Lecture 2 Overview of Light in Water

Transcription

1 Lecture 2 Overview of Light in Water Collin Roesler Department of Earth and Oceanographic Science Bowdoin College /01/underwater-light-beams.jpg 10 July 2017

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3 Inherent Optical Properties Radiative Transfer Equation Radiometric Quantities Apparent Optical Properties

4 Tracing light from the Sun into the Ocean

5 The Source What is the intensity and color of the Sun? The bright sun, a portion of the International Space Station and Earth's horizon are featured in this space wallpaper photographed during the STS-134 mission's fourth spacewalk in May The image was taken using a fish-eye lens attached to an electronic still camera. credit: NASA

6 Black body radiation Any object with a temperature >0K emits electromagnetic radiation (EMR) Planck s Law : The spectrum of that emission depends upon the temperature (in a complex way) Sun T~ 5700 K So it emits a spectrum of EMR that is maximal in the visible wavelengths

7 Blackbody Radiation

8 Blackbody Radiation

9 Earth's atmosphere

10 Spectrum of energy that we measure is different from Planck s Law predictions at Earth surface Atmospheric gases (O 3, O 2, H 2 O) beneath Ocean surface Water Particulate and dissolved constituents

11 In the absence of the atmosphere What is the color of the sun? What is the color of the sky? What is the angular distribution of incident light?

12 In the presence of the atmosphere What is the color of the sun? What is the color of the sky? What is the angular distribution of incident light? So the atmosphere Reduces the intensity Changes the color Changes the angular distribution Consider Natural variations in E solar (l) Measurement-induced variations in E solar (l) Try it for yourself in the radiometric properties lab

13 Impact of clouds on E solar (l) Intensity Color Angular distribution Impact on remote sensing

14 This photograph of the Bassas da India, an uninhabited atoll in the Indian Ocean, has an almost surreal quality due to varying degrees of sunglint. credit: NASA/JSC Now we are at the Ocean surface Surface effects

15 As light penetrates the ocean surface and propagates to depth, what processes affect the light transfer? Absorption Scattering Re-emission

16 Case study 1: Consider an ocean that has no particles but does have absorption Is there a natural analog? The Rio Negro in 2010 Credit: MODIS Rapid Response Team NASA GSFC

17 Case study 1: Consider an ocean that has no particles but does have absorption

18 Case study 1: Consider an ocean that has no particles but does have absorption

19 Case study 2: Consider an ocean that has no absorption but does have particles Is there a natural analog?

20 Case study 2: Consider an ocean that has no absorption but does have particles Is there a natural analog? objects/ jpg

21 While these examples have generally considered the whole visible spectrum, it is important to realize that within narrow wavebands, the ocean may behave as a pure absorber or pure scatterer and thus appear nearly black or white in that waveband Pure absorber in near infrared (water absorption) Close to pure scatterer in the uv/blue (clear water)

22 From space the ocean color ranges from white to black generally in the green to blue hues All of these observed variations are due to the infinite combination of absorbers and scatterers MODIS image of phytoplankton bloom in the Barents Sea observed on August 14, 2011 (image credit: NASA)

23 Now consider the process of absorption and scattering in the ocean As you look down on the ocean surface, notice variations in color, clarity and brightness These are your clues for quantifying absorption and scattering Color: blue to green to red Clarity: clear to turbid Brightness: dark to bright

24 IOPs: Inherent Optical Properties Absorption, a Scattering, b Beam attenuation, c (a.k.a. beam c, ~transmission) easy math: a + b = c IOPs are Dependent upon particulate and dissolved substances in the aquatic medium; Independent of the light field (measured in the absence of the sun)

25 Photo credits: Clark Little

26 Before measuring IOPs it is helpful to Review IOP Theory Incident Radiant Power F o F t Transmitted Radiant Power No attenuation

27 IOP Theory Incident Radiant Power F o F t Transmitted Radiant Power If F t <F o there is attenuation

28 Loss due solely to absorption F a Absorbed Radiant Power Incident Radiant Power F o F t Transmitted Radiant Power

29 Loss due solely to scattering F b Scattered Radiant Power Incident Radiant Power F o F t Transmitted Radiant Power

30 Loss due to beam attenuation (absorption + scattering) F b Scattered Radiant Power F a Absorbed Radiant Power Incident Radiant Power F o F t Transmitted Radiant Power

