3.1.3 Structural colors in nature
Examples of structural colors are abundant in nature; iridescent colors in insects
and marine animals, and study on the mechanisms of the structural colors has a long
history, dating back to Newton and Hooke in the 17th century. Structural colors are based
on three possible mechanisms of optics, (i) reflection, (ii) diffraction and (iii) scattering.
Initially, reflection of light through a multi-layer laminate was studied by Lord Rayleigh
(1917 ,1923). Later on the reflection in biological structures was studied by Land (1966,
1972) and Denton(1970, 1971). We shall review the optical wave reflection,
transmission and diffraction first.
The mechanism of structural color can be cast into two different interferences,
one by reflection , the other by diffraction. Let us review briefly the normal reflection
and transmission of electromagnetic waves. Consider the interface between medium 1
with ε1 and µ1, and medium 2 with ε2 and µ2 , Fig. 5.1 where ε and µ are dielectric
constant and magnetic permeability, respectively. Solving the propagation of transverse
electromagnetic wave (TEM) along the z-direction through the interface using the
interface boundary conditions, one can arrive at ( Taya, 2005, p.109) the reflection and
transmission coefficients as
R = (η2 − η1)/(η2 + η1)
(1)
T = 2η2/(η2 + η1)
(2)
where η is intrinsic impedance which is the ratio of Ex (electric field along x-direction)
to Hy (magnetic field along the y-direction) given by
η = √ µ/ε
(3)
For non-magnetic materials, η is reduced to
η = η0/n
(4)
η0 = 120π Ω ,
n =√ εr,
(5a)
(5b)
where
and where εr is relative dielectric constant and n is refraction index. With Eq.(4) and (5),
the reflection and transmission coefficients of Eqs (1) and (2) are written as
R = (n1 − n2)/(n1 + n2)
(6)
T = 2n1/(n1 + n2)
(7)
1
x
Fig. 5.1 Transmission and
reflection of transverse
electromagnetic waves for
media composed of medium
1( ε1, µ1, η1 ) and
2(ε2, µ2, η2 ) where ε, µ and η
are dielectric constant,
magnetic permeability and
intrinsic impedance,
respectively.
z
y
transmission
reflection
Medium 2
(ε2, μ2, η2)
Medium 1
(ε1, μ1, η1)
Let us consider two cases of normal incident, case (a) wave propagating medium 1 to 2
and reflection at the back interface, Fig. 5. 2(a), and case (b) incident from medium 1 to 2
and reflection at the interface between medium 2 and 3, Fig. 5. 2(b)
d
n1
d
n2
n1
n1
n2
n3
n1< n2< n3
n1< n2
(a)
(b)
Fig. 5.2 Two cases of normal incident optical wave through, (a) media 1-2-1 with n1<n2,
(b) media 1-2-3 with n1<n2<n3
Case (a): medium 2(n2) located in air(n1), see Fig. 5. 2(a)
As the wave propagating in medium 1 enters the interface of medium1-2, then two
phenomena take place, reflection back to medium 1 with reflection coefficient given by
Eq.(6) which we term as R12 , and transmission into medium 2 with transmission
coefficient, T12. As the transmitting wave in medium 2 then takes two routes, one
reflection at the 2-1 interface with reflection coefficient R21 and transmission into
2
medium 1 with transmission coefficient T21. The wave reflected at the 2-1 interface (back
surface of medium 2) will propagate through medium 2 then hit the 2-1 interface causing
another set of two waves, one reflected at 2-1 interface (front surface of medium 2) with
reflection coefficient R21, and transmitting into medium 1 with transmission coefficient
T21. This process of reflections and transmissions at the front and back surfaces of the
medium 2 continue as shown in Fig. 5. 3
T12 T21e-jθ
T12
T12 R21e-jθ
R12
T12 R212e-j3θ
-j2θ
T12 R21T21e
T12 R212T21e-j3θ
T12 R213e-j4θ
T12 R214e-j5θ
T12 R213T21e-j4θ
T12 R214T21e-j5θ
T12 R215e-j6θ
T12 R215T21e-j6θ
n1
d
<
n1
n2
Fig. 5.3 Reflections and transmissions of normal incident waves through media 1-2-1
with n1<n2
The total reflection of optical waves (Rt) into medium 1 is the summation of all waves
reflected into medium 1, thus it is given by
Rt = R12 + T12R21T21 e-j2θ + T12R213T21 e-j4θ + T12 R215 T21e-j6θ + ∙ · · · · · · ·
(8)
where
R12 = (n1 – n2)/(n1 +n2), R21 = - R12,
(9a)
T12 = 2n1/(n1 +n2), T21 = 2n2/(n1 +n2)
(9b)
T12=1 + R12, T21 = 1+R21, R12 = -R12
(9c)
3
θ = 2πdn2/λ
(9d)
And where d is the thickness of medium 2, λ is the wave length.
