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Underwater laser detection system
Article in Proceedings of SPIE - The International Society for Optical Engineering · March 2015
DOI: 10.1117/12.2080181
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Underwater laser detection system
a,b
Walid Gomaaa, Ashraf F. El-Sherifb, Yasser H. El-Sharkawyc
Eng. Physics Dept., c Biomedical Eng. Dept., MTC University, Cairo,Egypt.
ABSTRACT
The conventional method used to detect an underwater target is by sending and receiving some form of acoustic energy.
But the acoustic systems have limitations in the range resolution and accuracy; while, the potential benefits of a laserbased underwater target detection include high directionality, high response, and high range accuracy. Lasers operating
in the blue-green region of the light spectrum(420 : 570nm)have a several applications in the area of detection and
ranging of submersible targets due to minimum attenuation through water ( less than 0.1 m-1) and maximum laser
reflection from estimated target (like mines or submarines) to provide a long range of detection. In this paper laser
attenuation in water was measured experimentally by new simple method by using high resolution spectrometer. The
laser echoes from different targets (metal, plastic, wood, and rubber) were detected using high resolution CCD camera;
the position of detection camera was optimized to provide a high reflection laser from target and low backscattering
noise from the water medium, digital image processing techniques were applied to detect and discriminate the echoes
from the metal target and subtract the echoes from other objects. Extraction the image of target from the scattering noise
is done by background subtraction and edge detection techniques. As a conclusion, we present a high response laser
imaging system to detect and discriminate small size, like-mine underwater targets.
Keywords: Blue-green lasers, laser absorption, reflection, target detection, laser imaging system, and image processing.
.
1.
INTRODUCTION
Water is one of the most important molecules on Earth, which makes our planet different from other known celestial
bodies. Water covers approximately 70% of the earth, mostly as saltwater in the ocean (97%) [1]. Water is unique in that
it exists in all three states of matter: solid, liquid, and gas within the range of temperature and pressure at Earth’s surface
[2, 3].
Sonar technique is usually used in detecting underwater target. Though modern sonar detection improves detecting
capability through all kind of advanced techniques, it cannot satisfies needs of fact underwater target detecting because
of limitation itself and noise elimination and disturbing techniques using. Therefore, other means such as underwater
laser target imaging detection are investigated.
Underwater range finders have historically used acoustic radiation due to their long range propagation characteristics.
However, acoustic systems have limitations in the range resolution and accuracy which they can provide under certain
conditions [4]. The distance meters utilizing acoustic waves use frequencies in the ultrasonic range from 20kHz to some
MHz. The advantages of ultrasonic methods are relatively low frequencies, which are easy to handle with electronics,
simple and cheap structure of the device. The main disadvantage is the large divergence of the beam (the beam width
being some tens of degrees) which apply limitations in the range resolution and accuracy, slowness, multiple reflections
and large attenuation in long distances. The ultrasound methods are most suitable in short distances (some tens of meters
at the maximum) [5]. Optical techniques are currently being investigated to detect and imaging objects underwater due to
the advantages that lasers provide, including high-directionality, high response, high-precision, and high range accuracy
[4].
Solid State Lasers XXIV: Technology and Devices, edited by W. Andrew Clarkson, Ramesh K. Shori,
Proc. of SPIE Vol. 9342, 934221 · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2080181
Proc. of SPIE Vol. 9342 934221-1
2.
WATER OPTICAL PROPERTIES THEORY:
The optical attenuation spectrum of liquid water has been intensively studied, its structure is very complicated since its
molecules are connected by hydrogen bonds; the structure of liquid water gives rise to several anomalies when compared
to other liquids [2].
The bulk optical properties of water are conveniently divided into two mutually exclusive classes; inherent and apparent.
Inherent optical properties (IOP’s) are those properties that depend only upon the medium, and therefore are independent
of the ambient light field within the medium. The two fundamental IOP’s are the absorption coefficient and the volume
scattering function. Other IOP’s include the index of refraction, the beam attenuation coefficient and the single-scattering
albedo. Apparent optical properties (AOP’s) are those properties that depend both on the medium (the IOP’s) and on the
geometric (directional) structure of the ambient light field, and that display enough regular features and stability to be
useful descriptors of the water body. Commonly used AOP’s are the irradiance reflectance, the average cosines, and the
various diffuse attenuation coefficients [6].
