Amplitude-Modulated Laser Imager

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inda Mullen, Alan Laux, Brian Concannon, Eleonora P. Zege, Iosif L. Katsev, andlexander S. Prikhach

Laser systems have been developed to image underwater objects. However, the performance of thesesystems can be severely degraded in turbid water. We have developed a technique using modulatedlight to improve underwater detection and imaging. A program, Modulated Vision System �MVS�, whichis based on a new theoretical approach, has been developed to predict modulated laser imaging perfor-mance. Experiments have been conducted in a controlled laboratory environment to test the accuracyof the theory as a function of system and environmental parameters. Results show a strong correlationbetween experiment and theory and indicate that the MVS program can be used to predict future systemperformance. © 2004 Optical Society of America

OCIS codes: 010.4450, 120.4820, 280.3420, 010.3310, 010.3640, 290.7050.

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. Introduction

aser systems have been and are continuing to beeveloped to detect and identify objects in turbid me-ia �seawater, clouds, tissue�. Operating a laser im-ging system in such an environment is challengingecause light is both absorbed and scattered. Al-hough the optical wavelength is typically selected toinimize absorption, the scattering experienced by

n optical signal can severely degrade image quality.n highly turbid media, there may be plenty of lightcattered back from the object of interest, but it isuried in the signal returning from the surroundingnvironment. A method for separating the unscat-ered �or minimally scattered� image bearing photonsrom the multiply scattered background light can besed to improve object detection and identification.he focus of this paper is to explore techniques to

mprove the sensitivity of underwater imaging sys-ems.

Several techniques have been developed to reducehe detrimental effects of scattered light. These ap-roaches can be categorized according to the type of

L. Mullen �[email protected]�, A. Laux, and B. Concannonre with the Electro-Optics and Special Mission Sensors Division,aval Air Warfare Center, Naval Air Systems Command, 22347edar Point Road, Patuxent River, Maryland 20670-1161. E. P.ege, I. L. Katsev, and A. S. Prikhach are with the Institute ofhysics, Belarus Academy of Sciences, Scaryna Avenue 68, Minsk20072, Belarus.Received 30 September 2003; revised manuscript received 11arch 2004; accepted 5 April 2004.0003-6935�04�193874-19$15.00�0© 2004 Optical Society of America

874 APPLIED OPTICS � Vol. 43, No. 19 � 1 July 2004

aser source and receiver combination and the scan-ing method used to create the image. All systemsre capable of creating an image, whether it is aynchronously scanned narrow beam and narrow re-eiver field of view �narrow–narrow� or a flood-lluminated scene with a multiple-pixel receiverwide–narrow�. The decision as to which configura-ion provides the best performance depends directlyn the task at hand �i.e., above-water or below-waterperation, size and depth of underwater object, waterptical properties�.The Laser Line Scan system consists of a well-

ollimated continuous-wave source and a narrow-eld-of-view receiver that are synchronously scannedver the object of interest.1 The bistatic configura-ion limits the common volume created by the sourcend receiver field-of-view overlap and reduces theontribution from scattered light. However, be-ause the system uses a continuous-wave source, nonherent time �depth� information is present in theetected signal, and postprocessing with triangula-ion methods must be used to obtain target rangenformation.

Pulsed laser sources are also used in several un-erwater laser imaging systems to temporally dis-riminate against scattered light and to providearget range information. In the operation of a typ-cal range-gated imaging system, a short �10–20 ns�ulse is transmitted to a distant object, and the re-eiver is timed to open only when the reflected lighteturns from the object. A typical configuration isroad-beam illumination of the scene and a gatedntensified camera receiver,2 although systems thatse photomultiplier tube receivers in both single- and

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ultiple-pixel configurations have also been demon-trated. The Streak Tube Imaging Lidar uses aulsed laser transmitter in a scannerless configura-ion.3 Instead of scanning the laser beam, a fan ofight is used to illuminate a volume of water. Thetreak tube receiver can measure both the amplitudend the range �time� of the collected slit of light, andthree-dimensional image is created when the sys-

em is operated from a moving platform. Althoughhe range-gated and Streak Tube Imaging Lidar ap-roaches are effective to minimize background light,he sensitivity is ultimately limited by small-angleorward-scattered light that induces image blurring.

A final category of underwater imagers encom-asses those that use temporal modulation of theransmitted light and subsequent synchronous detec-ion of the modulation envelope at the receiver. Thenderwater scannerless range imager uses a radio-

requency modulation source that is coupled to bothhe timing of the laser transmitter and the gain of themage-intensified CCD receiver.4 Target range in-ormation is obtained by measurement of the phaseifference between the transmitted and the reflectedignals simultaneously for each pixel of the receiver.owever, multiple frames are required by use of dif-

erent modulation schemes to extract the range in-ormation and to differentiate changes due to rangeariations from those due to intensity variations inhe scene. Previous configurations used continuous-ave sources, but a recent configuration implementspulsed source and a range-gated receiver to mini-ize the volumetric backscatter signal.5Researchers at the Naval Air Systems Command

re also developing a system that uses temporal mod-lation of the transmitted optical signal.6 However,

n this approach, the optical receiver consists of ahotodetector with sufficient bandwidth to recoverhe modulation envelope encoded on the optical sig-al. The resulting radio-frequency signal is thenrocessed by traditional radar signal processing tech-iques. This approach reduces the contribution byolumetric backscatter by use of a modulation fre-uency that becomes strongly decorrelated with re-pect to the transmitted signal because of multiplecattering. A gain in image contrast is achievedhen the modulation envelope emanating from annderwater object remains coherent relative to theriginal modulation signal. The phase informationncoded on the detected modulation signal is pro-essed to obtain target range information. Thismplitude-modulated imaging approach has been ap-lied to a pulsed laser configuration.7 However, theurrent focus is to study the application to aontinuous-wave synchronous scan configuration be-ause of the availability of off-the-shelf components.8lthough not compatible with realistic scan rates, theurrent configuration has enabled us to test the effectf modulation frequency on the volumetric backscat-er and target signals and the resulting image con-rast.