By using the following relation:
Σ xn = 1/(x-1)
(10)
The total reflection formula of Eq.(8) is reduced to
Rt = R12(1 – e-2jθ ) / (1 – R122 e-2jθ )
(11)
If 2θ = 4πdn2/λ = 2mπ
(12)
Then, e-2jθ =1, Eq.(11) is reduced to
Rt = 0
(13)
This case is called “destructive”, thus, no reflection at the medium 1-2 interface takes
place.
If 2θ = 4πdn2/λ = (2m + 1)π
(14)
Then, e-2jθ = -1 and Rt is reduced to
Rt= (n12 – n22)/(n12 + n22)
(15a)
It is noted in Eq. (15a) that the total reflection , Rt becomes negative for n1<n2, which is
not convenient for optical reflection , thus, we normally take its absolute value as the
total reflection, i.e.,
Rt = |(n12 – n22)/(n12 + n22)|
(15b)
This case is called as “constructive”, all waves are reflected by the 1-2 interface.
Case (b), media 1, 2 and 3 located in this order with n1<n2<n3 , see Fig. 5. 2(b)
Using the above procedure of step by step reflection and transmission at each interface,
i.e interface 1-2 and interface 2-3, see Fig. 5.4,we can obtain the total reflection into
medium 1 as
Rt = R12 + T12R23T21e-j2θ + T12R23R21R23T21e-j4θ + T12R23R21R23R21R23T21e-j6θ + · · · · · · ·
······
(16)
By using the formula of infinite series given by Eq.(10), we can simplify Rt as
Rt = R12 + T12R23T21e-j2θ / (1 – R23R21e-j2θ )
(17)
4
If 2θ = 4πdn2/λ = (2m +1)π,
(18)
e-j2θ = e-jπ = -1, then Eq.(17) is reduced to
Rt = R12 –T12R23T21/( 1 + R23R21 )
(19)
Upon substitution of R’s and T’s defined by Eq.(9a) and (9b), and use of
R23 = (n2 – n3)/(n2 + n3)
(20)
we obtain the total reflection as
Rt = (n1n3 – n22) / (n22 + n1n3)
(21)
Therefore, If n22 = n1n3, and 2θ = (2m +1)π
(22)
Then, the total reflection Rt becomes
Rt = 0
(23)
On the other hand, if 2θ = 4πdn2/λ = 2mπ,
(24)
e-j2θ = 1
(25)
then, Rt is reduced to
Rt = R12 + T12R23T21 / (1 – R23R21 )
(26)
Upon substitution of Eqs.(9a), (9b) and (20), we obtain Rt as
Rt = (n1 – n3) / (n1 + n3)
(27
Rt vanishes if n1=n3, which corresponds to the case (a), i.e., medium 2 is sandwiched by
air, Eq.(13) above.
