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#'''Receiver and receiver electronics''' — Receivers are made out of several materials. Two common ones are [[silicon|Si]] and [[InGaAs]]. They are made in either PIN diode or [[Avalanche photodiode]] configurations. The sensitivity of the receiver is another parameter that has to be balanced in a LIDAR design. |
#'''Receiver and receiver electronics''' — Receivers are made out of several materials. Two common ones are [[silicon|Si]] and [[InGaAs]]. They are made in either PIN diode or [[Avalanche photodiode]] configurations. The sensitivity of the receiver is another parameter that has to be balanced in a LIDAR design. |
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#'''Position and navigation systems''' — Lidar sensors that are mounted on moving platforms such as airplanes or satellites require instrumentation to determine the absolute position of the sensor. Such devices generally include a [[Global Positioning System]] receiver and an [[Inertial Measurement Unit]] (IMU). |
#'''Position and navigation systems''' — Lidar sensors that are mounted on moving platforms such as airplanes or satellites require instrumentation to determine the absolute position of the sensor. Such devices generally include a [[Global Positioning System]] receiver and an [[Inertial Measurement Unit]] (IMU). |
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==History of LASER== |
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In 1921, Albert Hull invented the magnetron, which produced radio microwaves, and in 1951, Charles Towns started working on the production of stronger microwaves. In 1953, he built the MASER, a device, which produced amplified radio microwaves. Townes felt that this concept would work with light as well, but the technical problems proved more difficult. The on November 11, 1957, a Columbia graduate student by the name of Gordon Gould invented the first laser. This was followed in May 1960 when Theodore Maiman built the first working laser model, using a ruby cylinder. |
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The laser followed naturally from the concepts of radio waves as found in radar. Since the laser could deliver energy with great accuracy, it was used in industry as a welding or cutting tool. It wasn’t long before the medical field too saw a use for lasers, and in 1962, lasers were used for the first time in eye surgery. Today lasers are a common tool in medicine, replacing scalpels in delicate procedures such as the repair of retinas. Lasers are not used commonly for a variety of highly scientific purposes, from making survey measurements for improved highways to detecting surface movements of the earth in order to predict earthquakes. In fact, the most common application of lasers are found in the local supermarket, where they read bar codes on our purchases. |
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Well, now that we know who invented the laser and what some of its uses are, we need to find out what laser light is. Just as the radar operator should be familiar with the properties of radio signals, the laser operator will need to be familiar with the properties of light. Actually, the properties used to describe radio signals are often applicable to light as well, although some will not be relevant to laser for speed-detection use. We now need to cover those properties of light that will help us better understand how a laser works. |
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Light is what enables us to see. We see an object because light from a nearby source reflects off that object, making the object visible, or we may see the light source itself, such as a flaming match or the sun. When light reflects from an object, the light actually scatters in all directions. Our eye captures only a portion of the light, the porting being reflected from that part of the object that we are looking at from our present angle of view. If we change our location, we see the object by means of different rays of light. This example gives us an idea of how light can reflect in may directions from the same point on an object, and will help us better understand reflection and refraction later. |
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Light is composed of waves and manifests properties similar to the waves of microwave radar. Therefore, light will reflect and refract, as do radio waves. Light, however, has two other properties: intensity (brightness) and color. Each color of light has a certain wavelength, measured in nanometers (billionths of a meter.) Since light travels at a speed of 186,000 miles per second (approx 300,000 km/sec,) it travels about one foot (30.5 centimeters) in one billionth of a second. In the same way that the part of the electromagnetic spectrum occupied by the various radio waves is determined by their frequency, the part of this spectrum occupied by the various colors of light is determined by their number of wavelengths per meter. Visible light is found starting at approximately 750nm and ending at approximately 400 nm. Within this range we find the visible colors, which when merged together form the white light we normally see. When a prism is used to separate white light into all its constituent colors, we see the colors of the visible spectrum. We may get the same effect with light passes through water from a hose or through a rain shower, the latter of course giving us the word rainbow. |
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There are other kinds of light that we can’t see. All light waves are part of the spectrum; visible light occupies only a small portion of it. Above the visible light range and having wavelengths of less than 400 nm, we find ultraviolet light, x-rays and gamma rays. Below the visible light range, and having wavelengths greater than 75 nm, we find infrared light and microwaves. |
