Monday, December 29, 2008

Persistance of Vision

Interesting, low part count, relatively simple devices, called Persistence of Vision displays, utilize the exceptionally slow response time of the retina to optical variations. Exploiting these effects make it possible to build a graphical matrix display with far fewer light emitting pixels than one might first intuitively expect. For example, this first picture illustrates my attempt of a hand built rotating POV clock without the need of any specialty machining tools. Using only seven LEDs, a 0.1 Horse power AC electric motor, 16Mhz microcontroller, and a solidly constructed aluminum frame, a virtual seven by 120 pixel display appears to come into existance. Electric power is supplied to the rotating circuit board by an isulated rotary contact and spring brush. The motor's rotor provides the necessary grounding connection and reduces the otherwise increase in friction that would result with a second brush. Alternative methods for supplying power to the board are numerous and probably would result in a quieter, longer lived design. Of these variations, the the simplest, quietest, and possibly least electrically efficient is to use an air core resonant transformer. This method consists of a primary coil mounted stationary to the motor which induces an electromagnetic field upon a secondary coil which travels with the rotating display platform providing a contactless power bridge to the control circuitry.

Since the display contains only minimal LEDs and weight is a factor, resolution can be improved simply by a faster control circuit and finer pitched bulbs. With LEDs spaced closer together and with the ability to control them more precisely, the designer can create images with better quality without resorting to spending extra money. Some have taken this concept to the extreme and produced full HD quality video using only about a 1000 emitters. Since rotation speed directly affects picture quality, it is important to minimize system components and mass. An aerodynamic design or placing the device in a vacuumed chamber can greatly improve refresh rates.

Rather than rotating the light source, a specially angled type mirror can be used to automatically reflect the light source into scanning rows upon a projection screen. The motor simply spins this 7 sided cylindrical mirror (each side having a slightly different tilt). When a stationary mounted laser is bounced off the rotating mirror a raster pattern is produced on the screen. By keeping track of rotational indexes and other timing information, a processor can determine whether to turn the laser on or off at extremely high speeds. The end result is the stable illusion of an image on the wall.

A still better approach is to use a flat mirror on a double axis, spring suspended mount. Electromagnets can force the mirror into a scanning pattern where rows are generated more often than columns. Very similar to a television's electron beam control, but far superior in its ability to project images beyond a vacuum chamber and zero requirement for a phosphorescent doped screen, a mechanically scanned laser projector can generate images of any size, with any resolution, and is only limited by the intensity of the laser's beam and the reaction speed of the electronics.

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