5
T12 T23e-jθ
T12
T12 R23e-jθ
R12
T12 R23 R21e-j3θ
-j2θ
T12 R23T21e
T12 R23 R21T23e-j3θ
T12 R23 R21 R23e-j4θ
T12 R23 R21 R23 R21e-j5θ
T12 R23 R21 R23T21e-j4θ
T12 R23 R21 R23 R21T23e-j5θ
T12 R23 R21 R23 R21R23e-j6θ
T12 R23 R21 R23 R21R23 R21e-j7θ
T12 R23 R21 R23 R21R23T21e-j6θ
T12 R23 R21 R23 R21R23 R21 T23e-j7θ
n1
<
n2
<
n3
Fig. 5. 4 Reflections and transmissions of normal incident waves through media 1-2-3.
Case (c) alternative layer with medium 1-2-1-2 type, see Fig. 5.5
The total reflection of this case may be calculated by using the procedure used for case
(a). Land(1972) gave the following formula for the alternative layer of medium 1 and 2,
with number of p pairs of media 1 and 2.
Rt = |(n12p – n22p)/(n12p + n22p)|
(28)
This case corresponds to “ideal multi-layer reflection, i.e. from Eq.(14) we obtain this
condition with m=0 for both media, 1 and 2.
n1d1 = n2d2 =λ/4
(29)
6
Land calculated the values of Rt based on Eq.(28) for three cases of ideal multilayer of 12-1-2 type : (i) n1=1.33(water or cytoplasma) and n2=1.88(guanine),(ii)
n1=1.33(cytoplasma) and n2=1.56(dry protein, i.e keratin) and (iii) n1=1(air) and
n2=1.56(dry protein like chitin). Fig. 5.6 show the predicted results of the total reflection
for the ideal alternative multi-layer for these three cases. It is clear from Fig. 5.6 that the
the larger the gap of two different layers’s refraction indices, the higher the total
reflection coefficient, which is more enhanced with increasing number of multi-layer(p).
So far we used the minimum number of integer m in Eq.(14) for the calculation of
constructive wave interferences. If we consider all integer m, m= 0,1,2,3….., then, the
constructive case of interferences occurs at
λ = 4n2d2, 4n2d2/3, 4n2d2/5, …..
(30)
Similarly from Eq.(12), the destructive case of reflection interferences, takes place at
λ = ∞ (4n2d2/0), 4n2d2/2, 4n2d2/4,……
d2
d1
(31)
d2
d1
d2
n1
n2
light
n1
n2
n1
n2
Fig. 5. 5 Normal incident wave propagation through alternative media of 1-2-1-2 with
n1<n2
It is noted that the maximum reflections from multi-layer composed of alternatives of n1n2 media (Fig. 5.5) takes place at
λmax = 2 (n1d1 + n2d2 )
(32)
Eq.(29) is a special case of Eq. (32) for single layer. By using Eq.(32) we can estimate
the type of colors reflected on the skins of animal which has the multi-layer structure of
Fig. 5.5. For example, if the order of n1d1 and that of n2d2 are 100 nm, then, the wave
range reflected by such multi-layer structure is blue color, see Fig. 5. 7 where different
colors are shown with corresponding wave lengths.
7
Fig. 5.6 Reflection coefficient of normal optical waves through three different types of
alternative media of 1-2-1-2 as a function of repeating plates (p) which are considered
media 2 with higher reflection index(n2)(Land, 1972).
Fig.5.7 Color spectrum with corresponding wave length.
By using the formula derived by Huxley (1968), Land calculated the spectral distribution
of reflectance of ideal case (medium 1 and 2’s optical length is equal to quarter wave
length) and non-ideal case for three different combinations of n1-n2 alternative laminates.
Fig. 5.8 (a), (b) and (c) show reflectance as a function of incident wave(λ) for the case of
n1=water(or cytoplasma) and n2=guanine, n1 = water (cytoplasma) and n2=dry
protein(keratin), and n1= air and n2= dry protein(chitin), respectively where number of
medium (plate) is p for three figures. It is clear from Fig. 5. 8 that increasing number of p
increases the reflectance (%), and also that the larger the gap between n1 and n2, wider
the band-width of the principal peak becomes. For example, the case of (c) associated
8
with insect wings, wider range of colors in the environment of the insect can be reflected
by such ideal nanostructured wing of the insect.
n2=1.83
n1=1.33
(a)
n2=1.56
n1=1.33
(b)
n2=1.56
n1=1.00
(c)
Fig. 5.8 Spectral reflection curves for ideal multilayers with number of repeated plates(p),
for three different cases (a) guanine-water ,(b) chitin –water and (c) chitin-air(Land,
1972).