Preliminary experiments were conducted in both initu and laboratory tank environments. In both

ases, interesting features were observed in the dataollected in relatively turbid water �beam attenuationoefficient �1�m� and at short ranges �2–3 m�. Aodel was developed concurrently with the experi-ents and was incorporated into a program, Modu-

ated Vision System �MVS�, to investigate thepplication of this approach to other system configu-ations and to understand the underlying physicsnvolved with modulated light beam propagationhrough water. The MVS program was also used tonvestigate the origin of the interesting but unex-ected experimental results. Our purpose in thisaper is to briefly describe the theory that was devel-ped to simulate amplitude-modulated imaging per-ormance and to illustrate its utility in interpretingxperimental measurements. Comparisons of ex-erimental and simulated data are shown to demon-trate the accuracy of the theory and its ability toredict future amplitude-modulated system perfor-ance.

. Theoretical Approach

he challenge in the development of an accurate the-retical model for modulated light beam propagations that it is intimately related �through Fourier trans-orm� to short-pulse propagation, which requires aolution of the nonstationary radiative transfer equa-ion. The characteristic parameter of the nonsta-ionary light field is the mean photon residence in theedium, or the mean free time between two scatter-

ng events, t � 1�c�, where c is the extinction �oream attenuation� coefficient and � is the velocity ofight in water. If the changes in light source inten-ity �i.e., pulse width or modulation wavelength� oc-ur in time scales that are much longer than thisean photon residence time, the optical field essen-

ially follows the source power fluctuations and isaid to be quasi-stationary. However, if the pulseidth or modulation wavelength are on the order ofr smaller than this characteristic time, then theight field is essentially nonstationary and must bealculated by means of solving the nonstationary ra-iative transfer equation. Fortunately, we have de-eloped techniques that help simplify the problem sohat a semianalytical solution can be found. In Sub-ection 2.A the approximations and assumptions thatere used to derive this solution are discussed. Theasic equations that describe the modulated lasermaging system are then described. Finally, the fea-ures of the MVS program that incorporate this the-ry to predict modulated laser imaging systemerformance are discussed.

. Approximations

ultiple scattering in ocean water contributes signif-cantly to both the backscatter and the object signals.ncluding the effects of multiple scattering is theain challenge when we compute backscattered ra-

iation. Therefore a technique was used that essen-ially simplifies the estimations of the backscatterignal. This approach includes the effect of multiplecattering in both the object signal and the backscat-

1 July 2004 � Vol. 43, No. 19 � APPLIED OPTICS 3875

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er signal components and the effect of almost all theeatures of the scattering phase function of water.he one assumption that is made regarding the scat-

ering phase function of water is that it has a sharpeak in the small forward angles, which is character-stic of typical ocean water. It is also assumed that

ultiple scattering occurs in small angles, and onlyingle scattering into large angles is used to computehe radiance distribution.9

Other approximations used in the theory includehe multicomponent approach10 and the small–angleiffusion approximation.11 The multicomponent ap-roach focuses on simplifying the complex nature ofhe scattering phase function p�� so that the radia-ive transfer equation can be broken into smaller,ore simple equations that are easier to solve. The

mall-angle diffusion approximation can be used withhe assumption that the phase function is stronglyorward peaked and has a sharp maximum at 0 degnd that the angular radiance distribution is de-cribed by smooth functions and is not closely corre-ated with the phase function �e.g., there are enoughcattering events so that the radiance distributionoes not follow the phase function�. The combina-ion of these two approaches provides the transferharacteristics of the scattering medium that includehe optical transfer function and the point-spreadunction. A new model of the point-spread function,hich includes the inherent point-spread function

ingularity at the beam axis, was also developed andmployed in the theory.12 In the current version,he temporal stretching of the forward-propagatingptical signal is not included.

. Basic Equations

n the modulated laser imaging system, the sourceower P�t� can be described by the following equation:

P�t� � P0�1 � exp�i2�ft�, (1)

here f is a specific modulation frequency and t isime. At the receiver end, a high-speed optical de-ector and a microwave receiver record and processhe signal with frequency f for the frame recordingime tfr. If there are no objects in the volume inter-ected by the transmitted beam and receiver field ofiew, the power of an optical signal at the modulationrequency f at the input of the receiver with the axisirected to any point r at the object plane z � 0 is theower Pb�r, t� of the background. When an object isresent, the total signal power at the input of theeceiver is

P�r, t� � PVS�r, t� � Pb�r, t�, (2)

here PVS�r, t� is the power of the valid signal due tohe underwater object. Here

Pb�r, t� � Pb�r�expi�2�ft � �b�r��, (3)

PVS�r, t� � PVS�r�expi�2�ft � �VS�r��, (4)

here Pb�r�, PVS�r� and �b�r�, �VS�r� are the ampli-udes and phase shifts, respectively, of the modulated


ptical signals. The component Pb�r� consists of twoarts:

Pb�r, t� � PBSN�r, t� � Pbot�r, t�, (5)

here

PBSN�r, t� � PBSN�r�expi�2�ft � �BSN�r��, (6)

Pbot�r, t� � Pbot�r�expi�2�ft � �bot�r�� (7)

re the optical powers due to the volumetric back-catter signal and to the diffuse reflection by the seaottom, respectively.As can be seen from Eq. �2�, the power of a valid

ignal, PVS�r, t�, is the difference between the totalower at the receiver input when an object is present,�r, t�, and the power of the background signal thatxists when no object is present, Pb�r, t�. With al-owance for the object reflection and a shadow behindhe object, we obtain

PVS�r, t� � Pob�r, t� � Psh�r, t�. (8)

ere Pob�r, t� is the signal power reflected by thebject. The object partially shields the water layeretween it and the bottom. This shielding changeshe valid signal and is included by the term Psh�r, t�n Eq. �8�.