Determination of the spectral beam attenuation c(λ) for natural waters is a difficult task for several reasons. First, water
itself absorbs only weakly at near-UV and blue wavelengths, so that very sensitive instruments are required. The total
spectral beam attenuation:
c(λ) = a(λ) + b(λ )
(1)
Where a(λ) is the spectral absorption coefficient and b(λ) is the spectral scattering coefficient. The attenuation
coefficient c(λ) with unit m-1of the liquid water for electromagnetic radiation, at a particular wavelength is defined by
Lambert-Beer law illustrated in Eq. (2):
I=I e
( )
(2)
Where I is the transmitted intensity of light, I0 is the incident intensity of the light and L is the path length.
In studies of electromagnetic wave propagation at the level of Maxwell's equations, it is convenient to specify the optical
properties of the medium via the electrical permittivity ԑ, the magnetic permeability µ, and the electrical conductivity σ.
The effects of ԑ, µ and σ on electromagnetic plane-wave propagation are compactly summarized in terms of the complex
index of refraction, m = n + ik, The real part n of m is usually called "the index of refraction"; the imaginary part of the
index of refraction k(λ) is electrodynamic absorption coefficient, Although n(λ) and k(λ) are collectively called the
optical constants of water, they depend strongly on wavelength. The explicit dependence of m on ԑ, µ and σ is given by:
[7,8]
m = μԑc − i
m = (n − ik) = n − k − i 2nk
(3)
(4)
The optical constants are convenient because they are directly related to the scattering and absorbing properties of water.
The spectral absorption coefficient a(λ) is related to k(λ) by[6] :
a(λ) =
( )
Proc. of SPIE Vol. 9342 934221-2
(5)
Figure 1. The optical connstants of pure water.
w
The leftt axis gives the imaginary part of m, and the rright axis gives the real
part of m,, where m is thee complex indexx of refraction.
ws the waveleength dependence of the opptical constan
nts n and k foor pure water. The extraorrdinary featuree
Figure 1 show
seen in this figure
f
is the narrow "windoow" in k(λ), where
w
k(λ) deccreases by oveer nine orderss of magnitud
de between thee
near ultravioolet and the visible,
v
and then
t
quickly rises again in
n the near innfrared. Thiss behavior in
n k(λ) gives a
correspondingg window in the
t spectral abbsorption coeffficient a(λ), as
a shown in figg.2. The shape of the absorrption curve of
pure water can
c be explainned as follow
ws. At blue-ggreen wavelen
ngths, photonns are barely energetic eno
ough to boosst
electrons intoo higher energgy levels of thhe water moleccule, and the photons
p
do noot have the rigght energy to interact easilyy
with the wateer molecule ass a whole (althhough some vibrational modes of the moolecule can be excited). so, the
t absorptionn
coefficient a(λ)
a
has a minnimum valuess in that bandd. As the wav
velength decreeases toward the ultravioleet, the photonss
become suffiiciently energgetic to excitee atomic transitions, and the
t absorptionn rapidly incrreases. At ex
xtremely small
wavelengths, processes succh as Comptonn scattering (sscattering of high-energy
h
phhotons by elecctrons) come in
i to play, andd
the relevant parameter is just the denssity of the material,
m
not itts structure ass a water moolecule. As th
he wavelengthh
increases from
m blue to redd and beyond, the photons begin having
g just the righht energy to eexcite first thee fundamentaal
vibrational annd then the rootational moddes of the waater moleculess, and absorpttion once agaain increases rapidly in thee
infrared. Thee prominent peaks in a(λ) in
i the infraredd result from these resonannt excitations of molecularr motions. At
A
very long waavelengths, thee photons are not energeticc enough to ex
xcite moleculaar motions, annd the absorption decreasess
[7].
Figure 2 Spectral
S
absorpttion coefficientt of pure water (solid
(
line) and
d of pure sea waater (dotted line) as a function of
wavelengtth.