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This spectrum we have been speaking of is called the electromagnetic spectrum and gives us a relationship of all electromagnetic waves in respect to their wavelengths. Due to the fact that light shares the properties of waves, light will travel indefinitely unless reflected, refracted or absorbed. Even light from a flashlight pointed at the sky will travel though the atmosphere until the water vapor in the atmosphere absorbs it. Absorption takes place at different distances for different wavelengths of light and is somewhat dependent on the light’s intensity – the amount of energy – in terms of both brightness and heat – transferred by the light onto a given area per unit of time. |
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Prior to being absorbed, light may be refracted. An excellent example of the refraction of light can be seen when we look at an underwater object where the light rays bend due to the change in the density of the medium they are traveling through and cause the object to appear in a different place than it really is. |
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Light may also be reflected off an object. Although some of the light will be absorbed by the object, and in some cases transferred through the object (glass), a high percentage of the light will be reflected if the object is very smooth. When light strikes an object, the rough surface of the object will scatter the rays in many directions. Even though the object seams smooth to us, it may have tiny ridges that help scatter the light. When light shines on this type of surface, the light is reflected at many different angles. This is called diffused reflection. But if light strikes a truly smooth flat surface, such as a mirror, the rays stay together better and all the light will reflect at the same angle. Unless our eye is in a position to see the light that is reflected at that particular angle the light will be difficult to see. |
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Things to remember concerning light are that it may be considered to consist of waves and that it is described in terms of wavelength. Also, that because of its wavelike composition, light will be affected by its environment – being reflected, refracted or absorbed. Finally, that the visible light spectrum is very small in comparison to the entire electromagnetic spectrum. With this knowledge, let’s now move on to lasers. |
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LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. |
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==LIDAR – How it works== |
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Now that we have a speed-measuring laser, we need to know that it can determine not only speed but also direction of travel (toward or away from the laser) and distance. In order to understand how the laser makes these determinations, lets look first at the relationship of time and distance. |
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In everyday life we often use time-distance expressions and don’t even know we are doing so. When a speeding ticket is written the driver is charged with exceeding the speed limit by a certain number of miles per hour. This is actually an arithmetic statement that says that the distance in miles is divided by a time (1 hour). |
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So we now have miles/hour – let’s incorporate this into an algebraic equation solving for speed: Speed = Miles/hour |
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If we replace speed with the letter S, miles with the letter D (for distance) and hour with the letter t (for time), we get the basic speed formula: |
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S = D/t |
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We need to start with the three variables in this formula in order to understand the way a laser makes a speed determination. |
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When we point a laser and transmit a beam, infrared light is sent out in a series of predetermined pulses. Each pulse has a set duration and a set interval and is traveling at the set speed of light, or 186,000 miles per second. As we will recall, this speed is equivalent to about a foot every billionth of a second. As the pulse of light is about to leave the laser, it enters a beam splitter; a device which causes a portion of the beam to go in a different direction from the rest of the beam. This second beam will enter a detector at the same time the first beam leaves the starting point, or lens of the laser. When the second beam hits the detector, it starts a timer, and the laser will now determine how long the first beam is gone from the laser. |
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Since our laser beam is covering a foot every billionth of a second (every nanosecond) it is a simple matter of math to determine the distance of a reflecting object from the laser. Let’s say that a single pulse from the laser travels to a car and back in 800 ns (nanoseconds.) Upon return of the pulse to the laser, the timer is shut off and the elapsed time is fed into a processor. The laser then divides the elapsed time of 800 ns total time by 2 – since only a one-way distance is needed – and converts the result 400 ns into an actual distance of 400 feet. |
