http://www.diva-portal.org
This is the published version of a paper presented at SPIE European Defence and Security,
Amsterdam, September 19-23 2014.
Citation for the original published paper:
Andersson, K., Åkerlind, C. (2014)
A review of materials for spectral design coatings in signature management applications.
In: Douglas Burgess; Gari Owen; Harbinder Rana; Roberto Zamboni; François Kajzar; Attila
A. Szep (ed.), Optics and Photonics for Counterterrorism, Crime Fighting, and Defence X; and
Optical Materials and Biomaterials in Security and Defence Systems Technology XI (vol. 9253)
SPIE - International Society for Optical Engineering
https://doi.org/10.1117/12.2067167
N.B. When citing this work, cite the original published paper.
Copyright: 2014 Society of Photo Optical Instrumentation Engineers. One print or electronic copy
may be made for personal use only. Systematic reproduction and distribution, duplication of any
material in this paper for a fee or for commercial purposes, or modification of the content of the paper
are prohibited.
Permanent link to this version:
http://urn.kb.se/resolve?urn=urn:nbn:se:fhs:diva-5009
A review of materials for spectral design coatings in signature
management applications
Kent E Andersson
*a
, Christina Åkerlind
b
a
Finnish National Defence University/Swedish National Defence College, Box 278 05, SE-115 93
Stockholm, Sweden;
b
Swedish Defence Research Agency SE-581 11 Linköping, Sweden
ABSTRACT
The current focus in Swedish policy towards national security and high-end technical systems, together with a rapid
development in multispectral sensor technology, adds to the utility of developing advanced materials for spectral design
in signature management applications. A literature study was performed probing research databases for advancements.
Qualitative text analysis was performed using a six-indicator instrument: spectrally selective reflectance; low gloss; low
degree of polarization; low infrared emissivity; non-destructive properties in radar and in general controllability of
optical properties. Trends are identified and the most interesting materials and coating designs are presented with
relevant performance metrics. They are sorted into categories in the order of increasing complexity: pigments and paints,
one-dimensional structures, multidimensional structures (including photonic crystals), and lastly biomimic and
metamaterials. The military utility of the coatings is assessed qualitatively. The need for developing a framework for
assessing the military utility of incrementally increasing the performance of spectrally selective coatings is identified.
Keywords: Spectral Design, Signature Management, Camouflage, Military Utility, Military-Technology, Coating
1. INTRODUCTION
In Swedish defense policy
1
focus has again widened from the focus in recent years on force protection in asymmetric
expeditionary scenarios, to include national security. Military capabilities are assessed to be evolving towards high-end
technology and a need for more competent personnel - at the cost of volume. In turn, fewer and more expensive
platforms mean an increased interest in effect, of course, but also mobility. Mobility is often achieved by decreasing
weight at the expense of less armor and the decreasing force protection capability is in turn compensated for with active
Electronic Warfare systems and camouflage and deception means – i.e. systems for Signature Management. In parallel
there is rapid development in multispectral sensor technology and a proliferation among non-state actors of advanced
optronic, infrared and microwave sensors - increasing the threat to Swedish Armed Forces in international operations.
2
Both factors add to the utility of developing new more effective solutions for signature management. The need to
camouflage soldiers is high in any event, and the requirement to conduct both expeditionary missions and be prepared
for national protection scenarios drives the need for adaptive signatures of platforms and hence the need for Signature
Management Technology (SMT).
Any property, or a combination of properties, of an object, that makes it distinguishable from its immediate background
by a sensor defines the object signature.
3
However, this study is limited to the signature originating from electromagnetic
interaction with the surface of an object. In addition, now that the need for SMT increasingly includes the infrared
spectrum, there is a great demand for solutions meeting the requirements from visible through to long wavelength
thermal infrared regions simultaneously. Furthermore, the need to adapt the signature to the full range of missions, in
*
kent.andersson@fhs.se; phone 46 8 55342836; fax 46 8 553 425 98
Invited Paper
Optics and Photonics for Counterterrorism, Crime Fighting, and Defence X; and Optical Materials and Biomaterials in
Security and Defence Systems Technology XI, edited by Douglas Burgess, Roberto Zamboni, et al., Proc. of SPIE
Vol. 9253, 92530Y · © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2067167
Proc. of SPIE Vol. 9253 92530Y-1
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international
control – mu
l
Spectral des
ig
interest by c
h
optical filter
design for S
M
some contex
t
however, is
c
parts of the
s
threat sensor
the remainde
r
Figure 1.
needs an
d
materials
engineeri
n
object ha
s
effective
n
accompli
s
requirem
e
However, th
e
p
roperties, it
p
olarization,
controllabilit
y
operational
e
construction
p
rotection m
e
have real mil
to adapt it ra
p
starting fro
m
figure also il
l
properties.
as well as n
a
l
tispectrally –
ig
n is the abi
l
h
oosing suita
b
on head-up
d
M
T
p
urposes
t
s a military
c
amouflage, i
s
pectrum wh
e
capability an
r
of the spect
r
The illustratio
n
d
design choice
s
with favorabl
n
g activities b
a
s
been integrat
e
n
ess could be
a
s
h the missio
n
e
nts, the proces
e
signature o
f
also depend
s
and also o
n
y
, with the
a
e
nvironment.