Proc. of SPIE Vol. 9342 934221-3
Figure 3 .Absorption andd Scattering of water
w
as a functtion of wavelen
ngth (200:750 nm).
n
Studies on the visible bandd to investigatte the band thaat has lowest attenuation
a
in water is blue-green light as illustrated inn
Fig.3, due to low absorptioon and scatteriing coefficientt[8],[9].
At the most fundamental
f
a scattering arises
all
a
from innteractions bettween molecuules or atoms and photons. Neverthelesss,
scattering in natural waterrs is convenieently viewed as being cau
used by small-scale (<<λ) density fluctu
uations due too
random molecular motionss, by the large particles (>λ)).The forward
d scattering efffects in the seaa water for waavelengths λ =
400nm is 0:00076m-1 and att λ = 550nm iss 0:06m-1 whicch is the loweest values of sccattering in thhe electromagn
netic spectrum
m
[10]
3.
EX
XPERIMEN
NTAL SETU
UPS AND RESULTS:
R
w
selection:
3.1 Laser wavelength
In the next work
w
we have to select the Laser waveleength that pro
ovides minimuum attenuationn through watter, maximum
m
laser reflectioon of estimateed target (minne or submariine) to provid
de a long rangge of detectioon, and also comparing
c
thee
beam profile of the laser soources.
3.1.1 Water optical
o
absorrption coefficiients:
Although diffficulties of obbtaining accuraate measurem
ments of the op
ptical absorptioon of water inn the visible an
nd UV regionss
due to very low
l
values [33], we obtain the absorptioon coefficientt of water in that band using the experrimental setupp
illustrated in Fig. (4a) thhat consists of
o tungsten haalogen light source (HL-22000), water sample, a hiigh resolutionn
2
1100nm
m, Ocean opticcs Inc.) equip
pped with ann integrated spphere. The sy
ystem used too
spectrometer (USB4000, 200
measure the absorption cooefficient of water
w
by compparing the speectrometer spectral intensitty of referencce light sourcee
after propagaating through the water witth its correspoonding valuess if propagatinng through em
mpty glass co
ontainer at thee
same distancee, the dark sppectral intensiity was subtraacted from bo
oth reference and
a sample inntensities; Du
ue to the small
variation of absorption
a
coeefficient valuees, it convertedd to logarithm
mic scale to disstinguish its vaariation.
The measured spectrum of
o tungsten haalogen light source
s
(HL-20
000) shown inn Fig. (5a), aactually it is not
n the wholee
spectrum of the
t source, buut it is the thee limit of the used spectrom
meter spectrum
m band. The unit of vertical axis of thaat
spectrum is counts
c
( i.e. noot the power or
o energy ) buut it refers to th
he variation of
o the spectral power of sou
urce versus thee
wavelength, and
a the measuured water’s absorption
a
coeefficients of th
he visible speectrum shown in Fig. (5b). It is clear thaat
the absorptionn coefficient of
o water has itts lower valuee (less than (0::1m 1) in thee band of visibble wavelength
hs.
Proc. of SPIE Vol. 9342 934221-4
(a) tungsten halogen light source (HL-2000)
(b) Cool Red Light Source (Ocean Optics)
Figure 4: Photographic image of the experimental setup consisting of light source, watersample, a high resolution
spectrometer equipped with an integrated sphere.
For the experimental setup shown in Fig. (4b). The measured spectrum of Cool Red Light Source (Ocean Optics)
measured by the spectrometer ( NIR Q512 Ocean optics Inc.) is shown in Fig. (6a), and the corresponding measured
water’s absorption coefficient of Cool Red Light Source (Ocean Optics) is shown in Fig. (6b).
It’s clear from the absorption coefficient graphs ( Fig. (5b), and Fig. (6b)) that the blue–green region has the lowest
absorption coefficient and similar to .Fig. (3). This lower values of absorption coefficient provides a high propagation
range and suitable for the applications of underwater detection and imaging systems.
(a) Measured spectrum of tungsten halogen light source
(HL-2000)
(b) The experimental measurements of water’s
absorption coefficient of (HL-2000)
Figure 5 (HL-2000) tungsten halogen light source.
Proc. of SPIE Vol. 9342 934221-5
(a) Measured spectrum of Cool Red Light Source (Ocean
(b) The experimental measurements of water’s
absorption coefficient of the Cool Red Light Source
Figure 6: Cool Red Light Source (Ocean Optics).