31 Conservation of radiant power F b Scattered Radiant Power F a Absorbed Radiant Power Incident Radiant Power F o F t Transmitted Radiant Power F o = F t + F a + F b

32 Beam Attenuation Theory Attenuance C = fraction of incident radiant power attenuated F b F a F o F t C = (F b + F a )/F o C = (F o F t )/F o

33 Beam Attenuation Theory Beam attenuation coefficient c = attenuance per unit distance (m -1 ) c = C/Dx F b F a cdx = limit -DF/F Dx 0 F o F t integrate x x 0 c dx = - 0 df/f Dx c x x x 0 = - ln F 0

34 Beam Attenuation Theory Beam attenuation coefficient c = attenuance per unit distance (m -1 ) c x x x 0 = - ln F 0 F o F b F a F t c (x - 0) = - [ ln(f x ) - ln(f 0 )] c x = -[ ln(f t )-ln(f o )] c x = - ln(f t /F o ) c (m -1 ) = (-1/x) ln(f t /F o ) Dx This provides a guide towards measurements (lab 2)

35 Following the same approach Absorption Theory A = absorbance a = absorbance per unit distance (m -1 ) A = F a /F o F b F a a = A/Dx F o F t a (m -1 ) = (-1/x) ln(f t /F o ) Dx How is this measurement different from beam c?

36 Scattering Theory B = scatterance b= scatterance per unit distance (m -1 ) B = F b /F o F b F a b = B/Dx F o F t b (m -1 ) = (-1/x) ln(f t /F o ) Dx How is this measurement difference from beam c, a?

37 Scattering has an angular dependence described by the Volume Scattering Function (VSF) b(q, f) = power per unit steradian emanating from a volume illuminated by irradiance = df 1 1 dw dv E E = F/dS [mmol photon m -2 s -1 ] ds dr E dv = ds dr b(q,f) = df 1 ds dw dsdr F o = 1 df F o drdw

38 A note about solid angles q r r dq = arc length Arc length of a circle = r dq Area on a sphere, da r dq r sinq df sinq dq df

39 Volume Scattering Function (VSF) b(q, f) = power per unit steradian emanating from a volume illuminated by irradiance b(q,f) = 1 df F o drdw b = 4p b(q,f) dw What is dw? 2p p b = o b(q,f) sinq dq df

40 Calculate Scattering, b, from the volume scattering function b = 4p b(q,f) dw If there is azimuthal symmetry p b = 2p 0 b(q,f) sinq dq b f = 2p b b = 2p p/2 0 p p/2 b(q,f) sinq dq b(q,f) sinq dq ~ Phase function: b(q,f) = b(q,f)/b These are spectral!

41 Inherent Optical Properties Absorption, a Scattering, b, and volume scattering function, b Beam attenuation, c

42 Apparent Optical Properties Derived from Radiometric measurements Above or within ocean Ratios or gradients Depend upon light field IOPs AOPs describe: Depth of sunlight penetration (diffuse attenuation) Angular distribution of sunlight (average cosine) Ocean color and brightness (reflectance)

43 Now that we have some vocabulary Trace a beam of sunlight through the ocean Describe the beam of sunlight as radiance, L, traveling along a path described by the zenith and azimuth angles, q and f What processes impact the beam?

44 Radiative Transfer Equation Consider the radiance, L(q,f), as it varies along a path r through the ocean, at a depth of z dz = dr cosq d L(q,f), what processes affect it? dr absorption along path r -a L(z,q,f) scattering out of path r -b L(z,q,f) scattering into path r 4p b(z,q,f;q,f )L(q,f )dw

45 Radiative Transfer Equation Consider the radiance, L(q,f), as it varies along a path r through the ocean, at a depth of z d L(q,f), what processes affect it? dr cosq d L(q,f) = -a L(z,q,f) - b L(z,q,f) + 4p b(z,q,f;q,f ) L(q,f ) dw dz If there are sources of light (e.g. fluorescence, raman scattering, bioluminescence), that is included too: a(l 1,z) L(l 1,z,q,f ) (quantum efficiency) L(l 2,z,q,f)

46 Radiative Transfer Equation relates the IOPs to the AOPs

47 Now you will spend the next four weeks considering each of these topics in detail