# Far IR light intensity as a function of altitude

## Homework Statement

Hello, so I'm in a class that is building sensors and sending them up in a weather balloon. For my project, I am wanting to quantify the greenhouse effect by measuring the intensity of infrared light emitted as thermal radiation from the Earth as a function of height-- the idea is that as the balloon rises, some of the IR light is absorbed by water vapor and other greenhouse gases, causing intensity to go down.

## Homework Equations

From Wein's displacement law, I know emitted radiation will be about 10 μm. I calculated the change in intensity due to spherical geometry causing an increase in area.

## The Attempt at a Solution

I have a couple questions about the experiment and one problem.

1. It seems like the intensity of far-IR light from the sun is negligible compared to that radiated from the Earth. Is this a reasonable assumption?

2. I am unable to find data on emission of water vapor. Will the light absorbed be re-emitted at the same wavelength, skewing our data? This is my biggest concern, because it would basically invalidate the experiment.

Finally, we have been searching for a circuit component that detects light intensity in the 10 μm range. Have any of you used one in this range? It must detect intensity of the wavelength.

Thanks you so much!

mfb
Mentor
I calculated the change in intensity due to spherical geometry causing an increase in area.
For the same solid angle, that should cancel with the inverse square law. Without absorption or emission in the air the mean intensity should not change. You just average over a larger area (make sure you measure a homogeneous area).
1. It seems like the intensity of far-IR light from the sun is negligible compared to that radiated from the Earth. Is this a reasonable assumption?
You can check the emission values and compare them. A sensor that is not in direct sunlight avoids that issue.
2. I am unable to find data on emission of water vapor. Will the light absorbed be re-emitted at the same wavelength, skewing our data? This is my biggest concern, because it would basically invalidate the experiment.
Infrared will be scattered in the atmosphere, sure, and you have to think how this influences your measurements.
Measurements in different directions could help to distinguish between different infrared sources.
Finally, we have been searching for a circuit component that detects light intensity in the 10 μm range. Have any of you used one in this range? It must detect intensity of the wavelength.
There are many commercial detectors for this wavelength range. You might have to filter out other wavelength ranges first if the detector doesn't do that.