OS12A-141 Comparison of Monte Carlo Model Predictions with Tank Beam Spread Experiments Using a Maalox Phase Function Obtained with Volume Scattering Function Instruments

Jennifer E. Prentice1, Alan E. Laux1, Brian M. Concannon1

Linda J. Mullen1, V. Michael Contarino1, Alan D. Weidemann2

1Naval Air Warfare Center Aircraft Division Code 456 Bldg. 2185 Suite 1100 22347 Cedar Point Road Unit 6, Patuxent River, MD 20670

2 Naval Research LaboratoryOcean Sciences, Code 7330, Stennis Space Center, MS 39529-5004

Research

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Measure, Verify, & Compare – Maalox Phase Functions

BackgroundIntroductionOceanographic 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:

The Forward ProblemInput of IOP’s into a predictive model to generate example waveforms that match actual waveforms

The Inverse ProblemDeriving 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.

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.

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.

The Monte Carlo Model

Matching Monte Carlo Models with Experiment – History & Progress

Previous Models : IOP’s guessed at and adjusted to match experiments

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

Our Model : a, b, c, VSF )measured independently and used as inputs to the Monte Carlo

Subsequent Attempts : Improvements in measured IOP’s and matching of tank geometry leads to better agreement in single scattering

What We’ve Learned : The VSF must be accurately determined to obtain good agreement with experiment especially in the multiple scattering regime

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.

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NAVAIR + Extrapolated (2000)

NAVAIR + Petzold (1972)

NAVAIR + HydroBeta (2001)

a = 0.005b = 0.089c = 0.094

Maalox additions were made in increments of 0, 2, 5, 10, 20, 40, 80, and 160 ml in approximately 2,600 liters of water.

The transition from a single to a multiple scattering regime occurred around 20 ml of solute, when bl = 1.

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.

The detector was scanned laterally in 1 cm increments out to 10 cm and in 5 cm increments out to 50 cm .

NAVAIR TankExperimental Measurement of Maalox VSF

An Accurate VSF – Essential to Predicting Multiple Scattering Behavior

Maalox Phase Functions

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Polystyrene Microspheres (25 diameter)

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.

Acknowlegements & References

Phase Function Validation 0.1–2º 50 Polystyrene Spheres

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Phase Function Validation 1–17°10 Polystyrene Spheres

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PROTOCOLConstruction of Table Top VSF Meter.

Measurement of extremely small angle (0.1º to 2º) scattering – 50 polystyrene microspheres

Measurement of small angle (1º-17º) scattering – 10 polystyrene microspheres

Comparison of experimentally measured and Mie Theory predictions

Measurement of Maalox – NAVAIR Table Top & HydroBeta

Measurement of Maalox Beam Spread Function – NAVAIR Tank

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.

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.

SIGNIFICANCEUsing 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.