It is necessary to stress that the sum of all compo-ents of P�r, t� at the modulation frequency f includesontributions of both the amplitude and the phase ofhe background and object signal components. Theower of the total signal is

P�r� � � Pb�r, t� � PVS�r, t��

� Pb�r��1 �1

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here

�r� � Pb�r��PVS�r�, (10)

��r� � �b�r� � �VS�r�. (11)

f Pbot�r� �� PBSN�r�, we have Pb�r� � PBSN�r� andorrespondingly �r� � PBSN�r��PVS�r�, ��r� �

BSN�r� � �VS�r�. This is the case when the water isurbid or the bottom depth is large so that the back-catter signal dominates the background signal. Its this case that we examine further in Sections 3 andto study particular features of images produced byodulated laser systems.

. Modulated Vision System Software

he theoretical approach described above provideshe backbone for the MVS program, which simulateshe performance of underwater, modulated laser im-ging systems. This software runs under Windowsnd has an interactive, user-friendly interface. Thenputs to the MVS program include

1. system configuration �modulated or nonmodu-ated, focal-plane array, or synchronous scan�,

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2. system geometry �source–receiver separation,eceiver–object distance, object–bottom distance�,

3. source parameters �optical power, wavelength,perture size, divergence, modulation frequency�,4. receiver parameters �aperture, field of view, op-

ical transmission, detector quantum efficiency, num-er of pixels�,5. target characteristics �size, shape, reflectivity�,

nd6. water characteristics �beam attenuation coeffi-

ient, single-scattering albedo, scattering phase func-ion�.

ith these inputs, the program simulates imagesbserved by a modulated or a nonmodulated lasermaging system, including the amplitude and phaseignals from both backscatter and targets. Addi-ional program outputs include plots of the frequencyependencies of the total signal, backscatter noise,nd image contrast that are useful to understand theffect of modulation frequency on system perfor-ance. Results from the MVS program are used inection 3 to illustrate interesting features of imagesroduced by a modulated laser imaging system.

. Features of Modulated Vision System Images

n the past, it was observed in both experimental andomputer simulation results that under certain con-itions maxima and minima were observed in theependence of signal power on the modulation fre-uency.8 An explanation for these results was thathe reflection of the modulated optical signal from thearget interacted with the backscatter signal to pro-uce both constructive and destructive interferencef the modulation envelope at the receiver. To un-erstand and explain these interference effects andheir influence on the images created by a modulatedaser imaging system, two main factors must be con-idered:

1. dependencies of the backscatter noise phasend amplitude �BSN�z, f � and PBSN�z, f � and the validignal phase and amplitude �VS�z, f � and PVS�z, f � onhe target depth z and the modulation frequency f forfixed water clarity �beam attenuation coefficient c�;nd2. dependencies of �BSN�r�, PBSN�r�, PVS�r�, and

VS�r� on the coordinate r at the image plane.

lots of the dependencies of �BSN�z, f � and �VS�z, f �n modulation frequency and depth for a fixed c �.2�m are shown in Figs. 1�a� and 1�b�, respectively.he data for these graphs were obtained with theVS program, and we used the values in Table 1 as

nputs. In Fig. 1�a� the depth was fixed at z � 2.74, and in Fig. 1�b� the modulation frequency was set

o f � 33 MHz. It is evident from Fig. 1 that theunction �VS�z, f � shows much stronger dependenciesoth on the depth and on the modulation frequencyhan the function �BSN�z, f �. Also shown in Fig. 1 ishe dependency of �BSN�z, f � and �VS�z, f � on theeam attenuation coefficient c for z � 2.74 m and f �

3 MHz �Fig. 1�c�. It is evident that �VS�z, f � isonstant whereas �BSN�z, f � changes only slightlyith increasing c. The plots of PBSN�z, f � and PVS�z,� corresponding to the phase data in Fig. 1 are shownn Fig. 2. It is evident that PVS�z, f � has a strongerependency on depth and water clarity than PBSN�z,�. However, PVS�z, f � is constant whereas PBSN�z,� decreases as a function of modulation frequency.his is due to the fact that, as the modulation fre-uency increases, the backscatter signal becomes

ig. 1. �a� Dependency of �BSN�z, f � and �VS�z, f � on modulationrequency for z � 2.74 m and c � 2.2�m. �b� Dependency ofBSN�z, f � and �VS�z, f � on depth for f � 33 MHz and c � 2.2�m.

c� Dependency of �BSN�z, f � and �VS�z, f � on the beam attenuationoefficient for z � 2.74 m and f � 33 MHz.


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ecorrelated relative to the transmitted signal be-ause of multiple scattering. The effect of �BSN�z,�, PBSN�z, f �, �VS�z, f �, and PVS�z, f � on the totalower received by the modulated system is now ex-mined in more detail.

