kean@tank.uchicago.edu (Keane Arase) (02/17/89)
In article <3167@stiatl.UUCP> john@stiatl.UUCP (John DeArmond) writes: > >Perhaps between this and the laser radar, we'll be motivated to do what ^^^^^^^^^^^ There's been a lot of talk over laser radar lately. Is this the term we should be using for this? (radar=RAdio Detecting And Ranging). Since we're quite a bit shorter than radio, shouldn't this be LIDAR? (LIght Detecting And Ranging) Of course we could make it generic and call it EMDAR (Electro-Magnetic Detecting And Ranging). Or we could just call it BULLSHIT for what it really is. :-) (Sorry, no acronym for this one. It is left as an exercise for all those rec.humor readers out there.) Remember... You heard it here first! :-) (Where's that caffeine?) --- Keane Arase | Thought for the Day: kean@tank.uchicago.edu | Drive safely, Drive Offensively. syskean@uchimvs1.uchicago.edu | -- An old Chicago rush hour motto :-) * Please file the usual disclaimers here *
bnick@aucis.UUCP (Bill Nickless) (02/17/89)
In art <1868@tank.uchicago.edu>, kean@tank.uchicago.edu (Keane Arase) writes: > There's been a lot of talk over laser radar lately. > [ Conserving bandwidth (and getting it through postnews!) ] > > Or we could just call it BULLSHIT for what it really is. :-) Maybe it's not such a bad idea. Microwave radar has a tendancy to pick up on the strongest reflection it gets--and we all hope it's the vehicle being pointed at! Laser light could be focused into a narrow beam and we'd KNOW which vehicle was being clocked. 'Course there's no leakage for Cincinnati Microwave to pick up on.... -- Bill Nickless Andrews University Computer Science Department ...!sharkey!aucis!bnick or bnick@aucis.UUCP Unix Support Group "Help! I'm locked up in this .signature factory!"
kean@tank.uchicago.edu (Keane Arase) (02/18/89)
In article <142@aucis.UUCP> bnick@aucis.UUCP (Bill Nickless) writes: >pointed at! Laser light could be focused into a narrow beam and we'd KNOW >which vehicle was being clocked. 'Course there's no leakage for Cincinnati >Microwave to pick up on.... The laser beam will spread 3.5 feet in 1000 ft (1/5 mile!) and register your speed in 1/4 to 1/2 second. (I know *I* don't have reaction times this fast!) And of course there will be no *radio* leakage. BTW, they aim the device with a telescopic scope. The same type used on hunting rifles. (Let's see... Put that red jag in the crosshairs and squeeze the trigger... :-) -- Keane Arase | Thought for the Day: kean@tank.uchicago.edu | Drive safely, Drive Offensively. syskean@uchimvs1.uchicago.edu | -- An old Chicago rush hour motto :-) * Please file the usual disclaimers here *
h16@homxc.ATT.COM (D.JACOBOWITZ) (02/21/89)
In article <1895@tank.uchicago.edu>, kean@tank.uchicago.edu (Keane Arase) writes > > The laser beam will spread 3.5 feet in 1000 ft (1/5 mile!) and register > your speed in 1/4 to 1/2 second. (I know *I* don't have reaction A laser will spread 3.5 feet in 1/5 mile? Are you sure? That doesn't sound like coherent light. ----------------------------------------- Dave J. homxc!h16 usual disclaimer implied
bishel@mcnc.org (Geoffrey R. Bishel) (02/21/89)
In article <5632@homxc.ATT.COM> h16@homxc.ATT.COM (D.JACOBOWITZ) writes: > >A laser will spread 3.5 feet in 1/5 mile? >Are you sure? >That doesn't sound like coherent light. You've got to remember that the beam is passing thru the air, and dust, dirt, water molecules, etc. will all diffract the beam to some extent. Thus you get a spreading of the beam. -- Geoff Bishel "Parlez en anglais, for Christ's sake," said the (bishel@mcnc.org) Corporal. "Je ne parle pas francais." Standard Disclaimers apply "Neither do I," answered Yossarian... (Picture your ad here!) (Joseph Heller - _Catch 22_)
consult@osiris.UUCP (Unix Consultation Mailbox ) (02/22/89)
