EODSarge
Well-known member
For what it's worth- here's my take on explaining how radar and lidar work. I'm not an electronics guru; while I have a ham radio license, I've only got a hobbyist's understanding of electronics and radio theory. I'm approaching this from the law enforcement angle as a more practical, rather than theoretical, explanation. I'm certified in my state as a general law enforcement instructor and hold specialty instructor certificates in, among other things, speed detection devices to include radar and lidar; and I've taught a fair percentage of the law enforcement in my corner of the state as well as certified others as instructors. This post is condensed, in fact, from my lesson plans for both. This post will cover lidar.
Some in law enforcement may become agitated at seeing another cop explaining to the public how speed detection works. Really, there's nothing here you can't get from other websites or NHTSA itself. There's no "top secrets" being given away.
LIDAR
LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Lidar stands for LIght Detection And Ranging. Lidar instruments are designed to measure target vehicle speeds using laser light, a part of the electromagnetic energy spectrum similar to sound, radio, and microwave energies. Laser light is somewhat different from other types of light, however.
In its simplest form, laser energy is produced by sandwiching a piece of active material, known as a laser medium, between two mirros. The laser medium is put in an excited state by an external energy source. When another light source is directed at the laser medium, the excited material is stimulated to release its own light energy. This energy reflects back and forth between the mirrors, increasing in intensity, until it is strong enough to exit the end mirror as a laser beam. The laser light itself is monochromatic- that is, of one color or frequency- and coherent- that is, all of the light rays are traveling in the same direction. Because of this fact, laser light spreads out much more slowly than light from a light bulb. A perfect laser would produce a beam that did not spread at all in a perfect vacuum. Lidar beams are approximately 3 to 4 feet wide at 1000 feet. Laser light can be produced by many different materials (solids, liquids, gasses) and different intensities and wavelengths. Different materials are used for visible laser pointers, industrial and military lasers, communications lasers, and the infrared laser used in police Lidar.
Laser light can be reflected, refracted, or absorbed. The Lidar’s reflective capability is influenced by the color of the target vehicle. Generally, lighter colors reflect more of the signal than darker colors. A darker colored vehicle’s ability to reflect the signal may affect the operational range; however, it will not affect the speed or accuracy of the Lidar unit. The relative size of the target has no affect on the Lidar unit. Rain or fog may scatter a portion of the Lidar signal, reducing its operational range. Very hot weather may result in mirages caused by heat waves, which may also distort the Lidar signal and reduce its effective range. Finally, Lidar signals may be absorbed by some types of materials or surfaces, allowing less energy to be reflected by that object. Again, this absorption only affects the range of the unit, not its accuracy.
This is because light waves are electromagnetic waves- just like AM and FM radio signals, microwaves, radar, and X-rays. Electromagnetic waves can be reflected off of objects; refracted through them- like sticking a pencil in an aquarium; the pencil appears broken because the water bends the light waves more than the air- or absorbed. A green t-shirt is green because the material of the shirt absorbs most of the colors of light but reflects the green light at the viewer. All electromagnetic waves share three properties that are connected by a simple equation.
1) Speed- all electromagnetic waves travel at the speed of light, or 186,000 miles per second.
2) Frequency- if we could see these waves traveling through the air, they would look like ocean waves:
If we counted the number of waves that passed us in one second, that would be the wave's frequency. Frequency is measured in cycles per second, or Hertz (Hz).
3) Wavelength- If we measured the distance from the peak of one wave to the peak of the next, or the start of a peak to the end of the valley, this distance would be the wavelength.
The formula that relates these three things together is frequency x wavelength = speed of light; or FxW=c. Since the speed of light is a constant, if the frequency goes up, the wavelength must get shorter, and vice versa. You can see in this illustration that as we go from 1 Hz to 2 Hz to 4 Hz, the distance between peaks gets shorter.