T
methods wit
h
e
asures, such
itary value y
o
p
idly to the ta
m
a spectral d
e
l
ustrates the
e
a
tional scenar
the signature
l
ity to create
a
b
le materials
d
isplays, lase
r
the objective
object shoul
d
.e. trying to
m
e
re the threat
d is often me
a
r
um should, i
f
n
is a model o
s
in several act
i
e
p
roperties t
o
a
lance surface
p
e
d into a capab
i
a
chieved. Ulti
m
n
militarily.
O
s is traversed i
n
f
an object in
s
on several
f
n
shadows a
n
a
id of Syste
m
T
he aim is t
o
h
chosen sur
f
as physical a
n
o
u must know
ctical situatio
n
e
sign activity,
e
qually comp
l
ios, also incr
e
of an object,
a
desired spe
c
and structur
e
r
protective
c
is to tune th
e
d
be seen, e.
g
m
inimize the
sensors are
s
a
sured in dis
t
f
possible, be
o
f the spectral
d
i
vities at differ
e
o
obtain desir
e
p
roperties with
i
lity, with
p
rop
e
m
ately
p
roper t
a
O
nly then do
e
n
the other dire
c
a real enviro
n
f
actors such
a
n
d atmosphe
r
m
s Engineeri
n
o
give the sy
s
f
ace propertie
n
d electronic
your signatu
r
n
. Thus the
pr
is a comple
x
l
ex process o
f
e
ases the be
n
such as a co
m
c
tral optical
r
e
s. The techn
i
c
oatings on o
p
e
spectral ap
p
g
. for show o
contrast bet
w
s
ensitive. Th
u
t
ance to dete
c
o
ptimized for
d
esign leg of a
e
nt system leve
e
d structural
p
othe
r
design
m
e
r techniques a
n
a
ctical utilizati
o
e
s spectral de
s
c
tion.
n
ment is com
p
a
s the size o
f
r
ic phenome
n
n
g, the requ
i
s
tem designer
s
s.
3
Here one
warfare prot
e
r
e; you must
h
r
ocess of desi
x
one and is
c
f
for
m
ulating
r
n
efit of contr
o
m
bat vehicle, i
r
esponse fro
m
i
que is used
i
p
tics and hea
t
p
earance of a
n
f force or fo
r
w
een an objec
t
u
s,
t
he militar
y
c
tion or time
b
emission of t
h
signature ma
n
ls. When desig
n
p
roperties of s
u
m
easures to obt
a
n
d procedures
f
o
n of the syste
m
s
ign have mi
l
p
lex. Apart f
r
f
the object,
t
n
a. In order
i
rements pro
c
s
the freedo
m
must also b
a
e
ction.
5
Finall
y
h
ave a tactica
l
gning a syste
m
c
aptured in F
i
r
elevant requ
i
o
llable signat
u
s thus sought
m
a surface in
i
n several mi
t
radiation c
o
n
object in su
p
r
deception.
T
t
and its im
m
y
effectivene
s
b
efore detecti
o
h
e excessive
h
n
agement
p
roce
n
ing a capabili
t
u
rfaces in a t
e
a
in a desired o
b
f
or its use, desi
r
m
’s capability
g
l
itary utility.
W
r
om the surfa
c
t
he waveleng
t
to design t
h
c
ess has to s
m
to optimize
a
lance signat
u
y
, for a signa
t
l
concept for
h
m
with signat
u
i
gure 1. Turn
i
i
remen
t
s for
s
u
res. The abil
after,
b
ut ver
y
favor of the
litary applic
a
o
ntrol. When
p
port of mili
t
T
he most ch
a
m
ediate backg
r
s
s of SMT is
o
n. Meanwhi
l
h
eat.
4
ss seen as a b
a
t
y, spectral des
i
e
chnical syste
m
b
ject signature
.
r
ed signature
m
g
ives effective
W
hen analyzi
n
c
e, reflectanc
e
th of the inc
i
h
e signature
tart with the
the signatur
e
u
re managem
e
t
ure manage
m
h
ow to use it
a
u
re manage
m
i
ng the proce
s
pectral desig
n
ity to activel
y
y
limited.
application o
a
tions: e.g. th
e
using spectr
a
t
ary tactics. I
n
a
llenging cas
e
r
ound in thos
e
related to th
e
l
e, emission i
n
a
lancing of
i
gn utilizes
m
. System
.