. Constructive and Destructive Interference

he total power P�z, f � received from an underwaterarget at r � 0 corresponding to the data shown inigs. 1�a� and 2�a� is plotted in Fig. 3�a� as a functionf modulation frequency. In Fig. 3�a� it is evidenthat P�z, f � is not a smooth function of modulationrequency, but rather it has a certain periodic depen-ency with several extrema. Because it has alreadyeen determined that both PVS�z, f � and PBSN�z, f �re relatively linear functions of modulation fre-uency �see Fig. 2�a�, the fluctuations observed inig. 3�a� must be due to the interaction between these

wo signal components. It is well known that elec-romagnetic waves that are overlapped in the sameegion of space can either reinforce �constructive in-erference� or cancel each other �destructive interfer-nce�.14 The condition for constructive interferences that the two waves are in phase �0-deg phase dif-erence� or that the phase difference between the twoaves is a multiple of 360 deg. Destructive inter-

erence will occur for waves that are opposite in phase180-deg phase difference� or have phase differencesqual to an odd multiple of 180 deg. The phase dif-erence between the backscatter noise and valid sig-als � � �BSN�z, f � � �VS�z, f � corresponding to theata in Fig. 3�a� is shown in Fig. 3�b� to illustrate thisoint. Also plotted in Fig. 3�b� is the amplitude ratioetween the backscatter and the valid signals �

�z, f ��P �z, f �. At the frequency correspond-

Table 1. System Configurationa and Environmental Parameters Usedas Inputs to the MVS Program to Produce the Results Shown in

Figs. 1–11

Parameter Measurement

System geometrySource–receiver separation 0.289 mReceiver–object distance 2.74, 3.5

Source parametersWavelength 532 nmPower 5 WAperture size 0.01 mDivergence 0.3 degModulation frequency 10–100 MHz

Receiver parametersAperture �diameter� 0.0508 mField of view 1.0°, full angle

Target characteristicsSize �diameter of white� 0.1 mReflectivity �white�black� 0.8�0.01

Water characteristicsBeam attenuation coefficient 2.2, 2.6�mSingle-scattering albedo 0.85Phase function Maaloxb

aModulated and nonmodulated, synchronous scan, continuousave.bRef. 13.

BSN VS


ng to � � 180 deg � f � 33 MHz�, the total power P�z,� reaches a minimum value because of destructiventerference between the backscatter and the validignal components. The effect of constructive inter-erence is also evident in the plot of P�z, f � where theower reaches a maximum at the frequency corre-ponding to � � 360 deg � f � 62 MHz�. Although aecond minimum is observed at the frequency corre-ponding to � � 540 deg � f � 88 MHz�, it is lessronounced than the minimum occurring at � � 180ecause the ratio between the two waves, , de-reases from � 0.6 to 0.2. The effects of destruc-ive interference are the most prominent when the

ig. 2. �a�–�c� PBSN�z, f � and PVS�z, f � corresponding to the phaseata in Fig. 1.

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The effect of object depth on P�z, f �, �, and ishown in Fig. 4 where the data from Fig. 3 are plottedlong with the data corresponding to an increasedbject depth of z � 3.25 m. Because the effect of thencreased depth is to change the phase and amplitudeelationships between the backscatter and the validignals �as shown in Fig. 4�b�, the frequencies athich constructive and destructive interference are

bserved change as well as the relative signal ampli-ude. The effect of a change in water clarity c on P�z,� is shown in Fig. 5. Here it is evident that with anncrease in c the locations of the extrema change onlylightly, but the signal level decreases relative to theignal for c � 2.2�m. This is due to the fact that anncrease in c produces only a slight change in theelative phase between the backscatter and the validignals but results in an increase in the ratio betweenhe backscatter and the valid signal amplitudes �ashown in Fig. 5�b�. Furthermore, the destructiventerference at f � 88 MHz is enhanced because of the

ig. 3. �a� Total power P�z, f � as a function of modulation fre-uency for the data in Fig. 1�a� and 2�a�. The power was normal-zed relative to the power at f � 10 MHz. The dashed linesndicate the frequencies at which destructive and constructive in-erference occurs. �b� � � �BSN�z, f � � �VS�z, f � �left axis� and �

BSN�z, f ��PVS�z, f � �right axis� as a function of modulation fre-uency calculated from the data in Figs. 1�a� and 2�a�. Theashed lines indicate the frequencies at which destructive andonstructive interference occurs.

act that at this frequency the backscatter and validignal amplitudes are approximately equal � � 1�,nd almost complete cancellation occurs. To sum-arize, the relative phase between the backscatter

nd the valid signals � determines the frequency athich constructive and destructive interference oc-

urs, whereas the amplitude ratio between the twoignals affects the overall magnitude of P�z, f �.he effect of these dependencies on both the targetontrast and the images produced by the modulatedaser imaging system is discussed in Subsections 3.Bnd 3.C.

. Target Contrast

figure of merit used to quantify the effect of theackscatter and the valid signals on the system sen-itivity is the contrast of the target at the targetenter �at r � 0�:

k� z, f � �P� z, f � � PBSN� z, f �

P� z, f � � PBSN� z, f �, (12)

here P�z, f � and PBSN�z, f � are the powers of theotal signal and the backscatter signal at the center ofhe target at depth z and modulation frequency f,espectively. Plots of k�z, f � for the data shown inigs. 1 and 2 are shown in Fig. 6. From this graph

t is apparent that the plots of k�z, f � as a function ofodulation frequency exhibit similar characteristics

ig. 4. �a� and �b� Data from Figs. 3�a� and 3�b� plotted along withhe data corresponding to an increased object depth of z � 3.25 m.


