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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
Patrick wrote ...
yes, agreed. but near objects dont seem to be a problem. far objects with beam divergence is.
AFAICS the decrease in magnification with distance makes that irrelevant.
I was trying to confirm this, but im worried i might have to light up 20-40 pixels, of just 497 useful pixels. im not sure what 2spoons thinks is a "good" amount of ir blur, versus what would be too much, i was hoping for 3-5 pixels being lit at a time...
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615
5 pixels is enough to get over the quantization jumps you would other wise get. Your method of finding beam mid point works, but will be prone to jumps. A better method is to use a weighted average technique, so brighter pixels are weighted heavier than dim ones. Are you using ambient light cancellation?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
2Spoons wrote ...
5 pixels is enough to get over the quantization jumps you would other wise get. Your method of finding beam mid point works, but will be prone to jumps. A better method is to use a weighted average technique, so brighter pixels are weighted heavier than dim ones. Are you using ambient light cancellation?
Oooooo! glad i asked. i guess i will figure out what ambient light cancellation is... and as long as the DLIS-2k is good at analog, then yeah ill look at the btighter pixels, can you elaborate on the math for that?
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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if each pixel gets a number in the array (0 - 2047), and the pixel value is how bright it is then weighted average: sum(pixel value x pixel number) / (sum(pixel number)
To make this calc faster you can use thresholding to find the start and end of the laser image, and go a few extra pixels either side.
An 8-bit ADC is plenty of resolution - more than that and you are really only getting noise bits.
Ambient subtraction: capture a frame with the laser off, and store it. capture another frame with the laser on and subtract the first frame from it . This gets rid of everything in the image except the laser line, making the line much easier to find in software. Its not a perfect technique, motion and time delays between frames can reduce its effectiveness, as can flickering light sources (halogen lamps are really bad: lots of IR modulated at 2x mains). Ideally you want a very bright laser, for short shutter times, and a fast ADC to speed up capture and get the two frames as close in time as possible.
I know you've been looking at using a simple lens for imaging, but it might be worth considering the small lenses used for CCTV and webcams. These come in a large range of focal lengths, have a handy fine threaded (12mm) barrel for mounting and focusing, and are ridiculously cheap.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
2Spoons wrote ...
if each pixel gets a number in the array (0 - 2047), and the pixel value is how bright it is then weighted average: sum(pixel value x pixel number) / (sum(pixel number)
ill code this for sure...
2Spoons wrote ...
To make this calc faster you can use thresholding to find the start and end of the laser image, and go a few extra pixels either side.
yep i was going to do this 1 or so uS before the main measurement.
2Spoons wrote ...
An 8-bit ADC is plenty of resolution - more than that and you are really only getting noise bits.
Agreed.
2Spoons wrote ...
Ambient subtraction: capture a frame with the laser off, and store it. capture another frame with the laser on and subtract the first frame from it . This gets rid of everything in the image except the laser line, making the line much easier to find in software. Its not a perfect technique, motion and time delays between frames can reduce its effectiveness, as can flickering light sources (halogen lamps are really bad: lots of IR modulated at 2x mains). Ideally you want a very bright laser, for short shutter times, and a fast ADC to speed up capture and get the two frames as close in time as possible.
yes ill subtract a first initial frame from the second actual frame, then use a minimum value for lit pixel count, like 50% brightness is on(and goes to the averaging algorithm), less is a dark pixel. im thinking a fast sample of 3uS-500nS time interval will be fast enough to avoid flicker noise for unwanted sources. but i dont think a 9$ IR laser line can be modulated that fast.
2Spoons wrote ...
I know you've been looking at using a simple lens for imaging, but it might be worth considering the small lenses used for CCTV and webcams. These come in a large range of focal lengths, have a handy fine threaded (12mm) barrel for mounting and focusing, and are ridiculously cheap.
Ill look at this then, any sources? EBAY is where ill start.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
I still think the 50mm FL lens with it's centre 50mm away from the laser would be a good place to start (covers around 10 mm of the sensor). It has the advantage of nice round numbers.
This is with the setup carbon rod linked to with the laser perpendicular to the plane of the lens/sensor, not with the laser at an angle like in the original post.
PS. the diode itself in that 9$ laser almost certainly can be modulated with 500 ns pulsewidth, you'll have to rip out the existing electronics though ... I'm debating myself what's the easiest/best/cheapest way to pulse a laser diode is, there are reasonably priced specialist ICs, you could use an opamp voltage to current converter, but I'm leaning towards having an inductor with constant current and simply paralleling the diode with a RF BJT/MOSFET, turning it off to push current through the diode.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
...unfortunately turning that into a decent line will be quite hard...
why? is it the package? is everything inside that plastic case? so we just pulse power to its pins and it works at nS speed? becuase it seems its the driver most people build wrong and kill there diode lasers.
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Optics and laser beam width, need help please.