In article <1895@tank.uchicago.edu> kean@tank.uchicago.edu (Keane Arase) writes: >The laser beam will spread 3.5 feet in 1000 ft... That's wonderful news for all us motorcyclists. All this fine new technology and it'll be as fundamentally unreliable (assuming ignorance of the principles involved, which seems a safe assumption) as existing radar equipment. >BTW, they aim the device with a telescopic scope. The same type used >on hunting rifles. So now they not only have to calibrate the device, but scope it in too? Sure. (btw, motorcycles ain't got no front plates! :-) Phil Kos Information Systems ...!uunet!pyrdc!osiris!phil The Johns Hopkins Hospital Baltimore, MD
kluksdah@enuxha.eas.asu.edu (Norman C. Kluksdahl) (02/22/89)
In article <5632@homxc.ATT.COM>, h16@homxc.ATT.COM (D.JACOBOWITZ) writes: > In article <1895@tank.uchicago.edu>, kean@tank.uchicago.edu (Keane Arase) writes > > > > The laser beam will spread 3.5 feet in 1000 ft (1/5 mile!) and register > > A laser will spread 3.5 feet in 1/5 mile? > That doesn't sound like coherent light. Beware the trap of incorrect terminology! There is a great difference between COHERENT light and COLLIMATED light. A laser is pretty much coherent light in that the spectrum of output is nearly delta-function like. However, LED's are also nearly coherent in that their output is at nearly a single frequency, discounting some small amount of higher and lower energy photons arising from trap states within the energy gap and higher-energy radiative recombinations. Unlike a laser, an LED is not COLLIMATED. I.e., it's output is not even close to being aligned. Photons are emitted in many directions. But even a laser is not perfectly collimated. If you examine the near-field and far-field patterns of lasers, you quickly come to the conclusion that they do possess some spread. A collimating lens minimizes the spread. (Now gas lasers? That I'm not sure of. My field is semiconductors.) Norman Kluksdahl Arizona State University ..ncar!noao!asuvax!enuxha!kluksdah standard disclaimer implied Useful criticism always appreciated. Senseless flames always discarded.
depolo@eniac.seas.upenn.edu (Jeff DePolo) (02/22/89)
In article <4057@alvin.mcnc.org> bishel@mcnc.org.UUCP (Geoffrey R. Bishel) writes: >In article <5632@homxc.ATT.COM> h16@homxc.ATT.COM (D.JACOBOWITZ) writes: >> >>A laser will spread 3.5 feet in 1/5 mile? >>Are you sure? >>That doesn't sound like coherent light. > >You've got to remember that the beam is passing thru the air, and dust, dirt, >water molecules, etc. will all diffract the beam to some extent. Thus you get >a spreading of the beam. >-- >Geoff Bishel "Parlez en anglais, for Christ's sake," said the >(bishel@mcnc.org) Corporal. "Je ne parle pas francais." >Standard Disclaimers apply "Neither do I," answered Yossarian... >(Picture your ad here!) (Joseph Heller - _Catch 22_) Another thing to think about - a beam a few feet wide is going to reflect a beam a bit bigger than that, but not much. If the copper is on the side of the road, he's going to have to "wiggle" the laser around until it hits a perpendicular plane on your car. If, for example, he hits the hood, most of the beam is going to be angled upwards into the sky. His best bet is to aim somewhat low, since it isn't going to bounce off of the windshield or any other part of the car. The grill/radiator/bumber would probably be the best bet. As far as the scope - it had better be pretty damn accurately aligned. If it's off by only 4 or 5 feet, it would be very easy to hit a car next to you. Or what if a semi was coming up from behind you at a pretty good pace - if he aimed at your windsheid, it would go through the front windshield, through the passenger's compartment, through the rear windshield, and bounce off the semi. Kind of scary. --- Jeff +----------------------------------------------+------------------------------+ | Jeff DePolo [depolo@eniac.seas.upenn.edu] | o The best things in life | | => The University of Pennsylvania <= | come in six-packs. | | Class of 1991 - Computer Science Engineering | o Life begins at 85 MPH. | +----------------------------------------------+ o It's not illegal if they | | DISCLAIMER: Someone else used my account. | don't catch you. | +----------------------------------------------+------------------------------+
