[cantharis]

People get very worried about LEO using heat detectors to detect their grow, and I have read quite a bit of mis-information on the subject, so I hope this post may clarify matters.
LEO fly around in helicopters aiming heat detectors at people and buildings - these devices detect infrared radiation. All objects warmer than absolute zero will emit some infrared radiation, and the warmer the object the more infrared radiation it will produce, and the spectrum of that radiation changes as well.
So what the heat detectors do very well is showing objects that are warmer than their surroundings. We have all seen the police pursuit videos where the fleeing villains (who are warm) are glowing white and all laughed when the perp hiding in the bushes stands out like a dog´s balls on the heat detector. Also heat detectors are inexpensive and widely used by all police departments, and for many uses as well as detecting possible grows.
So how does this affect us growers? First of all, cannabis plants growing in fields or woods are at exactly the same temperature as all the other plants around them, so do NOT show up on heat detectors. Good news for the outdoor growers then.
Indoor growers have more of a problem, as their grow room is often hotter than the rest of the house, and any exterior walls will show up as being warmer than the surrounding brickwork. But the same is true for a warm bathroom, the wall supporting your central heating boiler or hot water tank. In fact, it is perfectly normal to see a whole load of hotspots in a street of houses. Where you will run into problems is if your ENTIRE house is a growing area, and the whole building is glowing on the detector. A small grow in one room, better still if there are no external walls to that room, or in a cupboard is unlikely to attract attention. So keep it small, and you are almost certainly OK.
I should mention airborne spectrometers as well. These are only bad news for SOME outdoor growers. All growing plants reflect visible light from the sun, and the spectrum of that visible light is characteristic of the species of plant. You can see this yourself, your cannabis plant is clearly a different green from, say, a holly bush. Airborne spectrometers are tuned to the spectrum of light reflected from Mary Jane plants, and can indicate whether a particular plant is cannabis or not. The good news for us is that these devices are expensive, not all police departments have them. And they are most effective at detecting a large number of plants all together. An operator would have to get quite lucky to find a single plant growing in a wood. The message is keep your outdoor grow spread out if you can, not all in one spot.
I hope this keeps a few more plants safe from LEO.