Why Do Aerial Images of Earth Appear Blue Despite Equal RGB Scattering?

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Discussion Overview

The discussion revolves around the appearance of aerial images of Earth, particularly why they tend to look blue despite claims of equal RGB scattering of light by the atmosphere. Participants explore the physics of light scattering, the role of camera sensors, and the conditions under which images are captured.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asserts that blue photon scattering is more significant than for other colors, leading to a bluish appearance in aerial images, supported by a graph from MODTRAN.
  • Another participant questions the definition of the lambda function mentioned in the calculations.
  • A participant seeks clarification on the sun's position relative to the detector in the simulation, emphasizing the importance of angles in light scattering.
  • It is noted that the sun is at the zenith and the detector looks straight down, prompting further inquiry about the angle between the sun-ground axis and the detector-ground axis.
  • A participant clarifies that the angle is zero, indicating that the sun is directly behind the detector, which raises questions about the implications for backscattering of light.
  • Another participant suggests that the sky appears most blue when viewed at a right angle to the sun and recommends running MODTRAN simulations at different angles while focusing on the 400 - 700 nm range.
  • There is an assumption that particulate matter is not included in the simulation, which could affect the results.

Areas of Agreement / Disagreement

Participants express differing views on the factors influencing the bluish appearance of aerial images, with no consensus reached on the underlying reasons or the validity of the calculations presented.

Contextual Notes

Participants highlight the importance of angles in light scattering and the potential impact of simulation parameters, such as the inclusion of particulate matter and the specific wavelength range analyzed.

AlexVM
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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
 
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What is lambda function? Function of what?
 
AlexVM said:
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.

Where was the sun relative to the detector in this simulation?
 
The sun is at the Zenith and the detector is looking straight down towards the ground.
 
AlexVM said:
The sun is at the Zenith and the detector is looking straight down towards the ground.

Ok, but what is the angle between the sun-ground axis and detector-ground axis?
 
All three points are on the same vector - the angle is zero.
 
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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