This Christmas Tree Is a Massive POV Marvel

Persistence of vision (POV) is a visual illusion that relies on our eyes’ (or brains’) inability to keep up with extremely fast-moving objects. For the same reason that we see motion blur, something moving fast enough will create the appearance of a solid object. Rotate something really fast, add lights, and sync those lights to the rotation speed and you get a POV display. If all of the lights remain set to a single color and brightness, you will see a solid surface. But by modulating the color or brightness, one can display images and animations, which is how Sean Hodgins’ Christmas tree works.

This is essentially just a massive POV display that shows a Christmas tree or any other image/animation that Hodgins chooses. But that doesn’t mean that it was easy to build; scaling a POV to this size introduces a number of challenges. The most obvious are related to weight, which increases the momentum to overcome and the friction on the bearings. The size of the rotating frame also increases drag, while the wind resistance simultaneously intensifies as the expanded radius of the frame results in faster speeds. When compared to a small POV display, like the Sharper Image clocks that were popular around the turn of the millennium, this over-sized version required much more robust construction.

To make a POV display work at this scale and spin at around 10Hz, Hodgins built a sturdy frame out of aluminum and steel. The spinning portion rides on heavy duty ball bearings and a beefy slip ring, which allows for power and data transmission through the rotating joint. A large DC motor spins the inner frame and a Hall Effect sensor keeps track of the rotational speed. The triangular spinning frame has a strip of NeoPixel-style RGB LEDs running down one side, which a Raspberry Pi single-board computer controls those according to the input image. The software can also display a series of images in sequence to produce animated videos.

As you can see, this works really well. It isn’t exactly high-resolution, but the images look quite nice. To capture those for his video, Hodgins had to lower his camera’s frame rate to match the display’s refresh rate while also allowing more light to enter the camera’s sensor.