3.1.2 Target Reflection coefficients:
Figure 7.
Figure (7a) illustrated Photographic image of the experimental setup consisting of halogen light source (DH-2000) with
spectrum shown in Fig. (7b), water sample, reflecting target ( shinny metal, black coated metal, wood, and plastic), and a
high resolution spectrometer (USB400, Ocean optics Inc.) equipped with an integrated sphere. The reflection spectrum
of the shinny metal target shown in Fig. (8a) illustrate that the reflection from that target almost 30% at that conditions,
and its highest value is at blue-green region of the visible band spectrum. The reflection spectrum of the black coated
metal target shown in Fig. (8b) illustrate that the reflection from that target almost 20% at that conditions. The reflection
spectrum of the wood target shown in Fig. (8c) illustrate that the reflection from that target less than 10% at that
conditions. and approximately the same for the reflection from plastic targets shown in Fig. (8d).
Its clear that the reflection from metal target along the visible band spectrum is higher than other targets (wood, and
plastic), so we can distinguish the metal targets (like mines) due to its high reflection and ignore the reflection from other
targets that will be illustrated later in this thesis.
Proc. of SPIE Vol. 9342 934221-6
Figure 8: The experimental measurements of reflection of halogen light source (DH-2000) from different targets.
Figure 9: Photographic image of the experimental setup of reflection from different targets
.
Now we will study the reflection of different laser sources in the visible band ( blue laser source (457nm), green laser
source (532nm), red laser source (632nm) ) with the experimental setup shown in Fig. (9) that consisting of laser source,
water sample, reflecting target ( shinny metal, black coated metal, wood, and plastic), and a high resolution spectrometer
(USB400, Ocean optics Inc.) equipped with an integrated sphere. In order to compare the reflection from different laser
sources although they have different power, we normalize the reflection spectrum to its corresponding maximum output
from each source.
Selection of laser source must provide low absorption coefficient of laser in water and also a high reflection from targets.
So, three laser were used to distinguish the normalized reflection from different targets with respect its reference as
illustrated in Fig.(10b), Fig.(10d), and Fig.(10f). for the blue laser source (457nm), the reflection from different targets
(metal, wood, plastic, and rubber) plotted in Fig. (10b), the reflection from metal target (38%) is higher than from the
other targets. But the reflection of green laser source (532nm) from different targets (metal, wood, plastic, and rubber)
plotted in Fig. (10d). The reflection from metal target (44%) is higher than from the other targets. The reflection of red
Proc. of SPIE Vol. 9342 934221-7
laser source from metal target (31.8%) is higher than from the other targets as plotted in Fig. (10f), but the reflection
from red laser is lower than its corresponding from blue and green laser sources from different targets; so red laser
source (632 nm) will be excluded from our measurements. The reflected laser from metal target is higher than other
targets (wood, plastic, and rubber), so we can distinguish the metal targets (like mines) due to its high reflection.
Figure 10: Reflection measurements setup and results.
Proc. of SPIE Vol. 9342 934221-8
3.1.3 Laser beam shape:
For the laser sources used in the system measurements, the beam profile of the laser sources were measured using the
experimental setup shown in Fig. (11a).
Figure 11
The beam profile of the red laser source shown in Fig. (11b).it is clear that the beam of red laser source is Gaussian
beam, but this source were excluded from the system because it doesn’t provide lower attenuation in water and higher
reflection from the targets.
The beam profile of the green laser source is shown in Fig. (11c) and it is also a good Gaussian beam shape but Fig.
(11b) illustrate that the beam profile of blue laser source is TEM11, that provide the reason that the reflected laser from
the blue laser is lower than the green laser source although the absorption coefficient of the two wavelengths are the
same approximately.so, The Gaussian beam shape of the green laser source provides the precedence to be used for the
underwater imaging system.
3.2 Imaging system:
The concept of an underwater laser imaging system was initially proposed in the early 1970’s. And the first airborne
underwater laser imaging system was used for detecting mines in the period of the second Gulf War and its maximum
detection depth can reach to 30m under the water. Especially in the last decade, the airborne underwater laser imaging
systems has been wildly used in fish detection, rescue, oceanic mineral explorations, and oceanic research because of its
flexibility and high efficiency [11].
