Transcript Document

OS12A-141 Comparison of Monte Carlo Model Predictions with Tank Beam Spread Experiments
Using a Maalox Phase Function Obtained with Volume Scattering Function Instruments
1
Prentice , Alan
1
Laux ,
1
Concannon
Jennifer E.
E.
Brian M.
1
1
2
Linda J. Mullen , V. Michael Contarino , Alan D. Weidemann
1Naval Air
Warfare Center Aircraft Division
Code 456 Bldg. 2185 Suite 1100 22347 Cedar Point Road Unit 6, Patuxent River, MD 20670
Naval Research Laboratory
Ocean Sciences, Code 7330, Stennis Space Center, MS 39529-5004
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Introduction
PROTOCOL
The Monte Carlo Model
Input of IOP’s into a predictive model to generate example waveforms that match actual waveforms
The Inverse Problem
Deriving a unique set of IOP’s from a single LiDAR return.
Advances in the development of in situ IOP instrumentation over the past decade has provided for the routine
measurement of a, c, ) at discrete angles, and the determination of b.
This presentation is focused on The Forward Problem. It addresses the accuracy of measuring the VSF under
single and multiple scattering regimes by matching Monte Carlo Model Predictions with Beam Spread Function
Measurements under controlled laboratory conditions.
Virtually nothing is known about the variation in the VSF in the marine environment. New in situ instrumentation
based on different methods are emerging to fill the void. As a first step in evaluating the in situ technology, VSF’s
were measured in the small forward angle direction from 0.1 to 17° with a NAVAIR custom table top instrument
and from 5° to 170° using HydroBeta, developed by HOBI Labs, Inc.
Measure, Verify, & Compare – Maalox Phase Functions
cw doubled YAG
mirror
ND’s
spatial filter/
beam expander
0.1– 2.0º
Comparison of NAVAIR Tank observation and Monte Carlo Simulation using the various ’s
RESULTS
1) Successful prediction of the experimental results verifies that both the Monte Carlo
simulation realistically predicts photon scatter and that the shape of the experimentally
measured phase function ( ) is acceptable for the attenuating agent used.
Initial Attempt : Inaccuracies in the
measured IOP’s lead to poor agreement
Today : Latest VSF ) measurement
yields excellent agreement in both
single and multiple scattering
What We’ve Learned : The VSF must be
accurately determined to obtain good
agreement with experiment especially in
the multiple scattering regime
pivot
cell
SIGNIFICANCE
Using this new measured VSF in the Monte Carlo Model demonstrates the substantial importance
of having accurate VSF measurements out to 90º in order to simulate LiDAR system performance
in the forward direction under conditions of multiple scattering as is characteristic of natural
coastal and ocean waters.
CONCLUSION
Accurate in situ characterization of the VSF may play a
dominant role in LiDAR performance prediction.
20 ml Maalox (0.0076 ml/liter)
5 ml Maalox (0.0019 ml/liter)
1E+0
1E+0
Tank Measured
a = 0.005
b = 0.089
c = 0.094
Petzold (1972)
NAVAIR + Extrapolated (2000)
1E-2
Optical instruments that measure a and c were placed in the
tank so as not to interfere with the beam path or any singly
scattered photons; b was calculated according to b = c - a.
DSA
2) New measurements made with HydroBeta spanning the range from 5º to 170º under parallel
conditions of single and multiple scattering with Maalox accurately match NAVAIR
results at angles less than 20º. NAVAIR tank results were combined with the HydroBeta to
yield a measured VSF between 0.1º and 90º.
An Accurate VSF – Essential to Predicting Multiple Scattering Behavior
1E-1
The transition from a single to a multiple scattering regime
occurred around 20 ml of solute, when bl = 1.
motion
controller
PC
Measurement of Maalox Beam Spread Function – NAVAIR Tank
Subsequent Attempts : Improvements
in measured IOP’s and matching of
tank geometry leads to better
agreement in single scattering
Maalox additions were made in increments of 0, 2, 5, 10,
20, 40, 80, and 160 ml in approximately 2,600 liters of water.
mirror with
pinhole
1m
Measurement of Maalox  – NAVAIR Table Top & HydroBeta
Our Model : a, b, c, VSF )
measured independently and used as
inputs to the Monte Carlo
Relative Amplitude
Polystyrene Microspheres (25  diameter)
pmt
mirror
Comparison of experimentally measured  and Mie Theory predictions
Previous Models : IOP’s guessed at
and adjusted to match experiments
NAVAIR Tank
Experimental Measurement of Maalox VSF
motorized
translation
stage
lens
cell
Measurement of small angle (1º-17º) scattering – 10 polystyrene microspheres
Matching Monte Carlo Models with Experiment – History & Progress
Actual measurements of the complete optical volume scattering function (VSF, )), describing the angular
distribution of light scattered from a collimated beam are extremely limited, both in the laboratory and in situ, due
to the high degree of difficulty in making the measurement. The primary challenge is that light scatter relative to
propagation direction is highly peaked in the forward direction; therefore, measurement across the necessary
angular range from 0-180 degrees requires a method that is sensitive over a dynamic range spanning at least 4 to 5
orders of magnitude.
spatial filter &
beam expander
Measurement of extremely small angle (0.1º to 2º) scattering – 50 polystyrene microspheres
We simulate the experiment by launching a large number of photons through a virtual setup.
