Traffic control by digital imaging cameras - Emerging Trends in Image Processing, Computer Vision, and Pattern Recognition - page 191

decrease their inductance. The decreased inductance actuates the electronics unit output relay
or solid-state optically isolated output, which sends a pulse to the traffic signal controller sig-
nifying the passage or presence of a vehicle.
4.2 Microwave Radar
Radar is an object-detection system which uses radio waves to determine the range, direction,
for speed of objects. It can be used to detect motor vehicles. The radar antenna transmits pulses
of radio waves which bounce of any object in their path. The object returns a small part of the
wave's energy to the receiver antenna which is usually located at the same site as the transmit-
ter.
The basic use of the traffic radars is the measurement of the speed of the vehicle. Traic
radar calculates speed from the reflections it receives. It uses a phenomenon of physics known
as the Doppler principle. The classic example is heard along roads. As the car approaches, you
hear the high pitch sound of the car horn. The instant the car passes and begins to move away,
you hear a lower pitch sound. The car itself is making the same sound for both coming and
going, but for a stationary listener, the speed of the car adds to the pitch of its sound as it ap-
proaches, and subtracts as it departs. This change from true pitch is called the Doppler shift,
and the magnitude of the change depends upon the speed of the car. The Radar compares the
shifted frequency of the reflection to the original frequency of the beam it sent out and from
the difference it calculates speed. Figure 3 shows how microwave radar detects the speed of
the vehicle.
FIGURE 3 Microwave radar.
4.3 Infrared Sensors
Active and passive infrared sensors are manufactured for traffic flow monitoring applications.
Active infrared sensors illuminate detection zones with low-power infrared energy transmit-
ted by laser diodes operating in the near infrared region of the electromagnetic spectrum. A
portion of the transmited energy is relected or scatered by vehicles back toward the sensor.
The diodes operated in the near infrared spectrum at 880 nm. The signal modulation preven-
ted interference from other sources of infrared energy, including sunlight. Two transmiter-
receiver systems measured the vehicle speed and one measured the vehicle height. When
trucks susceptible to rollover or jackknifing were encountered, flashers were activated to warn
drivers to reduce speed [ 5 ] .
Passive sensors do not transmit energy; they detect energy from two sources:
1. Energy emited from vehicles, road surfaces, and other objects in their ield-of-view.
2. Energy emited by the atmosphere and relected by vehicles, road surfaces, or other objects
into the sensor aperture