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Path radiance in atmosphere

  1. Nov 2, 2015 #1
    It is common knowledge that the scattering of blue photons by the atmosphere is more significant than for other colors hence aerial images of earth look bluesh.
    The graph below obtained from MODTRAN 'allegedly' supports this fact.

    While trying to quantify this effect, I've preformed several calculations
    According to my calculations, since the camera counts photons and since one has to divide energy by hc/lambda to obtain #photons we obtain a monotonically decreasing scattering function multiplied by the monotonically increasing lambda function, which yields a relatively constant product (high energy at low wavelengths represent same photon number as low energy at high wavelengths).

    This product, multiplied by each of the 3 color bands (R, G and B) gives a similar result for all channels.
    In other words, the sun scattering is relatively equal for all three RGB channels of a standard CCD camera.


    My question is why are the images bluesh?
    Am I missing something??

    Thanks!




    SolarScattering.jpg
     
  2. jcsd
  3. Nov 2, 2015 #2
    What is lambda function? Function of what?
     
  4. Nov 2, 2015 #3

    Andy Resnick

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    Where was the sun relative to the detector in this simulation?
     
  5. Nov 3, 2015 #4
    The sun is at the Zenith and the detector is looking straight down towards the ground.
     
  6. Nov 4, 2015 #5

    Andy Resnick

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    Ok, but what is the angle between the sun-ground axis and detector-ground axis?
     
  7. Nov 4, 2015 #6
    All three points are on the same vector - the angle is zero.
     
  8. Nov 4, 2015 #7

    Andy Resnick

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    So the sun is behind the detector- thus, the light scattered from sunlight is backscattered to the detector. Angles matter- the sky is most blue when you are looking approximately at a right angle to the sun. Try running MODTRAN with a few different angles and compare the curves, and only plot the range 400 - 700 nm, since digital cameras have IR cut filters.. I'm assuming you don't have any particulate matter included in your simulation...
     
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