Thanks Uwe. I had left time aside today to catch up on some reading, but based on that treatise, I don't think I'll have to. Even a limited worst-case analysis of what was intended shows me Infrared sensing is inadequate, and misleading in the circumstances in which I expect to place myself. The idea goes back onto the back burner, until another good approach suggests itself.
It could still be done, I am sure. But the size of the project changes, and it is certainly out of range for the guys involved. On Wed, Nov 19, 2003 at 12:44:48PM -0800, Uwe Zimmermann enlightened us thusly > > What I am looking for is reading to check the feasability. I need > the freauencies that everything emits. We all enjoy discussing the > design strategy, but I haven't given out enough info to get much > help on that. Where I need the help is a treatment of what > frequencies things emit (Particularly people and the sun). > > Hi Declan, > > in optics people rather talk about wavelengths than frequencies, > because micrometers and nanometers are much easier to grasp than > terrahertz. Many have already cited the black-body radiation and I > don't have the time or energy to check the links you already have. > Some values: > > -for a human body we talk about wavelengths of 5-10um for the peak of > the emitted thermal radiation, that's in the far infrared > > -for the sun we talk about 0.5um, which is in the yellow part of the > spectrum > > The problems with far infrared are mainly: - very few detector > materials are able to "see" these wavelengths. There have recently > been progresses with certain semiconductors, such as InAs, InP, PbTe > which can be used in 2D imaging devices (e.g. FLIR cameras). Silicon, > germanium, gallium arsenide are all blind in this part of the > spectrum. - the refractive index of almost all common materials for > lenses is too low to be useful - this means it is hard to make > focusing optics for this wavelength range. Some materials used are > certain plastics for Fresnel-type lenses, magnesium fluoride and > germanium. - The fact that the thermal radiation of a 37 centigrade > body does not differ too much from a 20 centigrade surrounding makes > it necessary to cool the whole system - otherwise the thermal > radiation of the electronics and optics itself will drown your signal! > > Of course if the surrounding on a summer's day is about as warm as the > human body it will be impossible to detect the human body from its > thermal signature! And the 37 centigrade only counts for the uncovered > parts of the body! Even the thinnest layer of clothes is a very > effective heat shield and the surface of clothes can be assumed to > have ambient temperature! > > Alternatives to sensitive pn-junctions or PIR detectors are > thermopiles which consist of several to several hundred of small > thermocouple junctions connected in series. Combined with good thermal > shielding and a focusing optical system these appear to give quite a > good signal. > > The sun. Well the sun is a bright and hot spot in the sky, about half > an angular degree in diameter. If it happens to be in the direction of > the sensor, the only chances for you to detect a much cooler human > being are: - the angular size of the human being might be > significantly larger - the signal of a moving (living?) human being > does change faster than the sun - but we are probably talking time > constants in the order of a few seconds for the necessary filters here > > > A great thermal source is the homepage of Omega. They offer a very > thick "handbook" on temperature measurement technology on their web > page which is an extraordinary resource - even though it is their own > product catalog at the same time.... http://www.omega.com/ -- With best Regards, Declan Moriarty. -- Author: Declan Moriarty INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
