Laser target shooting (finding laser spot location)

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Hello Everybody,

I want to get expert opinions about how we can find/measure the position of
a few millisecond long laser spot on an A4 size of target area.

Although my application notting to do with shooting, using the "laser target
shooting" analogy will help. In these days we can buy small red laser diodes
just for few dolars. Assume we have one of them.  The Laser diode will be
connected to a microcontroller which will trigger (turn on and off for few
millisecond) the diode. (If required, I think we can also modulate the laser
diode for few KHz ).

We will point this laser and trigger to a target area of approximatelly
20x30 cm in size and at a distance of 25-50meters. Now the question; on the
target, how we can find the position of this laser spot?

I know there are some laser shooting targets which practically doing this.
How they work?

Thx.

Favne Reas




Re: Laser target shooting (finding laser spot location)

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http://www.philohome.com/sensors/lasersensor.htm


Re: Laser target shooting (finding laser spot location)

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Thank you for the link. I liked their "Laser Target Finder Sensor". The
content of this link is fantastic. It explains their system in very good
details. Also the small movie file demonstrates its use.

But, my application is slightly different. I need to know the location of
the laser beam pulse on the target with 5-10mm accuracy. As one suggested
may be a high speed camera would be usefull. But if we try to identify only
the coded/modulated laser rather than any red light, than I tink we need to
have something else or something additional to high speed camera.

Any suggestion?



Re: Laser target shooting (finding laser spot location)
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1. Turn the laser on.

2. Capture a frame from the camera, call this image A.

3. Turn the laser off.

4. Capture a frame from the camera, call this image B.

5. C = A - B (pixel by pixel difference)

6. Apply a little fuzzy matching to identify the red spot in C.

Essentially, what I'm suggesting is to forget about modulating or
coding the laser signal, except to the extent that turning it on and
off in sync with the frame rate of the camera can be considered
modulation.  Step (6) is where the magic is, but if your camera is
decent and the elapsed time between (2) and (4) is minimal, it should
be pretty easy.  If you put a filter on the camera that's matched to
the wavelength of your laser, you're almost certain to get it right in
the absence of malicious attempts to fool the sensor.  You won't be
able to have multiple sensors operating at the same time with the same
field of view, unless they are tightly coordinated or use different
wavelengths.

--
Randall

Re: Laser target shooting (finding laser spot location)

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In fact the exact 'inverse' of this approach, was what was used for the
early raster scan display 'light pen' systems, where a bright spot was
scanned across the display, and when the pen 'saw' the spot, the X,Y
coordinates where the detector in the pen was pointed could be estimated.
The approach outlined, lends itself to simply synchronising the laser to
the frame sync pulse, activating the laser on alternate frames. Then the
point with the largest change between alternate frames, is the image of
the point where the beam is pointing. The faster the camera sync rate, the
faster the detection can be, and the better the rejection of other
sources. At the 'crude' end of the design, you could even ignore
complexities in matching, and do a direct frame to frame compare.

Best Wishes



Re: Laser target shooting (finding laser spot location)

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Using a filter will be a key element here.  A filter having only a
narrow (10 A to 50 A) bandpass at the laser wavelength will make the
camera think it's the dead of night, except for the laser spot.  At a
guess, you might get one of these for $50 new -- or maybe Sam will have
leads to surplus sources.

Re: Laser target shooting (finding laser spot location)
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Good explanation, but I think he needs a coordinate, not just being
able to identify the point in the frame.

This might involve recognizing some fiducial marks. In the absense of
marks, this problem may be unsolvable.

If an absolute vector is acceptable, the high res rotary encoders measuring
elevation and azimuth may be OK. This is getting expensive.

There is a company in Austin that makes 3d digitizers using this method. I
think they use time of flight to get the distance, which makes this even
more exotic (=expensive).


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Re: Laser target shooting (finding laser spot location)
On Sun, 11 Sep 2005 00:05:49 +1000, in article
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Well whatever you use is going to need some form of optics in front, so
is this going to be a fixed distance or any distance position finding?

Look at Hamamatsu Position Sensitive Detectors, that give an XY on a
sensor that is focused as required. Primarily meant for cameras and
industrial applications.

<http://sales.hamamatsu.com/en/products/solid-state-division/position-sensitive-detectors/catalog.php

They have various application notes, which are quite detailed, I was
looking at for one application.

A google search on "Hamamatsu PSD" will give various other references
including a Circuit Cellar project for laser level using a 1D sensor.
You obviously need a 2D sensor from their range.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Laser target shooting (finding laser spot location)

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Can you be more specific about your actual application?
Could the target for example be a television screen?
If a television screen you could use the method that
shoot em up games use to determine what part of the
screen the "gun" is pointed.


Re: Laser target shooting (finding laser spot location)
THe typical TV camera has about 600 pixels per line and there are 480 lines.
With a target area of 300x400mm this means that you'll easily be able to get
1mm accuracy in the point which is much better than the 10mm that you need.
Illuminate the target to get about 10% gray and the spot will probalby
saturate the camera which means that you just need to know where the more
than 50% illumination is and the 10% will give the outline of the target.
If you're running a CCD or CMOS accay and are able to access the imageer
direct, I've got a simple program in QuickBasic on my website
http://bobmay.astronomy.net/misc/software.htm that will show you where to go
with the software.  The program is GUIDE and it uses an early B&W webcam
that the commands are available for to do the imaging.  For additional help,
there is another astro camera that used the parallel port to directly acces
the imager chip called the Cookbook Camera for how the circuits in the
camera work for such an application.
Today, if I was to do a job like yours, I'd probably do a PIC processor type
processor and drive the imageer chip from that.

