Re: [Phys-L] Infrared

[John Denker <jsd@av8n.com>]

On 12/18/19 2:16 PM, Forinash III, Kyle via Phys-l wrote:

> I’m a bit confused by the following New York Times article. They 
> claim to be detecting methane leaks using IR cameras in the 3.2 to 
> 3.4 Micrometer range. Wouldn’t such an image be overwhelmed by 
> thermal IR? In other words the pictures are of heat, not methane 
> specifically?

Rather than framing the question in terms of "overwhelmed"
perhaps it would be better to ask whether the foreground
(methane) stands out from the background (whatever that
may be).

Thermodynamics says that IF (big IF) everything were in
thermal equilibrium, then no photography of any kind
would be possible.  Everything would look the same in
all directions.

Instead, of course, typically we illuminate ordinary
scenery (300 K) with sunlight (6000 K color temperature)
so the situation is very far from equilibrium.

As for IR photography, the situation is a bit trickier.
I'm not an expert, but based on what I see in the Times
article, here's what I think is happening:

 -- The methane plume is optically thick.  It has a
temperature on the order of 300 K, and glows accordingly.

 -- The camera angle is such that the background is
the sky.  The night sky is black, and the daytime
sky is blue, which is the same as black at 3.3 μm.
To say the same thing another way, the atmosphere
is optically thin at 3.3 μm, so you are looking out
at outer space, i.e. 4 K background, so the methane
in the foreground definitely stands out.

The Rayleigh scattering that makes the sky blue on
a dry day scales like λ⁴ so it's really small at
3.3 μm.  Even if the air is humid (but not cloudy)
it is optically thin for a different reason, namely
the absorption spectrum of water vapor:
  https://en.wikipedia.org/wiki/File:Water_infrared_absorption_coefficient_large.gif

The worst case scenario is where you have a cloud in
the background.  That's gonna be optically thick, but
if it is a /high/ cloud it will be cold, so once again
the warmish methane in the foreground will stand out.

If the site is fogged in, then yes, sure, the method
will fail -- but nobody should be surprised by that.
Photography of all kinds fails under such conditions.

Aerial reconnaissance, where you are looking down on
the plume with little if any control of the background,
might be quite a bit more difficult, perhaps impossible.

Here's the article.  The link works fine for me:
https://www.nytimes.com/2019/12/13/reader-center/methane-infrared-camera.html