9
Oblique incident waves
When incident light is no longer normal, i.e oblique wave into a thin film(n2, d2), Fig.
5.9(a), then, the above formulas need to be changed, for example, constructive
interference , the first term of Eq.(30) is changed to
λ = 4n2d2 cos ν
(32)
The peak reflection that occurs at the wave length given by Eq.(32) is function of incident
angle, ν. In other words, larger the value of ν, the peak reflection wave length decreases.
This affects the visible colors reflected from the animal skin with such nanostructure as
Fig. 5.9(b) , shifted toward shorter wave length. Fig. 5.10 show how the peak reflection
wave length under oblique incident light (with angle of incident, ν1) is changed for three
cases of n1-n2 laminates.
ν2
ν1
d2
n2
n
n
n
n
n
d
d
d
d
d
(a)
(b)
Fig. 5.9 Oblique incidence of light to (a) n1-n2 media, (b)
Fig. 5.10 Change
in λmax with angle
of incidence(ν1)
for three cases of
multi-layers
(Land , 1972)
10
Fig. 5.11 Change in
λmax with angle of
incidence(ν ) for
Land sorted out the biological functions of multilayer structures into 5 different1 cases, (1)
three cases of multitapeta for light-path doubling and image-forming, (2) camouflage, (3) display , (4)
layers
optical filters and (5) anatomical accidents. Land then discussed in details three cases of
(Land , 1972)
multi-layer structures of biological reflectors, (i) guanine-cytoplasma, (ii) chitin-water
and (iii) chitin-air.
Three cases of reflector designs observed in animals
(i) Reflector made of guanine and cytoplasm
The reflector based on alternative layers of guanine and cytoplasma are found in a
number of biological species, particularly in fish scales and skins. Fig. 5.11 (a) and (b)
show the nanostructure of cross section of the belly of a clupeoid fish where the guanine
crystals of thickness of 100 nm and cytoplasma of 135 nm are alternatively stacked,
forming nearly ideal alternative multi-layer reflector. If the refraction indices of guanine
(n2=1.83) and cytoplasma (n1=1.33) are used, the peak reflection wave based on Eq.(32)
is calculated as λmax = 726 nm, which corresponds to red color. Another example of
reflection multilayer associated with fish is the iris of Neon Tetra whose crystals(made of
guanine) are shown in Fig. 5.11(c) where two different crystal thicknesses are used, thick
crystal with its thickness of 98 nm and thin one with 67 nm, resulting in the peak
reflection wave (4nd) being 709 nm and 482 nm, which correspond to orange and blue
color, respectively. Fig. 5.11(d) shows the case of camouflage if another fish viewing this
fish from slightly lower level , it can see the reflected light from B angle, thus, the color
recognized by another fish is the color of the surrounding, as the intensity of the light
from A angle is the same level as the reflected light from B, thus, the fish located at the
lower level is deceived by the surrounding color.
11
Fig. 5. 11 reflectors in fish ;(a) irridophores of the belly of a clupeoid which is composed
of hard tissue of guanine crystals (hatched) and soft tissue (redrawn from the SEM
photos made by Kawaguchi and Kamishima (1966), (b) diaphragm (taken from Denton,
1970) of a reflecting platelet from a ventral a sprat with approximate dimensions of hard
and soft lamellae are shown, (c) surface views and thickness of crystals from two
reflecting systems in the iris of he Neon Tetra with thickness d and 4nd values also
shown, and (d) principle of camouflage using a reflecting surface (modified from the
paper by Denton, 1970), in sea, light from A-direction is similar to that from B-direction,
is reflected perfectly by the vertically oriented reflectors, thus, the fish is invisible from
the viewer from C.
