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Optics and laser beam width, need help please.

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Pinky's Brain

Wed Nov 28 2012, 04:32PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

A polygon mirror won't get him 360 ... for 360 degrees he'll simply need to put the sensor/laser horizontally and rotate a mirror above them at a 45 degree angle.

Patrick

Wed Nov 28 2012, 05:02PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Pinky's Brain wrote ...

A polygon mirror won't get him 360 ... for 360 degrees he'll simply need to put the sensor/laser horizontally and rotate a mirror above them at a 45 degree angle.

Im hoping its that easy, one or two 45 mirrors. im just not sure i can transmit the vertical beam and recive it too, without the geometry changing with rotation...this may be why the XV-11 uses slip rings (for both power and signal) and rotates the whole head.

EDIT:
with the senosr now at the focal plane, can i assume im getting a IR beam on a few pixels at a time? how can i calculate this?

edit: it looks like a spinning mirror wont work...

Shrad

Thu Nov 29 2012, 10:24AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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hmmm...

using a polarizing beam splitter and a wave plate could do the trick

a rotating periscope with the emitted beam and received image in the same head

what could be done is based on designs from CD reader heads

see image :

in this example, the beam changes polarisation from linear to circular, reflects, then changes from crcular to linear again, but of opposite angle than when first entering the quarter wave plate

basically there are two solutions

off/axis so the emitted beam separates from its image at beamsplitter output (tilted and translated objective lens)

a hole in the center of the objective lens for the beam to pass through

if you wish, I have some objective lenses I'd be willing to trade, but I have absolutely no idea of their precise geometrics

I think that with this method, you can make a spinning periscope objective/mirror out of a classic brushless motor and a piece of tubing, especially if you use something like a floppy PCB motor

Pinky's Brain

Thu Nov 29 2012, 01:49PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

wrote ...

with the senosr now at the focal plane, can i assume im getting a IR beam on a few pixels at a time? how can i calculate this?

No, only a spot from infinity is focused to a spot ... everything short of that is blurred, but the smaller the focal length (and thus the size of the lens) the less it's blurred for closer objects.

As for calculation, I'm sure there is a formula ... but I have no idea, I can do it with a drawing though :P Just draw a line through a centre of the lens to the proper image distance which is 1/(1/f-1/o) (as I said before, the thin lens equation is all you really need). Then draw lines to that final point from the lens edges, then look where those lines intersect the focal plane ... that will be the width of the blurred image.

Or you could google the proper formula (look for circle of confusion).

wrote ...
edit: it looks like a spinning mirror wont work...

You're right ... how about this, it's perfectly possible to make a rotating powered assembly without brushes nowadays.

Shrad

Thu Nov 29 2012, 03:16PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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I dont know it you have knowledge of this utility, but Mxedgas from photonlexicon reminded me of its existence

you can simulate gaussian optics and laser cavities with a sufficient accuracy for our uses, which might be nice to have

Pinky's Brain

Thu Nov 29 2012, 03:25PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Even for a time of flight sensor that isn't usually relevant shrad. Only if you are using self-interference do you care about the properties of the laser to that degree and that's only used for ridiculously high precision.

Shrad

Thu Nov 29 2012, 06:28PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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that was just an idea ;) trying to contribute with what I know

anyway the PSST! software could be useful for optics calculation

Patrick

Thu Nov 29 2012, 08:36PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Pinky, yes i could make the whole head spin, using that non-contact power system, then using a 10mbps irDA pair, send the signal to the stationary base. (i wonder if this is what Velodyne does?)

the problem is, im seeing on aUTOcad, THE 1.9mm wide beam at 2.9m equalling .43mm at the senor plane... and at 10-15m it .3mm wide, it should be 0.010~ish mm

nevermind i botched the mrad divergence rate!!!

EDIT: with 6mm beam width its 137 micrometers... 15meters

im late for class, but if i do better math, it looks like 72um is possible with a 3mm beam... thats 18 pixels lit...

i dont know if a second optic would help or just complicate matters.

Pinky's Brain

Fri Nov 30 2012, 02:32PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

What a second lens allows you to do mostly is collect light from a larger surface while still getting a small focal length (which you need to keep defocusing down for close objects). You really need a prototype before you can determine you need that.

Patrick

Fri Nov 30 2012, 07:45PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Pinky's Brain wrote ...

What a second lens allows you to do mostly is collect light from a larger surface while still getting a small focal length (which you need to keep defocusing down for close objects). You really need a prototype before you can determine you need that.

yes, agreed. but near objects dont seem to be a problem. far objects with beam divergence is.

2Spoons wrote ...

I think you need to take a step back and work out what optical resolution you need at what distance, so that your ranging calculations give you the accuracy you require. Once you work out what your field of view needs to be you can work out the lens focal length from similar triangles. Don't worry about trying to match the laser line width to the pixel size - at infrared wavelengths there will be noticeable crosstalk between pixels, so the digital image will always be slightly fuzzy. THIS IS A GOOD THING - it allows you to interpolate across several pixels to get sub-pixel resolution of the laser line position.

yes i plan to do this, in C code:
start count at the lowest dark pixel, increment count for every dark pixel until first lit pixel is found... then count number of lit pixels, and divide by 2.
thus {[(# of dark pixels) + (# lit pixels divided by 2)] times the distance each pixels represents}

I may also use a distance schedule from near to far, so the pixels arnt linearly interpolated.

unless 2spoons can describe a better math procedure.

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