johng@ecrcvax.UUCP (John Gregor) (02/23/89)
In article <1895@tank.uchicago.edu> kean@tank.uchicago.edu (Keane Arase) writes: >The laser beam will spread 3.5 feet in 1000 ft (1/5 mile!) and register >your speed in 1/4 to 1/2 second. (I know *I* don't have reaction >times this fast!) And of course there will be no *radio* leakage. Doesn't the authority of the FCC end at a few GHz? So, what's the possibility of doing some sort of ECM in the visible and near visible end of the spectrum? For the automated-take-your-picture-and-send-you-the-bill toys, I've thought of strongly illuminating the license plate with either near-IR or near-UV to fog the film. Also coating the windshield with something that's clear at visible wavelengths but a diffuse reflector at the near-UV would also be nice. Any ideas for such a material? As for the laser rangefinders/speedometers, what's a good reference for and/or the basic theory behind coherent light detectors? Could they perhaps be defeated by a very bright flash peaking near the frequency of the laser without violating state laws on lights (no flashing blues, maximum energy, etc.). Do the lasers require a recharge time? Would the time required for the officer to look at his readout, figure out that he needs to re-target and try again, and actually do it be enough for the driver (who was alerted at the first firing) to slow down, change lanes, or whatever? NOTE: This is a theoretical discussion only. I'm not advocating the use of these things to drive like hell (if I get a Ruf Porsche I might feel differently :-) anymore than I recommend using items gleaned from the Anarchist's Cookbook as a means of dealing with the IRS (but it does make me smile thinking about it). If you have a burning desire to talk about the ethics of speeding, Road Warrior-ish ECM, etc. Go hold a vote for sci.ethics (seriously, it could be very good, if moderated). -- John Gregor johng%ecrcvax.UUCP@pyramid.COM
hollombe@ttidca.TTI.COM (The Polymath) (02/23/89)
In article <4057@alvin.mcnc.org> bishel@mcnc.org.UUCP (Geoffrey R. Bishel) writes: }In article <5632@homxc.ATT.COM> h16@homxc.ATT.COM (D.JACOBOWITZ) writes: }>A laser will spread 3.5 feet in 1/5 mile? }>Are you sure? }>That doesn't sound like coherent light. } }You've got to remember that the beam is passing thru the air, and dust, dirt, }water molecules, etc. will all diffract the beam to some extent. Thus you get }a spreading of the beam. About 15 years ago, when lasers were relatively new toys, I was living in London, England. One night I went to a film festival on Oxford Street. Evidently, some other event was going on as well and, for decoration, they had what looked like two "neon" tubes, one red, one blue, hanging in the air from Regent Street to Charing Cross Road, a distance of about a mile. I soon realized that an unbroken, unsupported length of neon that long was impossible, and the effect had to be created by two laser beams aimed down the length of the street. There was no obvious spreading of the beams, in spite of a light mist in the air. A commonly quoted statistic, at the time, was that a one inch laser beam aimed from the earth to the moon would spread to a circle 50,000 ft in diameter. 50K ft / 250K mi = .2 ft/ mi spread, or about 2.4 inches per mile. Any way you look at it, 17.5 ft of spread per mile isn't even in the ball park. -- The Polymath (aka: Jerry Hollombe, hollombe@ttidca.tti.com) Illegitimati Nil Citicorp(+)TTI Carborundum 3100 Ocean Park Blvd. (213) 452-9191, x2483 Santa Monica, CA 90405 {csun|philabs|psivax}!ttidca!hollombe
jbayer@ispi.UUCP (Jonathan Bayer) (02/23/89)