Lidar uses infrared lasers at about 330 terahertz, or 330 trillion cycles per second. It uses infrared, which is invisible to the human eye, because... well, imagine seeing a guy on an overpass pointing something that looks like a scope at you and seeing a red dot on your chest!
Another thing to understand is that laser only sees that portion of the signal that is directed straight back at the unit. If the vehicle is headed straight for the unit, it will show us the vehicle's true speed. However, the greater the angle of the unit to the vehicle, the lower the speed the lidar will show. For example, if you're walking up a flight of stairs whose steps are one foot tall and one foot deep (making a 45 degree angle- just go with it), your net motion is diagonally up the stairs, but we can break that motion into a vertical part and a horizontal part. If you're zooming up the stairs at 50mph, you're also going 25mph up and 25mph horizontally. If the lidar is placed so it's shooting horizontally, it will only see that motion coming straight at it, or horizontally; and will only display 25 mph. The same thing happens with a lidar unit that's at an angle to the road- the car is doing 50mph, but only 25mph of that motion is straight at the unit.
This is called the cosine effect. In lidar, which is always operated in stationary mode where the unit is sitting still, it will always be in the favor of the motorist; because the greater the angle, the less the read speed. It doesn't become an issue- more than tenths of a mile per hour- until the angle exceeds 10 degrees. So, as long as the lidar is pointed straight down the road, and is no more than 10 feet off the road for every 100 feet down the road, the reading is good.
The narrow Lidar beam allows the instrument to be operated with pinpoint accuracy in selecting specific vehicles on crowded roadways. Most if not all Lidar units employ some sort of “heads-up display”, or HUD, to aid in targeting.
Lidar does not use the doppler principle, as radar does. Instead, it relies on the formula speed=distance/time, or s=d/t. The lidar unit sends out a series of laser pulses and is capable of identifying each pulse. It sends out a pulse and measures the amount of time it takes for that pulse to hit the target and return. If it takes 50 nanoseconds to hit the target and come back, it knows that it took 25 nanoseconds (ns) to reach the target and 25 ns to return. At the speed of light, that means that 1 ns roughly equals 1 foot; so it knows the target is 25 feet away. If the next pulse comes back in 48 ns, and was fired one second after the first pulse, it knows that the target covered one foot in that one second, and is traveling at 1 foot per second. The lidar sends out many pulses per second and measures the changes in distance during that time period to determine the vehicle's speed.
So, what if the first pulses hit the grille, one pulse hits the windshield, and the next pulses hit the grille again? The distance changed, won't that change the speed calculated? Normally it would, but the lidar employs an error checking routine called the sum of least squares equation. basically, it "plots" the readings and checks to ensure they follow a smooth line. Any readings that don't are thrown out.
Lidar also requires a tracking history as radar does. The operator must first make a visual estimate of the vehicle's speed, then trigger the laser while keeping the hud centered on a point on the vehicle. Some lidars include a telescopic monocular that makes this targeting even easier. When the lidar is getting good pulses, it will sound a confirmation tone and display the vehicle's speed and distance in the hud. The operator then verifies that the speed matches his visual estimate.
Lidar is less likely to be influenced by outside interference. It is obviously not subject to the problems encountered with instruments that allow moving mode operations. The Lidar instrument itself may be subject to radio frequency interference (RFI) and should be equipped with an RFI indicator. Generally, the RFI signals are weak and are ignored when the Lidar instrument received a stronger signal reflected from the target vehicle.
Random RFI: The patrol vehicle itself may contain several sources of RFI that may result in anomalous speed readings. These include the vehicle’s computerized electronic ignition, radios, personal electronic devices, or transmission components. Avoid transmitting on vehicle radios while operating Lidar and develop a good tracking history.
Lighting Devices RFI: Certain types of lighting equipment such as mercury vapor, neon, or fluorescent lights are capable of producing RFI. Avoid areas where this type of interference is found and develop a good tracking history.
Electrical Lines RFI: High voltage electrical lines, electrical transformers, or electrical substations may produce RFI. Avoid areas where this type of interference is found and develop a good tracking history.
