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View Full Version : Measuring Distance With a Laser.

Jim B.
05-12-2010, 09:48 PM
Lasers can apparently be used for measuring distance, and quite accurately too apparently. When I was still young, I remember, they were going to measure the distance to the Moon with a laser beam.

My question: How on earth does that work? I mean, how do they "know" when the laser has struck its target? Or more correctly, how does the laser "know"? What specifically happens when the laser strikes its target that makes it so perfect for measuring distance?

Shmendrik
05-12-2010, 09:53 PM
Lasers can apparently be used for measuring distance, and quite accurately too apparently. When I was still young, I remember, they were going to measure the distance to the Moon with a laser beam.

My question: How on earth does that work? I mean, how do they "know" when the laser has struck its target? Or more correctly, how does the laser "know"? What specifically happens when the laser strikes its target that makes it so perfect for measuring distance?

It hits a reflector (http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment)placed there during one of the Apollo missions and bounces back. You measure how much time that takes and calculate the distance based on that.

Derleth
05-12-2010, 09:53 PM
What specifically happens when the laser strikes its target that makes it so perfect for measuring distance?It bounces back. You measure the time between when you send the pulse and when you receive the pulse at your device, then divide by two (to get the one-way distance) and use the fact the speed of light is a constant (about 186,000 miles per second, not accounting for the slowdown caused by air) to determine the distance from that one-way trip time.

All they had to put on the Moon was a mirror.

Shmendrik
05-12-2010, 09:57 PM
It bounces back. You measure the time between when you send the pulse and when you receive the pulse at your device, then divide by two (to get the one-way distance) and use the fact the speed of light is a constant (about 186,000 miles per second, not accounting for the slowdown caused by air) to determine the distance from that one-way trip time.

All they had to put on the Moon was a mirror.

Not just a mirror. You need mirrors arranged into a retroreflector, otherwise the beam won't bounce straight back at you.

engineer_comp_geek
05-12-2010, 09:57 PM
You measure the time between when you send the pulse and when you receive the pulse at your device

Yep. The key here is that you pulse it. You aren't just sending out a constant laser beam. You send out a pulse and you time how long until your receiver gets the pulse back. 186,000 mph is roughly a foot per nanosecond, so if you want to measure short distances you need a really accurate high speed timer.

Mangetout
05-13-2010, 02:28 AM
You can do it without a mirror at the other end, as long as the object you're trying to measure isn't completely non-reflective.

When you shine a laser pointer at the wall, the spot you see is reflected light - so without a mirror, you could direct your laser at the target and just measure the interval between turning it on and 'seeing' the spot (this might not be so easy with a really distant object, and of course, I'm not talking about naked-eye measurement).

Mangetout
05-13-2010, 02:34 AM
Alternatively, you can use triangulation - set your laser and your sensor apart from one another - if you know the length of one side and the two adjacent angles in a triangle, you can calculate the rest.

05-13-2010, 06:57 AM
http://www.rp-photonics.com/distance_measurements_with_lasers.html
http://www.acroname.com/robotics/info/articles/laser/laser.html#e4

Surprisingly, laser distance measurement is actually fairly inexpensive, as it has been reduced to almost commodity levels by industrial sensor companies. I routinely use some extremely high precision laser rangefinders in the 1m range, that I can get for about \$300-\$500.

CalMeacham
05-13-2010, 08:32 AM
There are other methods od laser measurement as well. the Keyence company makes laser distance meters that rely upon imaging of a reflected laser spot onto a position sensor. The device claims micron accuracy (You can buy individual cips that do the same thing, and build your own).

And, of course, there's laser interference measurement, which gives you sub-micron accuracy.

There's a host of other laser distance techniques, relying on wavefront sensing, moire deflectometry, and other esoteric methods as well.

Quartz
05-13-2010, 05:35 PM
I routinely use some extremely high precision laser rangefinders in the 1m range, that I can get for about \$300-\$500.

That's 1 mile, right, not 1 metre?

They're commonly used by estate agents over here for measuring rooms.

sevenwood
05-13-2010, 06:20 PM
That's 1 mile, right, not 1 metre?

Nope, that's one meter (or one yard for us Americans). I regularly use a laser rangefinder on the golf course that I purchased a couple of years ago for about \$200 that measures distances up to roughly 300 yards with accuracy +/- one yard. The beam "bounces" off of whatever is handy (a flag, a tree, the edge of a bunker).

Google "golf rangefinders" for a look at the current competition.

Unfortunately, the laser device is a lot more accurate than my subsequent golf swing is...

commasense
05-13-2010, 06:23 PM
They're commonly used by estate agents over here for measuring rooms.Right. Here's (http://www.amazon.com/Bosch-DLR130K-Digital-Distance-Measurer/dp/B001U89QBU/ref=sr_1_2?ie=UTF8&s=hi&qid=1273792645&sr=1-2) one that has an accuracy of 1/16th of an inch and a range of 130 feet for less than \$100.