According to the ideal reflection model based on Eq.(32), the colors of herring and sprat
are red, but actually they look silver. This is mainly due to overlap of scales which have
different color region (Denton and Nicol, 1965). This white color could be induced by
active spacing of cytoplasma as it can swell in taking water, larger spacing of d1, leading
to the longer reflected waves. Fig. 5. 12 shows such an example of effect of osmotic
pressure observed in teleost Ringer where open circles show the real case of optical
density of this fish in normal sea water environment, while the dark circles are the optical
density of the fish skin in condensed sea water for which the spacing of the cytoplama
layer is narrowed, thus, reflecting mainly shorter waves, shifting to blue color region
(Denton, 1970).
Fig. 5. 13 the reflector made of
alternative chitin(dark) and
water(white) with their
thicknesses being 100 nm,
redrawn from the photos of
Kawaguchi and Ohnish(1962)(
Land, 1972)
Fig.5.12 Effects of osmotic pressure on
reflectance (or equivalently optical density,
open circles: scale in marine teleost
Ringer, closed circles: the scale in
condensed solution
12
(ii)Reflectors made of chitin or protein in water
Kawaguchi and Ohnishi(1962) examined the Cephalopod reflector which is composed of
soft protein (Chitin or keratin) layer embedded in water-like medium where the thickness
of the alternative layer of chitin and water is about 100 nm, Fig. 5.13 , ideal geometry of
reflector system, by which the surrounding colors can be reflected back to viewers, i.e
camouflage skin.
(iii) Reflectors made of chitin and air
In insects, the multi-layer reflectors can not have water, in stead the lower index layer is
air, between hard tissue , typically chitin. Thus, this alternative layer of chitin (n=1.56)
and air (n=1.0) makes another ideal reflector design in nature, see Fig. 5.8 which is
notably found in a number of insect wings, such as Morpho cypris first observed by
Anderson and Richard (1942) , then reexamined by Lippert and Gentil(1959) , followed
by more recent researchers (Vukusic, et al, 1999; Kinoshita et al, 2002;Watanabe et al,
2005).
Vukusic et al(2001) categorized two classes of reflector design inherent in butterflies,
class I (or Morpho class) and class II(or Urania class). The first class comprises layering
within discrete ridged structures on the surface of scales that cover the wing while the
second class is made of continuous multilaying within the body of the iridescent scales.
Vukusic et al focused two species of butterfly with the class II which has modulation in
the profile of the multi-layering, P. palinurus and P. ulysses which are shown in Fig.
5.14(a) ,and (b), respectively.
Fig. 5.14 (a) P. palinurus (green color )
(b) P. Ulysses (blue colar)[ the scale bar is
1.5 cm] (taken from the paper by Vukusic
et al (2001)
Fig. 5. 15 SEM photos of P.palinurus, (a)
several iridescent scales, (b) a part of a
iridescent scal and (c) single concavity[scale
bars, (a) 10 µm, (b) 5 µm and (c) 1µm] (taken
13 from the paper by Vukusic et al (2001)
SEM photos of the magnified views of P. palinurus’s iridescent scales are shown in Fig.
5. 15 exhibiting concavity of scale surface. If the scales are cut and viewed from side by
using TEM , then Fig. 5. 16 shows such TEM photos of the cross section of P. palinurus
, periodic angulation with periodicity of 6 µm(inset photo ) and each concave portion is
also multi-layering vertically composed of cuticle and air whose thicknesses are
approximately 100 nm range. It is noted that the dimensions of the multi-layer of P.
palinurus are smaller than those of P.ulysess. This leads to the fact that the color
reflection from the flat bottom of P. palinurus is green while that from the bottom of P.
ulysess is blue.
Fig. 5. 16 TEM photos of the cross section of P. palinurus iridescent scale (insert shows
several concavities across a single scale)[the scale bars, 1 µm (inset 3 µm) (taken from
the paper by Vukusic et al,2001).
