Category Archives: Tech

Little Infrared Riding Hood

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My, what bright, glowing optical fibers you have.

One of my online compatriots recently explained a quick and easy way to do some IR photography. I felt compelled to try, and it was pretty easy. Cheap webcams are the most direct way to do this for a few reasons:

— they’re cheap. If you mess it up, you’re only out a few simoleans.

— they have manual focus. Modifying an autofocus camera requires you replace the IR filter with a glass plate, because removing it changes the optical path length. It’s a much trickier operation.

— it’s usually a fast modification

Just remove the lens — some of them simply unscrew — and check to see if the filter is mounted on the back. (If not, you’ll have to take the assembly apart. No biggie, though, it’s likely just one or two screws. You’ll need a jeweler’s screwdriver, probably phillips-head). Pop the filter off with a small screwdriver or equivalent; the filter may not survive in one piece, so don’t go into this expecting it to survive. Reassemble. You’re done. If the filter isn’t there, it’ll be covering the CCD/CMOS chip, but my extensive data (three points) says that it’s mounted on the back of the lens.

Plug it in to your computer and start taking pictures.

Expectations: This isn’t thermal imaging, so don’t expect bodies to show up glowing. Silicon, the element of choice, has a pretty sharp cutoff starting at about 950 nm, so what you’ll see in the near-IR. Something would have to be about 3000 K to be peaked at that wavelength and thermal images of body temperature targets peak between 9 and 10 microns. Also, the images will be small, since cheap webcams generally run only about a megapixel.

I just happen to have access to several infrared lasers (852 nm and 780 nm, the images use the latter), to give extreme examples of what you can see. This first picture is a laser table with the room lights off. You can see scattered light from several optical components, as well as light emanating from two optical fibers — not all of the light gets coupled into the fibers, and you’re seeing some of what leaks out (some probably in the wrong mode, since these are single-mode fibers, and the bending probably contributes)

IR laser table photo

In this second photo, there are two images of the same scene, taken with the room lights on. On the left, some shutters are shut, and on the right they are open, and you can see two fibers lit up. Also note the cylinder to the left — that’s a vapor cell with rubidium gas in it, set up for spectroscopy for servo-locking the laser. The laser is on resonance, so you can see the fluorescence as the beam passes through it.

As you can see, there’s quite a lot of scattered light, so normally this is encased in opaque plexiglass. None of the bright features shown are visible with the naked eye.

The LEGO vault

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Lego Secret Vault Contains All Sets In History

You know what I’m talking about, those were the days and all that jazz. But for real. Feelings and moments from times when everything was innocent and your only concern was your bike, a big carpet full of Lego bricks, and the amount of cocoa in your cereals.

After that, it was one wave after the other, jumping from Lego Space to Lego Technic to Lego Town to Lego Castle and Lego Pirates and Lego Star Wars. Each set a memory, a particular Kodak moment blurred by the occasional teary eye

Probably no Fun Snacks, though

via Kottke

Lab Tricks

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No, not that kind of trick, you pervert*.

I was doing some homemade wiring, and whether it’s power or signal, you generally want to use twisted-pair (or triplet, or quad, etc.). It’s faster than running all of the single-wire, so there’s a labor-saving aspect to it, but there’s a data quality aspect to it as well.

Any time you have a pair of wires that completes a circuit, you have to worry about ground loops and other signal pickup. If the wires are separated, and the magnetic field that runs through them changes, Faraday’s law tells you that you’ll add some current or a potential difference to the loop. The bigger the loop, the more flux you’ll be capturing. If you write this onto the common ground for your experiment, you will be putting this signal onto all of your equipment. And this is not just the earth’s field — everything radiates. A loop is an antenna for picking up 50 or 60 Hz power and also any other frequency equipment you use in the lab. (Early on in my current job, in the dark days before I had a CD-burner, much less an iPod, I tried listening to the radio in the lab. At one point we added an Acousto-optic modulator and started driving it a smidge above 100 MHz, which was almost the same frequency as the local oldies station, and I couldn’t get that station anymore because of the interference). This will get written on to your signal lines, and will get picked up by power lines, which then writes it on to all of that precision equipment you soldered together, and forgot to add bypass capacitors everyplace you needed them) . Chasing down ground loops is a big pain, as is filtering out noise. Twisting the wires means that the net current flow of any power signal is zero, as current input is as close as it can be to the return path, so the far-field radiation — basically anything further away than the diameter of the wire bundle — is nonexistent. If it’s a data line, it doesn’t look like an antenna, except perhaps for extremely high frequencies — the area for magnetic flux is vanishingly small, so it has no opportunity to pick up a signal.

