Biomedical Engineering Reference
In-Depth Information
11.6.3 Nacreous Pigments
Paints containing bright materials such as alumi-
num flakes and mica have been on the market for
quite a while. These paints are used to cover sur-
faces in buildings. Nacreous paints, also called
pearly luster paints, exploit the multilayer-inter-
ference mechanism of labradorite's iridescence.
Nowadays, most nacreous paints contain flakes
of titanium-dioxide-coated mica (TiO
2
-mica),
their greatest advantages against dyes being
their high stability with respect to pH (acidity
and/or alkalinity), ambient light, and tempera-
ture
[130, 131]
. Titanium dioxide (refractive index
2.5-2.7) is the primary ingredient in white paint
used for walls, since it has the highest opacity of
all known pigments. Small quantities of titanium
dioxide are used to lighten other pigments.
To increase the reflectance and assure wave-
length selectivity, the thickness
t
of the titanium-
dioxide coating is adjusted according to the rule
thicknesses of the coating and the correspond-
ing interference colors are given in
Table 11.2
.
Because of the wide bandwidth and the low
reflectance of each flake, a dense pile of flakes is
needed for high reflectance. These paints exhibit
white color with a high degree of pearly luster,
and various interference colors--such as yellow,
red, blue, and green--are obtained by tailoring
the coating thickness.
When a paint containing titanium-dioxide-
coated mica flakes is deposited on a white back-
ground, only a pearly white luster, is visible.
This is because the light transmitted through the
flakes onto the white background is reflected
back to the eyes of the viewer, along with the
light directly reflected from the flakes. When the
TABLE 11.2
Thickness and interference colors of mica
coated with titanium dioxide
[131]
.
Interference Color
Approximate Thickness (nm)
N
TIO
2
−
SIN
2
θ ,
(2
M
−
1)λ
=
4
T
(
M
=
1, 2, 3, ...),
Yellow (gold)
75
Red (magenta)
90
(11.5)
Blue
120
where
t
is the thickness,
N
TIO
2
is the refrac-
tive index of titanium-dioxide coating, and
θ
is the angle of incidence. The color selection
in transmission and reflection resulting from
Eq.
(11.5)
for a particular wavelength is illus-
trated in
Figure 11.27
, and examples of different
Green
145
Yellow (gold)
160
Red (magenta)
185
Blue
210
Green
245
FIGURE 11.27
Reflected and transmitted colors depend on the thickness
t
of the titanium-dioxide coating of mica flakes,
after Ref.
131
. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
this topic.)
