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  • Writer's pictureKristjan Salev


Updated: Jun 28, 2022

What does laser mean on a radar detector?

Radar and laser detectors are electronic devices placed in cars thatyk can help detect when the speed of a car is ubeing measured. In addition to radar detection, some of these devices alsotu can detect laser light from the lidar guns, which is why they are also called laser detectors. If both radar and laser are mentioned in the description of the detector, means that the device can detect radio waves generated by the radar gun and light waves generated by the lidar gun.

Although both RADAR and LIDAR (light detection and ranging) guns, which can measure speed, are sensing devices that can detect stationary and moving objects using electromagnetic waves, their difference is due to the electromagnetic waves used by these devices.

How does a police LIDAR gun measure the speed of a car?

As mentioned, both radar and lidar guns use electromagnetic waves to determine the speed of a car. However, while radar guns use radio waves, lidar guns use focused beams of light waves. Radio waves have a much longer wavelength and lower frequency than light waves. That makes them much easier to detect and alert the driver when police radar is nearby. When the police lidar gun is aimed at the target car and emits short pulses of infrared laser light, these light pulses are reflected back. When the light wave is reflected back to the lidar device, its frequency changes depending on the speed and moving direction of the car, that is being targeted. This phenomenon is known as the Doppler effect and is used by all police radar and lidar guns. The Lidar gun instantly calculates the difference between the frequency of the transmitted and received light waves and determines the speed of the moving car.

How an approaching vehicle changes the frequency of the measured laser light?

If you listen carefully, you will notice that when the car moves towards you, it will make a sound high in pitch. However, as the car moves away from you, its sound becomes low in pitch. This is the Doppler effect with sound waves. Sound waves in front of the car are being squashed together when the car is moving towards you, and the wavelength decreases. That also means that the frequency increases because the wavelength of the sound wave is now shorter.

Approaching vehicle changes the frequency of the measured laser light

A similar situation arises when the police measure the speed of an approaching car with a lidar gun. A Lidar gun sends out a beam of laser light at a certain frequency. Some of that light gets reflected from the target car. The device then estimates the difference between the emitted and observed frequencies and calculates the speed of the approaching vehicle. The simplified equation below shows that the Doppler frequency (fd) is proportional to the speed of the moving vehicle (v). The higher the speed of the approaching car, the higher the frequency of the Doppler shift. C in this equation indicates the speed of light, fe is the frequency of the emitted laser light, and fc is the frequency of the observed laser light.

Doppler frequency formula, and speed of the approaching vehicle.

How effective is a laser detector compared to a radar detector?

Since the radar beam diverges from the radar gun to a much greater extent than the laser beam, it is much easier for radar detectors to detect it. Police radars use certain radio frequencies, and radar detectors are very good at detecting the presence of a police speed radar, without the police being able to target them. The radar beam may already be 200 feet (60 m) in diameter at a distance of 1,000 feet (300) from the police radar gun. How much the laser beam diverges, depends on the lidar gun used, but the average is about 3 milliradians (0.17 degrees). By doing a little bit of trigonometry, we can see that at a distance of 1000 ft from the lidar gun, the diameter of the laser beam is only 3 ft (0,9 m).

A laser beam from the lidar gun diverges

At a distance of 500 ft (152 m), it is 1.5 ft (0.45 m) in diameter, and so on. As can be seen, the collimated laser beam diverges significantly less than radio waves. As the collimated laser beam scatters significantly less than radio waves, their early detection is also much more difficult. Usually, when a laser detector responds, it is too late. So laser detectors can certainly warn you right away when the police are measuring the speed, but it must be borne in mind that the warning from the laser detector is often too late, and the speed has already been recorded.

Is a laser radar detector a good investment?

Technology is constantly evolving, and just as the police have better speed measurement equipment today than they did yesterday, radar laser detectors are getting better. The police often use radar guns because the lidar gun must be stationary at the time of measurement and depends on weather conditions, for example.

Whether to use a detector or not depends largely on where you drive the most and what are your driving habits. In heavy urban traffic with many traffic lights, they are of little use. However, they can be most effective if you have to travel a lot on the highway. For a few hundred dollars, which can be the cost of a speed ticket, you can already buy a pretty good high-range detector with Bluetooth connectivity for software upgrade and the ability to pick out the wrong signals.

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