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To determine the speed a target vehicle is moving, this kind of measurement would have to be repeated to get a new distance at a new time. In laser speed measurement a series of such measurements will be obtained. In our example we will have our laser send out a pulse of light 300 times per second, a rate fast enough to determine the speed of a moving vehicle in about 1/3 of a second, the period required for about 100 pulses. We can see that the vehicle is moving toward the laser, so we would expect the distances to become smaller. |
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Each pulse that is sent out must be converted from a reading in nanoseconds to a distance in feet because the laser operator may stop sending at any time and the last distance calculated, which ever one it turns out to be, is needed. In our example, we’ll assume that the sending period lasts exactly 1/3 of a second and included exactly 100 pulses. The final pulse converts to a distance of, say, 378 feet that, in accordance with our expectations, is smaller than the initial distance of 400 feet. From the decrease in distance the laser will sense the direction of the target vehicle as being toward the laser. Since the vehicle travels 22 feet, i.e., 400 feet – 378 feet, in the 1/3 of a second during which the 100 pulses are being sent out, the laser will calculate the vehicle’s speed to be 66 feet per second, i.e. 3 x 22 feet, which the laser converts to 45 mph. |
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In actuality, the calculations are made using each separate piece of data fed into the processor, so the computation is based on something a bit more complex than simply the difference between two distances over a given period of time, but the result is the same. |
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The laser will compute the movement of the vehicle as CHANGE IN TIME OF FLIGHT instead of change in distance. Both processes yield the same results, and the math is similar. In applying time of flight to the situation above, the laser takes the 800 ns required for the initial pulse to leave the device and return and divides it by 2, giving a one-way time of flight of 400 ns. By the 100th pulse, the time of flight is calculated to be 378 ns, so the resulting change in time of flight is 22 ns. We can see that the process is still the same as with change in distance. Finally, the laser processor takes all the data and averages it over the number of pulses received. This is an important safeguard to prevent an erroneous reading on the target vehicle from placing the vehicle’s speed above the maximum speed the vehicle could have been traveling during this particular measurement. The process by which the current lasers compute this average is called LEAST SQUARES, which we will discuss next. |
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==AVERAGE OF LEAST SQUARES== |
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Ok, so what is this new thing…least squares!? It is a mathematical way to determine that a group of data is consistent. It is really not that difficult to understand. Recall that the laser is sending out pulse after pulse of light and that each time a pulse leaves the laser a timer is started and then, on return of the pulse, is stopped. The elapsed time is known, and the interval between pulses (1 second divided by the number of pulses each second) is also known. |
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The relationship of these two known quantities can be illustrated in a graph where the vertical axis reflects the distances that the laser has determined and the horizontal axis reflects the time intervals. With the vehicle moving away from the laser, as in our example, each successive distance reading will become greater at each successive time interval. Any two distances will suffice to establish a line on the graph, so we could stop sending out pulses after a very short period, but we want to keep taking reading to obtain a group of points which the line would best fit. |
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Each laser unit is set with error percentage limits at the factory, and the operator cannot change these limits. But once a speed-reading has been obtained, the operator can be confident that the math has been worked out correctly. |
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In order for the operator to know that the vehicle intended as the target is the one being displayed, the laser uses a targeting element and a tone. It is VERY IMPORTANT TO REMEMBER THE LASER DOES NOT LOCATE THE VIOLATOR, IT IS ONLY THE TOOL BY WHICH THE OPERATOR WILL CONFIRM HIS OWN ASSESSMENT. To see how this tool functions, let’s look at a typical operation, correlating the speed-measurement process of the laser with its inner workings. |
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First the operator will observe the violator, or intended target vehicle, and make a visual estimation of the target vehicle’s speed. The operator then positions the laser to obtain a speed-reading and places the targeting sight (either a small dot as in the majority of cases or a circle) on the image of the target in the scope (99.9% of all lasers have ZERO magnification in this scope). This sight is designed to give the operator more than just a pinpoint to place on the target vehicle, however. It will also give the operator an idea as to how much of the vehicle is being targeted, inasmuch as the targeting sight will appear superimposed on the vehicle’s image and cover precisely that portion of the vehicle ACTUALLY COVERED BY THE BEAM. If the vehicle is at just under 1000 feet, for example, that portion will be a circle about 2.7 feet in diameter. |