Windshield Obstruction: Because of the shape and composition of many windshields, it is suggested that as a general rule that the Lidar instrument not be used through the windshield. It won’t affect the accuracy of the instrument, but it may reduce the operational range. Some Lidar units offer an “Obstructed” or “Inclement Weather” mode that ignores all readings made within 10 feet of the instrument, effectively ignoring the windshield; however, the range may still be affected.
Weather: Although the laser emissions used by Lidar instruments are outside the visible light spectrum, they are close enough to be affected in the same way that light is. Rain, smoke, fog, and dust will reduce the operational range of the instrument; and if dense enough, may prevent its operation.
Low Voltage: When a low voltage situation is experienced, the Lidar will be disabled in accordance with manufacturer’s specifications. If this is experienced, the operator should check the power source. In the absence of any loose connections or low batteries, the Lidar instrument should be returned for service and repair.
Jamming and detection of Laser
Everyone wants to avoid a speeding ticket, and many devices have been sold trying to capitalize on this fact. Jamming devices are attempts to create false or distorted Laser signals. They will not totally obscure the vehicle to lidar; but they may reduce the range at which the vehicle can be detected. Such jammers are not illegal under Federal law.
Other techniques for passively jamming Laser are out there; but like passive radar jammers, aren't very effective. There used to be a brand of car wax sold that advertised that it would make your car invisible; it's no longer available- probably because it didn't work. In fact, by making the car shinier, it probably increased the range at which the car was detectable.
Many different methods of detecting Laser exist, as well. Flashing headlights or using CB radio to warn of a police Laser unit ahead are the oldest methods. Electronic Laser detectors have been on the market for many years, gradually increasing in complexity and sophistication. However, to cover all of the areas in which a laser may be targeted requires sensors to be place in a lot of locations on the vehicle, increasing the cost of the unit as well as its installation. Human reaction times being what they are, most likely, by the time the unit displays a warning, the vehicle's speed has already been read.
Proper use of a Laser unit- not activating it to obtain a reading until tracking history is established, and proper site selection - lessen the effectiveness of Laser detectors.
In any case, all these devices and techniques accomplish is that the motorist slows down- which is what we are trying to accomplish in the first place. So let them spend their money- the result is the same.
Some in law enforcement may become agitated at seeing another cop explaining to the public how speed detection works. Really, there's nothing here you can't get from other websites or NHTSA itself. There's no "top secrets" being given away.
LIDAR
LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Lidar stands for LIght Detection And Ranging. Lidar instruments are designed to measure target vehicle speeds using laser light, a part of the electromagnetic energy spectrum similar to sound, radio, and microwave energies. Laser light is somewhat different from other types of light, however.
In its simplest form, laser energy is produced by sandwiching a piece of active material, known as a laser medium, between two mirros. The laser medium is put in an excited state by an external energy source. When another light source is directed at the laser medium, the excited material is stimulated to release its own light energy. This energy reflects back and forth between the mirrors, increasing in intensity, until it is strong enough to exit the end mirror as a laser beam. The laser light itself is monochromatic- that is, of one color or frequency- and coherent- that is, all of the light rays are traveling in the same direction. Because of this fact, laser light spreads out much more slowly than light from a light bulb. A perfect laser would produce a beam that did not spread at all in a perfect vacuum. Lidar beams are approximately 3 to 4 feet wide at 1000 feet. Laser light can be produced by many different materials (solids, liquids, gasses) and different intensities and wavelengths. Different materials are used for visible laser pointers, industrial and military lasers, communications lasers, and the infrared laser used in police Lidar.