When the
m
easures of
support to
n
g system
e
and specula
r
i
dent light, it
s
and possibl
e
object in it
s
e
by balancin
g
e
nt with othe
r
m
ent system t
o
a
nd the abilit
y
m
ent capabilit
y
ss around, th
e
n
and materi
a
y
f
e
a
l
n
e
,
e
e
n
r
s
e
s
g
r
o
y
y
,
e
a
l
Proc. of SPIE Vol. 9253 92530Y-2
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Research into materials with the potential to be used in spectral design for SMT applications in the optical and thermal
spectra is extensive. Apart from the military applications, it covers smart textiles, solar energy, “cool roofs”, decorative
coatings, heat control of satellites etc. By exploiting these advances in the future, it may be possible to use spectral
design to satisfy the new survivability requirements of signature management.
The work presented in this paper is a first effort in adding a military-technological perspective to spectral design for
signature management, i.e. to assess the military utility. The aim is to review what types of materials and coating
techniques are available for spectral design purposes and to get a rough idea what their utility is for SMT. Firstly, there is
an introduction of terminology used and the desired characteristics of a spectral design coating. In order to limit and to
focus the survey the use case is multispectral optical camouflage, i.e. reducing the contrast with the background. In the
next section of the paper results are presented and discussed. Finally a summary of results, conclusions and a discussion
on future work are presented.
1.1 Terminology and Physical quantities
Electromagnetic radiation (light) of some wavelength,
λ,
incident on an optically thin coating (thin film) is reflected,
absorbed or transmitted, hence I(
λ
)=R(
λ
)+A(
λ
)+T(
λ
)=100%, where I(
λ
) is the intensity of the incident light
,
R(
λ
) is the
reflectance, A(
λ
) is the absorptance and T(
λ
) is the transmittance. If Maxwell´s equations are used to solve the boundary
problem of a propagating wave at the surface between two media the Fresnel reflection coefficients are obtained,
=
1
cos
−
0
cos
1
cos
+
0
cos
,
=
0
cos
−
1
cos
0
cos
+
1
cos
and R(
λ
)=∙
∗
(1)
where
is the reflection coefficient for light polarized parallel and
for light polarized perpendicular to the plane of
incidence, respectively. These expressions show the relationship between the observable reflectance of a surface and
optical properties, since
= n
1
+ik
1
is the complex refractive index of the coating material and
is that of the ambient
air. In a dielectric (non-absorbing) material the refractive index is real valued.
is the angle of incidence to the
surface normal and
is the angle to the normal for refracted, transmitted, light.
Light reflected from a smooth surface, like a mirror, is specular, i.e. the angle of reflection is the same as the angle of
incidence. Light reflected from a rough surface is diffuse, i.e. scattered in all directions. In SMT context the former
surface is denoted glossy and the latter as low gloss.
If the x- and y- components of the electric field vector of a light wave propagating in z-direction are completely
correlated the light is said to be totally polarized. If there is no correlation the light is said to be unpolarized, like from
the sun or ordinary lamps.
6
Polarization features arise from the geometrical orientation, shape, shading and roughness of
an object surface. In general, man-made objects with smooth surfaces have more defined polarization signatures than
natural objects and tend to take on a polarized component in reflected or emitted radiation.
7
The ratio between the
intensity of polarized light and the total intensity of light irradiated from a surface is denoted the Degree of Polarization
(DoP).
6
The radiaton emitted from a perfect blackbody, i.e. a surface emitting maximum possible energy, depends on wavelength
and the absolute surface temperature T of an object. It is expressed through Planck´s radiation law as the spectral radiant
exitance, E
bb
(
λ
,T), through
(, ) =
1
5
(
2
−1)
(Wm
-2
μm
-1
) (2)
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where c
1 and
c
2
are not blac
k
wavelengths
denoted Emi
s
to that of the
1.2 Desired
From the de
f
justification
f
limit our inte
to design co
a
emitted radi
a
scattering of
greatly affec
t
optical prop
e
MWIR-regio
n
Figure 2
b
lackbo
d
shows t
h
and LW
I
ground t
a
In the UV-V
I
moon, the st
a
p
roperties. N
eye or any i
m
spectrally ad
a
management
R
VIS
<15%,
b
ackground
r
There are, ho
object regard
2
are radiatio
n
k
and hence
for a specifi
c
s
sivity, . He
n
background.
