Many invisible gases (such as propane) can only be seen with infrared cameras. They do not cast a shadow to the naked eye.
My question is whether they arelikely to cast a shadow to an infrared camera. This is a complex question, as I will explain shortly. The use case here isoptical gas imaging (OGI), so let's assume that's the type of camera we're using.
A quick run down on OGI cameras:OGI cameras work by converting the amount of infrared radiation exposed to its detector into a pixel brightness. Different materials emit different amounts of radiation based on their temperature, with higher temperatures emitting more radiation. Invisible hydrocarbon gases absorb infrared radiation better than air does. What that means is that infrared radiation coming off of a hot background will show a dark spot where the invisible gas is. The cooler that gas is, the darker the dark spot. The hotter the background is, the brighter the rest is. So the contrast is determined by the temperature difference between the gas and the background, which is the ground in this case.
Let's use propane as an example. If propane is being released, the OGI camera can see it because propane absorbs infrared radiation better than air, so the patches of ground around the propane are going to appear bright, while the patch of ground under it (assuming a top down view) is going to be darkened by the propane, because less infrared radiation is getting through it.
The question is whether the propane can cast a shadow to an observer looking at the ground below it from an angle,NOT through the propane itself.
As I understand it, this depends on
- Whether the propane blocking a spot on the ground from absorbing infrared radiation from the sun causes it to be cool enough to create a contrast with the other patches of ground.
- Whether a lot of the infrared brightness of the ground in general depends onreflected infrared from the sun, in which case that spot is being blocked by the propane.
There might be other factors which can cause a shadow, but if it's option 1, then I would imagine that the shadow takes time to adapt, it won't move quickly because it's based on heat transfer within the ground.
