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Quince
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Need machine vision camera
I need a machine vision camera (digital) that can output raw (unprocessed and uncompressed), 10 bits or better, monochrome is OK (non-demosaiced Bayer
if color), at least VGA resolution. Sensor quality and ADC linearity are my main concerns, as well as reasonable light sensitivity, so I can minimize
the exposure times for dim sources (I plan to be photographing a scintillation screen to capture X-ray projections). Obviously, an SDK should be
available so I can use the camera in my software. I prefer a used camera due to the price issue. I missed one such camera on eBay that auctioned for
$170 US a couple of months ago; I'm willing to spend up to $250 depending on the specs.
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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Polverone
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Canon digital cameras have SDKs available for free; you just need to sign up through a short online form IIRC. You should be able to get a RAW file
that has the unprocessed sensor information at 10+ bits depending on the camera model. I'd suggest signing up for the SDK first and then picking a
camera, as I seem to recall that SDK-available capabilities varied between cameras (read the docs). I'm sure you can find a used Canon digital camera
for under $250, and of course the resolution will be much higher than VGA.
I don't know how ADC linearity and sensor quality are. You won't be able to acquire multiple images in a very short timespan. If you don't think that
consumer digital cameras are up to it, you might want to post to sci.image.processing on Usenet as I'm sure there are many more machine vision people
there than here.
[Edited on 2-3-2006 by Polverone]
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Marvin
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Good points. If the pixel depth is too low and the object is stationary then multiple images can be integrated to get 10 or 12 bits from 8.
For a lower cost option, something to try that may work, remove the lamp from a scanner, and put the X ray imaging sheet directly on the glass. As
the screen is viewed via several mirrors the sensor can easily be protected with a metal foil. Some more foil around the electronics and just scan
the images in witht he software they provide...
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Quince
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The scanner won't work as it's a pusled X-ray; I can't run the setup continuously from mere house wiring.
Since I need many projections for tomography, taking multiple exposures would mean a couple of hours to capture a full dataset, during which I'd have
to stay out of my room...
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Marvin
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You only need a few watts actual X ray power for a decent image. Depending on the area illuminated. Carl Willis at the fusor.net forums has done a
few things with a portable unit you might want to look at.
Running an unprotected X ray device as a CT scanner is a moron solution, build a wooden box, line the inside with lead foil. If you use <30kev
rays this will be good sheilding. Test it for leaks.
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Quince
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30 kV is pretty dangerous, as almost all of these get absorbed in a few cm of flesh, not to mention if it hits the eye. That's why medical X-ray
tubes often have aluminum plate windows to block low energy photons. A 30 kV leak is more dangerous than the 60-120 kV range I'm interested in.
I don't have enough lead foil, but combining it with thick steel plates, and staying out of the room should be sufficient.
I'm curious what portion of the beam is absorbed by the scintillation screen.
But this project will take a while. The best camera quote I've gotten this far is $1000, including one lens, for a new camera (I didn't find a source
for a used one) 
[Edited on 7-2-2006 by Quince]
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Twospoons
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How big does your scintillation screen *need* to be? I ask because there are x-ray cameras already available from companies like Hamamatsu. These are
basically a CMOS image sensor coated directly with the scintillator. Used in modern dental x-ray machines because they're about 100x more sensitve
than film.
No idea what price though ...
Helicopter: "helico" -> spiral, "pter" -> with wings
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Quince
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I don't see how anything larger than the sensor can be imaged. That's a severe limitation. My scintillation screen is about 17 cm by 40 cm.
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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Quince
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Well, I've found nothing under $500 this far (only new cameras). In sci.image.processing I got no replies, the bastards.
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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Marvin
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"A 30 kV leak is more dangerous than the 60-120 kV range I'm interested in."
This makes no sense. For 20 - 30kev region you can get away with a couple of layers of lead flashing for roofs. The point is to stop it, not gawp
while you get sunburn.
100kev region will require several centimeters of lead for reasonable sheilding. Steel will not be useful, iron has a low Z and thus has no
absorbtion bands for high energy X rays. It is inefficiant, even for an equivalent mass. You need lead, or bismuth or depleted uranium.....
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Quince
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100 keV has less absorption in the body. Much of it will go through.
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Marvin
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You are guessing.
100kev will be about half absorbed in the first 4cm of flesh, half the remaining in the next 4cm. By moving to 100kev you save less than half, but
you cripple your ability to sheild easily.
Worse than that, this assumes you put the same total energy into the X rays. For a given picture you need the same number of impact photons to get
the image, so the total dose is a factor of three higher, canceling out any gain due to poorer absobption in the body. Oh! But whats this? The
sheet you are detecting with will also have poorer performance , so you need more photons being produced in order to have the same number of hits.
The reduction in absorbtion by human tissue is more than canceled out by other factors, but the move to 100kev ish does make it very very hard to
sheild. The materials your house is made of will be of no help at all.