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o the plots of P�z, f � in Figs. 3–5. In certain cases,estructive interference between the backscatter andhe valid signals produces negative contrast, whereasonstructive interference between these two signalomponents results in improved contrast relative tohe contrast at f � 10 MHz. To better understandhe effect of the relationships between the relativehase � � �BSN�z, f � � �VS�z, f � and the amplitudeatio � PBSN�z, f ��PVS�z, f � on the target contrast�z, f �, the two extreme points, where the phase shiftetween the backscatter and the valid signals isqual to � � 0 or � � �, is now examined in moreetail.

. Case 1: � � �BSN�z, f � � �VS�z, f � � 0hen the phases of the backscatter and valid signals

re equal �or a multiple of 360 deg�, constructive in-erference occurs:

kconstr �� PVS�

� PVS� � 2� PBSN��

11 � 2

, (13)

here

�� PBSN�� PVS�

. (14)

n this case, the contrast is positive �kconstr � 0� forny . The value of decreases and the contrast

ig. 5. �a� and �b� Data from Figs. 3�a� and 3�b� plotted along withhe data corresponding to an increased beam attenuation coeffi-ient of c � 2.6�m.


constr grows with decreasing depth, increasing mod-lation frequency or decreasing beam attenuation.his is shown in Fig. 6 where kconstr increases withecreasing depth and decreasing beam attenuation.

. Case 2: � � �BSN�z, f � � �VS�z, f � � �hen the two signals are opposite in phase �odd mul-

iples of 180 deg�, destructive interference occurs.wo situations are possible in this case:

1. When �PBSN� � �PVS� �i.e., � 1�, the contrastorresponding to destructive interference becomes

kdestr �� PVS� � 2� PBSN�

� PVS�� 1 � 2 . (15)

quation �15� shows that the contrast kdestr � 0 at� 0.5, which would occur at shallow depths or clearater when the valid signal is large or for high mod-lation frequencies when the backscatter signal istrongly decorrelated. The negative contrastdestr � 0 is produced when � 0.5, which requiresomparatively large depths, more turbid water, orow modulation frequencies. For example, for theata corresponding to c � 2.2�m and z � 2.74 m inigs. 3 and 6, kdestr � 0 at f � 88 MHz when � 0.19,hereas kdestr � 0 at f � 33 MHz when � 0.67.2. When �PBSN� � �PVS� �i.e., � 1�, the contrast

orresponding to destructive interference is

kdestr �� PVS�

2� PBSN� � � PVS��

12 � 1

. (16)

n this case, the contrast kdestr is negative for any �. This is the case in Figs. 4 and 6 for z � 3.25 m:destr � 0 when � 4.9 and kdestr � 0 for � 1.8.In summary, when � � �BSN�z, f � � �VS�z, f � � �,

kdestr � 0 at � 0.5, kdestr � 0 at � 0.5. (17)

In Fig. 7 the value of kdestr is plotted as a functionf . When the target dominates the signal, the con-rast kdestr is high and positive. This situationould occur for shallow target depths or for highodulation frequencies when the backscatter is

trongly decorrelated. However, as the backscatter

ig. 6. Contrast k�z, f � as a function of modulation frequency forhe data in Figs. 3–5.

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nd target signals approach one another because ofecreasing water clarity, increasing depth, or for lowodulation frequencies, the contrast becomes nega-

ive and approaches a minimum when � 1. As rows because of larger target depths, the contrastpproaches zero but remains negative.

. Object Images

he effect of these variances in target contrast on themages produced by the modulated laser system cane better understood when we study the dependencef the backscatter and valid signals on the spatialoordinate r in the target plane. Within the small-ngle approximation, the phases of the backscatternd valid signals are independent of the spatial co-rdinate r: �VS�r� � const and �BSN�r� � const.his is the case for a synchronous scan configurationhere the receiver field of view is narrow or the tar-et takes up a significant portion of the receiver fieldf view at the object plane. Therefore, within thisssumption, the difference in phases between theackscatter and the valid signals is

��r� � �BSN�r� � �VS�r� � const. (18)

gain, two extremes must be considered: ��r� � 0nd ��r� � �. The modulation frequencies that cor-espond to the phase difference ��r� � 0 when k �

constr and the phase difference ��r� � � when k �

destr can be found when we plot the contrast as aunction of modulation frequency �as shown in Fig. 6�.he MVS program can then be used to illustrate theffect of these interference effects on the object image.or example, the images corresponding to the datahown in Fig. 3 are shown in Fig. 8. Also shown inig. 8 are the normalized energy distribution and theatio of backscatter to valid signal energy as aunction of pixel number �spatial position r�. Herehe water-free and continuous-wave images arehown for reference, and the constructive and de-tructive images are those we obtained by using theodulation frequencies corresponding to kconstr and

destr in Fig. 6, respectively. The constructive imagehows an improved contrast between the white cir-ular object and the black background relative to theontinuous-wave image. The destructive imagehows a dark ring around the white object and a

orresponding dip in the energy distribution at theransition between the object and the background.lso, the contrast between the object center �which isominated by the white object reflectivity� and thelack background �which is dominated by backscatteroise� is negative. From the plot of as a function ofixel number in Fig. 8, it is evident that 0.5 � � 1n the center of the target at r � 0. The fact that �.5 explains the negative contrast between the whitebject and the black background because we foundhat kdestr � 0 for � 0.5 �see Fig. 6�. As the dis-ance increases from the target center, the valid sig-al begins to decrease and �r � 0� increases. Ashe valid signal level approaches the backscatter sig-al level, �r � 0� approaches a value of �r � 0� � 1hat produces the dark ring in the object image. Forncreasing r past the boundary of the white object,�r � 0� � 1 and the backscatter signal begins toominate the image.To study the effect of the value of �r � 0� on the