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When the trigger is pulled, a volley of pulses is sent to the target. Assuming the laser is tracking a surface on the target vehicle capable of reflecting the pulses adequately, the pulses will be returned to the laser and a tone will be generated. This tone IS NOT AN AUDIO Doppler. It ONLY purpose is to identify a good return from a bad return. If the return is good then you get the "good tone", then within a brief moment the data will be entered in the processor and fitted to a slope, which confirms the speed, and a speed-reading will be displayed. |
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As you can see, the process in obtaining a speed-reading on an intended target is similar to that with radar: Visual Estimation – Audio Tone – Digital Readout |
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The major difference here is that the tone does not correlate with the speed of the target, as it does with radar; it is the same regardless of target velocity with the exception of the STALKER Lasers manufactured by Kustom. But the tone is just as valuable because more precise aiming is needed due to the narrow beam of the laser and the tone lets the operator know that the beam is on target. The end result is that the target intended for a measurement is the target that is measured. |
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==POTENTIAL ERRORS== |
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But even with a laser there are some things that can go wrong or that lend themselves to a speed-reading that is different from the true speed of the vehicle. |
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===COSINE ERROR=== |
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The effect attributable to cosine error prevalent with radar also occurs with laser when the position of this speed-measuring device is not in true alignment with the target. Since the motion to be determined is relative to the device, any deviation from true alignment results, as with radar, in a DECREASE in the speed displayed. Ok… for those of you that are math junkies – To find the cosine of any angle, we need to determine the relationship of the two sides of the triangle that form that angle. For example, if a car is approaching us in the northbound inside lane of a 4-lane highway divided by only a double yellow line while we are in the southbound inside lane, then the perpendicular distance separating us from the path of the approaching car (from CENTER MASS TO CENTER MASS) would be, say, 12 feet. This distance becomes the opposite side of our triangle. If the target is 200 feet ahead of us, that distance becomes the adjacent side of our triangle. To find the length of the hypotenuse of our triangle, we first square 12, getting 144, then square 200, getting 40,000 and add these amounts: 1144+40,000 = 40144. After finding the square root of 40144, which is 200.359, we then divide the adjacent side by the hypotenuse to determine the cosine of the angle by which our device is out of alignment with the target: 200 / 200.359 = .998 |
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We should keep in mind that at present laser is still a stationary device and all differences between displayed speed and true speed are in favor of the potential violator. However, due to the narrow beam of a laser, the effect of cosine error will be less pronounced than with radar. |
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Compared to radar, laser does not show as low a speed from cosine error until the operator, in turning the device to continue tracking the target when it approaches very close, creates a larger angle between the target and the ground directly ahead than would be necessary with radar and its broader beam. |
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===SWEEP ERROR=== |
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This is another possible error that may manifest its self. The problem may arise when the laser is tracking a certain portion of a vehicle and, during this track, changes from tracking that portion to tracking another portion of the vehicle, giving the laser the impression of a greater or smaller distance covered during the clock and resulting in a higher or lower speed reading. If the sweep is toward the rear of a vehicle that is moving towards the laser, the distance will be increased and the speed will be read higher. A sweep toward the front of the vehicle would decrease the distance and cause the speed to be read lower. |
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EXAMPLE – I am clocking an approaching vehicle that is traveling at 60 mph or 88 fps. If I am aiming the laser at the front license plate and by accident I move the laser to the windshield, a sudden change in distance is realized. This change adds 4 feet to the calculated distance covered by the target vehicle over the 1/3 second period. In reality, the target vehicle will cover a distance of only 29.33 feet over that time. But to the laser the vehicle covers 33.33 feet and therefore, the speed computed by the laser will be 68 mph, not the actual speed of 60. |
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Laser manufactures admit to the possibility of a high reading but advise that the occurrences are rare, as the momentary error is usually thrown out of the lease square equation. But the potential for sweep error does exist and needs to be recognized. |
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When lasers were first introduced for speed measurement, the limits used for error trapping were somewhat loosely set. Today, laser manufacturers have tightened up greatly on this feature. The Kustom Pro-Laser II and the LTI Marksman, are very difficult to induce sweep errors. However in 1994, a Pro-Laser I or an LTI 20-20 could generate a reading up to 12 mph in error. Today the maximum error of any NEW laser would be only about 4 mph. |
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===REFLECTION ERROR=== |