--
Why do penguins walk so  far to get to their nesting grounds?



Re: Laser target shooting (finding laser spot location)

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How accurate do you need this position ? What sort of precision do you
need ? Must the target be portable ? At one end of the spectrum you
need lots of sensors in a grid, which can be read. On the other end of
the spectrum, you can use optics + mechanics to direct the beam into a
sensor. Based on time, and the mechanical position you can determine
position.

Regards
  Anton Erasmus


Re: Laser target shooting (finding laser spot location)
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With a webcam, (or video camera + frame grabber)  a cheap spotting scope
or telephoto lens, and a pc you can resolve the position of a light
flash on a screen to a resolution of about 1 part in 500 of the
dimensions of the screen in both axis.  The target can be any difuse
reflective  surface like a sheet of paper or even a completly random
background. I've written software which can measure the positon of a
spot on a video image to better than one pixel using centroiding but for
a different application and not available for distribution. Still, it's
within the capability of any decent programmer. Cost for all the
hardware except a typical P4 PC should be under $300.  Higher resolution
and faster camers are available with digital interfaces but cost more.

You can download source code for the program "gspy" from
http://gspy.sourceforge.net which runs under Linux.
While it is intended as a securiity camera program it has all the
routines to grab and analyze video images and to locate groups of
adjacent pixels which change.  It's close to what's needed for the above
application.


Re: Laser target shooting (finding laser spot location)

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[snip]

I can attest that finding centroids of edge-enhanced,
frame-differenced images can locate spots of interest
in extremely noisy images.

Re: Laser target shooting (finding laser spot location)
for a simple machine vision solution using an NTSC camera with appropriate
supporting hardware and software and lensing, you should be able to reliably
detect the pointer to within ~1/2 pixel
assuming that you digitize your frame at 640x480 pixels, and align it so
that the sensor is in the same orientation as the target, that should allow
you to achieve spatial resolution on the order of  30cm*1/(640*2)= ~.25mm.
Higher resolution cameras would allow for better spatial resolution

This assumes that your laser is bright enough to be noticable at 30
frames/sec--you may find that you need to go to a faster camera to detect a
short, weak pulse.  Also, cameras do not acquire images continuously--for an
exposure on the order of a couple milliseconds, you may need two "out of
sync" cameras set up so that on is collecting an image while the other is in
its blank phase....  If you can increase the laser pulse time to something
greater that a full camera frame cycle, things get much easier....

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Re: Laser target shooting (finding laser spot location)
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Hmm depends if you want a 'spot is about here' or want to measure to better
repeatable accuracy, as most measuring algorithms rely on at least 5 pixels
per edge to avoid aliasing issues
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THe guy was going to be happy with 10mm accuracy.  Nearly filling the image
with the target will give near the accuracy of the full image size.  Even at
a 400x400 pixel size, it is merely a little math to get to find out where
the spot is iwth greater accuracy than is needed.  In addition, doing a
subtraction of the unlit image with the lit image will subtract out all of
the background quite nicely and even a simple centroidin of the spot will be
better than is needed.
The program on my website has all of the math needed to find out where the
spot is - after all, the program is designed to track a star for autoguiding
a telescope for photographing the star field of interest.  It downloads an
image from the camera and detects where the star is in the field and
provides an error correction to put the scope back to where it is supposed
to be.  Much of that logic isn't needed (the tracking part) but the finding
of the star in the image part will do a vey nice job for him.
Target illumination is more that of makign sure that the background around
the target is not going to be brighter than the target and the illumination
level is intended to find the position of the target in the image.  This is
an optional item tho as just knowing where the camera is pointed will be
sufficient probably for the original poaster.
These are all minor problems that can easily be solved by intelligent use of
the camera and laser pointer.  Get a positive attitude and all problems can
be solved.

--
Why do penguins walk so  far to get to their nesting grounds?



Re: Laser target shooting (finding laser spot location)

Please leave attributions in.

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That all depends on circumstances, I have seen many a 'simple subtraction of
xx image' screwed up by beat frequencies of lighting.

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....

Star tracking has different issues to daylight image processing, internal
or external.

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At all times the illumination has to be constant to be effective, otherwise
the background subtraction FAILS. At the original distance of 25-50m that
is not always guaranteed.

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MINOR problems, in over 20 years of dealing with image processing in lots
of market places (industrial, medical, security, leisure and others). the
most common problems are illumination, lenses, and wrong sensor (type or
use of it). These are the most overlooked aspects. Forgetting the classic
ones like "why can't I stream high resolution images as raw data to a
USB 1 hard drive".

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Helps if you know BEFORE hand what problems you are trying to solve and
what is going to bite you. Too many people think imaging is simple
and attempt applications that are way beyond the simple kit they think
will do anything.

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For many reasons including avoiding the worst winds.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Laser target shooting (finding laser spot location)
Yppe, the illumination of the target is indeed important.  I did assume that
if the guy saw such a problem, he'd solve it in the proper fashion.
I'm not here to provide the problems but rather answer his question and as
such, all I heve heard from you is how he can't do it.
I'll note that tracking a star does include the actual finding of that star
in the image and that is the part of the software that he should be looking
at.  The whole process of tracking isn't needed by him and that part of the
program should be ignored.  Yet, you ignorantly state that because the
program is designed to track a star, it isn't relavent at all to his
problem.

May you reinvent the wheel everytime that you need one!

--
Why do penguins walk so  far to get to their nesting grounds?



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