Greenhouse Gas Camera
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# Greenhouse Gas Camera

We are building cameras which help make pollution visible. Our goal is to create high-quality color video footage of CO2, NOx, SO2 and other gas emissions from from vehicles, power plants, gas flares, and container ships, as well as to understand gas sensing cameras and how both their video quality, and durability can be improved.

## Inspiration

This liquid nitrogen tank was left outside in our backyard for a few months. It was on a concrete slab, far away from any dirt or dust.
Every container ship that passes into the port of Oakland releases enormous amounts of carbon dioxide and soot into the air. A significant percentage of U.S. imports from China are delivered on these ships, but what is not shown next to the cheap prices in big box stores, are the health consequences from ships and diesel trucks that pollute Oakland. We set out to build pollution-sensitive cameras to help spread awareness of the impact that consumerism is having on our town.

## How It Works

Normally you can't see CO2, which makes it hard to understand the impact that it has on the environment. But, CO2 absorbs light at a very specific wavelength (4.26 micrometers), so with a camera that is sensitive to this type of infrared light, and a filter designed to specifically block out all light except 4.26um, we will be able to see CO2 emissions.

## How do we see this forbidden light?

There are a number of ways to see infrared light, but most often you need an exotic material that's not silicon, as silicon's band gap energy is way too large for an infrared photon to overcome. All of the common infrared detector technologies are shown here on this handy chart.
As you can see, some viable candidates for 4 micrometer light are InSb, HgCdTe, and PbSe -- all very specialty and somewhat toxic materials. Fortunately, we don't need to eat them.
InSb is by far the most common material used for this,  so we will have the best luck looking for an InSb "focal plane array" camera.
Now 4 micrometers is very far into the infrared, so much so, that everything that's warm emits light at 4 micrometers, including cameras themselves! This means that in order to see anything, you need to have a very special camera that is cooled to 77 kelvin so that the sensor itself does not emit any light. Keeping the sensor inside a vacuum vessel is the easiest way to reach this temperature, as vacuum is a great insulator.
The "cold finger" depicted in this photo is usually the business end of a Stirling engine cryo-cooler, which uses high pressure helium as a working gas to extract energy out of the sensor and bring it down to liquid-nitrogen temperatures.

## Woah there that looks expensive...

A camera that uses a rare semiconductor, stirling engine, and vacuum dewars? Yes, it definitely is. The only people who can afford cameras like this are the oil drilling companies and well-financed militaries. Just a look online for "Mid Wave IR" cameras shows draw dropping prices north of $40,000. This is really unfortunate, as the absorbance bands of most gasses fall into the mid-wave IR spectrum. You can buy cameras specially built to see methane and other gasses, but the prices start at$100k and go up from there. I reached out to some OEM groups to see if they'd spare an MWIR camera core for the greenhouse gas camera project, but there wasn't much interest in parting with one for less than the price of a luxury car ¯\_(ツ)_/¯.

## Where do we go from here?

It is very fortunate that in the early 2000s digital video was not common place. This means that most video equipment, including these MWIR cameras, used good ol' composite video that has no DRM or vendor-specific protections to prevent you from using it without authorization. Thus, if you can get your hands on one manufactured during this time frame there is a non-zero chance that you can get it working. They are very rare, but every so often you can find one for prices affordable by mere mortals, like this one I just recently purchased from an army surplus vendor!
Naturally it will come with no information, and it's up to you to figure out which of the jumble of wires are power wires, video wires, serial communication wires and the like. You can usually do this by process of elimination and careful examination of the circuit board (for example, ground and power usually go directly to some big capacitors).
In this case, I was able to find out what the wires of interest were fairly easily, and was greeted shortly after with some grainy composite video, in which I was able to see my breath.
Clearly there is some hope here! It does look like there is going to be a lot of work to do, to get to a cinema quality video though.

## Filtering out everything but CO2

As I mentioned previously, nearly everything room-temperature emits light at 4 micrometers, gasses included. This is why my breath on the video above was white --it was hot, and emitting light. This isn't what we are trying to see though, as we are interested only in seeing CO2.
Since CO2 has very strong absorbance at 4.26 mircometers, if we were to include a filter in the camera's optical path that only passes this wavelength then we will be able to see "blackness" where there is CO2 gas, since it will absorb all the 4.26um light.
Fortunately for us, an optics company Thorlabs sells filters that do exactly this function (FB4260-105), and are standard stock items for about \$250. Considering it would cost more than a thousand dollars to get a custom filter made, this is a stroke of good luck!
We need to insert this filter into the optical path of the camera, ideally behind the lens, and keep it held very still so that it won't move on us. It is important to keep it held in the same place, because this filter will introduce irregularities in the picture which the camera will need to calibrate out, and if it moves, these irregularities move too.
I laser cut a holder out of plastic and paper for this filter, so that it can be held in place with neodymium magnets and carefully installed meccano set pieces.
After a couple of hours playing around with the laser cutter, this assembly was found to work the best. It holds the filter flat against the rear of the camera's lens mount, but spaced just far enough from the dewar's germanium window and the rear of the lens such as to not scratch either of them. Still, it scares me every time I insert it, as doing so risks damaging these optics!
To further prevent damage, I built a wooden and acrylic frame with a door to keep out dust, as dust is the worst enemy to a camera like this and needs to be avoided at all costs. There's not very many ways to get dust out of optics except to use high pressure nitrogen, but even then this doesn't always work. Just as with environmental damage, prevention is the best cure!

## First Tests!

The first tests of this camera have gone well. The filter blocks out most of the light the camera would normally see, so the gain of the sensor has to be turned up a lot to for us to be able to see anything. This means the dynamic range is pretty low, and there are a lot of "stuck" pixels ruining the video. You can absolutely see the black haze of CO2 though, even from your breath as you exhale. This is very exciting, as it means that the filter is absorbing the unwanted light just as we expected!
I suspect that the video can be greatly enhanced with post processing of the stream using OpenCV on a linux computer, to enhance the video contrast and eliminate the dead pixels with a calibration map and nearest-neighbor interpolation. I also have a suspicion that a wider-band spectral filter can improve the image too, but it's not clear yet how wide you can go before you lose the ability to see only CO2.

## Detecting Natural Gas

We did some experiments recently detecting propane gas! Have a look here: https://civilpedia.org/p/?pid=355&h=Seeing+Propane+Gas
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