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Without a careful tracking history, reflection can also give rise to an erroneous reading. Often when the day becomes very hot, especially in those southern and western states where the atmosphere does not always provide for adequate cooling of the pavement, a visible mirage (mirror image) or reflection may be formed on the ground by the rising head. In this way a mirror image of a vehicle may be created on the pavement. When the laser beam travels out under such conditions and it is aimed at the lower front of a vehicle, there is potential for at least a portion of the beam to be returned from the reflection. Actually, the laser beam is reflected first from the vehicle to the mirror image and then back to the laser. As in sweep error, the added distance increases the speed of the vehicle in the display. The same effect might occur on rainy days when there is standing water. |
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==LASER STEALTH WHAT YOU REALLY READ THIS FOR== |
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Of course you the motorist is interested in how you can beat a laser. Old successes in radar detection and warning systems are repeated with LASER. The problem that drivers using laser detectors are facing is that the beam of the laser is so small it CAN be tracking the car next to the driver and the detector may not give a warning of the presence of a beam. HOWEVER, this is in a perfect world. Laser light is reflected and refracted in all directions, as there is not a single perfectly oriented surface on a target vehicle. This allows for the possibility that a driver with a laser detector could be warned ahead of being measured – provided the laser detector is a 360 degree capable and has very good sensitivity! |
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One company has come up with a laser jammer, called the Defuser, that transmits from a front license plate holder (for states that have no front license plate). It is essentially an infrared light bulb like the one on a television remote control. The idea is to confuse the laser by dumping the infrared light back at the laser and this DOES WORK UNDER CERTAIN CIRCUMSTANCES. If the vehicle is close enough and the LASER beam is aimed near enough or on the front license plate holder, then truly no reading is received. Of course, the operator can avoid the jammer by aiming at the headlight of the target vehicle instead of the license plate. Or the operator can just tag the car from the rear. |
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VEHICLE COLOR AND RETURN SIGNALS – Since laser light is transmitted to an object and then bounces back from that object to the laser, the ability of the object to reflect this light is obviously crucial, and some colors reflect light better than others. On the other hand, anyone who has walked along a beach on a bright day can testify that white beach sand creates such a bright reflection that without sunglasses a person has difficulty seeing at all. The question of what role color plays in returning the laser signal is very legitimate. |
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The color of the target does affect the intensity of the returned laser light and therefore the range, or distance, at which a speed determination can be made. However, it turns out that the ranges for the various colors are not dramatically different because factors other than reflectivity (e.g., absorption) are involved. Although a white car at 1000 feet has 10 times the reflective ability of a black car at the same distance (white returns 60% of the light as opposed to 6% returned by black), the range for a speed determination would be about the same for each. A white car with its 60% reflectivity would nevertheless prove EASIER and require LESS time to acquire as a target than a black car. |
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While tests have shown that color has little effect in actual practice, cars with hidden headlight systems are harder to hit. A Pontiac Firebird with its headlights closed may be picked up at 600 to 800 feet, but with them open, may be picked up at 1500 feet, or about twice the distance because of the greater reflectivity of a headlight. |
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In a controlled environment, the effect of reflectivity on range can be even more positively demonstrated. Scientists at NASA conducted an experiment with a police laser placed on a bridge and pointed at a retro-reflector mirror. The mirror returned close to 100% of the light and the scientists received the return signal from an impressive 25,000 feet – just shy of 5 miles. Of course the mirror was lined up perfectly and they used a computer to give the range, since police lasers cannot indicate a distance greater that 9999 feet on their current displays. |
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It should also be mentioned that heat waves rising from the pavement on a hot day might also disrupt the laser beam, causing a decrease in effective range. Likewise, a laser beam passing through the windshield of the patrol car may result in range loss. For maximum range, the laser device should be operated free of windshield glass. |
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TESTING OF LASER DEVICES – I will break these tests of police laser into three classes: THE INITIAL CERTIFICATION tests (Federal), the SIX-MONTH tests, and the DAILY tests. The operator will not have contact with some of the testing equipment but should be aware of the purpose of each test and be able to apply this knowledge in court testimony. When presenting the certification documents to the court, the operator should be able to explain the entries on the documents. |
