Home-brew high speed photography

I posted this information to the studio and lighting technique forum as well, but it didn't seem to generate much interest, so I thought I'd post here as well. Apologies to those that already saw it in the other forum:

This summer I posted a thread about my efforts with high speed photography.

I was using home-made blow darts to pop a balloon, and a vibration sensor attached to the balloon to trigger the camera.

Since then I've bought a ballistic sensor (a pair of photo gates that detect the passage of a projectile) and written software for an Arduino microcontroller that calculates the speed of the projectile and the delay needed fire a flash when the projectile is at the desired location. (The ballistic sensor is made by Maurice Ribble and sold by his company, Dreaming Robots. He also sells a ready-made device called the Camera Axe that offers lots of features in addition to timing shots with his ballistic sensor. I also borrowed some of the (open source) code from the Camera Axe for my project.

Instead of blow darts I'm now using an air rifle and lead pellets.

I designed and built a trigger switch and attached it to the trigger of the gun.

The trigger switch is a strip of thin flexible plastic (like from a blister-pack package) folded in half into a slight V shape, with copper foil tape taped to the the inside. When you squeeze the V together, the 2 pieces of foil touch, closing a switch. I soldered a piece of wire to each piece of foil, and hot-glued the assembly to the trigger of my BB gun. Now as you begin to squeeze the trigger it closes the switch, grounding a pin to the Arduino. The Arduino then opens the camera shutter, turns off the lights, and starts waiting for the projectile to pass through the photo sensors.

It takes a significant amount of time (around 50 MS (milliseconds) for the camera shutter to open, so the system starts opening the shutter as you pull the trigger. If you wait until the ballistic sensor detects the projectile then you don't have enough time to open the shutter.

I bought some 12 volt LED landscape lights and have 1 pointing at my work area and one pointing at the target. When the arduino detects the trigger being pressed, it turns off the LED lights just before opening the camera shutter. The light from LEDs stops almost instantly when the power is removed, so the room is dark by the time the camera shutter is open.

I did some tests to fine-tune the timing so the pellet is exactly at the specified distance when the flash fires. I can then add extra delay to that so the balloon is at various stages of popping. There's a certain amount of variability in the timing, so there is an element of luck to it.

I'm using a Nikon D600 with a shutter release cable, and a Nikon SB800 flash dialed back to 1/128 power for the shortest possible flash duration. That doesn't give me very much light, so I have to use a larger aperture than I'd like (I managed to get f/8) and a higher ISO (I'm shooting at ISO 200, and then pushing the finished exposures some in post)

For the most recent shots I bought a black project board (The trifold boards they sell for school kids to make displays) and put it in front of my home-made bullet trap. It provides a nice flat black background. The bullet trap made for distracting backgrounds that were a pain to edit out.

I've also put a medium sized sheet of white foam-core on either side of the ballon, angled slightly toward the flash to provide some fill.

For most of my newer shots I put a few ml of cornstarch in the balloon before inflating it, and the rub it on my hair to make it stick to the inside of the balloon. The cornstarch makes really interesting patterns as the balloon pops. I'm assuming the patterns are from the shock waves as the balloon rips apart explosively.

Here is the best shot I've gotten so far:

View: original size

And here is a shot much earlier in the pop:

View: original size

Note that balloons apparently fly apart at faster than the speed of sound. That's what makes the loud boom as the pop - a mini sonic boom. You'll notice that the edges of the balloon are always blurry, even though I'm shooting at f/8 and a flash duration of less than 1/40,000.

What I really need is a sub-microsecond flash like an air gap flash. There's a company here in the US called Prism Science that makes them and will even rent them for a month at a time. I also found a link today to a company that has a kickstarter to sell a half-microsecond flash from a bank of 9 high intensity LEDs over-driven with a very strong, short pulse of current. They can get away with overdriving the LEDs because the pulse is so short that the LEDs don't have a chance to overheat and burn out.