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LED Advertisement, Persistence of Vision

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LED signage has become one of the most common forms of digital signage and advertising, usually from giant display screens to simple scrolling text displays.
We’ve seen several forms of LED signage during the shows we’ve visited, some of the most innovative ways of LED signage we’ve seen are city advertisements.

We had the chance to experience and see the Shanghai metro tunnel LED advertising, you would think that the entire wall is covered with LED panels; instead, a technic used is quite innovative: persistence of vision (POV).

LED advertising found in the Shanghai Metro tunnels.

 

Persistence of Vision

Persistence of vision or POV, can be understood as a visual illusion, POV means the eye averages what it sees over a short period of time, it’s somewhat analogous to a camera where you have a long exposure time, this provides with more light to the eye, increasing sensitivity but at the cost of a slower response, or in other words, anything moving faster is blurred out.

A sequence of images printed on a revolving barrel or drum deliver the illusion of continuous motion when viewed through a hole in the drum. The hole acts as a shutter momentarily freezing the image. A traditional cinema projector works on the same illusion by actually stopping the film frames.

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The zoetrope used the basic principles of persistence of vision to provide the illusion of animation during the 19th century.

For example, on our daily routine we use screens (computer monitors, laptops, cellphones and TV’s) have a refresh rate averaging in the 60FPS range (frames per second), meaning every 16 milliseconds a different image or frame is displayed. This sequence of static frames is interpreted in our brain as motion or animation; our brain only needs around 16 FPS to 24 FPS to create the illusion of motion.

Back to the LED advertisement on the metro tunnel walls: having already understood the concept of POV, you can now understand the way it works, and how it’s set up.

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Here is a picture of the LED display technology placed in the metro walls.

Vertical strips of LEDs are placed on the tunnels with a horizontal separation of 1 meter from each other, the metro is moving constantly at around 35-40km/h, meaning it passes the strips at certain speed, this combined with the amount of LED strips generates the illusion of a continuous image or motion.

Each strip in the tunnels display a static array of RGB LEDs, meaning this LEDs don’t change color, they only turn off and on at a certain rate (hertz), depending of the train’s speed, this in order to avoid the black lines usually seen when there is not a synchronization among them.

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LED strips placed in the Metro tunnels walls.

If you were to place all the strips together side by side, you would see the image displayed as the users observes it from inside the moving train.

Another signage application of POV in LED that we have seen during our tradeshow visits are LED fan displays, which use the principle of persistence of vision to create the effect of animation.

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Full color LED fan displays.

These displays have a rotating blade with an integrated RGB LED strip, when it rotates the integrated controller sends the signal to the strip telling which LEDs to turn on and what color each LED should be, this depending on the angle of the blade. The resolution of the final image depends on how many degrees (angle) the LEDs are told to change and the amount and size of the LEDs.

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Close up of the LED strip on the fan POV display.

LED signage has evolved and has become more accessible over time, leading to new forms and uses for digital signage, you can notice it by simply looking at modern cities, the amount of signage and advertising that comes displayed in LED has increased significantly.

LED fan display exhibited at APPPexpo 2018.

LED fan display exhibited at APPPexpo 2018.

Source: Tradeshow visits, personal experience, a few books and some informative sites and videos.
https://paulbakaus.com/tutorials/performance/the-illusion-of-motion/
http://service.shmetro.com/en/operation/965.htm
https://www.youtube.com/watch?v=Df2RHvwdt5M

 

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