Laser light can be reflected, refracted, or absorbed. The Lidar’s reflective capability is influenced by the color of the target vehicle. Generally, lighter colors reflect more of the signal than darker colors. A darker colored vehicle’s ability to reflect the signal may affect the operational range; however, it will not affect the speed or accuracy of the Lidar unit. The relative size of the target has no affect on the Lidar unit. Rain or fog may scatter a portion of the Lidar signal, reducing its operational range. Very hot weather may result in mirages caused by heat waves, which may also distort the Lidar signal and reduce its effective range. Finally, Lidar signals may be absorbed by some types of materials or surfaces, allowing less energy to be reflected by that object. Again, this absorption only affects the range of the unit, not its accuracy.
This is because light waves are electromagnetic waves- just like AM and FM radio signals, microwaves, radar, and X-rays. Electromagnetic waves can be reflected off of objects; refracted through them- like sticking a pencil in an aquarium; the pencil appears broken because the water bends the light waves more than the air- or absorbed. A green t-shirt is green because the material of the shirt absorbs most of the colors of light but reflects the green light at the viewer. All electromagnetic waves share three properties that are connected by a simple equation.
1) Speed- all electromagnetic waves travel at the speed of light, or 186,000 miles per second.
2) Frequency- if we could see these waves traveling through the air, they would look like ocean waves:
If we counted the number of waves that passed us in one second, that would be the wave's frequency. Frequency is measured in cycles per second, or Hertz (Hz).
3) Wavelength- If we measured the distance from the peak of one wave to the peak of the next, or the start of a peak to the end of the valley, this distance would be the wavelength.
The formula that relates these three things together is frequency x wavelength = speed of light; or FxW=c. Since the speed of light is a constant, if the frequency goes up, the wavelength must get shorter, and vice versa. You can see in this illustration that as we go from 1 Hz to 2 Hz to 4 Hz, the distance between peaks gets shorter.
Lidar uses infrared lasers at about 330 terahertz, or 330 trillion cycles per second. It uses infrared, which is invisible to the human eye, because... well, imagine seeing a guy on an overpass pointing something that looks like a scope at you and seeing a red dot on your chest!
Another thing to understand is that laser only sees that portion of the signal that is directed straight back at the unit. If the vehicle is headed straight for the unit, it will show us the vehicle's true speed. However, the greater the angle of the unit to the vehicle, the lower the speed the lidar will show. For example, if you're walking up a flight of stairs whose steps are one foot tall and one foot deep (making a 45 degree angle- just go with it), your net motion is diagonally up the stairs, but we can break that motion into a vertical part and a horizontal part. If you're zooming up the stairs at 50mph, you're also going 25mph up and 25mph horizontally. If the lidar is placed so it's shooting horizontally, it will only see that motion coming straight at it, or horizontally; and will only display 25 mph. The same thing happens with a lidar unit that's at an angle to the road- the car is doing 50mph, but only 25mph of that motion is straight at the unit.
This is called the cosine effect. In lidar, which is always operated in stationary mode where the unit is sitting still, it will always be in the favor of the motorist; because the greater the angle, the less the read speed. It doesn't become an issue- more than tenths of a mile per hour- until the angle exceeds 10 degrees. So, as long as the lidar is pointed straight down the road, and is no more than 10 feet off the road for every 100 feet down the road, the reading is good.
The narrow Lidar beam allows the instrument to be operated with pinpoint accuracy in selecting specific vehicles on crowded roadways. Most if not all Lidar units employ some sort of “heads-up display”, or HUD, to aid in targeting.
Lidar does not use the doppler principle, as radar does. Instead, it relies on the formula speed=distance/time, or s=d/t. The lidar unit sends out a series of laser pulses and is capable of identifying each pulse. It sends out a pulse and measures the amount of time it takes for that pulse to hit the target and return. If it takes 50 nanoseconds to hit the target and come back, it knows that it took 25 nanoseconds (ns) to reach the target and 25 ns to return. At the speed of light, that means that 1 ns roughly equals 1 foot; so it knows the target is 25 feet away. If the next pulse comes back in 48 ns, and was fired one second after the first pulse, it knows that the target covered one foot in that one second, and is traveling at 1 foot per second. The lidar sends out many pulses per second and measures the changes in distance during that time period to determine the vehicle's speed.
