6.3 Laser Radar Technology

• Library: Defense of Serious Traffic Cases in Virginia (Virginia CLE) (2020 Ed.)

6.3 LASER RADAR TECHNOLOGY

6.301 How It Works.

A. Theory. Lasers (Light Amplification by Stimulated Emissions of Radiation) are extremely pure or "coherent" light waves, similar to only one pure color of light. Note that white light consists of multiple wavelengths or what we perceive as colors with random (noncoherent) phases. Laser radars are sometimes referred to as ladars (LAser Detection And Ranging) or lidars (LIght Detection And Ranging).

Laser units radiate in the upper infrared (IR) band and have extremely narrow beams of 3 or 4 milliradians (0.17E or 0.23E). Laser radars function from a stationary position only (there is no moving mode) and measure approaching and/or receding targets; some also display the target range. Most laser radars can also measure the range of stationary objects. Best performance is for short ranges, typically less than about 700 or 800 feet.

Laser radars transmit pulsed laser light to measure the target range. The time that it takes for a laser-light pulse to travel at the speed of light from the ladar unit to the target and back is used to compute the distance from the unit to the target and back (distance pulse travel equals speed of light multiplied by time). This return trip means that the target range from ladar is really half of this distance (range equals 0.5 multiplied by speed of light multiplied by time). The change in target range over time (1/3 second is typical) equals the target's velocity. Laser radar must transmit a minimum of two pulses and get at least two range measurements at two different times to compute speed. In reality, laser radars transmit tens to hundreds of pulses per second, so this does not present any difficulty.

B. Operation. Ladars have narrow beams and must be aimed very steadily and with care; an operator cannot simply set up the ladar unit and wait for a violator but must aim the ladar at a single target. Ladars use a scope that is similar to a rifle scope or a heads-up display (HUD) as an optical aiming device. A HUD allows the operator to simultaneously observe a target and see target data (speed and/or range) by projecting a circle and target data onto a glass plate that the operator can see through.

C. The Cosine Effect. The cosine angle has the same effect on laser radar as on microwave radar. ⁵

[Page 374]

D. Government Regulations. The FCC regulates radiated emissions from high-speed circuits such as processing circuits inside a ladar, but not light frequencies. The Federal Drug Administration Center for Devices and Radiological Health regulates laser products sold in the United States. Traffic ladars meet the...