Find distance using laser ?

Thread Starter

CVMichael

Joined Aug 3, 2007
419
I bought a laser pointer, and I dismantled it to get the laser diode (including it's circuit).

I was thinking if it's possible to use the laser diode to calculate distance between the laser diode and some object ?

I would probaly need to record the feed back time, but the speed of light is very fast, so the time will be extremely short...

I know there are devices out there that do this, but can it be done using a laser diode ?

Anyone knows how it's done ?

[Edit]
Also, is there a laser pointer with invisible light ? (not in the huma's vision spectrum)
I know that laser pointers are very dangerous to the eyes, so I don't want to make anyone (or myself) blind while playing with it...

Also...
Is there a photo resistor that is like a laster (or opposite of later) to read the light from only one direction ? Because if I am to read the laser feedback, I don't want to read any other light from other sources.
 

SgtWookie

Joined Jul 17, 2007
22,210
Photoresistors wouldn't be nearly fast enough.

Look in the area of phototransistors.

Light travels at the speed of 299 792 458 meters/sec, or roughly 299.79 meters/uSec. (microsecond)

Since you'd have to wait for the beam to return, that would cut the time in half, or 149.895 meters/uSec.

You would need very good optics to collimate the returning beam of light. Propagation delays would have to be precisely accounted for, both through the optics, and in all of the electronics; from powering up the diode sufficently to emit light, turn-on time of the phototransistor, delays throught the rest of the circuitry.

Sure, it can be done - it'll take work. Your pointer probably is underpowered for such a task, if it's a standard red laser around 650nm. The green lasers in 530nm to 535nm range seem to be quite a bit more visible; of course it depends on your phototransistor and what it's optimum detection frequency is.
 

Mike M.

Joined Oct 9, 2007
104
Light travels at the speed of 299 792 458 meters/sec, or roughly 299.79 meters/uSec. (microsecond)
Wouldn't the speed depend on not only the presence of air (different speed than a vacuum) but also it's pressure, water vapor content, and temperature to properly compensate and be precisely accurate?
 

HarveyH42

Joined Jul 22, 2007
426
Nothing said about accuracy. Working with a cheap laser pointer, guessing not to much precision...

Anyway, measuring the time would be tough, expensive, and most likely not worth the results. A laser pointer would be kind of short on range, so you would need very fast circuits and programming. Why not measure the size of the dot? It should spread at an even amount.
 

bloguetronica

Joined Apr 27, 2007
1,372
Wouldn't the speed depend on not only the presence of air (different speed than a vacuum) but also it's pressure, water vapor content, and temperature to properly compensate and be precisely accurate?
Yes, It depends on the material biased by the beam.

You should consider that different materials will refract/deviate light differently. So you device acuracy will be vary greatly in consonance with the material.
 

SgtWookie

Joined Jul 17, 2007
22,210
Why not measure the size of the dot? It should spread at an even amount.
Actually, that would be rather difficult to do if you're attempting to measure it from the point of origin.

Or are you volunteering to jog down 500 meters or so and measure the size of the dot? :D
 

Voltboy

Joined Jan 10, 2007
197
Maybe you could use a photocell and measure the voltage it produce, I guess that the nearest is the laser from the cell, it will produce more voltage.
 

niftydog

Joined Jun 13, 2007
95
Also, is there a laser pointer with invisible light ? (not in the huma's vision spectrum)
I know that laser pointers are very dangerous to the eyes, so I don't want to make anyone (or myself) blind while playing with it...
Whether the laser is visible or not won't make a scrap of difference to it's safety. Non-visible lasers are just as dangerous, if not more dangerous partly BECAUSE they aren't visible!!!

Visible laser beams pass through the eyes' cornea and lens and strike the retina whereas UV and most IR lasers are absorbed by the cornea. So the only difference is where in the eye the damage occurs.

Just keep the laser power below the standard described in the relevant local law and you'll be fine.
 

ixisuprflyixi

Joined Sep 16, 2007
52
what about using trig. to triangulate the beam in some way and then use the PYTHAGOREAN THEOREM to calculate the distance to the object? just a thought but I am not sure how it would work exactly.
 

