Scattering: The Other Half of Optical Oceanography Curtis D. Mobley University of Maine, 2007 Motivation Terminology and Definitions Example Instrument Designs and Data

Light and Matter Interactions absorption: radiant (light) energy disappears and is converted to chemical (e.g., photosynthesis) or thermal (e.g., heats the water) energy scattering: the light changes direction (elastic scattering) and/or wavelength (inelastic scattering, e.g., water Raman, chlorophyll fluorescence)

Once again… Curt, Lecture 1 Collin, Lecture 2 Collin, Lecture 10 Emmanuel, Lecture 18 Curt, Lecture 6 Emmanuel, Lecture 8 Curt, Lecture 9 Collin, Lectures 2 & 10 Collin, Lecture 2 Mary Jane, Lecture 7 Curt, Lecture 12 Collin, Lectures 3 & 4 Mary Jane, Lecture 5

Why is Scattering Important? The world with scattering

Why is Scattering Important? The world with scattering The world without scattering

Why is Scattering Important? Determines the angular distribution of the radiance The basis for ocean color remote sensing Enhances absorption effects Degrades visibility Diagnostic for particle properties (size, composition) Lots of math, so lots of fun! Mobley’s Fundamental Law of Optics: If the light doesn’t go in your eye, you don’t see it.

Terminology All visible-light scattering is fundamentally the same: EM radiation interacting with electrons bound to atoms. Terms like Rayleigh scattering, Mie scattering, surface scattering (reflection and refraction), volume scattering, etc., all refer to mathematical models of scattering, which are valid for various situations.

3 Places Scattering Occurs at the sea surface: reflection and refraction by the wind-blown sea surface within the water: light is scattered by water molecules, phytoplankton, mineral particles, etc. at the sea bottom: light is reflected by sediments, sea grass, coral, shipwrecks, etc. How do we understand, measure, model, and predict scattering in these places? For this lecture, we’ll discuss only scattering within the water.

What Causes Scattering? Scattering is caused by a change in the real index of refraction n, i.e., by a change in the speed of light when going from one medium to another. Recall: c(in medium) = c(in vacuo)/n

Scattering within the Water Absorption coefficients vary by orders of magnitude, depending on the types and concentrations of water- column constituents (phytoplankton, CDOM, mineral particles) and, for a given particle type, on its current state (pigment suite, pigment packaging, etc). The same holds true for scattering.

The Volume Scattering Function (VSF) s(s()s(s() tttt iiii aaaa  VV rr  The VSF tells you everything you need to know about how a volume of matter scatters light

Other Measures of Scattering (1) The total scattering coefficient: The scattering phase function: angular dependence of the scattered light strength of scattering strength and angular dependence of scattering b tells how much light is scattered, without regard for the direction of the scattering The phase function gives the angular pattern of the scattered light, without regard for the magnitude of the scattering [m -1 ]

Other Measures of Scattering (2) The backscatter fraction: B = b b / b B gives the fraction of the total light scattered that is scattered through 90 to 180 deg The backscatter coefficient: [m -1 ] b b tells how much light is scattered through  = 90 to 180 deg The albedo of single scattering:  o = b / (a + b) ω o gives the fraction of the light scattered (vs. absorbed) in any interaction with matter; also called the probability of photon survival

Example b Data ac9 profiles for San Luis Bay, CA and averages over 0-15 m

Example b Data ac9 profiles for Sequim Bay, WA and averages over 1-4 m

Example b Data ac9 profiles for Sequim Bay, WA and averages over 1-4 m absorption and scattering may or may not be correlated. You MUST measure both!

Measurement of the VSF How do you implement the definition of the VSF in an instrument? Several example instruments (angle range; era; application): ALSCAT (very small angles < 1 deg; 1960’s; visibility and imaging) LISST (small angles < 20 deg; 1990’s; inversion of VSF to get PSD) GASM (intermediate angles, 10 to 170 deg; 1960’s; general optical oceanography) VSM (wide angle range, 0.5 to deg; 2000’s, general optical oceanography and remote sensing) HydroScat and EcoVSF (backscatter; 1990’s; ocean color remote sensing) BetaPi (backscatter near 180 deg; 1990’s; LIDAR remote sensing)

Basic Optics Lesson The fundamental idea for measurement of small angle (near forward) scattering

Basic Optics Lesson As implemented in the ALSCAT and LISST instruments

ALSCAT = ALpha (beam c) and SCATtering; developed by Petzold, et al., at Scripps Visibility Lab (figs from Petzold, SIO Ref )

GASM = General Angle Scattering Meter; developed by Petzold, et al., at Scripps Visibility Lab (fig from Petzold, SIO Ref )

Petzold’s VSF Data (fig from Petzold, SIO Ref ) 5-6 orders of magnitude from 0.1 to 180 deg Two orders of magnitude in VSF values for a given 

An Idea It would simplify optical oceanography if most of the variability in the VSF were contained in the scattering coefficient b, so that a common phase function could be used for “particles.” highly variable maybe not too variable Is this a good assumption?

Variability in Phase Functions 62 phase functions measured in coastal New Jersey waters,  = 530 nm (VSM data courtesy of E. Boss, M. Lewis, et al.) Petzold “average particle” Order-of- magnitude variability Note: Petzold may be fine on average, but way off in any particular instance.

(from Maffione and Dana, Appl. Optics 36, ) HOBILabs HydroScat 6 

(stolen from Mike Twardowski) WETLabs ECO-VSF

(from Maffione and Dana, Appl. Optics 36, ) VSF and b b 2  /b b  0.9 at   120 deg, so b b  2  (120)/0.9 2  /b b Mobley et al., AO 2002: b b values from integration of the VSF, HydroScat6, and ECO-VSF agreed to within 8% of the average b b

Variability in Particle Backscatter Fraction Petzold average particle From Mobley et al., AO, 41(6), 2002

Beta-Pi, β(  ), for VSF at deg from Maffione and Honey, SPIE , 1992

β(  ) data, deg Enhanced backscatter very near 180 deg A crucial measurement for LIDAR models unpublished data by R. Maffione

Take-home Message Scattering is just as important as absorption in understanding and predicting the optical properties of the ocean Scattering is just as complicated--but even less well understood-- as absorption A revolution in understanding scattering is now underway, enabled by recently developed instruments (VSM for the full VSF, LISST for small angle VSF, ac-9 for b, HydroScat and EcoBB & ECOVSF for b b, LISST-Back for large angles (under devel.), etc.) Finally, you can measure only absorption and call yourself a biological or physical or geological oceanographer, but you aren’t an optical oceanographer if you don’t also measure scattering!

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