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Quince
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You better correct this Wikipedia article then, as it says up to 200 kV penetrates tissue no more than 5 mm: http://en.wikipedia.org/w/index.php?title=Superficial_X-rays
And then from http://www.sprawls.org/ppmi2/RADPEN/
| Quote: | | photons with energies less than approximately 40 keV. These are the photons with a low probability of penetrating a typical patient and contributing
to image formation. They do, however, contribute to patient exposure. |
And from http://www.utoronto.ca/safety/RadTraining/Module10.htm
| Quote: | | Diagnostic rays are designated for tissue penetration and are carefully filtered to avoid X-ray damage to the skin caused by the longer, more readily
absorbed wavelengths. |
<-- 1 to 3 mm Aluminum filter depending on kV
So the fact is that to image well a thick piece of flesh, you need higher kV. A collimator minimizes the amount of shielding needed.
ALSO, MODERATOR, WHY DOES IT STILL SAY I'M ON PROBATION???
[Edited on 28-2-2006 by Quince]
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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Marvin
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Well, we could believe a wikipedia article that was changed to add that faulty data just 3 days before your post. Or we could look at something
reliable.
"The Australian Radiation Protection and Nuclear Safety Agency " website on X ray safety.
http://www.arpansa.gov.au/basics/xrays.htm
Which backs up virtually everything Ive said. It does seem to indicate half a centimeter or so of lead sheild might be ok for 100kev, I'm uncertain
if they are talking about impacted photons with reduced energy or total removal. Either way, lead is slightly better than I expected, concrete is a
lot worse. Lead Foil is not good enough unless it can be layered to similar thickness.
"A collimator minimizes the amount of shielding needed."
Wrong again, as stated on that website about 1/3rd of the X rays bounce off the object typically and your beamstop will glow in the X ray region with
a significant fraction of the power in the beam even if your target doesn't. The sheilding you need is not significantly reduced.
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Quince
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A significant fraction? 1/3 is a small fraction. Squared-falloff with distance is an even bigger attenuator (for small target, which I'm assuming).
The wikipedia articles have logged history of edits.
I also put more weight on the second article I quoted. Maybe you should address that.
By the way, your own reference defeats you:
Table 2
Dianostic X-rays, CT: Accelerating potential 80 - 140 kV giving average photon energy 60 keV
[Edit:] Rudeness removed.
[Edited on 3-3-2006 by Quince]
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Marvin
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I see, Your the one part way through building a machine that will give your whole family lukaemia on the unchecked assumption that 100kev photons will
pass right through people without doing any damage and *I'm* the idiot?
On my 'self defeating reference', sure. You want to build a whole body CT, you'll need 60kev X rays and sheilding right round the room to match.
Thats what it says on the page, thats what you'll need.
Of course if you don't want to CT a whole person....
"I also put more weight on the second article I quoted. Maybe you should address that."
Yes, and the quote is out of context unless, again, you are taking an image of a large living animal. In those circumstances its absorbed dose for a
given quality of image. If you are X raying something inanimate everything on that page supports what Ive said allready and it contains data that
contradict all your assumptions about sheilding, though most of the graphs do end before the 100kev energy you wanted to use. I guess you'll crayon
those in after printing them out right?
"A significant fraction? 1/3 is a small fraction. "
Do the math. If you can reduce the dose to 1/3rd that saves less than 0.2mm on your sheilding. Will you even be making your sheild to an accuracy
better than that? Factor of 3 lower dose is useful in certain ranges, but the sheilding you need is not significantly reduced, exactly as I said.
"for small target, which I'm assuming"
Oh right, the small target you won't need 100kev for, or even the full 60kev sort of energy they use in whole body CT machines? That small target. I
guess you could use lower energies that are sheilded more easily with lead foil, or did I suggest that allready?
The funny thing is, I think you won't get as far as producing a 3D image with this setup even if it works. I think you'll hit the problem with multi
angle fan projections applied to fourier slice theorum and give up. I think at best you'll end up using the 90 degree fan projection the medical CT
scanners use and your image would be best captured with a linear rather than area sensor, say for example a home desktop scanner! Now wouldn't that
be ironic if I hadn't actually thought that far ahead at the time I suggested it. Another pearl cast.
Oh, incidentally the X ray beam will also be scattered by the air, the term for this is 'Sky shine'.
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Quince
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Who's using Fourier slice? I'm using SART reconstruction for cone beams per Mueller's paper. I need only as many projections as about 2/3 the
horizontal voxelgrid resolution, and it's quite tolerant of noise and such. He also gets rid of the non-uniform sampling artifacts of a cone beam by
stretching the Kaiser-Bessel filter based on ray density. I already have the software implemented and optimized because I had to do it for an optical
project (glass object geometry acquisition immersed in colored fluid with matched refractive index; since it's in a cylinder that refraction is known
and so are the raypaths).
I'd never have even considered the X-ray project if I had to do scanline based. Instead, I can just use a turntable with stepmotor and just take
projections from a bunch of angles.