esulting destructive images, the albedo of the whitearget in Fig. 8 was varied while all other parametersemained unchanged. The results are shown in Fig.where the destructive images and the correspond-

ng normalized energy distributions and �r� arehown for target albedos ranging from 1 to 0.4. Ashe albedo decreases, the valid signal also decreasesnd �r � 0� increases. However, only when �r �� 1 does the dip in the energy distribution occuromewhere in the vicinity of the white object bound-ry. These results show that, for destructive imageshere �r � 0� � 1, the effect of a change in targetlbedo for r � 0 �i.e., change in reflectivity betweenhe white object and the black background� is that ainimum in the received photon energy is observedhen � 1. This explains the outline-emphasizingffect observed in some images produced by the mod-lated laser imaging system.Other factors that affect the value of �r � 0� in-

lude the object depth and the water beam attenua-ion coefficient. The images produced for anncreased object depth �data in Fig. 4� along with theorresponding normalized energy distributions and�r� are shown in Fig. 10. The data in Fig. 10 showhat in this case �r � 0� � 1 for the images producedith modulation frequencies corresponding to kdestrf � 26 and 72 MHz�. Therefore these images lackhe dip in the energy distribution and correspondingark ring in the object image at the target boundary.owever, the contrast between the white target and

he black background is negative because kdestr � 0or � 0.5. A similar situation exists for the imagesbtained with an increased beam attenuation coeffi-ient �data from Fig. 5�, which are shown in Fig. 11.ere, because �r � 0� � 1 for f � 32 MHz, the

orresponding image also lacks the outline-mphasizing features observed in Fig. 9. However,ecause �r � 0� � 1 at f � 88 MHz, a slight discon-inuity is observed in the normalized energy profile

Fig. 7. Plot of k as a function of � P �z, f ��P �z, f �.


watg

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3

hen � 1. It is important to note that in Figs. 10nd 11 the constructive images have improved con-rast between the white target and the black back-round relative to the continuous-wave images.We have shown that the interference between the

ackscatter and the valid signals produces interest-

ig. 8. Images corresponding to the data shown in Figs. 3 and 6.omparison. The graphs below the computer-generated images��PVS�z, f � �gray curve, right axis� plotted as a function of positio


ng features in images produced by a modulated lasermaging system. These features can be explainedhen we examine the amplitude and phase relation-

hip between the backscatter and the valid signals.he two extreme cases, when the relative phase be-ween the two signals is equal to 0 or �, lead to the

, continuous wave. The water-free �WF� image is also shown forthe normalized energy �black curve, left axis� and � PBSN�z,

CWshown r.

miamnt

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ost interesting results, including outline emphasiz-ng of the target albedo patterns, contrast inversion,nd contrast enhancement. These features are theost pronounced when the backscatter and valid sig-

als are close in amplitude, which is the situationhat typically results in poor image contrast in a tra-

ig. 9. Effect of a change in target albedo on the object image. Tarameters remained the same as in Fig. 8.

itional, unmodulated laser imaging system. Inection 4 an experimental setup is described that haseen used to validate the MVS program predictions.esults are then shown that compare the experimen-

al and theoretical data for a certain range of envi-onmental and system parameters.

lbedo of the white portion of the target was varied while all other
he a

4

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. Validation of Modulated Vision System Results

ontrolled laboratory tank experiments were con-ucted to validate the MVS program results for axed set of system and environmental parameters.he experimental setup included a 5-W, 532-nm laser

hat was modulated by an external electro-optic mod-lator at frequencies from 10 to 100 MHz. The op-

Fig. 10. Images corresponding to the data


ical receiver was a photomultiplier tube with an S20hotocathode and a gain of approximately 40,000. Aias T was connected to the output of the photomul-iplier tube so that both the dc current and the acodulation could be measured separately. A net-ork analyzer was used to drive the electro-opticodulator, to monitor the modulated signal power,

n in Figs. 4 and 6. CW, continuous wave.
show

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nd to measure the difference in phase between theransmitted and the detected modulated optical sig-als. The Aqua Tunnel water tank facility at theaval Air Systems Command was used for the labo-

atory measurements. The scattering properties ofcean water were simulated by the addition ofaalox antacid, and a WETLabs AC-9 instrument

Fig. 11. Images corresponding to the data

as used to measure the optical properties of theater for each Maalox concentration. The scatter-

ng phase function of Maalox was measured indepen-ently and was also used as an input to therogram.13 Other details of the experimental setupave been described elsewhere.8 A diagram of thexperimental setup is shown in Fig. 12, and the val-

n in Figs. 5 and 6. CW, continuous wave.
show

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es of the experimental parameters used as inputs tohe MVS program are listed in Table 2. In the ex-eriment, the target shown in Fig. 12 was scannedcross the plane of intersection of the source andeceiver to obtain one slice of the target image �rep-esented by the large dotted line� at each modulationrequency. Because the MVS program has the ca-ability to process a slice of the target image, the twoata sets were compared directly.

. Target Contrast

n the experiment we calculated the target contrast�z, f � by using Eq. �12�. The relative phase differ-nces between the modulator drive signal and bothhe total signal and the backscatter signal were alsoeasured. To calculate the phase difference be-

ween the backscatter and valid signals � � �BSN�z,� � �VS�z, f �, the phase of the target in clear wateri.e., no Maalox added to the tank water� at eachepth was measured and subtracted from the re-orded backscatter phase. The results are shown inig. 13 for three different water clarities, c � 1.2�m,� 2.1�m, and c � 2.5�m, at a target depth of 2.74 m.lso shown in Fig. 13 are the MVS program resultse obtained by using the relevant experimental pa-

ameters as inputs. For the cleanest water �c � 1.2��, both the experiment and the model show high

ontrast that is relatively independent of modulationrequency. However, for c � 1.2�m and c � 2.5�m,he contrast shows evidence of constructive and de-tructive interference effects. The correspondinghase data in Fig. 13�b� show that the frequencies athich constructive and destructive interference is ob-

erved in the experimental and model data correlatesith the conditions when � � 180 and � � 360, re-

pectively. The agreement between the model andhe experiment is quite good, especially at modula-ion frequencies exceeding 50 MHz.