In article <5632@homxc.ATT.COM> h16@homxc.ATT.COM (D.JACOBOWITZ) writes: >In article <1895@tank.uchicago.edu>, kean@tank.uchicago.edu (Keane Arase) writes >> >> The laser beam will spread 3.5 feet in 1000 ft (1/5 mile!) and register >> your speed in 1/4 to 1/2 second. (I know *I* don't have reaction > >A laser will spread 3.5 feet in 1/5 mile? >Are you sure? >That doesn't sound like coherent light. Whether a laser spreads out or not is irrelevent. Coherent light refers to the wavelengths and wavepatterns. Think of how they are trying to get fusion reactors developed. They have huge lasers which they then focus through a lens onto a tiny pinpoint, hoping to force enough energy onto that pinpoint to force it to implode and fuse, thereby releasing energy. JB -- Jonathan Bayer Beware: The light at the end of the Intelligent Software Products, Inc. tunnel may be an oncoming dragon 19 Virginia Ave. ...uunet!ispi!jbayer Rockville Centre, NY 11570 (516) 766-2867 jbayer@ispi.UUCP
sukenick@ccnysci.UUCP (SYG) (02/24/89)
>> A laser will spread 3.5 feet in 1/5 mile? That doesn't sound like coherent light. > >Beware the trap of incorrect terminology! There is a great difference indeed! >between COHERENT light and COLLIMATED light. A laser is pretty much >coherent light in that the spectrum of output is nearly delta-function like. >However, LED's are also nearly coherent in that their output is at nearly Ahem! No - No: MONONCHROMATIC: narrow frequency range ( The minimum width of a line depends upon temperature, etc. (doppler shift uncertainty principle, etc) Also lasers may lase at different frequencies at the same time other than the advertised line (harmonics, etc) and don't forget dye lasers which have a BIG wavelength spread when used without a grating...... COHERENT: All them photons are in phase with each other for a while in and as the stuff leaves the laser (as in: when Bertha jumps up, Fred jumps up at the same time) Thats what them fangled lasers do. (then there's spatial & there's temporal coherence, let them doggies lie for now) ) Collimated: the output is directional, and there is no such thing as perfectly collimated... (Dr. Heisenberg wouldn't let you get them perfectly straight and even if you did your best, the photons'll feud amongst themselves and spread out) LEDs are pretty much monochromatic but not very coherent at all.....
kean@tank.uchicago.edu (Keane Arase) (02/24/89)
[I've directed all follow-ups to rec.autos] In article <3930@ttidca.TTI.COM> hollombe@ttidcb.tti.com (The Polymath) writes: > >Any way you look at it, 17.5 ft of spread per mile isn't even in the ball >park. > Okay, I'm the one who reported it would spread 3.5 feet in 1000ft. From Autoweek, 12/12/88: David Williams, president of the Littleton, Colo., company that also makes other lasers for uses such as underwater surveys, said that the four pound device, which is seven inches long, uses an invisible laser beam and is aimed with a scope. It is fired like a pistol and registers the speed of it's target in 1/4 to 1/2 second. The device, which has not been given a formal name, has a range of 1500ft, although that may be expanded to 2000 to 2500 ft. How could a car be targeted at that distance? Williams said one option on the laser gun would be a magnifying scope. The unit is expected to cost about $3,500, roughly $500 more than the most expensive police radar unit and more than twice as much as what most radar units cost. Currently, there are no lasers being used for speed enforcement, although the idea has been around for two decades, said Jerome Dennis, the chief of laser product division of the federal government's Center for Devices and Radiological Health. Dennis said he is not aware of any other devices that shoot lasers at humans, other than those used for medical purposes. He said the games that children play use infrared beams, not lasers. Dennis said before the unit can be tested by police or sold, it must be reviewed by the Center to make certain that it meets safety standards. He said the review HAS NOT YET TAKEN PLACE. (emphasis mine - ka) One attraction of a laser gun would be that it cannot be detected by radar detectors, said Capt. Ken Casperson, the commanding officer of the traffic services division for the Michigan Department of State Police. The radar used by police sends out a radio beam that may cover two or three lanes of highway at 1000 ft. Some of the beam