So you want twisted-pair, but the commercial pickings can be slim for the exact wire type you might want to use, and besides, you want to color-code what you’re doing. So here’s the trick: use a drill. Clamp on to the wires with the chuck, pull taught and squeeze the trigger. Wind up to a reasonable pitch and — I cannot emphasize this enough — release the chuck before lessening the linear tension on the wires. You’ve added a lot of “twist” to the wires, and they will untwist. If you release the linear tension first —trust me on this — it will jumble up like a telephone cord. (If you’re under 30 and don’t understand the phrase “telephone cord,” it’s the phone you’ve seen at your grandparents’ house, perhaps in the basement. The phone might even have a round disc on the front, with ten holes in it around the perimeter)

*my conclusion after perusing the somewhat disquieting search-engine stats. Suffice to say that using “animal robo-p*rn” in a title isn’t leading to searches that are attracting science-minded folk to the post.

Ghostly Visages

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No, not of an alien at the window.

Here’s a little movie showing atoms being trapped and mistreated. What you’re seeing is a video of the monitor that’s hooked up to a little IR camera on the vacuum chamber. The really bright spot that’s squirming around a little are the atoms, or technically, the fluorescence from the atoms. There are probably more than a Sagan of them (i.e. biilliyuns) at about a milliKelvin or so in temperature (which is considered warm!) because it hasn’t been fine-tuned yet. The bluish circle is reflected light off of a flange, and there’s some other scattered light visible.

You need to a flashplayer enabled browser to view this YouTube video

About 10 seconds in, the trap’s magnetic field is turned off, and the atoms squirt off to the side. A second or two later the field is turned on again, and the trap fills up. If the lasers were properly aligned and balanced, and the earth’s magnetic field were either shielded or zeroed out with trim coils, then what you would see is a nice uniform expansion into a much colder (microKelvin-ish) optical molasses, but the earth’s field is still present here, so that gives rise to an imbalanced residual force which is small compared to the trapping force, so you only really notice it when the trapping field is turned off. The molasses impedes the motion of the atoms, but doesn’t technically trap them, i.e. it doesn’t define a point where the atoms should be, so when only the lasers are there, the atoms would normally just drift through, very slowly. But here they’re being shoved a little bit.

The accelerations involved here are large — these atoms can scatter a million photons a second, give or take, depending on the exact laser frequency, so even though an individual scatter changes the atom’s speed by about 6 mm/sec, when you scatter them that rapidly you can get accelerations of hundreds of g’s. But in the situation here, where the atoms are almost at rest, that’s balanced by an equally large acceleration from an opposing laser.

But Why is it so Hot in the Okefenokee?

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Evaporative (Swamp) Coolers

I was discussing this with our resident mechanical systems guru just a few days ago — really hot, humid weather had some of the HVAC systems gasping, and if you can’t reject heat anymore, the system stops cooling (a basic bit of thermodynamics lost on some people). He was reminiscing about when he could use swamp coolers, in the southwest part of the US.

Evaporation works as a cooling mechanism, which is why we sweat when we get hot, because the molecules that go to the gas phase take more than their share of energy with them — somewhere around 2300 J/g, depending on the temperature. And the energy to change that one gram of water’s temperature by a single degree is 4.18 J, so if I have 100g of water and lose one gram to evaporation, the remaining water will cool by 5.5ºC! (Assuming, of course, no other heat transfer to warm it back up. But hey, we’re physicists. Our cows are spherical and inclined planes frictionless)

You can use this cool things off without ice — put the beverages in a canvas bag and hose it down and let evaporation do the work (the canvas holds on to the water, so it doesn’t just run off). It won’t make the beer frosty, but as long as the water can evaporate, it’ll cool it off some. (rule of thumb — if your cold beverage containers tend to “sweat,” then this probably isn’t going to work very well. But here’s another trick for you, from my navy days aboard the USS Disneyworld — to keep that pitcher cold, fill a tall glass or cup with ice and let it float in the pitcher. Cold but no dilution.)