Nowadays lasers are no longer something that can only be
found in research laboratories, but are commonly encountered
in our everyday life: from general light sources to high-tech
consumables, like computers and VCD/DVDs. Because of the coherence
and small divergence, lasers are now also used in speed detection.
Speed detecting "Laser Gun" has been used around
the world for some years, and in May 1994, the Hong Kong Police
has also started using Laser Gun for speed enforcement. In
this article, we will discuss the principle and the practical
use of Laser Gun in speed detection.
The word ¡§LASER¡¨ stands for ¡§Light Amplification by Simulated
Emission of Radiation¡¨. In its simplest form, a laser is made
by placing a piece of active material, the lasing medium,
between two reflective mirrors. The two mirrors and the lasing
medium form an optical resonator (cavity) that allows the
generation of the laser light. Atoms in the lasing medium
are excited into a higher energy state by an external energy
source. The stored energy in the excited atoms is released
as light when they are triggered or by light emitted from
other atoms in the active medium. This is called stimulated
emission. The figure below [Fig. 1] shows a schematic diagram
for a laser cavity. Two mirrors are positioned at a fixed
distance apart such that standing wave can be formed when
light bounces back and forth between the mirrors. With this
setup, atoms in the active medium can be triggered to release
the store energy simultaneously giving out an intense and
coherent laser light. In order to allow light to come out
of the cavity, one of the mirrors is made to reflect partially
so that some of the lasing light can leak out of the mirror
for external use. The resonance cavity does not have to be
large, and in fact, it can be as small as a millimeter in
length. That is partially the reason why commercial laser
pointers are so small. The light source used in the Laser
Gun for speed detection is made from semiconductor material
and is known as laser diode.
(Fig. 1: Lasing cavity)
(Fig. 2: Laser Gun used by the Hong Kong
Police)
The Laser Gun used by the Hong Kong Police is shown above
[Fig. 2]. A laser diode inside the Laser Gun emits infrared
light of 900nm when triggered. The emitted light is focused
by the transmitting lens to form a narrow beam. The beam has
a 3 milliradians divergence giving the Laser Gun the pinpoint
targeting ability even at far distances. Since the emitted
light is in the infrared range of the electromagnetic spectrum,
it is invisible to human eyes and will not be a distraction
to drivers. Light signals reflected/scattered back from a
moving target are picked up by the receiving lens right below
the transmitting lens. One very important setup of the Laser
Gun is that the receiver can screen out light of other wavelengths
except the one emitted by the laser diode so that stray light
from other sources will not interfere with the speed detection.
The sighting scope mounted on the top of the Laser Gun is
for aiming purposes. Figure below [Fig. 3] shows a schematic
diagram of the operation of the Laser Gun.
(Fig.3: Operation of the Laser Gun)
The speed detection Laser Gun utilizes two simple facts in
Physics: 1) speed of light is much faster than the speed of
vehicle, and 2) distance can be found by the distance-time
formula we learn in HKCE mechanics. The Laser Gun determines
speed by measuring the time of flight of very short pulse
emitted by the Laser gun. This method is different from another
well known speed detection Radar that is based on the Doppler
frequency shift. Consider one laser pulse, the time of flight
for the pulse to travel to and reflect back from the target
vehicle can be measured by the time difference between the
pulse leaving the Laser and the time the returned pulse detected
by the receiver. Since the speed of light is constant (about
in air), the distance of the target from the Laser Gun can
be calculated using the distance-time formula. Now, if the
Laser Gun sends a series of pulses (as many as sixty pulses)
towards a moving target, the positions of the target at different
times can be mapped out. With this information, the speed
of the target can be calculated by taking ratio of the change
of distance over the change of time or from the slope of a
plot of distance vs time as shown in the figure below [Fig.
4]. Since the speed of light is much faster than that of automobiles,
the measurement can be finished within a very short time,
typically around 1/3 second, fast enough for almost instantaneous
speed measurement. In order to produce accurate speed results,
the Laser Gun has built in sophisticated software to perform
the speed calculation and to eliminate faulty data. Thus,
the software can be considered as the ¡§brain¡¨ of the Laser
Gun and dictates the performance of the Laser Gun.
(Fig.4: Distance vs. time plot)
In speed detection operation, the Laser Gun must be set up
with a clear line-of-sight to the targeted section along the
traffic. If there are obstacles, such as trees, telephone
poles or road signs, laser pulses could be reflected/scattered
from these obstacles causing confusions to the Laser Gun.
In this case, the Laser Gun will display an error message.
(Fig. 5: Clear Line-of-sight)
In addition, since it is impossible for a police officer
to aim at vehicles straight ahead, there is an angle between
the line-of-sight of the Laser Gun and the traveling direction
of targeted vehicle. Thus, the speed measured by the Laser
Gun will be less than the target's ¡§true¡¨ speed. The larger
the angle the lower the measured speed value. This phenomenon
is known as the ¡§Cosine Effect¡¨. Therefore, it is important
to keep this angle small so that the speed measured by the
Laser Gun is closer to the true speed.
Speeding is not the only cause of accidence in Hong Kong.
Statistics revealed that ¡§driving too close¡¨ to the front
vehicle was the main contributory factor in traffic accidents
in the years 2000 and 2001. Since the Laser Gun measures distance
directly, it is feasible to measure the distance between two
vehicles traveling in tandem using the ¡§Distance Between
Cars¡¨ (DBC) mode. This DBC mode is now integrated in recent
Laser Guns to enforce the law against driving too close to
front vehicle.
To conclude, Laser Gun is a very useful and accurate instrument
for measuring speed of vehicle. However, the usage of the
Laser Gun and other speed enforcement instruments is only
a preventive measure. The most effective method is to alert
drivers to observe the traffic regulations and to drive safely.
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