Biomedical Engineering Reference
In-Depth Information
a
b
Fig. 8.6
(
a
) CIE 1964 10
ı
color matching functions. (
b
) CIE 1931 2
ı
color space chromaticity
diagram. The outer curved boundary is the spectral locus which corresponds to monochromatic
light. The straight edge at the lower part represents the line of purples which have no counterparts
in monochromatic light. Mixed or less saturated colors appear in the interior with white at the point
.x; y/
D
.1=3; 1=3/
8.4.4
Structural Characterizations
Electron microscopes are indispensable tools for structural characterizations
because they have a much higher resolving power than optical microscopes. This is
due to the fact that electrons have wavelengths much shorter than those of visible
light. SEM and TEM are two types of electron microscopes commonly used to
characterize natural photonic structures. TEM may have a resolution of atomic
level, while the resolution of SEM is poorer by about an order of magnitude than
that of TEM.
SEM can image the surface topography of a sample by scanning the surface
with a fine high-energy electron beam and measuring reflected electrons. For many
samples, structural information on the substructure is needed. This can be done
by sectioning the samples. In SEM observations, a thin conductive layer of a few
nanometers (gold, platinum, tungsten, or graphite) is usually introduced on sample
surfaces in order to produce high topographic contrast and resolution.
Unlike SEM, TEM images samples by taking advantage of transmitted electrons.
This means that samples should be ultrathin in order to allow the transmission
of electrons. The ultrastructure or even the composition of samples may be
distinguished from the intensity of transmitted electrons.
Atomic force microscopy (AFM) can have a spatial resolution at the nanometer
scale and can also be used to scan the surface of a sample. AFM relies on a cantilever
with a sharp tip and a feedback mechanism that adjusts the tip-to-sample distance