The trajectory of each photon is calculated by ray tracing from a simulated source through
the tank to a simulated array of detectors. While in the tank the propagation and scattering
events are governed by probability distributions which are sampled at random. Photons
reaching the detectors are weighted by absorption and field of view and summed up to give
an intensity. This intensity is then divided by the number of photons launched to give a
relative amplitude which can be compared directly to the actual experiment.
The Forward Problem
beam dump
Construction of Table Top VSF Meter.
NAVAIR + Petzold (1972)
1E-3
NAVAIR + HydroBeta (2001)
1E-4
1E-5
mirror
The detector was scanned laterally in 1 cm increments out to
10 cm and in 5 cm increments out to 50 cm .
rail
PIN
photodiode
1E-6
1E-7
1m
DSA
Detector: 1 x 1 cm PIN Photodiode operated at 18 V reverse bias
Input Aperture: 8.4 mm diameter
Geometrical Acceptance Angle: 6.9° , half angle
Field of View: 3.6 , HWHM
ND’s
Phase Function Validation 0.1–2º
50  Polystyrene Spheres
Phase Function Validation 1–17°
10  Polystyrene Spheres
1E+0
1E+0
Relative Intensity
Relative Intensity
1E-3
1E-4
0.5
1.0
1.5
Tank
motorized
carriage
1E-2
motion
controller
1E-3
2.0
0
2
4
6
8
10
12
14
16
18
Angle (Degrees)
Maalox VSFs
Maalox Phase Functions
1E+3
1E+1
Petzold (1972)
Petzold (1972)
NAVAIR (2000)
NAVAIR + Extrapolated (2000)
1E+2
Amplitude (sr-1)
HydroBeta (2001)
1E+0
1E-2
1E-4
1E-3
10
20
30
40
50
Angle (Degrees)
60
70
80
90
0
5
10
15
20
25
30
35
40
45
0
50
5
10
15
20
25
30
35
40
45
50
Position (cm)
Position (cm)
80 ml Maalox (0.031 ml/liter)
3 ft
1E+0
ND’s
cw doubled
YAG
lock-in
amplifier
Laser: 170 mW CW doubled Nd: YAG (532 nm)
Beam Waist: 0.7 mm FWHM
Beam Divergence: 0.5 mrad, half angle
Neutral Density Filters control Beam Power
Beam is chopped at 1 Khz (Lock-in Amplifier reference)
1E+0
a = 0.031
b = 1.451
c = 1.482
1E-1
mirror
160 ml Maalox (0.061 ml/liter)
1E-2
1E-1
1E-3
1E-4
1E-5
1E-6
1E-2
1E-3
1E-4
1E-5
1E-6
1E-7
1E-7
1E-8
1E-8
5
10
15
20
25
30
35
40
45
a = 0.050
b = 3.065
c = 3.115
50
Position (cm)
0
5
10
15
20
25
30
35
40
45
50
Position (cm)
NAVAIR + HydroBeta (2001)
Acknowlegements & References
1E-1
1E-3
0
1E-8
0
1E+0
1E-2
1E-6
1E-9
NAVAIR + Petzold (1972)
1E+1
1E-1
rail
PC
Angle (Degrees)
1E+2
1E-5
1E-7
chopper
1E-1
1E-4
0.0
Amplitude (Norm. 5 Deg.)
PIN
photodiode
Mie Theory
1E-2
1E-4
mirror
Experiment
1E-1
1E-3
12 ft
Experiment
Mie Theory
1E-2
1E-8
Relative Amplitude
mirror
1– 17º
Relative Amplitude
cw doubled YAG
a = 0.011
b = 0.347
c = 0.358
1E-1
Relative Amplitude
Oceanographic researchers and the U.S. Navy need to quantify light attenuation in the sea and to understand the
mechanisms and environmental processes controlling its propagation. A primary area of investigation in the
NAVAIR Electro-Optics division is correlating the Apparent Optical Property (AOP) measurements made by
LiDAR systems with water column Inherent Optical Properties (IOP’s). Interpretation of the backscattered
LiDAR signal must be approached from two directions:
Small Forward Angle  Apparatus
Research
Background
0
10
20
30
40
50
60
Angle (Degrees)
70
80
90
NAVAIR Internal Research Grants in Electro Optic Sensors Division, the Middle
Atlantic Research Consortium (MARC) Program through the Office of Naval
Research (ONR), and PMA 264 provided the primary funding sources for this
research. The authors acknowledge the collaborative work throughout the program
of Dr. Richard Billmers of R.L. Associates, Langhorne, PA and the support of
AMPAC, Inc. North Wales, PA. We thank Dr. Jon Davis for assistance in the
theoretical development of the Monte Carlo Model and verification of the Mie code.
HydroBeta Data provided in conjunction with HOBI Labs, Inc. Marina, CA.
Bohren C. and Huffman D., 1998, Absorption and Scattering of Light by Small
Particles, John Wiley & Sons, pp. 82-129.
Concannon B. and Davis J., 1999, Results of a Monte Carlo investigation of the
diffuse attenuation coefficient, Applied Optics, Vol. 38, No. 24, 5104-5107.
Laux A., et al., (In Press), Closing the Loop-The a, b, c’s of Oceanographic Lidar
Predictions, Journal of Modern Optics, pp. 13.
Petzold T., 1972, Volume Scattering Functions for Selected Ocean Waters, Scripps
Institution of Oceanography, Visibility Laboratory, San Diego, California,
pp. 25-27, 38, 64-65.