I own one, and they're remarkably accurate, easy to use, and useful. They also have modes to allow you to measure area or volume, or measure a distance indirectly by using the Pythagorean Theorem.

Llama Llogophile
05-13-2010, 08:33 PM
Lasers can apparently be used for measuring distance, and quite accurately too apparently. When I was still young, I remember, they were going to measure the distance to the Moon with a laser beam.

My question: How on earth does that work? I mean, how do they "know" when the laser has struck its target? Or more correctly, how does the laser "know"? What specifically happens when the laser strikes its target that makes it so perfect for measuring distance?

Here it is on Mythbusters (http://www.youtube.com/watch?v=VmVxSFnjYCA).

There were/are three reflectors placed at Apollo landing sites, and I believe two on a Lunokhod rovers. Here is a closeup of the Apollo 15 landing site (http://lroc.sese.asu.edu/news/index.php?/archives/180-The-Apollo-15-Lunar-Laser-Ranging-Retroreflector-A-Fundamental-Point-on-the-Moon!.html) with the reflector visible and labeled.

05-13-2010, 09:32 PM
There were/are three reflectors placed at Apollo landing sites, and I believe two on a Lunokhod rovers. Here is a closeup of the Apollo 15 landing site (http://lroc.sese.asu.edu/news/index.php?/archives/180-The-Apollo-15-Lunar-Laser-Ranging-Retroreflector-A-Fundamental-Point-on-the-Moon!.html) with the reflector visible and labeled.

Was that pic taken from the ceiling of the soundstage?

Chronos
05-14-2010, 01:39 AM
And, of course, there's laser interference measurement, which gives you sub-micron accuracy.
Or sub-attometer accuracy, if you're really good.

Keeve
05-14-2010, 06:18 AM
I regularly use a laser rangefinder on the golf course ... The beam "bounces" off of whatever is handy (a flag, a tree, the edge of a bunker).My guess is that you can aim the rangefinder at a flag, tree, or bunker, because those things stick up from the ground, and are easy to aim the rangefinder at. But you would NOT be able to use it to aim at the golf ball itself, because even if YOU can see the ball clearly, the rangefinder would not be able to distinguish it from the grass around it. Am I correct?

sevenwood
05-14-2010, 06:47 AM
My guess is that you can aim the rangefinder at a flag, tree, or bunker, because those things stick up from the ground, and are easy to aim the rangefinder at. But you would NOT be able to use it to aim at the golf ball itself, because even if YOU can see the ball clearly, the rangefinder would not be able to distinguish it from the grass around it. Am I correct?

You are correct in that a golf rangefinder doesn't know or care what its laser is bouncing off of. However, given that the grass in the immediate vicinity of a golf ball is basically in the same spot as the golf ball itself (the accuracy of a golf rangefinder is typically only +/- one yard anyway) it really doesn't make any difference. Just use the rangefinder to look "at" the golf ball and use whatever it gives you as the distance.

Munch
05-14-2010, 07:41 AM
Was that pic taken from the ceiling of the soundstage?

Of course not - they dismantled the soundstage years ago to prevent it from being discovered by dedicated truth-seekers. There's a 1/1000th scale model of the "Moon" at Area 51 that is used for these types of photos.

Autolycus
05-15-2010, 03:03 AM
Yeah, but how well do lasers measure when they're attached to frickin' sharks?!

Napier
05-15-2010, 06:40 AM
I have a Total Station that measures out to about 800 feet, and over distances of 100 feet or so it reproduces its measurements to a millimeter or two on a timescale of minutes. It modulates a laser beam at several different frequencies and looks for a distance estimate that explains all the modulations in the return beam. This thing is a little mysterious to me, because I can sight through the eyepiece while it is measuring, and I can't see the laser beam through the eyepiece, though I can see it looking around the instrument. I don't know if there's a bandstop filter or what.

The Keyence company makes devices that measure distances with lasers by several principles. But if you try to deal with them, they will be all over you like a cheap suit. Their sales people are always changing and they spam their customers relentlessly. I had to enter them into my twit filter and make them send quotes on paper. Their web pages have little movements all over them, like a book covered in ants. And their user interfaces and manuals are the most baffling I have found anywhere. Any time it looks like they have a solution to some problem, I try like hell to find somebody else, just to avoid dealing with them.

Koxinga
05-15-2010, 07:22 AM
Lasers being used for one's golf game and by real estate agents to precisely ascertain the size of a room? I had no idea. This is the perfect rebuttal to "waah, I'm living in the future, where's my flying car?"

Napier
05-15-2010, 04:01 PM
Waah, I'm living in the future, where's my flying car? There's nothing but these lasers....