Thread Starter

CVMichael

Joined Aug 3, 2007
419
Hi, thanks for your replies

Actually I was thinking to find the distance using a web-cam like in one of thouse links, and I found Visual Basic code for that too, but the web-cam has 15 frames / second (at best), so if I want to scan the whole room (for example) it will take a long time to process everything....

That's why I was thinking to get the distance using a photo resistor (or phototransistor), but i guess that will be too difficult because of the speed being too fast (how ironic...)

I don't need a lot of precision, maybe 1 cm (or aproximatelly half an inch) should be fine, and I don't need it to go far also, maybe a maximum of 5 meters should be enough.

I want to do this for a robot by the way.
 

beenthere

Joined Apr 20, 2004
15,819
It's all in the timing. Light speed is close to 3 X 10^8 m/sec. So, a distance of 1 cm is traversed in something like 3 X 10^-12 sec. If you can't resolve the time interval, you can't measure the distance.
 

Thread Starter

CVMichael

Joined Aug 3, 2007
419
WAW, that is a lot....

So how do they do it then ? I saw it on TV (Discovery channel :) ) that they have devices out there that mesure distance using laser to impressive precision (like 1000'th of a centimiter or something like that...)
 

recca02

Joined Apr 2, 2007
1,214
instead of using the time of travel which will be real hard to get (IMO) why not use something simpler like tilt the laser with some angle and get the displacement along the plane of source when light gets reflected or even at the point where it strikes the object(the object will have to be a plane surface though) using sensors might work.
 

Thread Starter

CVMichael

Joined Aug 3, 2007
419
In that case, how would I find the "dot" on the objects that the laser strikes ?

Something like that I was talking about in the previous post (Post #12) where I was saying to use a web-cam to calcualte the angle of where I see the dot on the web-cam, but this is a slow process.

But I want to calculate the distance faster, so how would I go about finding the "dot" where it strikes on the surface of the object without using a camera ?
 

recca02

Joined Apr 2, 2007
1,214
ok i replied without taking into account the application,
the laser is going to be mounted on robot right?
i was thinking if somehow the laser is reflected back to robot , if we keep the tilting angle of robot to low values and since 5 meters is max distance(hence the distance between the point of incidence will not be great) may be we can mount a number of sensor on the robot or some large sensor which can detect distance between the point of incidence of reflected laser and the laser source i will need to figure out how that can be achieved.
 

hgmjr

Joined Jan 28, 2005
9,029
Distance measurement using ultrasonics is a much more practical approach. I believe this is the method of choice in robotics. The speed of sound is a bit easier to measure with generally available electronics.

Maybe you have considered it and tossed it out.

hgmjr
 

Thread Starter

CVMichael

Joined Aug 3, 2007
419
You are right, and I did consider it....

The problem is that ultrasound gives you the distance in general, not a specific point in space...

Actually, I was thinking to put that in the back of the robot so that it knows when "something" moves behind it, then when it detects that, then it's gonna turn it's attention to that section...

I want to assimilate into it, as many types of sensors as I can (as long as I can get it to process all information in a resonable speed)

I want to connect all the sensors through PIC chips, then in turn to a computer (the brains)

I know that I embarked into a huge project, but when I started, I knew that it's gonna take me years of work.

To me, it matters most what I learn through this.

Anyways... back to my question... does anyone have any idea how to do it with a laser ?
 

Mike M.

Joined Oct 9, 2007
104
Isn't there some way of accurately discriminating phase (instead of time) between transmit pulse and received pulse?
Isn't that how a 3D holograph is "recorded" based on the 3D target that it is trying to recreate? With beam splitting, you may be able to accurately measure distance by refocusing the blurred reflection pattern generated by the dual-beam interaction. The focal parameter would then be a function of the distance. Time and material type should then become irrelevant because the beams are referencing each other, both traveling the same distance and through the same material thus cancelling out any density fluctuations or movement of the target.
 
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