 10 of thereciprocal of spectral transmittance, (T).A 5 log101/T! 5 log10T3.2.1.1 DiscussionIt is often
21、expressed as a fraction ordecimal value and may be referred to as optical density.3.2.2 absorption, ntransformation of incident energy intolonger wavelength energy.3.2.3 bandwidth, noptical width of the monochromatorexit slit or the optical width of a semiconductor detectorelement that will vary wit
22、h monochromator design.3.2.3.1 DiscussionThis term can also refer to the wave-length interval over which radiant energy is greater than 50 %of the maximum intensity.3.2.4 charge-coupled device (CCD), na silicon-basedsemiconductor chip consisting of a two-dimensional matrix ofphoto sensors or pixels.
23、3.2.5 chromaticity, ndimensions of a color stimulus, ex-cluding luminous intensity, and expressed in terms of hue andsaturation (Commission Internationale de lEclairage (CIE) orredness-greenness and yellowness-blueness (L*, a*, b*).3.2.5.1 DiscussionIt is generally represented as a point ina constan
24、t luminance plane of a three-dimensional color space.3.2.6 chromaticity coordinates, CIE, nratios of each ofthe three tristimulus values X, Y, and Z in relation to the sumof the three designated as x, y, and z, respectively.3.2.6.1 DiscussionThey are sometimes referred to as thetrichromatic coeffici
25、ents. When written without subscripts,they are assumed to have been calculated for Illuminant C andthe 2 (1931) Standard Observer unless specified otherwise. Ifthey have been obtained for other illuminants or observers, asubscript describing the observer or illuminant should be used.For example, x10
26、Dand y10Dare chromaticity coordinates for the10 observer and Illuminant D. A “standard observer” refers tospecific numerical values that represent the nominal colorresponse of the human eye to different wavelengths of light. Itis based on a study of the average retinal response of the humanpopulatio
27、n.3.2.7 chromaticity diagram, CIE, ntwo-dimensionalgraph that describes a color as the locus of chromaticitycoordinates in a field of monochromatic light varying from 380to 780 nm in wavelength where X is the abscissa of thecoordinate system and Y is the ordinate and it is used todescribe and compar
28、e the colors of both luminous and non-luminous materials.3.2.8 CIELAB Unit E, n(L*a*b*) color difference equa-tion from Practice D2244.3.2.8.1 DiscussionThe color difference E between twosamples is given by:DECIEL*, a*, b*! 5 DL*!21 Da*!21 Db*!2#1/2whereL* (pronounced “L star”) = where a color falls
29、 on a verti-cal scale from light to dark,a* = where the color falls on thecontinuum of colors betweenred and green, andb* = where the color falls on thecontinuum of colors betweenblue and yellow.3.2.9 colorimetry, nconversion of instrumental light mea-surements into psychophysical descriptions or nu
30、merical nota-tions that can be correlated with visual evaluations of color andcolor differences.3.2.10 effect pigment, nany paint pigment that is designedto produce a significant change in color attribute(s) in a paintfilm when the film is viewed or illuminated from variedgeometries.3.2.11 error ell
31、ipse (visual limits), nplotted limits ofvariation in visually indistinguishable color coordinate valuesthat form an elliptical shape around a central color value whenplotted in a chromaticity diagram.3.2.12 grating, nparallel set of linear, regularly repeatingstructures that, when illuminated, produ
32、ces maxima andminima of light intensity as a consequence of interference.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe A
33、STM website.E2808 1123.2.12.1 DiscussionThese maxima and minima vary inposition with wavelength. This allows radiation of any givenwavelength to be isolated from a complex mixture of wave-lengths and allows the grating to be used as a monochromator.3.2.13 illumination aperture, nelement in the optic
34、al pathof a microspectrophotometric (MSP) system that limits the areaof illumination reaching the sample focal plane.3.2.14 measuring aperture, nelement in the optical pathof a microspectrophotometric (MSP) system that limits the areaof illumination reaching the detector focal plane.3.2.15 monochrom
35、ator, ndevice designed to isolate nar-row wavelength ranges of light from complex, broad-spectrumradiation.3.2.16 photomultiplier tube (PMT), nphotosensitivevacuum tube device that quantitatively converts photons oflight into electrical energy.3.2.17 reflectance, R, nthe ratio of the radiant powerre