Optical micrographs of P. palinurus and P. ulysess are shown in Fig. 5.17 (a) and (b),
respectively. In P. palinurus two different colors are seen, green in the flat center area
and blue from the sides of the concavities, while in P. ulysess the center flat region
showing blue and the sides of the concavitites showing somehow greenish blue, but on
the average, monocolor of blue is dominant, which agree with the macroscopic colors
seen in the wings of these butterflies, Fig. 5. 14. In P. palinurus , the green color that can
be incident to the 45 degree wall, can be seen as blue, see Fig. 5.11, and it passes over the
flat bottom then reflected at the 45 degree surface of the opposite concavity wall, back
into the direction of incident light (i.e vertical), thus, the same blue color is seen from the
14
observer sitting above the scale while the green color light is reflected from the bottom
flat surface, thus the reflected light is still seen as green. However in P. ulysess , the slope
of the concavities is less steep, say 30 degree or so, thus, the above phenomenon
observed with P. palinurus is not realized. Vukusic et al used the standard reflection
model from multilayer and predicted those green color for P. palinurus and blue color for
P. ulysess.
Fig. 5.17 Optical micrographs of two butterflies, (a) P. palinurus and (b) P. ulysess
(taken from Vukusic et al, 2001).
Fig. 5. 18 Four different Morph butterflies, (a) M. didius,(b) M. rhetenor, (c) M. Adonis
and (d) M. sulkowski (Kinoshita et al, 2002)
15
Morpho class reflector design
This class of reflector was first studied by Anderson and Richard (1942), who observed
the nanostructure of iridescent scale which is composed of periodically arranged
ribs(ridge), each ridge being a lamellar structure made of alternative chitin and air
elements. Kinoshita et al (2002) examined the nanostructure of four kinds of Morpho
butteries, M. didius, M. rhetenor, M. Adonis and M. sulkowski , Fig. 5. 18 the cross
sections of which are shown in Fig. 5.19 where the thickness of chitin and air layers are
55(d2=55 nm) and 150(d1=150nm) nm, respectively. If we use Eq.(32) and refraction
indeces of air (n1=1) and chitin(n2=1.56) , we can estimate of λmax = 480nm as the peak
of the reflected optical wave, which is light blue. It is noted that the reflected color peak
is shifted toward shorter if we look at the Morpho butterfly wing from oblique angles. So
the color should be looked at dark blue, however the color of the Morpho wing remains
light blue regardless of various angles of observation. This is based on simple reflection
model discussed earlier, i.e each chitin and air layers are extended infinitely horizontal
direction. However the widths of both chitin and air layers are finite, about 300 nm. The
numbers of repeating chitin layers(this the same as air layer) are 6-10 and the spacing of
the ridge is about 700-800 nm. Kinoshita et al considered that there are other mechanisms
operative in the case of Morpho wing color being light blue regardless of incident angles.
They considered the effects of finite width of each chitin layer, and the heights of the
ridges are not the same, i.e they change alternatively. Accounting of these, Kinoshita et al
considered the model shown in Fig. 5. 20. The predictions of their model suggest the
following mechanisms that exist in the Morpho wing color:
(1) the structural color is mainly from the reflections from chitin lamellae,
following the standard muli-layer reflection model
(2) The irregularity of the ridge height destroys the interference among
neighboring lamellae, resulting in diffuse of the reflected lights, thus more
uniformly distributed reflectance in angle.
(3) The high reflectivity is due to 6-10 layers of chitin-air lamella structure.
(4) The pigment at the bottom layer absorbs green and red , thus reflection of
blue light.
16
Fig.5.19 Nanostructures of ridges of four different Morpho butterfiies, (a) M.
didius, (b) M. rhetenor, (c) M. adonis and (d) M. sulkowskyi (Kinoshita et al,
2002)
Fig. 5. 20 Model for ridges which are not at the same height, and each ridge is composed
of lamellae (Kinoshita et al, 2002)
17
Watanabe et al (2005) attempted to synthesize the nanostructures inherent in Morpho
butterfly wings by using the nanotechnology route,i.e, focused ion beam and chemical
vapor deposition (FIB-CVD). The SEM photo of one ridge structure processed by the
FIB-CVD is shown in Fig. 5. 21 where (a) is the nanostructure of the synthetic ridge of
Morpho wing, and (b) is the blue color observed at different angles.