Proc. of SPIE Vol. 9342 934221-9
Applications requiring improved image quality and visual information, as well as military surveillance, identification,
and detection. Although conventional cameras, including disparity stereo imaging systems, can provide a means for
visualization of the work environment, they are limited by poor visibility. With regard to the ideal
monochromatic laser and its intense energy, as well as the transparent-windows effect for blue-green light propagating
through water, the development of underwater target laser imaging technology has been dramatically increased. The
imaging range of target laser imaging system is from tens to over one hundred meters [12].
3.2.1 Position of CCD camera:
When using conventional illumination systems, the amount of backscatter depends on the volume of water where the
light field and camera’s field of view intersect. Because of this, it is common to separate the light source as much as
possible from the camera in order to reduce the volume of water. Fig. (12a)show schematic drawing of the volume of
water producing backscatter when the camera and the light source are close, and Fig. (12b) show the difference when
increasing this distance, the volume of water producing backscatter when increasing this distance[11][12].
Figure 12: Schematic drawing of the volume of water producing backscatter
Optimization of the CCD camera position to provide higher reflection laser from target and lower backscattering from
the water medium. The measured reflection from the target was detected by photodetector (PDA36A-EC - Si Switchable
Gain Detector, 350-1100 nm, 10 MHz BW, Thorlab Inc.) from -90 degree to 90 degree and illustrated in Fig. (13) and
provide that the reflection reduces by increasing the angle between the incident beam and the CCD camera.
Proc. of SPIE Vol. 9342 934221-10
Figure 13: The measured reflection from the target was detected by photodetector from -90 degree to 90 degree
The images captured by CCD camera were applied to image processing processes; noise background first is subtracted
from the captured image and then reduce the probability of false target alarm by comparing the target image with
dynamic threshold and normalize the detected target to increase its contrast in the image, Fig.(15-20) illustrate (a) target
image, (b) background image, (c) subtracted image, (d) the normalized image, I mesh figure of target to illustrate the
intensity level of each pixel of image, (the contour image to determine the position of target in the image, (h) the mesh
figure for the target after normalization of the level higher than the dynamic threshold, and (i) the contour figure for the
normalized targets, for the target image capture at different angles 0, 10,30 ,50,70, 85 degrees.
The experimental results for the measurements at zero degree angle as shown in Fig. (15) provide that the scattering
noise of water and enclosure is very high and extraction of the target reflection is very difficult, so the systems cannot
detect the target, and that intensify the importance of CCD camera position as mentioned before. Fig. (16) for 10 degrees
angle images illustrate that the scattering reflection noise was decreased and the dynamic threshold reduce its value with
respect to the target reflection as the contour graph in Fig.16(i) provides. Measurements at angles 30 degrees shown in
Fig. (17) provides the optimum angle for minimum scattering noise and good reflection from targets. Fig. (18)and Fig.
(19) for 50 degrees and 70 degrees respectively illustrate that the scattering noise is very low but also the reflection from
target decreased but its level can be detected. Minimum reflection from targets occur at angle 85 degrees but the
dynamic threshold can also provide target detection and subtract the noise as shown Fig. (20), and this the maximum
angle because there is no reflection from targets at angles more than 90 degrees. It’s clear from the image at zero degree
and 10 degrees that the highest backscattered noise occur due to the distance between the camera and the laser source are
close, and this backscattering noise decrease by increasing the distance between them. The optimum distance between
the laser source and the camera is corresponding to the angles from 20 to 40 degrees for short distances of few meters.
The experimental underwater laser imaging system as shown in schematic in Fig. (14) consisting of green laser (l
=532nm) as transmitter via beam expander (BE15M-A, AR Coating Range 400 - 650 nm, Expansion 15X , Thorlab inc)
to scan the field-of-view, and CCD camera ( PL-B782U, 6.6Megapixel CMOS Color Camera, 3000 x 2208 pixels,
Pixel link Inc.) to extract and store reflected signal from target. Then these signals processed by computer to extract
image of object. Commonly transmitter device and receiver of underwater laser imaging system was designed as the
asymmetry in bi-static configuration to make the illuminating light and return light forming a stated angle to reduce the
backscattering and stray light noise.
