Biomedical Engineering Reference
In-Depth Information
BS1
BS2
1
G
R
G
R
B
G
B
G
0.8
B
λ =
532 nm
G
R
G
R
0.6
B
B
G
G
0.4
MO
Sample
L1
Bayer
pattern
color
camera
0.2
λ =
633nm
MO
0
400
BS3
500
Wave Length (nm)
600
700
(A)
(B)
Figure 14.6
(A) Simultaneous two-wavelength transmission QPM optical system
[18]
; (B) spectral response of the
camera color channels overlaid with the illumination wavelengths. Source: Adapted from Ref.
[43]
.
autocorrelation and complex conjugate terms. Wrapped phase maps at
λ
1
5
532 nm and
λ
2
5
633 nm were recovered by inverse Fourier transforming and taking the arctangent of
each pixel's resulting complex value.
To demonstrate the sensitivity of this system, we measured National Institute of Standards
and Technology (NIST)-certified polystyrene microspheres (
n
5
1.56,
d
5
19.99
6
0.20
μ
m)
that were immersed in index-matching oil (
n
5
1.515) to create a relative refractive index
(RI) difference of
0.045. Two-wavelength phase unwrapping resulted in a profile with
a maximum measured bead thickness of 20.105
Δ
n
5
μ
m and an RMS deviation from an ideal
sphere of 17.312 nm.
We also demonstrated the ability of simultaneous two-wavelength transmission QPM for
unwrapping high-aspect-ratio objects by imaging transparent 10
15
m tall microstructures
(
Figure 14.7A
D)
. These microstructures were printed in UV-cure epoxy using a maskless
holographic lithography process
[45]
. This procedure was also used to create a pattern of
bovine serum albumin conjugated with fluorescein isothiocyanate on substrates for use as
biosensor islands. Additionally, we demonstrated the utility of single shot two-wavelength
phase unwrapping with biological samples by imaging and unwrapping A431 human skin
cancer cells in culture (
Figure 14.7E)
.
μ
Multiple-wavelength phase unwrapping has technical drawbacks to consider. Since the
process amplifies the noise, the initial phase noise levels must be sufficiently low to keep
the amplified noise below
λ
m
/2 for successful implementation of this unwrapping
procedure. Wider spacing between the wavelengths, as what we have employed here, will
also help control the amplified noise to acceptable levels. Furthermore, highly dispersive
samples may suffer from wavelength-dependent RI differences when multiple-wavelength
phase unwrapping is applied to transmission-geometry systems. Nevertheless, the initial
results presented here demonstrate that this approach has the potential to extend the
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