Xref: utzoo sci.electronics:18482 sci.optics:138 Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!samsung!umich!csd4330a!newsserv!baskins From: baskins@csd460a.erim.org (Bob Baskins) Newsgroups: sci.electronics,sci.optics Subject: Re: Laser Listener - How does it work? Message-ID: Date: 14 Mar 91 21:13:27 GMT References: <1991Mar11.152746.19730@hobbit.gandalf.ca> <1991Mar11.163650.9794@fs7.ece.cmu.edu> Sender: jgarb@newsspool.erim.org (Joe Garbarino) Distribution: na Organization: Environmental Research Institute of Michigan, Ann Arbor, Michigan Lines: 26 In-reply-to: coast@faraday.ECE.CMU.EDU's message of 11 Mar 91 16:36:50 GMT In article <1991Mar11.163650.9794@fs7.ece.cmu.edu> coast@faraday.ECE.CMU.EDU (Douglas Coast) writes: > In the Oct 89 issue of Radio Electronics, an article by Richard L. Pearson > entitled "Laser Listener" appeared. This article describes the constructio > of a laser "bugging" device which works by bouncing a low-power laser > beam off of a reflective surface (like a window) and using a phototransisto > to measure intensity of the reflected beam. According to the article, > the reflected beam is amplitude modulated by vibrations of the reflecting > surface. > My question is: How does a vibrating reflector result in an amplitude > modulated beam? Is the effect simply due to the dispersion > of the beam (causing intensity to decrease) or is some other > phenomenon responsible? A question I've wondered about is do you have to be at (or near) the specular angle of the laser probe to detect the reflections? i.e. can you be at ground level and evesdrop on a second story window? -- ===================================================================== Robert Baskins - Environmental Research Institute of Michigan (ERIM) P.O Box 8618 NET: baskins@csd460a.erim.org Ann Arbor, MI 48107 PHONE: (313) 994-1200 (x2938) =====================================================================