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INITIAL CERTIFICATION – The International Association of Chiefs of Police (IACP) has taken on the responsibility of testing laser speed-measurement equipment. This organization has already been active in the testing of radar speed measurement equipment, to which the testing of lasers is parallel in style. |
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In January 1995, the IACP set up the first tests of lasers for speed enforcement. The University of California at the Davis Campus in Sacramento conducted the tests in cooperation with the National Institute of Standards and Technology. The California Highway Patrol provided the site for some of the field testes. Although the test results have been claimed by the IACP as proprietary, some of the test procedures are described below. |
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BEAM DIAMETER TEST – The laser needs to be tested to determine if the beam is of the diameter claimed by the manufacturer. If it is not, then the potential exists for the beam to spill over to other traffic and give rise to the question of validity of the target reading as it relates to the intended target vehicle. This test is done by pointing the laser along a clear distance and using a retro-reflector device and a glass rod to determine the point of contact of the beam with the reflector. The test is repeated seven times to give a reading at each of eight points on a circle. |
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DISTANCE TESTING RANGE – This is an important test in determining if the laser is functioning as it should. Since the laser is a time-of-flight device, all calculations are dependent on its ability to accurately convert time measurements to change-in-distance measurements and to do this consistently. The IACP conducted this test over a distance measured by a survey instrument capable of accuracy to the fourth decimal place. |
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SPEED COMPARISON TEST - In this test, the laser’s determination of target speeds is compared to the speed obtained by a time-distance device. The device used by IACP was a set of pneumatic hoses. The test was done in an area free of congestion on the California Highway Patrol Academy Driving Track. The goal was to have the laser automatically triggered as the target hit the first pneumatic hose so that the reading could be obtained between the two hoses. However, the lasers tested did not have the hardware configuration to allow them to be triggered from a remote source. The only alternative was to have a laser operator fire the laser manually. Members of the research team were given some instruction and were able to perform this task. |
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SIMULATED TARGET SPEEDS – Unique to this series of tests is a device built for the IACP. It is a target speed simulator. Since the laser is NOT a Doppler device, a simulated source of movement, such as tuning forks, cannot be used. With this new simulator device, the technician hooks up fiber-optic wires to the lens of the laser. When a laser pulse is fired, it travels through the fiber optics and upon entering the simulator device, it is absorbed. This prevents return to the laser. Then a second laser operation at the same wavelength sends back a pulse. The technician can adjust the time delay between pulses and by this technique can simulate movement of an imaginary target. |
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The purpose of the foregoing tests administered by IACP was to provide a list of lasers, which have passed testing and are recommended for purchase with federal funds. These tests are now included on the IACP’s Consumer Products List. |
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SIX MONTH TESTS – We can now assume that the laser to be tested has been given state acceptance of initial certification. The purpose of the Six Month Test is to determine that the device still meets the conditions for that certification and can still accurately determine the speeds of target vehicles. Every six months (in some states it is 6-12 months) the following tests should be performed on the laser by a local test lab. ALL TESTS SHOULD BE RECORDED, SIGNED, AND WITNESSED. |
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WAVELENGTH TEST – This test can be done with a spectrometer or narrow-band optical filter for the general purpose indicated above and the specific purpose discussed under the initial certification. |
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POWER OUTPUT TEST – The power output test needs to remain within certain limits to assure that it is maintaining its eye-safety classification. This test is to be done with a standard laser power meter. |
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PULSE REPETITION RATE, PULSE WIDTH, and DOUBLE PULSING TESTS – These performance characteristics need to be checked with a high-speed optical detector and oscilloscope to determine if the laser is making distance determinations with means that meet the standards required for accuracy. THESE TESTS ARE HIGHLY CRITICAL. |
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RANGE TESTS – Every six months the laser should be tested under actual conditions to determine if it can accurately determine a distance that has been measured from one known point to another. |
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SIGHT ALIGNMENT TEST – It should be determined that the optical sight is properly lining up with the selected target. This can be accomplished by applying the steps listed under the daily tests and can be carried out when the Range Tests above are performed. |
