Imaging Bats with an Invisible Strobe

Image of bat overhead

Looking and flashing straight up from my rooftop, this shot captures a bat in flight approaching the bat house. You can clearly see the fingers that support the membrane of each wing. The flash stops his (or her) motion completely.

The kind folks at Make Magazine published a short article I wrote about how I captured some of the images appearing on this blog. It’s in Make #20. Unfortunately they were looking for a maximum of about 750 words. When I’m waxing poetic about a subject that, like bats really interests me, I tend to overrun the editor’s limits. So here, with the permission of the magazine, is the full text from which the article was excerpted.


I’ve always been fascinated by flash photography. The strobe’s ability to freeze motion seemed magical, and one of the first electronic kits I ever assembled was a simple stroboscope. I think I burned out the strobe tube freezing the rotating motion of various fans and toys. But I was hooked.

When I began trying to document the bats flying around my backyard bat house, I turned to low light video. Driven in large part by security applications, the marketplace has exploded with inexpensive, sensitive monochrome CCD video cameras. In yet another example of Moore’s law, their sensitivity is increasing by an order of magnitude every few years. The first camera I bought off the Internet was a simple, bare board camera with a built-in lens and no housing. Its light sensitivity was 0.1 lux. Within a year, I bought a second camera rated at 0.01 lux, then another at 0.001 lux. The next one I buy will be 0.0001 lux. Even at the 0.001 lux level I found could get pretty detailed video at dusk and dawn, so I set up my newest camera, equipped it with a surplus lens I got at a swap-meet, and started recording the coming and goings of my winged friends on a Sony DVD recorder.

Is that a bat or a UFO? Clearly the 1/30th second video frame is too slow to freeze motion.

The flight speed of these little mammals was impressive. They streaked through the frame of a wide-angle lens in a fraction of a second. Feeling pretty smug about the images I’d recorded, I ripped the DVD to my PC with DVD Decrypter and started single-framing through the video segments with VirtualDub Mod. Then came the real disappointment. In each of the five or ten frames that held the images of a streaking bat, the bat itself was a mere blur (see image at left). Clearly the minimum 1/30th second exposure of each video frame was way too long to capture motion clearly; the bat moved a lot even in that short time.

One night, while pondering my rather disappointing frame grabs, I happened across a really cool TV show that reminded me of my early fun with the stroboscope. The Discovery Channel’s Time Warp featured really high speed, slow motion video, including water droplets, exploding light bulbs, bullets, pole vaulting, and on and on. Maybe I could use a strobe light together with the video to freeze the image of a bat in a video frame. While my thirty frame per second video would never rival the thousands of frames per second spat out by the cameras on Time Warp, a Xenon strobe flash only takes a millisecond and would stabilize the subject in the images which caught the flash. And while it wouldn’t be synchronized with the video frame rate, even a simple strobe could flash several times per second. The result was a shotgun approach for sure, but one statistically likely to freeze at least a few usable still images.

But what would the bats think about a strobe light? I consulted with a couple of bat experts on the Internet, and while they didn’t seem to think the bats would be harmed by a distant strobe flash, the consensus was that it would at least delay their exit in the evenings and perhaps alter their flight paths away from the light; not a good situation for imaging. So how could I make a subtle strobe light, and isn’t that an oxymoron? After all, these are advertised and sold as attention-getters, right?

Knowing that Xenon tubes can produce a very wide spectrum of wavelengths (thank you, Wikipedia), and also knowing that many tinted plastics block visible light but pass the near infra-red (NIR) spectrum to which monochrome cameras are very sensitive, I experimented with filter materials including smoky Plexiglas and exposed film negatives. But the cheapest and most versatile material I found was automotive window tint film. Designed to keep your car cool, and your identity a mystery, my favorite tint is the one called “Limo Black” which boasts that it blocks more than 95% of visible light. Layering five plies of this film over the front of the strobe light yields a flash that is just a dim purple spark to the eye, but which fully illuminates the video field of a sensitive camera from a distance of ten feet or more – an almost-invisible strobe.

Using the same camera, DVD recorder and software, I grabbed some much more impressive stills without disturbing the bats.

Here’s how to construct the strobe and use it with video.


Here’s what you need for the 12 volt version: a box of window tint film, a 12 volt strobe designed for vehicles, black duct tape, a 12 volt gel cell battery and a couple of clip leads. A sample of the tint film is shown just above the duct tape.

Step 1 – acquire a Xenon strobe.

Here’s the strobe, unpacked and ready to wrap. Be careful not to lose track of which wire goes to the positive (+) terminal and keep the wires free of the tint film when you’re wrapping.
You’ll need a real Xenon flash lamp, not LEDs here. (The white LEDs in some really inexpensive strobes don’t emit enough NIR to work well.) I’ve found two good sources for strobes. First, visit your local “party store”. In my part of the country, these pop up like mushrooms around Halloween, while a handful cater to the true party animal all year round. In amongst the rubber werewolf masks and naughty nurse costumes, you’ll often find inexpensive, imported Xenon strobes. These require access to 120 VAC house current, but feature a variable flash rate, a built-in mounting bracket, and even a ready-made holder for the window film IR filter.

