Re: (TFT) New File on Dwarves on Rick's web pages.

["Tapley, Mark" <mtapley@swri.edu>]

Rick,

On Feb 7, 2016, at 8:58 AM, TFT Digest <tft-owner@brainiac.com> wrote:

> The thing I don't like about 'infravision' is how does it work when the 
> dwarf him or her self is generating heat.  The sensors (the eyes) would
> drown out the signal with their own emissions.  (I suppose it could work 
> in the very near infrared, but then it wouldn't be much use as a night 
> vision ability.)  The D&D infravision, only made sense to me as a 
> magical effect.

	I had the same worry - when working on a design for an infrared telescope spacecraft. But, it actually can work. 

	First, the optics are emitting infrared energy isotropically (in all directions), wheras they are *focussing* down to a point incoming incoming energy from the target over a large solid angle (as seen by the target). Bigger optics (compared to the size of the detector) can concentrate on the detector more energy from the one direction of the target than they are emitting in that specific direction. 
	Second, many emitters emit preferentially at a few specific wavelengths which stand out against their normal black-body curve, and if the target emphasizes some wavelengths that are different from the optics, that can provide enough of a relative advantage for the detector to discriminate them. 
	Finally, even if the optics are overwhelming the target at a given frequency and for the given focussing, if the detector can still resolve the *difference* in intensity caused by target emissions, it can still form an image. (That is, the detector sees the small additions caused by the presence of the target compared to what it sees when looking at a cold, empty background).

	All this is theoretical, but night-vision (IR) goggles and IR telescopes and sidewinders (the snakes, not the missiles) are effective, and some of them work well without cryogenic detectors. It is also true that the more sensitive ones, particularly as you go down into far IR wavelengths, do use cryogenics, but remember that radio (and microwave, of course, is just far far far IR) receivers are usually not cryogenic. So clearly, there are techniques that will be effective even in the presence of detector-generated heat.

											- Mark
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