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DAILY TEST – Before a laser is used, it should be checked to determine that it is functioning properly. A user can set up an area at the police department or some other location in which to conduct these testes. To begin, locate an area free of obstructions where two distances can be determined. Even in an urban setting this can be accomplished since the distances only need to be line of sight distances. Keep in mind that the distances do not have to be completely horizontal as long as the laser is always in the same position each time the test is conducted. The location for the laser should be alongside a pole or at the corner of a building. A mark will be placed where the laser will rest. The receiving points to which the laser will measure the distance will be pre-measured by a steel tape. The two distances pre-measured in this manner must each be greater than 100 feet but not differ by more than 50 feet. (These distances are only as a guide…some states adopt statutory regulations, which mandate other distances – check local laws.) At each receiving point will be mounted a bicycle-type reflector at the same height as the laser. The reflector will help the operator find the exact point to measure to, thereby keeping the plane level between the laser and the point from which the laser beam will be reflected. |
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DISTANCE CHECK TEST – From the fixed point described above, the laser is aimed at the reflector and the distance read and compared to the known distance. An ERROR NO GREATER THAN 1 FOOT SHOULD BE THE STANDARD FOR EACH READING. |
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SIGHT ALIGNMENT TEST – The sighting element of the laser should be checked on at least one of the targets by placing the unit in the test mode and scanning the target both horizontally and vertically to obtain an audible tone indicating that the sight is lined up with the beam. If the laser does not have an audible sighting test, the reflector target described in the distance check test could be designed to the same diameter as the laser beam at a given distance. When the person performing the Alignment Test then sights onto the target set up at that distance, the laser beam would have to cover the entire target without any overlap in order for the laser to pass the test. |
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==See also== |
==See also== |
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LIDAR (Light Detection and Ranging; or Laser Imaging Detection and Ranging) is a technology that determines distance to an object or surface using laser pulses. Like the similar radar technology, which uses radio waves instead of light, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal.
The acronym LADAR (Laser Detection and Ranging) for elastic backscatter lidar systems is mainly used in military context. The term laser radar is also in use but somewhat misleading as laser light and not radiowaves are used and thus should be avoided.
General description
The primary difference between lidar and radar is that with lidar, much shorter wavelengths of the electromagnetic spectrum are used, typically in the ultraviolet, visible, or near infrared. In general it is possible to image a feature or object only about the same size as the wavelength, or larger. Thus lidar is highly sensitive to aerosols and cloud particles and has many applications in atmospheric research and meteorology.
An object needs to produce a dielectric discontinuity in order to reflect the transmitted wave. At radar (microwave or radio) frequencies a metallic object produces a significant reflection. However non-metallic objects, such as rain and rocks produce weaker reflections and some materials may produce no detectable reflection at all, meaning some objects or features are effectively invisible at radar frequencies.
Lasers provide one solution to these problem. The beam densities and coherency are excellent. Moreover the wavelengths are much smaller than can be achieved with radio systems, and range from about 10 micrometers to the UV (ca. 250 nm). At these sorts of wavelengths, a lidar system can offer much higher resolution than radar. The wavelengths are ideal for making measurements of smoke and other airborne particles (aerosols), clouds, and air molecules.
A laser typically has a very narrow beam which allows the mapping of atmospheric features with very high resolution compared with radar. In addition, many chemical compounds interact more strongly at visible wavelengths than at microwaves, resulting in a stronger image of these materials. Suitable combinations of lasers can allow for remote mapping of atmospheric contents by looking for wavelength-dependent changes in the intensity of the returned signal. Lidar has been used mostly for atmospheric research and meteorology. More recently a number of surveying and mapping applications have been developed, using downward-looking lidar instruments mounted in aircraft or satellites. Another newer use is to map the eye during LASIK eye surgery, in order to allow the main cutting beam to follow any movements of the eye.
Applications
In geology and seismology a combination of aircraft-based LIDAR and GPS have evolved into an important tool for detecting faults and measuring uplift. The output of the two technologies can produce extremely accurate elevation models for terrain that can even measure ground elevation through trees. This combination was used most famously to find the location of the Seattle Fault in Washington, USA. This combination is also being used to measure uplift at Mt. St. Helens by using data from before and after the 2004 uplift.
Similarly, airborne LIDAR systems are used to monitor glaciers and have the ability to detect subtle amounts or growth or decline. NASA's ICESat includes a LIDAR system for this purpose.
