Photographing fusors and near IR issues

This is bound to get mixed up with things in Electronics, check both. Physics-specific stuff here, mostly.

Photographing fusors and near IR issues

Postby Doug Coulter » Thu Nov 06, 2014 7:14 pm

MAybe we have someone who knows optics better than I (Peter?) here. I have a problem with our video remote viewing I'd like to really fix, and I'm almost there, but not quite. Near IR, even in CCD cameras that have the IR block filter, shows as purple in the image. It even looks like the D spectrum. This is a huge problem for remote monitoring of the fusor, as we are running the grid back end near the melting point of the copper feed through, and want to be able to judge temperature fairly accurately there - hard to do if everything in the picture is purple. (Edit, that was a high end webcam, and a single frame grab from a video, we can and do do a lot better)

So, I've been testing this and that, hoping to avoid paying $70 or so for an IR block that might or might not work well enough. Since it's close to the same temperature, and well, here where I'm working on a major raspberry pi project for streaming video (it has FAR less latency than the VLC/webcam -> network -> mplayer which is the fastest way to do a web cam latency wise), I'm working with a Mr Heater as my test source.

I had an old welding goggle glass, the kind that has a LCD "instant dark", which had gone bad, but I saved the many layer laminate for who knows what reason. Well, after some belt sanding around the edges to heat and remove some of the glue, I managed to pry off some small pieces of the thinner outer layer, which seems to be a pretty good IR block at the near end where it matters (the camera can't see my soldering iron cranked to 900f - that's far IR).

Doing this shattered the filter layer (that's what I assume it was since it works, kind of) into small pieces, but the pi camera lens is tiny...so I taped one over it. BTW, these are originally high rex pix that I severely cropped - the Mr Heater was 20 feet off.
At 10 feet, you can read the lables on packages with this thing, it's better than you'd think (around 4 megapix, though the glass probably isn't as good as that).

Before.jpg
Raw, no filter, IR blocking camera - well, not quite
Before.jpg (37.43 KiB) Viewed 9719 times

After.jpg
After a very thin layer of slightly green stuff...
After.jpg (36.12 KiB) Viewed 9719 times


Note that at least on my monitors here (they tend to be the best you can get FWIW) that's still not quite right - the real thing is orange at the top of the grille, and dull red/orange at the bottom. There's more work to do here. I think I need either a harder cutoff, or one closer to visible. I'm mainly interested in viewing fusor focus (man, this is going to rock at that range, about 15") and how hot things are getting. I don't want to be shutting down or rreducing power because this is showing me things hotter than they really are. Just one more little project to eat up time.

I turned up the brightness and the contrast on the second picture in the pi interface to make them as close as possible - the filter has a couple stops of attenuation at visible too.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Photographing fusors and near IR issues

Postby Peter Schmelcher » Thu Nov 06, 2014 8:51 pm

Doug I am no expert so caveat emptor. That established I would suggest a IR cutoff filter or hot mirror which will reflect IR. http://www.edmundoptics.com/ is a reasonable outfit for new stuff with specifications, pricing starts at $35 for <5% IR transmission for example parts #64-463 or #55-234.
I also like this outfit http://www.surplusshed.com/pages/item/l14561.html for inexpensive bits and pieces but they usually have very limited specifications.
Unfortunately silicon camera sensors are more sensitive to IR wavelengths than visible light so you may need multiple IR filters in series to capture what your eye sees. http://en.wikipedia.org/wiki/Photodiode.
-Peter
Peter Schmelcher
 
Posts: 26
Joined: Tue Sep 30, 2014 8:04 pm

Re: Photographing fusors and near IR issues

Postby Doug Coulter » Thu Nov 06, 2014 10:08 pm

Yes, I've been experimenting with a very thin, looks like an interference filter, carefully pried from a welding goggle "instant dark" LCD. Helps a lot with one layer. Thing is, these also block some normal visible - probably in the 50% range, and we have low light in the fusor, so multiple layers don't work out so well (and in fact I just tested that with the propane heater - it's a little better with two, but the "dark noise" goes way up). For fusion, any atom slow enough to still have an electron falling through the energy levels and giving off light is a lost one - this is something most people who look at poissers don't understand. You're only seeing the stuff that had no chance at all for fusion. The stuff that fuses has no electrons...thus no photons come off, and is moving so fast by comparison to everything else it won't pick up an electron either.
Interesting that a "perfect" fusor would be dark inside, eh?

