Tuesday, February 18, 2014

Breaking the speed of light

Everything has a speed. I was reading about somebody selling a laser trigger for high-speed imaging. Apparently the trigger had a frequency of 1/1,000th of a second. That set me thinking because I'm pretty sure that is just not fast enough. So, without further ado, let's dive into some calculations.

A 1/1000th frequency will mean that the laser will be emitting for half that time and not emitting for the other half. Thus we have 1/2,000th of a second when it will actually be active and productive. Any projectile passing through the beam cannot therefore travel any faster through the beam than 1/1000th of a second in order to make the thing work.

Let's break that down further. Projectiles have length which is beneficial for things like this. If something passes by in 1/1000th of a second, it has to be traveling at least at 166 feet per second. That's about a third of the speed of a slow-moving airgun pellet. Most airsoft pellets travel faster than that.

Given that we don't know exactly when the trigger frequency cycle begins, it could start at any point during the passage of an object through the laser beam. This means that even something traveling relatively slowly such as a dart (most darts are thrown with a forward speed of about 40mph), the 1000hz cycling of the trigger will mean that even something as slow as a dart (58 feet per second) has an inexactitude of 1 inch. This means the difference between taking a photo of a balloon with a nice pointed dart just above its surface, a photo of a balloon with a dart puncturing it or a photo of a shriveled piece of plastic with a dart somewhere in the picture. There is no predictability and guarantee of repeatability. This is not good for such an exact science as high-speed imaging.
The photo above is repeatable. This was done with an audio trigger, not a laser trigger. The sound of the gun firing triggered the exposure after a slight delay set by my delay unit. Thus I have a very nice photo of an airgun pellet sliced in two by a razor blade. Needless to say, this was not a single attempt, I had to keep adjusting the delay until the pellet was in the right place, I have a ton of photos of the pellets in different places on the scene, including one where it's just been cut by the razor blade and the two halves are on opposite sides of the blade.  This is the photo I choose to show, however. The plume of "smoke" is actually oil from the barrel of the air pistol - which is a pump-up Crossman. I did try using a pistol with a CO2 cylinder but the valve kept jamming so it would waste a complete cylinder of CO2 each time. I gave that one away. I needed dependability.

What would you do to improve the experience of high-speed photography?

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