Assessment	of	a	Portable	

Spectrophotometer	for	Measuring	Color	

of	Automotive	Paint	Trace	Evidence

Stephanie	Watson,	Li-Piin	Sung	and	Kimberly	Kha

Engineering	Laboratory

National	Institute	of	Standards	and	Technology

November	9,	2016

*Certain	instruments	or	materials	are	identified	in	this	presentation	in	order	to	adequately	specify	

experimental	details.	In	no	case	does	it	imply	endorsement	by	NIST	or	imply	that	it	is	necessarily	the	

best	product	for	experimental	procedure
Integrating	Sphere-Based	UV	Chamber

• Simulated	Photodegradation	via	High	

Energy	Radiant	Exposure	(SPHERE)

• 2	m	integrating	sphere

• High	power	mercury	lamps

– ~170	W/m2 (295	nm	- 400	nm)

– Up	to	10	x	the	outdoor	exposure

• 95%	exposure	uniformity

• Shorter	wavelengths	(<295	nm)	removed

• Visible	and	infrared	radiation	removed

• Exposure	conditions	for	32	chambers,	

individually	controlled	(UV,	RH,	T)

• Capability	of	mechanical	loading

• Martin	and	Chin,	U.S.	Patent	6626053

• Chin	et	al,	Review	of	Scientific	Instruments,	75(11),	4951-4959,	2004.

Why	Engineering	Laboratory?
Outline

• Background	and	Purpose	

– Color	measurements	of	paints

– Handheld	colorimeter	vs	microspectrophotometer

(MSP)

• Experimental

• Results

• Summary

• Future	Directions
Interactions	between	Light	and	Object

Standard	Observer

Three	types	of	cone	cells	responsible	for	color	vision:	

short	(420-440	nm),	middle	(530-540	nm)	and	long	(560-580	nm)

Human	Eye

Rod	cells	are	light	intensity	receptors

From	BYK	Additives	and	Instruments	2016	Catalog
Anatomy	of	Automotive	Paint

From	M.	Nichols,	Ford

(micrometer)
Color	in	Characterization	of	Paint	Trace	

Evidence

• History	of	visual	comparison

– Subjective	and	color	sensitivity	dependent

– Microscopy	guidelines	for	illumination	and	

background

• Visual	color	description/color-order	systems

– Classify	colors	in	3D	color	space		

• Munsell color	coordinate	system

• Standard	color	chart	DIN	6164

• Natural	color	system

• Methuen	Handbook	of	Colors
Munsell Color	Coordinate	System

• Human	perception	basis

• Color	assigned	values	for

– Hue	(H):	color

– Value	(V):	light	or	darkness

– Chroma	(C):	saturation

• Visual	comparison	of	samples	to	

standard	chips	in	Munsell Book	

of	Color	(X-Rite)

– Gloss	and	matte

• ASTM	1535-14

• Color	of	effect	(pearlescent,	

special	flakes	or	particles)	cannot	

be	adequately	analyzed Achromatic	(neutral)	=	N

Chromatic:	R,	Y,	G,	B,	P

Color	notation:	4.5R	4.2/6.4

Designative	System	for	Munsell Hue

from	ASTM	1535-14:	Standard	Practice	for	Specifying	Color	by	the	Munsell System
Commission	Internationale de	L’Eclairage

(CIE)- L*a*b*	System

• Mathematical	basis

• Color	is	appearance	using	three	components

– Illuminant:	spectral	power	distributions

– Observer:	standard	2o or 10o field	of	view	

functions

• Relative	response	vs	wavelength	curves	

for	red	(r),	green	(g)	and	blue	(b)	

response

– Object:	spectral	reflectance	distribution	from	

spectrophotometer

• CIE	color	spaces	combine	r,	g,	and	b	spectra	

into	color	values

– Accounts	ratios	of	long,	medium	and	short	

cones	in	the	eye	

• r,	g,	b	converted	to	tristimulus values			(X,	Y,	

Z),	which	are	then	converted	to			L*,	a*,	b*	

values

• Differences	between	two	colors,	DE*

– DE*	=	√ (dL*)2 + (da*)2 + (db*)2

L*	=	lightness,	luminosity	or	luminance

a*	=	red-green	chromaticity

b*=	yellow-blue	chromaticity

From	BYK	Additives	and	Instruments	2016	Catalog
Measurements										Colorimetry

• Measurements

– 400	nm	– 700	nm,	10	nm	resolution

– Illuminated	diffusely,	8° observation	

angle

– 11	mm	aperture	size

– Calibrate	monthly	with	color	

standards;	green	used	as	a	check

– Color	using	L*a*b*	(and	X,Y,Z)

• illuminant/observer	settings

– reflective mode

– 10° observation	fcn for	CIE	tests

• Data	Analysis	(ASTM	E1345-14)

– Sample	to	standard	comparison

– Modify	input	tolerances

– Data	processed	using	a	PC

From	BYK	Additives	and	Instruments	2016	Catalog
Colorimeter	Example	Data

Spectral	Curve Illuminants

From	BYK	Additives	and	Instruments	2016	Catalog
Measurements																microspectrophotometry

• Transmission mode

• Xenon	arc	source

– 200	nm- 900	nm

• Array	detector

• 15x		and	36x	reflecting	objectives

• (677	x	567)	μm	field	of	view

• Aperture	sizes

– (4.2	x	4.2) μm to	(79	x	79)	μm	for	15x	

objective

• Sample	preparation	

– Hand	section	(thin	peel)	versus	microtome	

(cross	section)

