What’s A Violet Wand?

At AbelCine, we offer sales, rental and service of all types of professional video cameras, and as part of that work, we also get a wide variety of technical-support questions. The majority of these questions are pretty straightforward, and we can answer them right away, but sometimes a question pops up that causes us to scratch our heads. Sure, these questions make our jobs tougher, but they also keep us on our toes.

One such inquiry I recently received was about shooting video of a little gizmo called a Violet Wand. Of course, my first response was, “What’s that?” so I’ll give you a little background info first. A Violet Wand is a handheld “wand” with an electrical transformer inside; you can attach various plasma-filled glass bulbs to it, and when high voltage is applied, the bulbs light up violet and produce small electrical discharges. The plasma-filled bulbs are identical to what you’d see in a plasma globe found in a science museum. Originally made in the 1900s as a medical instrument, the little device produces shocking results when held up to the body. Of course, it has no actual medical use, but it’s still used today for other purposes—just be careful not to Google it at work. (I never asked what the production company was doing with it.)

THE CLIENT’S ISSUE

When shooting close-ups of the device with a Sony PMW-EX1, the client was getting what they described as a glitch or jump in the video. At first, I was concerned with possible electromagnetic interference, but they were several feet away so that seemed unlikely. I asked to see an example of the problem, and the next day, they arrived with a couple of clips for me to check out. They showed me a short video of the wand going across someone’s arm, and whenever an electrical arc jumped from the wand to the person’s arm, a small white band was created in the video. Right away, I knew the problem: The bright sparks created by the wand were also creating “flash banding” in the image. If you’ve heard this term before, then you know that flash banding is an artifact found in cameras with a rolling shutter-type sensor. The Sony EX1 has a CMOS-type sensor with a rolling shutter, so this was the source of their problem. I let them know this, and they asked if a different shutter speed could help. My answer was, “Not really.” This brought up a discussion that I’ll share with you about sensors and shutters—one that I think every modern filmmaker should understand.

The terms “global” and “rolling” shutter are often thrown out there, but also often misunderstood. The terms are especially confusing when combined with shutter speed. So to help clear things up, let’s break down the terms.

SHUTTER

The word “shutter,” when used as a verb, means simply “to close,” and that’s exactly what it does in a film camera—the shutter blocks light from hitting the film. In a motion-picture film camera, the shutter is a spinning disc with a wedge cut out; that’s why shutter is often represented in degrees. Digital video cameras usually don’t have mechanical moving shutters (rare exceptions being the ARRI D-21, Grass Valley Viper and Sony F65); instead, shutter is a term used to describe when the value of each photosite on a sensor is read out and reset to zero, or simply turned off and back on again. Global/rolling shutter describes how a sensor is read out, while shutter speed describes the gap in time (exposure time) between each readout. This is an important distinction because, while the two terms are related, they shouldn’t be used interchangeably.

Global Shutter. A global shutter-sensor camera reads the value of every photosite on the sensor at the same time and then resets it back to zero—sort of like one big on/off switch. This means it can handle motion very well.

Rolling Shutter. A rolling shutter sensor doesn’t read each pixel of a sensor at the same time, but instead reads each line of the sensor in succession. The reading and resetting roll across the image in a sort of cascading effect. For example, when the first line of the sensor reads around 5% across, the next line starts and reads 5%, and then the third line starts and so on. This can result in warp and wobble motion artifacts where the camera (or subject) is moving so quickly that the readout of the sensor can’t keep up.

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