The effect of a change in target depth on the targetontrast is shown in Fig. 14. Here the depth was

ig. 12. Diagram of the experimental setup used to validate theVS simulation predictions.


ecreased to z � 1.83 m, and the water clarity wasncreased to achieve a value of cz � 6.77 within theange of the data shown in Fig. 13. The data fromig. 13 are also shown for reference. The effect ofhe decreased target depth was to change the slope of�z, f � as a function of modulation frequency, as ex-ected. This slope change affected the frequenciest which constructive and destructive interferenceccurred. Both the amplitude �contrast� and thehase data again show good agreement between theodel and the experiment.

. Images

s stated above, in the experiment we obtainedmage slices of the underwater target by scanninghe target across the intersection of the laser andeceiver field of view and measuring the total re-eived power at each position and at each modula-ion frequency. The image slices produced by theVS program were reduced in resolution to contain

nly those points measured by the experimentaletup. The results corresponding to the data inigs. 13 and 14 are shown in Figs. 15–18 where theonstructive and destructive images are those ob-ained with a modulation frequency correspondingo � � 360 and � � 180, respectively. The full-esolution, two-dimensional images �64 � 64 pixels�roduced by the MVS program are also shown foreference, as is the continuous-wave �no modula-ion� image. In the destructive image graphs, thealue of the amplitude ratio between the backscat-er and the valid signals �r� � PBSN�r��PVS�r� islso shown for reference. We obtained an estima-

Table 2. System Configurationa and Environmental ParametersCorresponding to the Experimental Setup Shown in Fig. 12 and Used as

Inputs to the MVS Program

Parameter Measurement

System geometrySource–receiver separation 0.289 mReceiver–object distance 2.74, 1.83 m

Source parametersWavelength 532 nmPower 5 WAperture size 0.01 mDivergence 0.3 degModulation frequency 10–100 MHz

Receiver parametersAperture �diameter� 0.0508 mField of view 1.0°, full angle

Target characteristicsSize �diameter of white� 0.1 mReflectivity �white�black� 0.8�0.01

Water characteristicsBeam attenuation coefficient 1.2, 2.1, 2.5, 3.7�mSingle-scattering albedo 0.85Phase function Maaloxb

aModulated and nonmodulated, synchronous scan, continuousave.bRef. 13.

t�

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Fwatal results in gray�.

ion of �r� for the experimental data by solving Eq.9� for :

�r� � �1 � ��r��1, PBSN�r� � PVS�r�

�r� � �1 � ��r��1, PVS�r� � PBSN�r� , (19)

here ��r� is the ratio between the total power andhe backscatter noise power at the modulation fre-uency corresponding to � � 180.The images obtained with c � 1.2�m and z � 2.74�Fig. 15� show high contrast between the black and

he white portions of the target for all three casescontinuous wave, constructive, and destructive�.he plot of �r� for the destructive data shows that� �r� � 0.5, which explains the positive destruc-

ive image contrast because kdestr � 0 for � 0.5.owever, for the data shown in Fig. 16 corresponding

o an increased beam attenuation of c � 2.1�m, theffects of constructive and destructive interferencere observed. The constructive image shows im-roved contrast relative to the continuous-wave im-ge. The destructive image shows the outline-mphasizing feature discussed above. For both theodel and the experimental results, �r � 0� � 1 and

he dip in the plot of P�r� occurs in the vicinity of �. These are the conditions described above thatead to the dark ring observed in the object image.his outline-emphasizing feature disappears when

ig. 13. Model �black curves� and experimental �gray curves� re-ults for the �a� target contrast, k�z, f � and �b� phase differenceetween the backscatter and the valid signals � � �BSN�z, f � �

VS�z, f � for three different water clarities.

he beam attenuation coefficient increases to c �.5�m �Fig. 17�. In this case, � 1 for all r, whichesults in kdestr � 0. Although the experimental de-tructive data are rather noisy, they still show theame trend toward negative contrast as is shown inhe model data. For both the model and the exper-ment, the contrast of the constructive image is en-anced relative to the destructive image. Theutline-emphasizing feature reappears when the tar-et depth is decreased to z � 1.83 m and the beamttenuation coefficient is increased to c � 3.7�m �Fig.8�. Here, �r � 0� � 1 and slowly increases for r �. The location of the dip in the plot of P�r� againccurs in the vicinity where � 1. The constructivemage again shows improved contrast relative to theonmodulated continuous-wave image.Although the model and experimental results

hown in Figs. 13–18 are a small subset of the datahat are possible with various system and environ-ental parameters, they do validate the theoretical

redictions of the effect of constructive and destruc-ive interference on the target contrast and the im-ges obtained with a modulated laser imagingystem. The good correlation between the modelnd the experimental data give confidence that the

ig. 14. Data from Fig. 13 plotted along with the results obtainedith a decreased target depth of z � 1.83 m and an increased beamttenuation of c � 3.7�m �model data results in black, experimen-


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VS program can be used to study the effect of otherystem and environmental characteristics on the sys-em performance.