Fig. 5. 21 (a) The SEM photo of the ridge of synthetic Morpho betterfly wing processed
by FIB-CVD route, and (b) solor reflected by the synthetic Morpho wing ridges observed
at different angles showing mostly blue color (taken from the paper by Watanabe et al,
2005).
Fig.5. 22 Ridge model used by Watanabe et al (2005) with defined dimensions and (b)
simulated reflected lights under various incident angles (50,200 and 300) indicating the
most visible color from the reflected lights is blue, matching with the experimentally
observed color of Fig. 5.21(b).
18
Watanabe et al used the model to explain the observed results of the reflected waves and
their intensities where they used the following formula.
 kdvM 
2  kaµ 
sin 2 

 sin 
2 
2 
k



2
⋅ cos 2  (aµ + dv) ⋅ sin 2 θ
⋅
I φ = I θ ⋅ 2a ⋅
2
 kdv 
2

 kaµ 
sin 2 



 2 
 2 
(33)
where Iθ and Iφ are the incident light intensity and the reflected light intensity,
respectively, M is the number of layer, θ and φ are the incident angle, and reflected light
angle (see Fig. 5. 22 (b)), respectively, µ and ν are given by
µ = cos θ + cos φ, ν = sin θ + sin φ
(34a)
and κ is wave number defined by
κ = λ /(2π)
(34b)
and d and a are periodic spacing of pitch and width of layer, see Fig.5. 22 (a). The
predicted reflected waves are plotted as a function of wave length for three different
angles of incidence, see Fig. 5.22 (b). This simulated results based on the model agree
well in that the reflected waves in Fig. 5.22 (b) is centered around 440nm, which is blue,
see the corresponding experimental data of Fig. 5.21(b).
Fig. 5. 23 shows the reflected wave intensities of real Morpho wing scales and synthetic
version processed by FIB-CVD on diamond like carbon material, respectively, indicating
that the peak wave of about 440nm(blue) is the same between the real and synthetic
Morpho wing scales and also the blue color peak is realized regardless of observation
angles.
Fig. 5. 23 Intensity of
reflected light by Morpho
wing scales at different
oblique angles, (a) real
Morpho wing scales, (b)
synthetic scales made of
diamond line carbon by
using FIB-CVD process
(Watanabe et al, 2005)
19
5.4.4 Camouflage skin
Camouflage skin in nature
Camouflage behavior of octopus is well-known, Fig. 5.29 illustates the skin color of
octopus in the background of sea weed, in camouflage mode, (a) and without camouflage
mode (b). Fig.5.29 (a) exhibits a good example of effectiveness of such camouflage skin
of octopus.
Fig. 5. 29 (a)Octopus in camouflage mode in the background of sea weed, and (b)
without camouflage mode.
The ability of cephalopods to create various color patterns has been studied by many
researchers. Earlier studies are made by several Italian biologists(Sangiovanni,
1819,1829;Chiaje, 1829) where they identified that chromatophores can be expanded by
their radial muscles.Hanlon and Messenger (1988) studied the adaptive color patterns of
young cuttlefish (Sepia officinalis). They identified that there are three key components
responsible for adaptive coloration, (i) chromatophores, (ii) iridophores and (iii)
leucophores, all of which are connected to local neuronal network which is then
connected to its brain. Among these, chromatophores play the key role in exhibiting
dynamic adaptive coloration. Chromatophores of cephalods are neuromuscular organs
with built-in pigments and they are controlled by a set of lobes in the brain organized
hierarchically (Messenger, 2001). The chomatophores located on top layer have fixed
color (red, yellow) but its size can be freely changed dramatically, while the irridophores
made of chitin and cytoplasma based multi-layer structure reflect mainly blue-green
colors and the leucophores made of chitin-cytoplasma type multi-layer but they are
broad-band reflector, thus creating any surrounding color including whitish color. Fig.