characterist
i
f
inition of si
g
f
or the resea
r
rest to partic
u
a
tings that c
o
a
tion. We wi
l
electromagn
e
t
the detecte
d
e
rties of spec
t
n
3-5μm, the
L
. The picture
t
d
y radiation sp
h
e transmittanc
e
I
R are indicate
d
a
rget in the sit
u
I
S-
N
IR regio
n
a
rs or from a
n
ow, the sens
o
m
aging devic
e
a
pted to that
purposes is,
t
R
NIR(green)
: 4
5
r
esponse indi
c
weve
r
, a furt
h
less of spectr
a
n
constants, c
1
are called
g
c
temperatur
e
n
ce, by desig
n
i
cs for coatin
g
g
nature we fi
n
r
ch is, howev
e
u
lar parts of t
h
o
ntrol emissi
o
l
l, however,
e
tic radiation
f
d
intensity in
t
rally selectiv
e
L
WIR 8-14μ
m
t
o the left illu
s
ectra of the il
l
e
spectra of the
d
in the lower
d
u
ation depicted.
n
the electro
m
n
artificial lig
h
o
rs in UV-VI
S
e
such as a ca
m
of the object
t
herefore,
s
pe
c
5
-60%, R
NIR(b
r
c
ated in Figur
e
h
er two para
m
a
l adaptabilit
y
1
= 3,7418 x
1
g
ray bodies.
8
e
. The ratio b
n
ing the surfa
c
g
s in multisp
n
d that it is p
er
, that sens
o
h
e electromag
n
o
ns resulting
assume that
t
f
rom the obje
c
some pa
r
ts o
f
e
coatings in
m
and the rad
a
s
trates a typica
l
l
uminating sun
atmosphere. T
d
iagram togeth
e
m
agnetic radia
t
h
t source. In t
h
S
-
N
IR are sen
s
m
era. Theref
o
background.
c
trally select
i
r
own)
: 10-25
%
e
2 is chlorop
h
m
eters to take
i
y
. Hence a se
c
1
0
8
Wm
-2
μm
4
The Emittan
etween a sur
f
c
e emissivity
ectral low si
g
artly defined
o
rs are beco
m
n
etic spectru
m
from a com
b
t
he sensors
a
c
t, due to int
e
f
the spectru
m
four wavele
n
a
r region >1
m
l
detection/ca
m
, the target ex
h
he atmospheri
c
e
r with the “id
e
t
ion from an
o
h
is interval it
s
itive to spec
t
o
re, low sign
a
Our first ch
a
i
ve reflectanc
e
%
and R
NIR(blac
k
h
yll-like in V
I
i
nto account i
n
c
ond required
and c
2
= 1,43
ce of a surf
a
f
ace gray bo
d
it is possible
g
nature appl
i
by the sensit
m
ing increasin
g
m
from the se
n
b
ination of i
n
a
re used at s
u
e
raction with
m
m
. From this
n
gth
r
egions;
m
m.
8,9
The sit
u
m
ouflage scena
r
h
aust and a s
o
c
“windows” i
d
e
al” emission s
p
o
bject is dom
i
is, therefore,
t
ral variations
a
ture requires
t
a
racteristic of
e
, close to th
a
k
)
: 5-10% in
I
S-NIR.
n
the UV-VI
S
characteristic
88 x 10
4
μm
K
a
ce is obtai
n
d
y emittance
to tune an o
b
i
cations
ivity spectru
m
g
ly multispe
c
n
sor sensitivi
t
n
cident solar
r
u
ch a distan
c
m
olecules or
point of vie
w
the UV-VIS
-
u
ation is illus
t
r
io. The right
u
o
ldier respectiv
e
d
entified for de
t
p
ectra of a coa
t
i
nated by refl
e
relevant to s
t
in the detect
e
t
he reflectan
c
a potentially
a
t of the
b
ack
g
accordance
w
S
-NIR region.
is low gloss.
3
K
. In practice
n
ed by integr
a
and that of
a
b
jects appare
n
m
of the thre
a
c
tral and hen
c
t
y point of vi
e
r
adiation an
d
c
e that the a
t
aerosols in t
h
w
it is useful
-
N
IR region
0
t
rated in Figu
r
u
pper diagram
e
ly. The midd
l
t
ection; VIS, N
I
t
ing used to ca
m
e
cted light fro
t
udy the mate
r
e
d radiation,
e
c
e of an objec
t
good coatin
g
g
round. This
u
w
ith earlier st
u
A glossy sur
f
3
(p83)
most surface
s
a
ting over a
l
a
blackbody i
s
n
t temperatur
e
a
t sensor. On
e
c
e we will n
o
e
w. The aim i
s
d
thermal sel
f
t
tenuation an
d
h
e atmospher
e
to discuss th
e
0
.2-2.5μm, th
e
r
e 2 below.
shows the
l
e diagram
I
R, MWIR
m
ouflage a
m
t
he sun, th
e
r
ials reflectiv
e
e
.g. the huma
n
t
surface to b
e
g
for signatur
e
u
sually mean
s
u
dies.
10–12
Th
e
f
ace reveals a
n
s
l
l
s
e
e
o
t
s
f
-
d
e
,
e
e
e
e
n
e
e
s
e
n
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