How fast is fast?

Several times a year I’m approached by people who want advice on buying a high-speed video recording system. They’ve all seen video of a bullet passing through a light bulb and believe that if they can capture a similar movie of their production or packaging process they’ll be able to figure out how to make some improvements.

I begin by explaining that price rises exponentially with speed, and then ask how fast they need to go. Most people haven’t thought to work this out, so in future I’m going to refer them to this first-rate article by Andy Wilson, “Capturing the Moment.” (Vision Systems Design, April 2009.)

Andy does an excellent job of explaining how to figure out the frame rate and exposure needed to capture an event, so I’m not going to paraphrase him here. I would however like to add a few hints and tips of my own.

First, you may not need a “high speed” camera at all. Most cameras can be made to run at a higher frame rate just by reducing window size and exposure time. This is especially true with CMOS sensors. In fact, if you can accept a small field of view – say 128 pixels square – you’ll be able to get over 500 fps from any number of cameras. (I’ve had good results with cameras from both Imperx and Photon Focus.) Make sure to use the CameraLink standard and a computer with a the PCI Express bus.

But second, remember that shorter exposure means more light is required. Much more light. I tend to use a halogen lamp with a fiberoptic goose neck to deliver it just where it’s needed.

And third, you’ll need somewhere to store all those images. Actually the problem is not so much storing them as writing them to disk as fast as you’re acquiring them. This means buffering the images in RAM, and then writing them to a hard drive, so it’s RAM that might be the problem.

The bottom line? Follow Andy’s tips to determine what frame rate and exposure your task requires, then decide if you really need a true high speed video system, (expensive,) or if you can put together something yourself to do the job, (cheaper.)