A second source is automotive supply stores. These sell various strobe lights intended to enhance vehicle safety, or at least curb appeal. These have a fixed flash rate, but function on 12 VDC, a real advantage for outdoor operation. Some have color filters, but for our application, the clear ones are best.

If you want a bit of an educational DIY side-trip, you can reprise my wasted youth by assembling a strobe light kit, like the ones from Electronic Goldmine. Spend some quality time freeze-framing rotating machinery when you’re not grabbing video.

Step 2 – add the filter.

The result is ugly, but effective.
For the Halloween-style strobe, note that the front cover of the strobe will unscrew, allowing you to remove a bezel over a clear plastic window. Cut four or five squares of window tint film using the bezel as a template. Ordinary safety scissors work fine here. The instructions for the tint film suggest peeling off the transparent backing, leaving just the very thin film of mylar that actually adheres to the car window. Throw these instructions away before they confuse you further. For our application, there’s no need to remove the transparent backing and its presence makes the film much easier to handle. After all, we’re not looking through the film, just shining a really bright light past it.

You may be able to lodge the film layers in the slot conveniently provided behind the bezel. (Probably this was intended to hold a colored gel for effect). Failing this, I use black duct tape to stick the film to the plastic housing. This is ugly, but highly effective and after all who’ll know, we’re using this in the dark, right?

For the automotive strobe, I just wrap the layers of window film around the transparent face of the strobe, and tape them together in back with duct tape. Be sure to leave the leads hanging out so you can connect a 12 volt battery as needed.

Step 3 – check out the strobe.

For the Halloween strobe, plug in the cord and rotate the flash rate control on the back. I always set it to the fastest rate, since this gives the most usable exposures per video. Looking at the front of the strobe, you should clearly see a dim violet flash. That’s all that’s left of the visible part of the spectrum, but when you see the video, you’ll realize that a lot of near infra-red is still getting through.

For the automotive strobe, connect a 12 volt battery to the leads provided, carefully noting the polarity. I use a surplus “gel cell” battery like those sold for replacement in UPS units. A cigarette lighter cable from your vehicle would work as well, as would various portable power supplies, or a 12 volt wall-wart transformer salvaged or re-purposed from a personal electronic device. The flash rate will be slower than the fastest setting on the Halloween strobe, but quite adequate. A single charge on a good size gel cell will flash one of these units for a long time.

Step 4 – shoot some video.

I’m usually snapping images of bats, but the NIR strobe will work fine for any nighttime subject, moving or not. If you want to see which of the neighbor’s cats is digging in your garden, or image a passing Opossum, the strobe and DVD recorder approach will work.

Tips

From left to right: A surplus zoom lens I modified with a new cable for remote control, a used camera I purchased for $5 which still yields acceptable pictures in conjunction with the 12 volt and a Halloween strobe, and several other surplus lenses which can be used to manually set the aperture and focus.

  1. Cameras – use monochrome, not color. Most monochrome security cameras are designed to be very sensitive to IR light. Not only are the CCDs used in color cameras much less sensitive, but they frequently have built-in IR filters that actually block the illumination for which we’re working so hard. Besides, IR is invisible to us anyway, so color is meaningless. (If you must, post-process the images with The Gimp and tint them green so they look like those nighttime war zone images from CNN.)
  2. Lenses – Use the best lens you can afford, but be sure to check the used market. Inexpensive imported lenses sold with security cameras will work, but if you check E-Bay, surplus outlets, flea markets, and yard sales you may be able to pick up cheap, good quality used lenses like those originally used with 8 and 16 mm movie cameras. You’re looking for lenses with a “C” or “CS” mount. The CS mount is best because a new monochrome camera is usually made for CS mount lenses, but the older C mount lenses can still be used with an inexpensive threaded ring adapter. (The ring moves them away from the CCD imager to correct for their increased focal length, relative to true CS lenses). C and CS mount lenses are easily recognized by their one inch (25mm) male thread. A manual adjustment for aperture (f-stop) and focus are best. Avoid electronic zoom/focus and auto-iris lenses if possible; they complicate operation and do little for nighttime videography where everything is wide-open anyway. (It’s still possible to hack the electronic zoom, focus and auto-iris lenses to work well, but that’s another article).
  3. Speaking of focus — two factors conspire to make focusing an IR-sensitive camera in darkness tricky. First, the correct focal point for near IR is a little bit different than the best focus for white light. This is because the wavelength for IR is longer than that for visible light. Think of it as the very deepest red you can imagine. If you focus the camera in white daylight, you will have to refocus for the monochromatic IR strobe. Second, with a sensitive camera, you will have to decrease the aperture to the minimum (highest f-number) in order to get a picture that is not completely washed out in daylight. But the small aperture minimizes focus errors because it maximizes depth of field; both distant and nearby objects will appear sharply focused. But when I open the aperture in order to get enough illumination from the strobe at night, I usually find that distant or nearby objects are no longer in sharp focus. This means refocusing once night falls.I’ve found two techniques helpful here. First, try taping a layer or two of window tint film loosely over the camera lens during daylight. This drastically reduces the amount of white light reaching the CCD and makes the daylight focusing a much more realistic simulation of nighttime conditions.Second, using your leftover tint film and an incandescent fixture, you can rig a temporary IR flood light to illuminate a garage or other darkened area for daytime focusing experiments. I sandwiched several layers of tint film between two small sheets of Plexiglas, and taped the edges together with aluminum duct tape. I hold this in front of a clip-on incandescent light fixture in my garage and get the rough focusing done during the day using real IR light. Halogen bulbs work well for this, but you may need to use a small dimmer to reduce the intensity so it doesn’t melt the tint film by emitting lots of far IR (heat).

