# Ladar **URL:** \ **Category:** Factual Questions\ **Created:** [October 29, 2005, 12:32am UTC](https://boards.straightdope.com/t/ladar/328398 "2005-10-29T00:32:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![ishamael69](https://avatars.discourse-cdn.com/v4/letter/i/b9e5f3/32.png) [@ishamael69](https://boards.straightdope.com/u/ishamael69)\ **Post date:** [October 29, 2005, 12:32am UTC](https://boards.straightdope.com/t/ladar/328398/1 "2005-10-29T00:32:15Z")
I was reading [this thread in Staff Reports](http://boards.straightdope.com/sdmb/showthread.php?t=340796) and wondering how Lasar “radar” works. (I posted this here, because it seemed offtopic from the Staff Report) > [@diogenes009](#): > > As to Ladar, or laser speed measurement tools, they are becoming more common. Laser has the advantage of a smaller target area, resulting in the ability to say “That blue car is going x mph.” as opposed to “A car in that cluster is doing x mph, and based upon my training and expertise, it is the blue one.” 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. ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [October 29, 2005, 12:37am UTC](https://boards.straightdope.com/t/ladar/328398/2 "2005-10-29T00:37:57Z")
WAG: 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. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 29, 2005, 12:44am UTC](https://boards.straightdope.com/t/ladar/328398/3 "2005-10-29T00:44:17Z")
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) ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 29, 2005, 2:10am UTC](https://boards.straightdope.com/t/ladar/328398/4 "2005-10-29T02:10:12Z")
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. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 29, 2005, 5:49am UTC](https://boards.straightdope.com/t/ladar/328398/5 "2005-10-29T05:49:01Z")
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 _li_ght + ra_dar_. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 29, 2005, 6:01am UTC](https://boards.straightdope.com/t/ladar/328398/6 "2005-10-29T06:01:42Z")
> [@dtilque](#): > > Also that it’s not an acronym so it doesn’t have to be spelled in all caps. Instead, it’s a blend of _li_ght + ra_dar_. Sorry, but [no](http://www.ghcc.msfc.nasa.gov/sparcle/sparcle_tutorial.html). LIDAR is an acronym of **LI** ght **D** etection **A** nd **R** anging. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 29, 2005, 6:23am UTC](https://boards.straightdope.com/t/ladar/328398/7 "2005-10-29T06:23:46Z")
> [@Q.E.D.](#): > > LIDAR works best when it’s aimed at the retroreflective parts of the vehicle. 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](http://www.mr2.com/TEXT/FAQonLidar.html), 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. ---
**Author:** ![ishamael69](https://avatars.discourse-cdn.com/v4/letter/i/b9e5f3/32.png) [@ishamael69](https://boards.straightdope.com/u/ishamael69)\ **Post date:** [October 29, 2005, 2:19pm UTC](https://boards.straightdope.com/t/ladar/328398/8 "2005-10-29T14:19:01Z")
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. 😃 > [@scr4](#): > > 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](http://www.mr2.com/TEXT/FAQonLidar.html), 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. That site also says: > [@](#): > > The lidar beam width at 250 meters is about 1 meter^2. 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, ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 30, 2005, 9:42pm UTC](https://boards.straightdope.com/t/ladar/328398/9 "2005-10-30T21:42:50Z")
> [@Q.E.D.](#): > > Sorry, but [no](http://www.ghcc.msfc.nasa.gov/sparcle/sparcle_tutorial.html). LIDAR is an acronym of **LI** ght **D** etection **A** nd **R** anging. I guess we’ll just have to have conflicting authorities, but when it comes to etymology, I’ll take [Meriam-Webster](http://www.m-w.com/dictionary/lidar) over NASA any day. American Heritage Dictionary agrees, by the way. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 30, 2005, 9:51pm UTC](https://boards.straightdope.com/t/ladar/328398/10 "2005-10-30T21:51:31Z")
> [@ishamael69](#): > > 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. 😃 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, [/QUOTE] 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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 30, 2005, 9:54pm UTC](https://boards.straightdope.com/t/ladar/328398/11 "2005-10-30T21:54:29Z")
