I was reading this thread in Staff Reports and wondering how Lasar “radar” works. (I posted this here, because it seemed offtopic from the Staff Report)
If this tightly focused laser beam hits my car, surely it won’t reflect straight back to the reciever unless the place it hits on my car is perfectly flat, right? I always thought radar worked because it blankets a huge area, and you are bound to get some of the signal back, so I don’t understand how a tightly focused laser beam can reflect back to the receiver.
Its not hitting the car, but rather two beams going out, hitting the ground, or whatever is across the street from the cop. And the gun measures the time it takes for both beams to be interrupted. The faster the car is going, the shorter the time between beam interruptions.
It’s not reflecting back to the reciever as a tight beam; the laser light is being dispersed by the target, but some of it makes it back to the device.
Just like when someone shines a laser pointer on a matt surface; you can see the spot from quite a wide range of angles - the light is dispersed by the surface, but some of that dispersed light happens to make it back to your eye (or else you simply wouldn’t be able to see it)
Nobody’s quite got it yet. LIDAR works best when it’s aimed at the retroreflective parts of the vehicle. The prime target are the license plates, which are coated with a material containing thousands of tiny glass beads embedded in it. These beads reflect the light directly back to the source by a process called total internal reflection. This same process is why driving in the fog with your high beams on will blind you–the spherical water droplets in the fog reflect the light right back at you. The next best targets are the taillights, turn signals, brake lights and other marker lights. These reflect light back to the source too, but use a different method. The lights’ lenses are molded with hundreds of little pointy projections called corner cubes–imagine a tiny cube cut in half diagonally. A corner cube is not quite as efficient as reflecting light back as the glass bead material (which is also used in road signs and street line paint), but it works well enough if the license plate is obscured, missing or covered with a plastic disperser cover in a vain effort to thwart the LIDAR.
I just want to note that the correct spelling is lidar. Also that it’s not an acronym so it doesn’t have to be spelled in all caps. Instead, it’s a blend of light + radar.
That would be the ideal target but as Mangetout said, it would work on any surface. Well, any surface that isn’t perfectly black or perfectly polished. According to this page, the lidar they tested detects a retroreflective licence plate from over 2500 ft away, while a sports car with retracted headlights and no licence plate can be detected from 800 ft away.
Thank you everyone for your responses. I did not know it was called LIDAR, I actually thought about making the title of the thread Laser Radar, but I knew someone would jump all over that.
That site also says:
That’s a bit bigger then I was thinking. I can understand how enough light can make it back to the receiver, especially if it is aimed at a license plate or taillights. When I read that it uses a laser, I was thinking like a tiny pinprick of light,
I guess we’ll just have to have conflicting authorities, but when it comes to etymology, I’ll take Meriam-Webster over NASA any day. American Heritage Dictionary agrees, by the way.
They shouldn’t, since that’s not an uncommon term for it.
[qote] When I read that it uses a laser, I was thinking like a tiny pinprick of light,
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Most people don’t realize that lasers diverge just like other light. For short distances, such as for typical uses of laser pointers, the amount of divergence is small which gives people the wrong impression.
No, it’s directly that it’s a combination of light and radar. Indirectly, you could say lidar is an acronym, but that’s not the way etymology works.
The word radar has long been incorporatated into the English language as an ordinary word and was at the time lidar was coined. Its etymology is not relevant to this situation. In other words, the ‘dar’ in radar doesn’t stand for anything anymore; it’s just the second syllable of the word.
Quite obviously. I wonder what Q.E.D. imagines that “gaydar” is an initialism for. Clearly, “radar” is enough of a word that it can get portmanteaued into other words. That’s how language works.
There is nothing obvious about it, or this discussion wouldn’t be taking place.
Nothing. It’s clearly a cutesy back formation from “radar”, in the same vein as “workaholic” is a back formation from “alcholic”. One could, admittedly, argue that “lidar” is a back formation from “radar”, but generally the folks who actualy deal with lidar technology are in agreement with me. I’d be curious what the ultimate authority on etymological matters, the OED, has to say about this.