So, what if the first pulses hit the grille, one pulse hits the windshield, and the next pulses hit the grille again? The distance changed, won't that change the speed calculated? Normally it would, but the lidar employs an error checking routine called the sum of least squares equation. basically, it "plots" the readings and checks to ensure they follow a smooth line. Any readings that don't are thrown out.
Lidar also requires a tracking history as radar does. The operator must first make a visual estimate of the vehicle's speed, then trigger the laser while keeping the hud centered on a point on the vehicle. Some lidars include a telescopic monocular that makes this targeting even easier. When the lidar is getting good pulses, it will sound a confirmation tone and display the vehicle's speed and distance in the hud. The operator then verifies that the speed matches his visual estimate.
Lidar is less likely to be influenced by outside interference. It is obviously not subject to the problems encountered with instruments that allow moving mode operations. The Lidar instrument itself may be subject to radio frequency interference (RFI) and should be equipped with an RFI indicator. Generally, the RFI signals are weak and are ignored when the Lidar instrument received a stronger signal reflected from the target vehicle.
Random RFI: The patrol vehicle itself may contain several sources of RFI that may result in anomalous speed readings. These include the vehicle’s computerized electronic ignition, radios, personal electronic devices, or transmission components. Avoid transmitting on vehicle radios while operating Lidar and develop a good tracking history.
Lighting Devices RFI: Certain types of lighting equipment such as mercury vapor, neon, or fluorescent lights are capable of producing RFI. Avoid areas where this type of interference is found and develop a good tracking history.
Electrical Lines RFI: High voltage electrical lines, electrical transformers, or electrical substations may produce RFI. Avoid areas where this type of interference is found and develop a good tracking history.
Windshield Obstruction: Because of the shape and composition of many windshields, it is suggested that as a general rule that the Lidar instrument not be used through the windshield. It won’t affect the accuracy of the instrument, but it may reduce the operational range. Some Lidar units offer an “Obstructed” or “Inclement Weather” mode that ignores all readings made within 10 feet of the instrument, effectively ignoring the windshield; however, the range may still be affected.
Weather: Although the laser emissions used by Lidar instruments are outside the visible light spectrum, they are close enough to be affected in the same way that light is. Rain, smoke, fog, and dust will reduce the operational range of the instrument; and if dense enough, may prevent its operation.
Low Voltage: When a low voltage situation is experienced, the Lidar will be disabled in accordance with manufacturer’s specifications. If this is experienced, the operator should check the power source. In the absence of any loose connections or low batteries, the Lidar instrument should be returned for service and repair.
Jamming and detection of Laser
Everyone wants to avoid a speeding ticket, and many devices have been sold trying to capitalize on this fact. Jamming devices are attempts to create false or distorted Laser signals. They will not totally obscure the vehicle to lidar; but they may reduce the range at which the vehicle can be detected. Such jammers are not illegal under Federal law.
Other techniques for passively jamming Laser are out there; but like passive radar jammers, aren't very effective. There used to be a brand of car wax sold that advertised that it would make your car invisible; it's no longer available- probably because it didn't work. In fact, by making the car shinier, it probably increased the range at which the car was detectable.
Many different methods of detecting Laser exist, as well. Flashing headlights or using CB radio to warn of a police Laser unit ahead are the oldest methods. Electronic Laser detectors have been on the market for many years, gradually increasing in complexity and sophistication. However, to cover all of the areas in which a laser may be targeted requires sensors to be place in a lot of locations on the vehicle, increasing the cost of the unit as well as its installation. Human reaction times being what they are, most likely, by the time the unit displays a warning, the vehicle's speed has already been read.
Proper use of a Laser unit- not activating it to obtain a reading until tracking history is established, and proper site selection - lessen the effectiveness of Laser detectors.
In any case, all these devices and techniques accomplish is that the motorist slows down- which is what we are trying to accomplish in the first place. So let them spend their money- the result is the same.