Looks like I want a sharp stop from about 700nm (don't care if it's not hot enough to glow at all) to about 1100 or 1200. UV doesn't seem to bother these guys, probably their own lenses stop that (we have plenty, enough to give you a burn quick as I learned when running with a sapphire window we got surplus). Longer than 1200 or so, it seems silicon just doesn't respond, so no worries there - for example, these can't see my soldering iron, even with the No IR stop filter.

I just did an interesting test with webcams, even though for real life I'm using the Pi stuff - much faster and higher resolution. It turns out that a pretty old logitech has a far better IR stop - nearly ideal - than a brand new top of line from the same guys. Interesting. I guess they "value engineered" the newer one?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Photographing fusors and near IR issues

Postby Peter Schmelcher » Fri Nov 07, 2014 12:03 am

Welding eye protection is UV and brightness but not so much IR. Humans have been staring at wood fires and sun sets for a few generations and our retinas reflect near IR. Not unlike some animal eyes do at human visible wavelengths. Hot mirrors don’t adsorb light as much as wavelength steer the light. The hot mirror I suggested has a visible light transmission of over 90%, and is probably what you want. To give a definitive answer and say that you need x number of these filters you would need to multiply the silicon sensor curve with the grid black body temperature curve (over the grid operating temperature range) and the filter attenuation curve given some residual IR leakage constraint. But what you actually want is a subjectively better image, so I would cut out the middle men and buy a filter or two and see the result :)
Peter Schmelcher
 
Posts: 26
Joined: Tue Sep 30, 2014 8:04 pm

Re: Photographing fusors and near IR issues

Postby Doug Coulter » Fri Nov 07, 2014 12:24 pm

Peter, things like eye protection regulations for welding have changed dramatically in the US in the past few years. Pretty much anything that can be focused by an eye lens onto a retina has to be attenuated X dB, which is one of the reasons you can now weld all day without sore eyes...In fact, the very thin greenish glass (looks like a mirror on one side) filter is IR stop only, taken from one of the newer welding helmets (about 3 YO) and it does nothing for UV at all, I checked.
Did you know the sale of the Nd glass lenses for glass blowers is banned now because it doesn't stop IR, just the sodium lines? Yup, only can find surplus. Now you have to pay a LOT more for ones that meet the new regs. <sarc> Gee, I'm so glad the government cares so much about someone's failure of common sense or inability to feel harm happening to them </sarc>

What's frustrating is that even the "pro" optics guys, like Edmund (well, they are pro at charging too much for everything) spec this stuff in percent, and the eye is logarithmic and you really want dB, which would show just how lousy most of these filters really are...sigh. At $40-$70 each....I'm not in a super hurry to experiment with something that amounts to not having a spec - pretty chart but with little usable info.

I call that specsmanship and we have seen it all over for the past few decades, especially in electronics. If there's no noise spec for an opamp, it's noisy. If there's no differential nonlinearity spec on an a/d...and so on. You have to look for the dog that didn't bark most of the time.

It looks like my problem is in the VERY near IR here, and most of the IR stop filters seem to start working at too long a wavelength vis the passband of the blue filter (or even the red one) in the cameras I can get. I really want to stop even dull red - from the very edge of what the eye can see, because I don't care if something is dull red hot or only visibly incandescent in total darkness, explaining my guess at 700 or so nM above as the short end. Silicon looks like it's really dead by ~1200, the reason for my other end of specification, but longer - no problem, I don't want any of that stuff, but the silicon itself won't let that stuff past the bandgap.

I believe the lower stuff in this picture, carefully separated from some defective welding goggles (well, I cracked it a little, it's very thin - .021") is more or less the "hot mirror" you are talking about, but I will go check specs on that too. This looks like a mirror on both sides at the right light angle, I think it's an interference type. Two layers are no better than one, but two block more of what I want to keep.
GoggleParts.JPG
Day-glow paint fluoresces merrily behind the light green stuff.


According to John Strong's "Procedures in Experimental Physics" what I want is about what a 2% solution of CuCl2 in water 1 cm thick gives me - very sharp cutoff at the edge of visible. I might just try that, I've got microscope slides I can make a cell out of. That blue-green glass at the top of the pic is pretty good too, but obviously messes with the color balance in the visible. I can't even find specs on this one anymore "Corning #4060" is what it's marked - it was in a set of boxes of filters my master scrounger friend (BillF) got awhile back - it's probably around the same age as I.

