COL 12(10), 101201(2014) CHINESE OPTICS LETTERS October 10, 2014
1671-7694/2014/101201(4) 101201-1 © 2014 Chinese Optics Letters
Linear polarization characteristics for terrain
identication at millimeter wave band
Xuan Lu (逯 暄), Zelong Xiao (肖泽龙)
*
, and Jianzhong Xu (许建中)
School of Electronic and Optical Engineering, Nanjing University of Science and
Technology, Nanjing 210094, China
*
Corresponding author: zelongxiao@mail.njust.edu.cn
Received May 7, 2014; accepted July 3, 2014; posted online September 28, 2014
We experimentally research linear polarization characteristics of various terrains at millimeter wave band for
image interpretation. We measure and discuss the polarization phenomena, and consider as well the incident
angle which also aects terrain radiometric temperature. An economic single-channel radiometer is used in
measurements, and changes to the linear polarization are produced by manually rotating its waveguide. We
demonstrate that the characteristic in polarization is a decisive advantage of terrain identication in ways
beyond that which can be achieved using an intensity radiometer alone.
OCIS codes: 350.4010, 120.5630.
doi: 10.3788/COL201412.101201.
Passive millimeter wave (PMMW) imaging system has
been widely applied in terrestrial remote sensing and
scene surveillance due to its low atmospheric attenua-
tion, all-weather working ability, and high contrast in
outdoor environment
[1,2]
. Additionally, the use of po-
larization provides an extra dimension of information
beyond intensity
[3]
. Hence a number of researchers are
focusing on passive polarimetric research at millimeter
wave band. Some of them successfully made the fully
polarimetric imaging systems
[4–8]
. On the other hand,
some researchers are more attracted by linearly polar-
ization dierence, especially horizontal and vertical po-
larizations
[9,10]
. A dual-polarization imager using a beam
splitter to analyze the scene by comparing two orthogo-
nal polarized images was developed
[11,12]
, and various
surfaces from desert environment using the polarization
dierence technique were test
[13]
. Kim et al.
[14]
obtained
a linear polarization sum imaging for target recognition.
For the economic single-channel PMMW system, the
waveguide direction determines the linear polarization,
that is, we can get all linear polarizations by manually
rotating the sensor. This is the aim of our letter and we
will carry out several experiments on linear polarization
characteristics of various terrains.
The millimeter wave energy of natural background is
generated in random polarization as the thermal energy
of emitting and absorbing molecules exceeds the ener-
gy of dipole alignment in natural electric and magnetic
elds
[7]
. However, the observed radiation reaches par-
tially polarized states after interaction with natural and
man-made objects, the degree to which is determined by
their dielectric properties, incident angle, etc. Assuming
the wave moves from a non-magnetic medium of a given
refractive index n
1
into a second non-magnetic medium
with refractive index n
2
, the reectivity of a smooth
dielectric substance, R, is governed by Fresnel equa-
tions. Specically, the reectivity for horizontally and
vertically polarized incidence are typically calculated as
2
1
12
2
2
1
12
2
cos 1 sin
,
cos 1 sin
H
n
nn
n
R
n
nn
n
−−
=
+−
2
2
1
2
2
1
2
1 sin cos
,
1 sin cos
V
n
n
R
n
n
=
(1)
where q
i
is the incident angle.
Here the incident wave for various terrains basically
comes from the air in outdoor environment. This im-
plies that n
2
> n
1
= 1 and total internal reection will
not happen. Consequently, the polarization dierence
can be observed in all reected angles from 0° to 90°.
Figure 1 shows the reectivity calculated in horizontal
0 20 40 60 80
0
0.2
0.4
0.6
0.8
1
θ
i
/[°]
Brewster angle
R
V
R
H
Fig. 1. Reectivity in horizontal and vertical polarizations for
water with n = 1.33.