Digital movies, perceptual psychology, and tricked out bicycles.

When you live in the Bay Area, you quickly realize that one of the standard status symbols is the bicycle.  It’s amazing what kinds of messages you can send on two wheels and a frame.  Are you the worn-down fixey hipster type?  Or maybe the svelte 2-pound frame roadster?  Whatever your look, Bay Area cultural cred requires that the bicycle fits the personality. Well, this situation is about to get a lot more complicated.

Enter “MonkeyLectric“, the bicycle accessory manufacturing group that boasts a creative vision that’s just as impressive as its electrical engineering skills.  Put simply, the company likes to put pictures on moving bike wheels – in fact, they just launched a Kickstarter campaign to build a device that would display a movie on your wheel as you ride.  That’s right, a moving picture on your wheel that’s rotating at more than 70 RPMs.  How do they do it?  Well, to understand this requires a brief foray into the world of perceptual psychology.

The spinning wheel products of MonkeyLectric are just one in a long line of inventions that take advantage of the Persistance of Vision Effect.  Basically, you begin by flashing an image 2 times a second.  It will obviously flicker on and off.  Now increase that to 4 times a second, now 8 times a second – the image starts to flicker faster and faster.  Now, once you hit somewhere around 16-24 flashes a second, a strange thing happens: the image stops flickering, and instead looks like a static image.  That is Persistence of Vision in action.  (you can even try it out yourself).  As an example, check out this video of a Thaumatrope, a popular toy in the 1800s that took advantage of this effect:

You’ve all probably experienced this at some point in your life, but why does this happen?  Well, it turns out that there are specific cells in the human eye that are responsive to photons of light – these are called “photoreceptors”.  When a photon of light hits one of these cells, it will respond for a certain “window” of time after the initial reaction.  If a second burst of light hits the receptor before that window is over, then the cell responds differently.  This is what allows our eyes to perceive things in the world as continuous movements.

There are many kinds of photoreceptors out there, and some are more sensitive to changes in time than others.  For example, at the center of our vision are “cone” photoreceptors that can detect very quick changes in time.  At the periphery are “rod” photoreceptors that are more sensitive to intensity, but less-sensitive to time.  In short, we are very good at seeing rapid changes we’re looking right at them, while things tend to be “smeared out” in time when we look at them out of the corner of our eyes.  These effects are even seen in animals too.

So, how is this useful?  Well, consider anything that displays a picture: TVs, movie theatres, computers – basically anything that has a screen.  Persistence of Vision tells us that if we display things fast enough, it will be perceived as one continuous picture, rather than a bunch of flickering images.  Our visual system then processes this information and “fills in the gaps” between the images in order to perceive movies as though they are fluid motion.

But why stop at theatre screens?  This is the idea behind MonkeyLectric’s new “movie wheel” technology.  They’re shooting for the same thing that movie theatres are: to create the illusion of a movie that’s being played with flickering lights.  However, they’ve got the extra trick of doing this on a rotating wheel.

Their solution: fit four rows of LEDs on the wheel, so they form a big cross.  Now, if they can figure out how fast the wheel is rotating, they can program the LEDs to blink at just the right moment so that an image is created (here’s a solution for simpler images).  As long as their LEDs can flash light faster than the threshold for Persistence of Vision, the images they display will look like a continuous movie.  And voila, you have cinema on your bicycle spokes.

You can click here to contribute to MonkeyLectric’s kickstarter campaign, and see a bunch of sweet demonstrations of their new device

 

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  1. Many people are doing persistence of vision projects using total Control LIghting, an off-the-shelf individually-addressable RGB LED pixel system available at coolneon.com, which happens to be owned by Benjamin James, a Berkeley alumnus. Each pixel has full 24-bit color and can be run at clock speeds up to 15 MHz. All the libraries are open source, including Arduino and Raspberry Pi. Cool Neon also sells MonkeyLectric products.

    Humbly submitted by said proprietor, Benjamin James.