LIDAR has also found many applications in forestry. Canopy heights, biomass measurements, and leaf area can all be studied using airborne LIDAR systems.
A world-wide network of observatories use lidars to measure the distance to reflectors placed on the moon, so measuring the moon's position with mm precision and enabling tests of general relativity to be done.
MOLA, the Mars Orbiting Laser Altimeter, used a lidar instrument in a Mars-orbiting satellite to produce a spectacularly accurate global topographic survey of the red planet.
In atmospherics, lidar is used as a remote detection instrument to measure densities of certain constituents of the middle and upper atmosphere, such as potassium, sodium, or molecular nitrogen and oxygen. These measurements can be used to calculate temperatures. Lidar can also be used to measure wind speed.
One situation where lidar has notable non-scientific application is in traffic speed law enforcement, for vehicle speed measurement, as a technology alternative to radar guns. The technology for this application is small enough to be mounted in a hand held camera "gun" and permits a particular vehicle's speed to be determined from a stream of traffic. Unlike RADAR which relies on doppler shifts to directly measure speed, police lidar relies on the principle of time-of-flight to calculate speed. The equivalent radar based systems are often not able to isolate particular vehicles from the traffic stream and are generally too large to be hand held.
Military applications are not yet in place, but a considerable amount of research is underway in their use for imaging. Their higher resolution makes them particularly good for collecting enough detail to identify targets, such as tanks. Here the name LADAR is more common.
Laser imaging systems can be divided into scanning systems and non-scanning systems. The scanning system can again be divided into sub-groups by the way the laser beam is scanned across the object. Beam-scanners scan a narrow beam, typically in lines on top of each other, therefore this type of system is called a Laser Line Scanner (LLS). Fan-beam scanners scan a fan-shape beam across the object.
3-D imaging is done with both scanning and non-scanning systems. "3-D gated viewing laser radar" is a non-scanning laser radar system that applies the so-called gated viewing technique. The gated viewing technique applies a pulsed laser and a fast gated camera. There are ongoing military research programmes in Sweden, Denmark, USA and UK with 3-D gated viewing imaging at several kilometers range with a range resolution and accuracy less than ten centimeters.
At the JET nuclear fusion research facility, in the UK near Abingdon, Oxfordshire, LIDAR Thomson Scattering is used to determine Electron Density and Temperature profiles of the plasma [1].
Design
In general there are two types of lidar systems, "high energy" systems and micropulse lidar systems. Micropulse systems have developed as a result of the ever increasing amount of computer power available combined with advances in laser technology. They use considerably less energy in the laser, typically on the order of one watt, and are often "eye-safe" meaning they can be used without safety precautions. High-power systems are common in atmospheric research, where they are widely used for measuring many atmospheric parameters: the height, layering and densities of clouds, cloud particle properties (extinction coefficient, backscatter coefficient, depolarization), temperature, pressure, wind, humidity, trace gas concentration (ozone, methane, nitrous oxide, etc.).
There are several major components to a lidar system:
- Laser — 600-800 nm lasers are most common for non-scientific applications. They are inexpensive and can be found with sufficient power but they are not eye-safe. Eye-safety is often a requirement for military apps. 1550 nm lasers are eye-safe but not common and are difficult to get with good power output. Laser settings include the laser repetition rate (which controls the data collection speed) and pulse length (which sets the range resolution).
- Scanner and optics — How fast images can be developed is also affected by the speed at which it can be scanned into the system. There are several options to scan the azimuth and elevation, including dual oscillating plane mirrors, a combination with a polygon mirror, a dual axis scanner. Optic choices affect the angular resolution and range that can be detected. A hole mirror or a beam splitter are options to collect a return signal.
- Receiver and receiver electronics — Receivers are made out of several materials. Two common ones are Si and InGaAs. They are made in either PIN diode or Avalanche photodiode configurations. The sensitivity of the receiver is another parameter that has to be balanced in a LIDAR design.
- Position and navigation systems — Lidar sensors that are mounted on moving platforms such as airplanes or satellites require instrumentation to determine the absolute position of the sensor. Such devices generally include a Global Positioning System receiver and an Inertial Measurement Unit (IMU).