I am sorry I called you an idiot, though, that was unreasonable.
As for target, I don't want to limit to flesh. I want to image such things as defects in soldering, and cracks in materials.
Air scattering? Sure, but the scattering attenutates even more because it increases the mean path length of the photon.
[Edit:] Here are Mueller's papers: http://www.cs.sunysb.edu/~mueller/research/ct.html
The one I was referring to is http://www.cs.sunysb.edu/%7Emueller/papers/ieeeCone.pdf
In another paper he implements it on GPU, but I didn't bother as it limits flexibility in my optical project to consider cases of some refraction
mismatch. Also, you can't use the high quality filter, and multithreading it on my dual core Xeon made it fast enough anyway. I get a few hrs for
512^3 grid and 360 images, 3 iterations over all images. 256^3 is a matter of minutes. Not nearly as fast as Fourier methods, but you can work with
sparse and crappy data.
[Edited on 3-3-2006 by Quince]
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Marvin
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I have to admit the cone based reconstruction does look very feasable with the hardware you are planning on. I'm really not a fan of the whole guess,
calculate error, modify guess, convergent method, it does solve the problem.
I do not think the resolution will be enough to see small cracks, I also think for solder defects a simple X ray will do fine, pushing this into CT
will make your X ray and sheilding requirements insane. If you were to stick with imaging household electrical items and the odd frozen fish this
would potentially be a much safer project.
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Quince
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Frozen fish? I was going to say the cat, but...
Algebraic methods are more general than Fourier slice ones. I've seen a paper where they get amazing results of tomography of binary densities (metal
defects) with only 7 projections. It is very easy to add a priori information to the error function you're minimizing. Check out this paper:
http://www.iop.org/EJ/abstract/0022-3727/32/19/306
BTW, I still haven't found any source of camera. I can't seem to find a quote for a digital machine vision camera even with just monochrome VGA
that's under $500.
[Edited on 4-3-2006 by Quince]
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Quince
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Is it very hard to convert a microscope camera for such a use? Or is something like aligning a different lens too much trouble?
[Edited on 7-3-2006 by Quince]
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Polverone
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| Quote: | Originally posted by Quince
BTW, I still haven't found any source of camera. I can't seem to find a quote for a digital machine vision camera even with just monochrome VGA
that's under $500. |
Did you look at the Canon consumer cameras? Were they unusable for some reason? It doesn't seem like you need industrial ruggedness or video frame
rates.
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Quince
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I'm asking about the microscope camera as I found a nice one for $200.
I'm wary of consumer cameras due to my concern for very high quality ADC and optics.
[Edited:] Corrected DAC to ADC as Polverone pointed out.
[Edited on 7-3-2006 by Quince]
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Polverone
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High-quality ADC, you mean? I think you gave up on that route too easily. You're more interested in quantitative image information than most users,
but consumer-level cameras can be pretty impressive nowadays. I would bet that there's a consumer camera with adequate sensor/ADC quality out there,
and as for the optics, the alternatives are so much more expensive and/or difficult that you should just try the consumer stuff first. I would imagine
that some optical flaws are worse than others here; an image that shows vignetting will be more troublesome than one that is uniformly a bit soft.
Some flaws might be corrected in software if they can be characterized from test images (which seems right up your alley). Really, I think you must
try before you buy, and that will be easier with consumer cameras than anything else. Don't you know anyone who owns a digicam? Does your university
have a media department or equipment center that will loan cameras out to students?
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Quince
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How would I choose among the Canon cameras? I can't find any listings on the details I'm interested in, such as what camera has how many bit ADC, or
the response curves, or how good the exposure control is.
And for these cameras, is the aperture adjustable? It is insufficient for me if I can only change exposure time.
BTW, I just got to the SDK downloads, and there are a bunch of SDKs, some of them listing the same cameras, and I can't figure out which one I'm
supposed to use WTF
[Edited on 10-3-2006 by Quince]
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Polverone
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According to this maker of RAW conversion software, the Canon G3, G5, G6, S45, S50, S60, D30, D60, 10D and 300D have 12 bit RAW and the G2, S30
and S40 have 10 bit RAW. The aperture is adjustable on all these cameras. I suggest you go to dpreview.net and reading reviews of the cameras. There will be a lot of information you don't care about, and not everything you do care about,
but they often discuss image quality issues like noise, low-light performance, chromatic aberration, sharpness, and vignetting in at least a
qualitative fashion (and with visual examples). When I did some more reading, I found that Canon cameras apparently already come with software that
allows you to control the camera and take pictures via a PC tether cable, so you probably don't need to touch the SDKs at all.
I'd suggest an older camera with 12 bit RAW, since you probably don't need super-high resolution and there will be a larger used market with better
prices. The G3, for example, has 12 bit RAW, very favorable reviews, and an ebay "buy it now" price as low as $225. I couldn't find a response curve
for it online with a couple minutes of searching, but once you've found a camera that looks acceptable in every other way, you can determine that
yourself.
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