36、flected by the specimen to the radiant power incident on thespecimen.3.2.17.1 DiscussionIn popular usage, it is considered asthe ratio of the intensity of reflected radiant energy from a testsample to that reflected from a defined reference standard.3.2.18 significant difference, ndifference between
37、 twospecimens that indicates their possible common origin cannotbe established.3.2.19 standard observer, CIE, nvisual color perceptiondata set adopted by CIE to represent the response of theaverage human eye when light adapted to an equal energyspectrum.3.2.19.1 DiscussionUnless otherwise specified,
38、 the termapplies to the data adopted in 1931 for a 2 field of vision. Thedata adopted in 1964, sometimes called the 1964 observer,were obtained for a 10 field of vision and are generally usedin industrial measurements.3.2.20 standard illuminant, CIE, nstandard sources forwhich the CIE specified the
39、spectral energy distribution asfollows:3.2.20.1 standard illuminant A, CIE, ntungsten filamentlamp operated at a color temperature of 2856 K, approximatinga blackbody operating at that temperature.3.2.20.2 DiscussionIt is defined in the wavelength rangeof 380 to 770 nm.3.2.20.3 standard illuminant C
40、, CIE, napproximation ofovercast daylight having a correlated color temperature ofapproximately 6770 K.3.2.20.4 DiscussionThis color of light can be obtained byusing a combination of Illuminant A and a color-correctingfilter. It is defined in the wavelength range of 380 to 770 nm.3.2.20.5 standard i
41、lluminant D, CIE, napproximation ofbright daylight illumination having a correlated color tempera-ture of 6504 K in the spectral range of 300 to 830 nm.3.2.20.6 DiscussionThe UV portion of Illuminant D, 300to 380 nm, is necessary to describe correctly colors that containfluorescent colorants or ultr
42、aviolet (UV) absorbers. The UVand visible portions are designated as UVD and VisD. Illumi-nant D is based on actual measurements of the spectraldistribution of daylight and is the most commonly usedstandard illuminant for spectral measurements.3.2.21 thermochromy, ncharacteristic of some materials,i
43、ncluding some pigments, to change color as temperaturechanges.3.2.22 step width, ndistance between two points of wave-length measurement in a spectrum, not to be confused withresolution although it can have an impact on resolution.3.2.23 transmittance, T, nthe ratio of radiant power trans-mitted by
44、the specimen to the radiant power incident on thespecimen.3.2.23.1 DiscussionBoth absorption and reflection influ-ence total transmittance.3.2.24 tristimulus values, CIE, namounts (in percent) ofthe three components necessary in a three-color additivemixture required for matching a color.3.2.24.1 Di
45、scussionThese components are designated asX, Y, and Z in the CIE system. The illuminant and standardobserver color-matching functions must be designated. If theyare not, an assumption is made that the reported values are forthe 1931 observer (2 field) and Illuminant C. The valuesobtained depend on t
46、he method of integration used, the natureof the sample surface, sample homogeneity, and the instrumentdesign. Tristimulus values are therefore not absolute values fora sample, but relative values dependent on the method used toobtain them. Examples of calculations of tristimulus values inthe CIE sys
47、tem can be found in Practice E308.3.2.25 Woods anomaly, neffect caused by a polarizationof diffracted energy as a result of non-uniform reflection as theangle of incidence varies on a holographically blazed mono-chromator.3.2.25.1 DiscussionThis effect can be observed as adistinct transmittance maxi
48、mum within the spectrum.4. Summary of Practice4.1 One of the most obvious decorative properties of paint isits color. Usually, one or more major pigments are used in apaint together with one or more minor pigments whose purposeis to modify color or other film properties. The interaction ofpigments w
49、ith light is very complex with light being scattered,absorbed, and reflected within the paint layer.4.2 The processes responsible for producing the color ofpigments include: crystal-field effects with transition metalcompounds (most of the inorganic pigments belong to thisgroup), electron transitions between molecular orbitals incompounds with conjugated double bonds (organic pigments),electron transitions in compounds with conduction energybands (metal pigments and pure semiconductors), and otherprocesses caused by geom