. Conclusions

technique with an amplitude-modulated laserransmitter has been developed to improve underwa-er imaging. The theory required to predict the ef-

ig. 15. Model �black� and experimental �gray� images for c � 1.2he destructive images and are indicated by filled diamonds. CW


ect of system and environmental parameters on theropagation of a modulated optical signal has beenerived and incorporated into a software program,odulated Vision System �MVS�. The MVS pro-

ram was used to gain insight into the origin of vari-tions in target contrast as a function of modulationrequency and other interesting features, such as out-ine enhancing and contrast inversion, that were ob-

nd z � 2.74 m �the values for �r� are plotted on the right axis forntinuous wave.

�m a, co

Ft

ig. 16. Model �black� and experimental �gray� images for c � 2.1�m and z � 2.74 m �the values for �r� are plotted on the right axis forhe destructive images and are indicated by filled diamonds. CW, continuous wave.


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3

ig. 17. Model �black� and experimental �gray� images for c � 2.5�m and z � 2.74 m �the values for �r� are plotted on the right axis for
he destructive images and are indicated by filled diamonds. CW, continuous wave.

Ft

ig. 18. Model �black� and experimental �gray� images for c � 3.7�m and z � 1.83 m �the values for �r� are plotted on the right axis for
he destructive images and are indicated by filled diamonds. CW, continuous wave.

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erved in experimental measurements. It wasound that the signal amplitude fluctuations as aunction of modulation frequency were caused by con-tructive and destructive interference of the modula-ion envelope between the backscatter and the targeteturn signals.

The accuracy of the MVS program with inputs wasalidated with controlled laboratory tank measure-ents. The good agreement between the experi-ental results and the MVS program predictions

ave us confidence in the accuracy of the theory de-ived for prediction of modulated laser imaging sys-em performance. In the future, other features wille added to the MVS program to improve its accu-acy, including realistic system noise and the effectsf the forward-scattered light temporal spreading.he program can then be used to quantify the relativeerformance of continuous-wave, modulatedontinuous-wave, pulsed, and modulated pulse con-gurations in terms of signal-to-noise ratio and tar-et contrast for a variety of environmental andystem parameters. This future research will helpetermine the benefits and limitations of themplitude-modulated imaging approach and will besed to optimize the system design for a desiredange of system performance criteria.

eferences1. M. P. Strand, “Underwater electro-optical system for mine

identification,” in Detection Technologies for Mines and Mine-like Targets, A. C. Dubey, I. Cindrich, J. M. Ralston, and K. A.Rigano, eds., Proc. SPIE 2496, 487–497 �1995�.

2. G. R. Fournier, D. Bonnier, J. L. Forand, and P. W. Pace,“Range-gated underwater laser imaging system,” Opt. Eng. 32,2185–2190 �1993�.

3. J. W. McLean, “High-resolution 3D underwater imaging,” inAirborne and In-Water Underwater Imaging, G. D. Gilbert, ed.,Proc. SPIE 3761, 10–19 �1999�.


4. M. W. Scott, “Range imaging laser radar,” U.S. patent4,935,616 �19 June 1990�.

5. J. W. Rish, S. M. Lebien, R. O. Nellums, J. Foster, J. W.Edwards, and B. T. Blume, “Performance of a gated scanner-less optical range imager against volume and bottom targets ina controlled underwater environment,” in Information Systemsfor Divers and Autonomous Underwater Vehicles Operating inVery Shallow Water and Surf Zone Regions II, J. L. Wood-Putnam, ed., Proc. SPIE 4039, 114–123 �2000�.

6. L. Mullen, V. M. Contarino, and P. R. Herczfeld, “Modulatorlidar system,” U.S. patent 5,822,047 �13 October 1998�.

7. L. Mullen, V. M. Contarino, and P. R. Herczfeld, “Hybrid lidar-radar ocean experiment,” IEEE Trans. Microwave TheoryTech. 44, 2703–2710 �1996�.

8. L. Mullen, E. Zege, I. Katsev, and A. Prikhach, “Modulatedlidar system: experiment and theory,” in Ocean Optics: Re-mote Sensing and Underwater Imaging, R. J. Frouin and G. D.Gilbert, eds., Proc. SPIE 4488, 25–35 �2001�.

9. I. L. Katsev, E. P. Zege, A. S. Prikhach, and I. N. Polonsky,“Efficient technique to determine backscattered light power forvarious atmospheric and oceanic sounding and imaging sys-tems,” J. Opt. Soc. Am. A 14, 1338–1346 �1997�.

0. E. P. Zege, I. L. Katsev, and I. N. Polonsky, “Multicomponentapproach to light propagation in clouds and mists,” Appl. Opt.32, 2803–2812 �1993�.

1. E. P. Zege, A. P. Ivanov, and I. L. Katsev, Image TransferThrough a Scattering Medium �Springer-Verlag, Heidelberg,Germany, 1991�.

2. E. P. Zege, I. L. Katsev, A. S. Prikhach, G. D. Ludbrook, and P.Bruscaglioni, “Analytical and computer modeling of the oceanlidar performance,” in 12th International Workshop on LidarMultiple Scattering Experiments, C. Werner, U. G. Oppel, andT. Rother, eds., Proc. SPIE 5059, 189–199 �2002�.

3. J. Prentice, A. Laux, B. Concannon, L. Mullen, V. Contarino,and A. Weidemann, “Comparison of Monte Carlo model pre-dictions with tank beam spread experiments using a Maaloxphase function obtained with volume scattering function in-struments,” presented at the 2002 Ocean Sciences Meeting,Honolulu, Hawaii, 11–15 February 2002.

4. F. W. Sears, M. W. Zemansky, and H. D. Young, UniversityPhysics �Addison-Wesley, New York, 1987�.

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Amplitude-Modulated Laser Imager