Text 395, 154 rader
Skriven 2004-08-24 13:44:50 av mark lewis (1:3634/12)
Kommentar till text 394 av Michiel van der Vlist (1:3634/12)
Ärende: laser/lidar stuff (was: Fido on the move via cell phone.)
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MvdV>> Maybe the cameras in North Amrica use infrared. Then it just
MvdV>> /may/ work/. In theory. I doubt it though. I suspect Roy is
MvdV>> feeding us urban legend too.
lidar is the infrared laser stuff... i thought i'd add some more to this thread
as well as changing the subject line... this one should thread (not subject
line threading!) in /after/ my first response to you...
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Contrary to what laser gun manufacturers would have us believe, the makers of
laser detectors claim the infrared is detectable. It only makes sense, they
say, that a similar receiver (to the gun that sends out the light beam) can be
built. The manufacturers also claim the beam has a diameter of approximately
four feet at 1,000 feet. The beam, however, has been proven to be wider, due to
atmospheric conditions. Warm and cold air pockets also can affect the beam. All
this makes it possible for a laser receiver to detect the infrared beam and to
provide warning.
A laser gun's beam is detectable from one to two miles away from the gun, while
the gun only measures speed with a range of 1,100 to 1,500 feet. The difference
in distance will provide the laser detector user plenty of notice.
The laser gun must be operated using a method of support to hold the gun steady
to properly "sight" a target. If the gun moves, the measurement will not be
accurate. The gun cannot be operated through a window, either. The glass will
impair the energy of the infrared beam and degrade its performance. Too much
sand, dust, snow, fog or other atmospheric changes also will affect the gun.
The durability of these guns is also in question.
In a regular situation, a laser gun operator doesn't have a "target" or know
which vehicle is equipped with a laser detector. The officer merely samples
traffic, taking speed readings of approaching cars. When a "target" vehicle is
clocked by the laser gun operator, advance warning at a range of one to two
miles will be given, in a real life situation. During testing, an "instant-on"
test has little validity. Using a radar detector regularly, you come to know
what "instant-on" is like.
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Well, it seems that although LIDAR is good at measuring speed and distance, the
guns are a bit of a pain to use. Fog, rain, dust, vehicle halogen headlights,
movement, (just to mention a few) make the guns more difficult to use than
radar. They can't be used inside a nice warm police car either. They won't work
through glass without error, and all that eye- damaging IR inside the car will
not be good for those in the vehicle. When used for distances over 400 yards
they usually have to be tripod mounted or steadied to stop beam shake and
instability. With handheld laser, a 1 degree movement of the operator's hand
moves the beam 6-7ft for every 100 yards the beam travels from the gun.
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The shape of the laser beam is very different from that of radar. First of all
it is not a pencil beam, as the operators believe, and many others would try to
have you believe. The beam starts from a small point (1 inch dia) at the laser
gun and fans out the further it travels from the source. The beam is
approximately 12 inches diameter at 100 yards range, 3 ft in diameter at 1000
ft, and increases in width and height the further it travels. (6 ft wide at
2000 ft distance)
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The laser beam is not visible to the eye. The beam may reflect off any
stationary or moving object and "may" be detectable by a laser detector
(sometimes). The laser will not go around corners or over hills. It does not
bend or spray the countryside like radar. The laser beam is only detectable
while the trigger is being depressed, and the detecting device is in a straight
line to the laser beam. If a vehicle is targeted at very close range, (1000 ft)
very little of the laser beam will travel beyond that vehicle.
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Laser can only read/measure the speed of an approaching or receding vehicle.
The vehicle must be driven directly into or away from the laser gun/beam.
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LIDAR
Laser speed measuring is called LIDAR (LIght Detection And Ranging) It is not
RADAR. Radar guns use microwaves transmitted to, and reflected from, a target
vehicle. LIDAR uses Infrared (3000 Ghz) pulses to measure the distance to the
moving vehicle. The effective range of handheld laser is up to about 1500 ft.
Beyond this point, the effectiveness begins to diminish due to the difficulty
in obtaining a stable reading, as a result of beam movement. The movement of
the laser beam at 1500 ft is quite substantial for very little movement of the
laser gun.
ERRORS
Laser guns are subject to the cosine effect, just like RADAR. That is, if a
vehicle comes into the beam at an angle, the speed displayed will be lower than
the true speed. LIDAR can't be used at any angle to traffic due to this effect.
It is a down-the-road device, and cannot be used across the road like the
camera radar. The Laser should not be used on sweeping bends, at corners or on
undulating roads. The operating site should not be elevated, as it is possible
to shoot over the tops of approaching traffic, targeting vehicles and trucks in
the background. It should only be used on long straight sections or highway.
Target error will occur where the Laser beam has multiple targets in the one
straight line!
THE LIMITATIONS
The laser beam cannot pass through opaque objects; signs, trees, scrubs, grass,
poles, leaves etc. The effective range of laser, and its performance, are
reduced by heavy rain, smoke, fog and any other airborne pollutants. Laser is
difficult to use at night, as halogen headlights and driving lights give off
substantial infrared light and this can reduce the performance of the laser
substantially. High beam or spot/driving lights will blank out most laser guns!
Targeting vehicles at night is difficult, the laser beam is not visible, and
the operator is looking into vehicle headlights.
IN-VEHICLE PROBLEMS
LIDAR cannot operate well through any automotive glass. The glass, and its
angle to the beam, may reduce its effective range. Tinted windscreens block
laser infrared emissions, but they can be received through glass to easily
trigger a laser detector. LIDAR cannot be used from a moving vehicle and
produce accurate speed readings, LIDAR is a stationary handheld device that
must be used outside the vehicle. LIDAR can be mounted on a tripod; this
steadies the beam allowing it to read the speed of vehicles over 4000 ft away.
This method of operation could only be used on long straight open roads with
very light traffic.
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this url, http://www.mr2.com/TEXT/FAQonLidar.html , is full of excellent useful
information... even some stuff on the reflectivity "quotient" used on license
plates... also info on the reflectors and other surfaces on cars...
almost seems to me that one would do good to cut down on their LIDAR
crossection by using "flat" paints instead of glossy/shiny stuff... the above
FAQ also talks about "stealthing" your vehicle... all in all, we can learn a
lot from the US' stealth fighter ;)
this url, http://www.m-99.co.uk/Cheetah_Radar_Speed_Trap_Detec/Camera_Types
/camera_types.html , seems to contain a bit of the info you've been saying but
with some additions as to how it may be thwarted... your statements have been
basically "this is how it is, period" without offering alternatives
until/unless they are pointed out... i guess this is called "debating"??
for more information, google for "lidar radar laser beam diameter distance"
sans quotes and refine from there ;)
)\/(ark
* Origin: (1:3634/12)
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