Laser target shooting (finding laser spot location)

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

Favne Reas wrote:
> 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


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

On Sat, 10 Sep 2005 13:18:00 +1000, "Favne Reas" <> wrote:

>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?

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

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

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?

On 2005-09-10, Favne Reas <> wrote:
[...]
> 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.

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

"Randall Nortman" <> wrote in message
news:W8CUe.10264$ link.net...
> On 2005-09-10, Favne Reas <> wrote:
> [...]
>> 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.
>
> 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.
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

In article <W8CUe.10264$ .net>,
Randall Nortman <> wrote:

> 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


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.

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....

"Favne Reas" <> wrote in message
news:43225059$0$22099$ u...
> 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
>
>
>

Anton Erasmus wrote:
> On Sat, 10 Sep 2005 13:18:00 +1000, "Favne Reas" <> wrote:
>
>
>>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?
>
>
> 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

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.

Randall Nortman wrote:
> On 2005-09-10, Favne Reas <> wrote:
> [...]
>
>>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.
>
>
> 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.
>

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).


> 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.
>