Re: [Phys-L] Infrared

[brian whatcott <betwys1@sbcglobal.net>]

On 12/18/2019 3:16 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?
>
> https://www.nytimes.com/2019/12/13/reader-center/methane-infrared-camera.html<https://www.nytimes.com/2019/12/13/reader-center/methane-infrared-camera.html?te=1&nl=climate-fwd:&emc=edit_clim_20191218?campaign_id=54&instance_id=14640&segment_id=19701&user_id=f3315a4f40ddaa91b46edcfed5881d14&regi_id=8509815120191218>
> /snip/
??Glass provides a green-house effect?? - to green-houses, of course. But 
how do I know that??????? The heating provided by a range of visible 
wavelengths in sunlight warms the interior that is illuminated, but the 
re-radiation is down in the infra red and the glass rebuffs it.?? In 
consequence, the greenhouse stays warmer during the night.
OK
If I fly by night, I expect the look-down greenhouse plan view will look 
cooler in my 3 to 3.5 um camera than its surroundings, hence darker - at 
least early on, until near dawn, the surrounds are so chill that the 
green house may no longer be a dark blob, I suspect.
How about if I look up? In the dark clear night sky, the temperatures 
seem rather low - a small number Kelvin. - at least, until I duck into 
the greenhouse and look up from there, where the glass is impeding 
radiation, but is warm along with the air below. My camera will see the 
up view warmer than it would outside.
Methane is a quite effective absorber at 3 - 3.5 um so I expect to see 
similar effects from methane plumes and clouds as from glass. That is, 
thermal IR does not overwhelm methane detectors - it enables them. Or 
does it?

Brian W