I don’t see how they used this to detect marijuana plants.
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They can see a room full of high intensity lighting.
No they can’t. When you use a true IR device to look at a bright light, all you’ll see is a little speck (or nothing for LED or cold lighting). IR is a line-of-sight-to-solid that only sees HEAT, not visible light. This is why you can’t see through a clear window. The IR device will see the heat signature of the window. You can see a handprint if someone palms the window, because of the heat transfer to the outside of the glass, but anything inside doesn’t transfer in a line like visible light. Even with plant lighting that generates a lot of heat, all you can detect from the outside with IR is that one room of the house is warmer than the others. That doesn’t imply marijuana. The only possible way they could see something inside the house would be if someone left a door/window open, in which case thermal signatures would only be more useful at night in the dark, than if they just looked in.
High-intensity lighting would be detected much easier by a normal camera (or naked eye), and still wouldn’t imply marijuana unless you actually saw the plants inside.