  4. Short digression: Just how dark is five layers of Limo Black tint film? The left picture shows an example. It’s high noon at my home in sunny central Florida. Yet the image, which I’ve even brightened a bit here, is very dark. Notice that the brightest object, after the clouds, is the foliage. Even though this digital camera, unlike the monochrome security camera I’m using for night work, isn’t very sensitive in the near IR, the high IR reflectivity of many plants is stills apparent.

    The right picture is a true daytime IR shot of the same area. The camera has had it’s internal IR-blocking filter removed and replaced with a single thickness of tint film. The white (well, pink here) leaves are very characteristic of daytime IR photos. If you’re interested in daytime IR digital photography, I recommend checking out this Geek Technique article on modifying your digital camera to be more sensitive to IR (not for the faint of heart) and Lifepixel, a commercial site sorts of IR add-ons for your digital camera. It also features a very nice video demo of daytime IR photography.

    By the way, there’s nothing magic about five layers; you should experiment with more or fewer thicknesses of tint film and check your results.

  5. Experiment with image processing. Video is not an inherently high-resolution medium. My frame grabs are currently 740 by 480, much lower than all but the poorest quality digital cameras. And since the these are figuratively, snapshots, their exposure is unlikely to be optimal. I use The Gimp to post-process frames I select from the video stream and the result is a greatly improved image.My standard method is to de-interlace, adjust brightness and contrast, and apply an unsharp mask.Even if you’re a Photoshop or Gimp weenie, you may not be familiar with the need to de-interlace unless you’re used to processing images from video, rather than still cameras. Each video frame is actually made up of two sets of scan lines, interleaved or interlaced into two sets, one consisting of odd numbered lines, and the other of even. Normally both of these sets of lines, or fields, flash by so quickly that our brains blend them into one image. But the strobe flash is so short that only one of the two fields in each frame is actually illuminated; the other is dark. When both are viewed statically, the result is ghostly semi-transparency. You might want this for artistic effect, I suppose, but for the sharpest and brightest image, we need to select the one field that has the subject illuminated and ignore the other. This is de-interlacing and The Gimp does this very nicely.
  6. Be patient. This approach depends on taking a lot of frames and throwing away the vast majority of them as empty, or not illuminated, or both. It takes patience to step through lots of black video occasionally punctuated by a bright flash to find a frame with your subject nicely centered. But then that has always been one of the secrets to photography anyway, hasn’t it – take lots of pictures and throw away most of them.

A note about safety

While I’m not keen on the “do not use in bathtub” or “caution: blade rotates when engine is running” variety of disclaimer, I do think it is important to keep a few safety tips in mind. First, the more obvious ones:

If you’re doing outdoor IR strobe photography, consider the automotive strobe. That way you don’t need extension cords and you won’t be plugging and unplugging wires carrying house current in the dark. (Most cameras will run on 12 VDC also, yielding a completely battery-powered setup). If you’re using the Halloween strobe, at least set everything up in daylight first. I’m sure you’re more dexterous than I, so you won’t duplicate my spectacular nighttime crash over that flower pot, right?

As one or more stickers will surely warn you, perhaps in broken English, never attempt to disassemble a strobe light. (Well, unless you know what you’re doing). Even the 12 volt types develop hazardous voltages inside and may retain a charge on internal capacitors even when powered off. The best way to experiment and learn about how these devices work is to purchase and assemble a kit, following all instructions.

While you have the bezel off the Halloween strobe to install the tint film, avoid touching the flash tube itself since oils from your fingers, combined with the extremely high temperatures reached during a flash, will shorten the life of the lamp.

Less obvious, but still important, is to avoid looking directly into the IR strobe when it is operating, and to avoid placing it where your subjects may do so. This is not so much related to the excessively publicized but very remote possibility of induced epileptic seizures as it is about possible negative effects on your vision. Just because we can’t perceive near IR light doesn’t mean it isn’t there – and it is very bright if you stare directly into the flash. To further compound the situation, your pupils will still be dilated in the dark, so more IR will pass into the eye than you’d get in daylight. Avoid looking at an operating IR strobe at close range. Light reflected from objects more than a few feet distant should pose no problem for you or subjects, however.

The camera and IR flash are mounted on the bat house, near the opening, looking down in this shot. Two bats are visible in flight, the closer and apparently larger one flying up towards the camera, and the more distant flying down and away.

Overall, be safe, have fun, and if you’re patient and lucky, freeze some illuminating images.

3 Responses to “Imaging Bats with an Invisible Strobe”

  1. BRETT says:

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  2. DENNIS says:

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