Edit: Yeah, I checked Edmund...I think I've already got what amounts to the same stuff. Stops cutting while still in the range of silicon...down at the long end, and not enough cut at the short end either - too narrow for this, or so it seems.
I guess you'd need two filters with overlapping stop bands or something.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Photographing fusors and near IR issues

Postby Peter Schmelcher » Fri Nov 07, 2014 7:25 pm

Keep in the back of your mind that an interference mirror will change its cutoff or pass band wavelength when it is rotated…
Peter Schmelcher
 
Posts: 26
Joined: Tue Sep 30, 2014 8:04 pm

Re: Photographing fusors and near IR issues

Postby Doug Coulter » Sat Nov 08, 2014 11:38 am

Ah, are they polarization sensitive? Looks to me like what I have stops OK, just not in a wide enough band to cover the whole are silicon senses that we don't want to see. I will try that rotation thing, perhaps with two layers? The source isn't polarized (much) except by the fact that we're looking at it through glass at at an angle slightly off normal.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Photographing fusors and near IR issues

Postby Peter Schmelcher » Sat Nov 08, 2014 1:39 pm

I used a poor choice for an angular description. The #64-463 hot mirror has a dielectric coating thickness designed for operation at 45 degree. http://www.edmundoptics.com/optics/opti ... rrors/3150
As the 45 degree angle is changed the photon travels through more or less dielectric coating thickness which scales the mirror design wavelength.

Alternatively a 45 degree cold mirror would reflect visible light and pass IR. By using some lead shielding this optical geometry might reduce camera pixel noise from fast neutrons and x-rays. Also if the protection worked well you could use a higher end camera with its digital video output.
Peter Schmelcher
 
Posts: 26
Joined: Tue Sep 30, 2014 8:04 pm

Re: Photographing fusors and near IR issues

Postby Doug Coulter » Sat Nov 08, 2014 5:26 pm

Gotcha, sure, all interference filters work like that. Perhaps I could use two at different angles to get more octaves band-stop? I plan to wrap this whole mess up in lead from the outside on general principle; it will be dark in there. But that has virtually zero effect on neutrons.
Lucky, they also have almost no effect on cameras unless they are slow, and we take care to not slow down a bunch near the fusor as that makes them more dangerous to me, too (capture gammas in all the hydrogen containing stuff around a wood house are 2.2 MeV - there's no stopping them either, the absorption of lead has a 5 order magnitude minimum right there...crap). We'd rather have fast neutrons just zip away - as far as possible for now. Later we'll want them for heat.

I'm not yet sure about the X rays. There's nothing that can stop the neutrons, period, that's why this project exists - I'm "getting outa Dodge" re being near it while running. It's driving me nuts doing all this prep, but let me tell you, that dose I got was quite the eye opener...so I'm willing to do all this off-topic junk to stay alive long enough to do the fun parts.

Our first test, which was pixel-noise free from X rays was shot through 1/2" thick leaded plexiglass. I might try without next time, as it adds some internal reflections, is slightly orange, old as heck, and scratched up. I'll only keep using it if I must, and in that case I have a heck of a buffing/polishing job ahead of me. I haven't yet found a source of new 3" round lead glass windows which is the size I have room for, and I'm loath to cut up the big piece I normally have in front of the big viewport. In case you weren't aware, cutting lead radiography glass is a good bit harder than regular glass cutting - it tends to shatter.

As far as I can tell so far - the pi camera is actually pretty near top of the line, only a many k$ DSLR might be better, and I don't want to sacrifice one of those. The pi cam is darn good - 2900-something on a side resolution, good in low light, and all important - I can control everything from exposure time, contrast, how it meters light - you name it, it's about 20 options (and do all these adjustments remotely), unlike a webcam which always self-adjusts just wrong for this - they seem to always make the light stuff saturate/bloom in the attempt to make the dark stuff visible, or the other way around, and focus on the wrong stuff in general - I'm too close for good focus with your average logitech camera as is, and would have to add a diopter or two....
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA


Return to Metrology

Who is online

Users browsing this forum: No registered users and 5 guests

cron