> [@dtilque](#): > > I guess we’ll just have to have conflicting authorities, but when it comes to etymology, I’ll take [Meriam-Webster](http://www.m-w.com/dictionary/lidar) over NASA any day. American Heritage Dictionary agrees, by the way. Indirectly, it _is_ a combination of light + radar. But what do you think the “dar” in radar stands for? ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 31, 2005, 1:21am UTC](https://boards.straightdope.com/t/ladar/328398/12 "2005-10-31T01:21:10Z")
> [@Q.E.D.](#): > > Indirectly, it _is_ a combination of light + radar. But what do you think the “dar” in radar stands for? 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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 31, 2005, 1:28am UTC](https://boards.straightdope.com/t/ladar/328398/13 "2005-10-31T01:28:34Z")
> [@dtilque](#): > > In other words, the ‘dar’ in radar doesn’t stand for anything anymore; it’s just the second syllable of the word. I disagree, but you are entitled to your opinion. ---
**Author:** ![Excalibre](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@Excalibre](https://boards.straightdope.com/u/Excalibre)\ **Post date:** [October 31, 2005, 1:46am UTC](https://boards.straightdope.com/t/ladar/328398/14 "2005-10-31T01:46:27Z")
> [@dtilque](#): > > 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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 31, 2005, 4:03am UTC](https://boards.straightdope.com/t/ladar/328398/15 "2005-10-31T04:03:36Z")
> [@Excalibre](#): > > Quite obviously. There is nothing obvious about it, or this discussion wouldn’t be taking place. > [@Excalibre](#): > > I wonder what **Q.E.D.** imagines that “gaydar” is an initialism for. 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. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 31, 2005, 4:50am UTC](https://boards.straightdope.com/t/ladar/328398/16 "2005-10-31T04:50:03Z")
> [@Q.E.D.](#): > > I’d be curious what the ultimate authority on etymological matters, the OED, has to say about this. There’s no ultimate authority on anything related to the English language. But the OED2 happens to agree with M-W that it’s light+radar. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 31, 2005, 5:01am UTC](https://boards.straightdope.com/t/ladar/328398/17 "2005-10-31T05:01:44Z")
> [@dtilque](#): > > There’s no ultimate authority on anything related to the English language. If that’s the case, then all your arguing has been for naught. We can BOTH be right. 😉 ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 31, 2005, 5:39am UTC](https://boards.straightdope.com/t/ladar/328398/18 "2005-10-31T05:39:59Z")
> [@Q.E.D.](#): > > If that’s the case, then all your arguing has been for naught. We can BOTH be right. 😉 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: > [@](#): > > [Invented word, associated with ‘Three quarks for Muster Mark!’ (Joyce Finnegans Wake (1939) ii. iv). > ‘I employed the sound “quork” for several weeks in 1963 before noticing “quark” in _Finnegans Wake_, which I had perused from time to time since it appeared in 1939… The allusion to three quarks seemed perfect… I needed an excuse for retaining the pronunciation quork despite the occurrence of “Mark”, “bark”, “mark”, and so forth in Finnegans Wake. I found that excuse by supposing that one ingredient of the line “Three quarks for Muster Mark” was a cry of “Three quarts for Mister…” heard in H. C. Earwicker’s pub.’—M. Gell-Mann, private let. to Ed., 27 June 1978.] 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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 31, 2005, 11:39pm UTC](https://boards.straightdope.com/t/ladar/328398/19 "2005-10-31T23:39:31Z")
> [@dtilque](#): > > 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. 🙂 ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [November 1, 2005, 1:29am UTC](https://boards.straightdope.com/t/ladar/328398/20 "2005-11-01T01:29:46Z")
> [@Q.E.D.](#): > > 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](http://en.wikipedia.org/wiki/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](http://www.m-w.com) 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 _li_ght + ra_dar_ 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. [Next page](https://boards.straightdope.com/t/ladar/328398.md?page=2)