hits the vehicle and bounces back to the radar gun, providing speed of the target. However, the rest of the beam, sometimes known as "scatter," continues down the road where it can be picked up by radar detectors. But there is little scatter with the laser gun, said Williams. He said the laser beam is aimed at one vehicle and at 1000 ft. is ONLY 3.5 FEET WIDE, so most of it would be reflected back to the laser gun. [End excerpt.] -- Keane Arase | Thought for the Day: kean@tank.uchicago.edu | Drive safely, Drive Offensively. syskean@uchimvs1.uchicago.edu | -- An old Chicago rush hour motto :-) * Please file the usual disclaimers here *
w-colinp@microsoft.UUCP (Colin Plumb) (02/24/89)
kluksdah@enuxha.eas.asu.edu (Norman C. Kluksdahl) wrote: > Beware the trap of incorrect terminology! There is a great difference > between COHERENT light and COLLIMATED light. A laser is pretty much > coherent light in that the spectrum of output is nearly delta-function like. > However, LED's are also nearly coherent in that their output is at nearly > a single frequency, discounting some small amount of higher and lower energy > photons arising from trap states within the energy gap and higher-energy > radiative recombinations. Funny, and I was always told that was monochromatic. Coherent also implies in-phase. That's why you can do all the fun interference pattern things with lasers. -- -Colin (uunet!microsoft!w-colinp) "Don't listen to me. I never do."
kluksdah@enuxha.eas.asu.edu (Norman C. Kluksdahl) (02/25/89)
Enough already!!!!!!!!! I've got a full mailbox of flames correcting
my incorrect correction!!!!
Let's see you do better at 2 a.m. without heavy doses of caffeine!!!!
I learned my lesson--check twice before posting!
'nuff said.
Norman Kluksdahl Arizona State University
..ncar!noao!asuvax!enuxha!kluksdah
standard disclaimer implied
Useful criticism always appreciated. Senseless flames always discarded.w-colinp@microsoft.UUCP (Colin Plumb) (02/25/89)
sukenick@ccnysci.UUCP (SYG) wrote: > Collimated: the output is directional, and there is no such thing as perfectly > collimated... (Dr. Heisenberg wouldn't let you get them perfectly > straight and even if you did your best, the photons'll feud > amongst themselves and spread out) Um.. don't I remember someone cracked this problem? Instead of a (statistical) normal distribution of intensity as you get away from the axis, someone used a Bessel function or something. Less than 10% efficiency, but then you can send it literally forever. If the light from the laser has a cross-sectional intensity distribution D, and 1m further out it has diffused to f(D), they solved f(X) = X. Doing it along the axis of beam propagation produces solitons - very useful for transatlantic fibres! -- -Colin (uunet!microsoft!w-colinp) "Don't listen to me. I never do."
siegman@sierra.Stanford.EDU (Anthony E. Siegman) (03/19/89)
Re: The safety of a police laser radar Laser surveying instruments that are in essence laser radars or lidars have been in common use for some time, and do not pose any eye (or other) safety problems. Re: Coherence and beam spreading Laser beams must be characterized both by their _temporal_ coherence, which means in essence how monochromatic, or spectrally pure, or "single frequency" the laser output is; and by their _spatial_ coherence, which means how much coherence there is between the optical signal, or optical wave, at different pairs of points across the output beam, or the output aperture, of the laser device. The two types of coherence are more or less independent of each other. A laser beam with high spatial coherence can be transmitted as a very narrow "diffraction-limited" beam, or focused into a very narrow spot, on the order of a single wavelength of light in diameter, using suitable optics. The smaller the starting diameter of a collimated beam at the transmitting aperture, the more rapidly it spreads in the "far field", which is why you use a large transmitting telescope if you want the smallest spreading anlge in the far field.