Proc. of SPIE Vol. 9342 934221-11
Figure 14: Underwater laser imaging system experimental setup .
4.
CONCLUSION
The major benefits of a laser-based underwater target detection are high-directionality, high response, and high range
accuracy. Lasers operating in the blue-green region of the light spectrum (420:570 nm) have potential applications in the
underwater detection and imaging of submersible targets due to minimum attenuation (less than 0:1m 1) through
water and maximum laser reflection of estimated target(mine or submarine) to provide a long range of detection. in this
paper we introduce the parameters of underwater laser imaging system by using blue-green laser as transmitter via beam
expander to scan the field-of-view, and the receiver (high resolution CCD camera) to extract and store reflected signal
from target. Then these signals processed by computer to extract image of like mine object and exclude other targets.
Commonly blue-green laser source and CCD camera of underwater laser imaging system was designed as the asymmetry
in bistatic configuration to make the illuminating light and return light forming a stated angle (20:40 degrees) to reduce
the backscattering and stray light noise, the images captured by CCD camera were applied to an image processing
processes to reduce the scattering noise and detect the shape of underwater target.
Proc. of SPIE Vol. 9342 934221-12
Figure 15: Target images captured at 0 degree.
Proc. of SPIE Vol. 9342 934221-13
Figure 16: Target images captured at 10 degree.
Proc. of SPIE Vol. 9342 934221-14
Figure 17: Target images captured at 30 degree.
Proc. of SPIE Vol. 9342 934221-15
Figure 18: Target images captured at 50 degree.
Proc. of SPIE Vol. 9342 934221-16
Figure 19: Target images captured at 70 degree.
Proc. of SPIE Vol. 9342 934221-17
Figure 20: Target images captured at 85 degree.
Proc. of SPIE Vol. 9342 934221-18
REFERENCES
[1] Alan Laux, Linda Mullen, Paul Perez, and Eleonora Zege, "underwater laser rangefinder", Proc. SPIE. no:83721B,
2012.
[2] LingWang, "Measuring optical absorption cofficient of pure water in UV using the integrating cavity absorption
meter", PhD thesis, Texas A&M University, 2008.
[3] Xiaoling Chen and Zhifeng Yu. "Geospatial Technology for Earth Observation", p.431. Springer Science, 2008.
[4] Ari. Kilpela, "Pulsed time-of-flight laser range finder techniques for fast, high precision measurement applications",
Technical report, OULU university, 2004.
[5] Risto Myllyla Markus-Christian Amann, Thierry Bosch and Marc Rioux, "laser ranging: a critical review of usual
techniques for distance measurement",. no.40(1),. Society of Photo-Optical Instrumentation Engineers no.40(1), 2001.
[6] Milton Kerker. "The scattering of light, and other electromagnetic radiation", Vol:16 of Physical chemistry,.
Academic Press, 1969.
[7] David R . Williams Michael Bass, Eric W . Van Stryland and William L . Wolfe, "HANDBOOK OF OPTICS",
McGRAW-HILL INC., 1995.
[8] Unai Ayala Fernández and Luis Manuel Hernández, "investigation of submerged maritime target detection using
lidar". Master’s thesis, Norwegian University of Science and Technology, 2009.
[9] Michael Bass. "Handbook of Optics: Optical Properties of Materials, Nonlinear Optics, Quantum Optics", Vol.4, The
McGraw-Hill Companies’, third edition, 2010.
[10] Zhou Renkui LV Pei, HE Junhue and Liu Haiying, “The application of underwater optics development”, Proc. Of
SPIE Vol. 6837, 2007.
[11] Zhang Qi-heng Wang Hua-chuang Yu Xue-gang Wang Lei, Xu Zhi-yong and Nie Rui-jie, “experimental
underwater scanning imaging system using pulsed blue-green lasers”,. Proc. of SPIE Vol. 8192, 2011.
[12] A.Burguera F. Bonin and G. Olivera, “imaging systems for advanced underwater vehicles”,. Journal of Maritime
research, Vol 8(1), pp.65-86, 2011.
Proc. of SPIE Vol. 9342 934221-19
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