– Quartz	slip	and	coverslip	with	glycerin	media

• Dark	and	reference	spectra	collected	for	

each	new	location	

MSP121
Experimental

• Color	Standards

– Metal	plates	(20	mm	x	30	mm)

– 3	plates	examined

– beige,	black,	blue ,	cinnabar,	

gray,	green,		white,	and	yellow

• Automotive	Samples

– Metal	and	plastic	substrates

• Chips	and	plates

• Black,	blue,	red,	and	white

– Ford	production	panel	(blue)

Color	Standards

• Colorimeter

– 20	measurements/plate

– Average	L*a*b*	values	for	

each	plate	used	to	calculate	

DE*	between	plates

• MSP

– Sectioned	samples	from	3	

plates

– 10	locations	from	each	

sample

– L*a*b*	values	from	

normalized	average	spectra	

used	to	calculate	DE*	

between	plates
MSP	Results		 Standards

Cinnabar	(C):	flat	dark	red	(3-0158-0)

15x,	aperture	(8.6	x	8.6)	μm

Notable	peaks:	514	nm,	551	nm,	604	nm

Transmission

Wavelength	(nm)

Optical	Image
MSP	Results																		Standards

Bright	Blue	(BB):	metallic	medium	blue	(5-0231-0)

15x,	aperture	(8.6	x	8.6)	μm

Notable	peaks:	455	nm	(shoulder),	529	nm,	658	nm Transmission

Wavelength	(nm)

Optical	Image
MSP	Results																Standards

Dark	Green	(DG):	metallic	dark	green	(7-0147-0)

15x,	aperture	(8.6	x	8.6)	μm

Notable	peaks:	503	nm	(shoulder),	579	nm,711	nm Transmission

Wavelength	(nm)

Optical	Image
MSP	Results																Standards

Medium	Gray	(MGr):	metallic	medium	gray	(2-0140-0)

15x,	aperture	(8.6	x	8.6)	μm

Broad	peak:	409	nm-560	nm

Transmission

Wavelength	(nm)

Optical	Image
DE*	for	different	colored	standards

compare	similar	colors

Colors:	cinnabar	(C),	bright	blue	(BB),	dark	green	(DG),	medium	gray	(MGr)

• Color	meter:

• MSP:

DE*	never	greater	than	0.1	between	the	same	colors;	

Uncertainty=	error	of	propagation	using	standard	deviations

DE*	less	than	3;microtoming	would	improve	repeatability

DE*	errors	not	calculated	due	to	limited	number	of	replicates

Rule	of	Thumb:

Above	5	units	is	significant
DE*	between	different	color	standards

Colors:	cinnabar	(C),	bright	blue	(BB),	dark	green	(DG),	medium	gray	(MGr)

• Comparing	plate	1	of	each	color:

• DE*	between	different	colors	generally	smaller	when	determined	by	MSP	

versus	colorimeter

– Low	transmission	from	MSP	samples	give	near	zero	L*a*b*	values

DE*	values	are	larger	when	

comparing	completely	different	colors

Rule	of	Thumb:

Above	5	units	is	significant
MSP	Results																Automotive	samples

Metallic	royal	blue,	metal/plastic	substrates

Same	car,	different	locations

15x,	aperture	(8.6	x	8.6)	μm

shoulder	at	550	nm	

in	2nd layer	indicates	repaint

Transmission

Wavelength	(nm)

Optical	Image
MSP	Results																Automotive	samples

Cross	Section	versus	Thin	Peel

Transmission

Transmission

Wavelength	(nm) Wavelength	(nm)

DE*	uncertainty	not	calculated	due	to	limited	number	of	replicates
MSP	Results																Automotive	Samples

Thin	Peel	Repeatability	on	Basecoats

TransmissionTransmission

Transmission

Wavelength	(nm) Wavelength	(nm)

Wavelength	(nm)

DE*	uncertainty	not	calculated	due	to	limited	number	of	replicates
Summary

• CIE	L*a*b*	system	has	a	quantitative	mathematical	

foundation	to	analyze	colors	in	trace	evidence	samples

– The	DE*	values	can	be	used	to	distinguish	between	

samples

– Handheld	colorimeter	and	MSP	can	accommodate	

measurements

• Samples	must	be	measured	in	reflective	mode

• The	handheld	colorimeter	has	portable	advantage	for	

field	measurements

– Minimum	aperture	size	is	limited	to	10	mm	and	so	small	

trace	samples	cannot	accurately	be	examined
Future	Directions

• Add	reflective	measurement	capabilities	to	

the	MSP

• Contact	NIST	Statistical	Engineering	Division	to	

better	examine	test	color	standards	

• Compare	Ford	production	panels	to	current	

Ford	paint	samples
Acknowledgments

• Sue	Ballou,	NIST	Forensics	Measurements	Program	

Manager

• Eric	Steel,	Project	Leader,	Materials	Measurement	

Laboratory	(MML)		

• Amanda	Forster	and	Julie	Bitter,	Project	Team	(MML)
Questions

stephanie.watson@nist.gov

National	Institute	Standards	and	Technology

Engineering	Laboratory

100	Bureau	Drive,	Stop	8615

Gaithersburg,	MD	20899

301-975-6448

https://www.nist.gov/topics/forensic-science

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