I should amend that statement. For specific words where we know who coined them, we can sometimes find out what the actual etymology was. So we have an “ultimate authority” on those specific words. For instance, the word quark was coined by Murray Gell-Mann coined it and they’ve asked him how he did it. This is what the OED has for its etymology:
However such words are fairly rare. If you could track down who came up with lidar and either ask him how he coined it or find the first place it was written down, then we might have something approximating an ultimate authority for this question. Etymologists have no doubt looked for such, although I can’t say how hard.
The earliest cite in the OED is from 1963 (Bulletin of the American Meteorological Society XLIV. 568/1) and does not indicate an etymology. It glosses lidar as “laser radar” in the text and that’s all. It’s unlikely this is the earliest use of the word, so feel free to hunt for an earlier one. Perhaps you can find one that will support your position.
There probably won’t be an earlier usage, since it appears that LIDAR was first used in 1963 by Myron Ligda to detect volcanic gasses in the troposphere, shortly after the invention of the laser. It’s certainly possible for a device or process to be given a name before it’s actually invented, but I imagine such occurences are rare. I’ve been trying, to no avail, to find an instance of Mr. Ligda explaining his coining of the term. We shall probably have to agree to disagree on this one.
No offense, Q.E.D., but you have provided essentially little of value to support your assertion of the derivation of the term “lidar.” The only thing you have provided is a web site from NASA which describes LIDAR as being an Acronym of similar derivation from RADAR. But the site doesn’t provide any indication of why it makes that assertion. Further, it isn’t like NASA is uniquely involved in uses of LIDAR, such that a basic understanding of the term’s origins would be assumed on their part. On the other hand, dtilque has provided two sources of etymology for the term, neither of which supports your theory.
While the NASA site may assert that the term derives as an acronym, that assertion, in the absence of attribution, is meaningless. At least as likely as not, some one writing for NASA thought that the derivation had to be the same as the original derivation for “RADAR.” This is hardly authority or evidence of the derivation.
Now, having said that, I’ll provide some actual evidence of the fact that the term is not simply a word derived as a similarity to radar, but an actual acronym. A perusal of Wikipedia on LIDAR shows a derivation for LIDAR of either LIght Detection And Ranging or Laser Imaging Detection And Ranging, and compares the term LADAR (LAser Detection and Ranging) for specific types of LIDAR systems. Wikipedia being what it is, one looks quite skeptically at such an assertion, but one finds several quite scholarly linked websites, of which all that make an assertion on the subject, assert that the term is an acronym. This is not conclusive, especially since most of the sites then proceed to use “lidar” as if it were a word, rather than an acronym. This, in itself, would not be of much worry, given that society in general, and scientist in specific, use the term “radar” as if it were a word and not an acronym as well. And the Merriam-Webster Online cannot be used as a reference on the issue given that it does NOT refer to radar as an acronym, although it gives an acronym-like etymology.
Still, in favor of the idea that “lidar” is really a way of getting a concept across substituting “light” for “radio” is the fact that some sites still use the combinative term “laser radar” for LIDAR, despite the obvious inaccuracy of such a term (since radio waves are not involved at all. This obviously raises as a point in favor of “lidar” being light + radar as a real probability.
In the end run, it seems likely that “lidar” was made up to be used to describe a technique that mirrored “RADAR,” but which used light instead. By 1963, I’d have to believe that radar as a word, rather than an acronym, was pretty common. My recollection of science fiction works of the time is that they used it thusly. But scientists can be sticklers for correctness, and quite possibly the common assertion that “LIDAR” is an acronym is simply an assumption on the part of such sticklers that the “proper” etymology and usage for “RADAR” should apply to “LIDAR” as well. Not that it matters one whit, given that modern usage has accepted the “lidar” concept.
Clearly, “radar” is enough of a word that it can get portmanteaued into other words. That’s how language works.