5.30 shows the vertical section of the skin of octopus vulgaris where chromatophores(CP)
shown in black color is located near top layer, and irridophores(IR) in gray color with
lamellar structure is in the middle layer and leucophores clubs(LC) are located at the
20
bottom layer (Messenger,2001). Fig. 5.31 illustrates idealized sketch of this multi-layer
structure of cephalopod where (a) is the case of relected lights by mainly
chomatophores(ch) in their expanded shape which provide darker colors(red and yellow),
(b) lights are reflected by mainly irridephores(IR) and leucophores(LP) which provide
green-blue color and whitish color respectively while the size of the chomatophores are
shrunk so as to passing incoming lights to deeper region.
Fig. 5. 30 Vertical cross section
of the skin of Octopus vulgaris
showing superficial
chromatophores in dark
spherical shape (CP),
iridophores (IR) in the middle
layer and leucophore clubs(LC)
at the bottom layer (Messenger,
2001).
Expanded
chromatophores
Ch
IR
LP
Ch
Shrunk
chromatophores
IR
Green-blue
reflector
White reflector
LP
Fig. 5. 31 Mechanism of camouflage skin patterning by using multi-layer structure of
skin of octopus , (a) lights are reflected by chromatophores in their expanded mode, thus
no lights passing through to the middle and bottom reflectors, (b) superficial layer of
21
chromatophores in shrunk mode so that lights are reflected by iridophore(IR) layer which
reflect mainly green/blue colors and reflected by leucophores (LP) which is broad-band
reflector, thus reflecting white light, (modified from the figure in the paper by Messenger,
2001).
Fig. 5. 32 shows top down view of arm skin of Octopus vulgaris, where darker colored
chomatophores (red, orange, yellow) are seen along with green-blue colored irridephores
of very small size as background, and also whitish leucophores (Messenger, 2001).
Fig. 3.32
Chiao and Hanlon (2001 a and b) studied the effects of background patterns on the
camouflage skin of cuttlefish. They used young cuttlefish (Sepia pharanis) inside a sea
water tank , the bottom of which is composed of predetermined black-white checker
board pattern with various sizes. They found several notable camouflage behaviors: (i)
there exists the range of size of white spots in the black background, that the cuttlefish
can respond, i.e. if the size is too small or too large relative to the size of the cuttlefish, it
would not respond to such background patterning, This size dependence camouflage
behavior is demonstrated in Fig. 5.33A-E and the size dependence summary is shown in
Fig. 5. 33F from which the size range for which the cuttlefish would respond to is 5-20
mm, (ii) for a given size of the checker board pattern, the cuttlefish can respond to certain
range of contrast ratio. The experimental data of this behavior is shown in Fig. 5.34 A-E,
and the range of the contrast ratio of white and black patterning is summarized in Fig. 5.
34 F which indicates that the larger contrast ratios give rise to the strongly responsive in
the camouflage skin, finally (iii) the effects of number of white spots in the black
background are examined, as shown in Fig. 5. 35 A-E and its summary given in Fig. 5.35
22
F which provides the range of number of white spots for which the cuttlefish would
respond to.
Fig. 5.33 Camouflage skin patterns of cuttlefish in various checker board patterns, (A) to
(E) in order of increasing size of checker , (F) the patterning grade as a function of size of
checker (Chiao and Hanlon, 2001b)
23
Fig. 5.34 Camouflage skin of cuttlefish in checker board with various contrast ratios, (A)
to (E) in order of higher contrast ratio between light and dark checkers, (F) patterning
grade as a function of percentage contrast (Chiao and Hanlon, 2001b).
Fig. 5.35 Camouflage skin of cuttlefish in checker board background with various
number of while checkers, (A) to (E) in order of increasing number of the white checker
and (F), patterning grade as a function of number of white squares (Chiao and Hanlon,
2001b).
24