Red digits on this photo appear to float in the air in front of the clock. This illusion is based on inertia of a human eye. If LED-formed digits will periodically and frequently enough flash, they will appear solid and steady. And since the matrix of digits is formed by a mechanically scanned single line of LEDs, and the fast rotating clock body is not visible, it leaves digits “suspended” in the air.
The first clock using this concept was built (and PIC microcontroller code written) by Bob Blick, please visit his page for yet more photos of his original clock and clocks built by other people. The heart of this clock is PIC16F84 microcontroller. (Older PIC16C84 version works just fine). The microcontroller is programmed with the code provided below. If you don’t have a PIC programmer, it isn’t difficult to make one. A few parts (for a couple bucks) connected to the parallel port of your PC will program the PIC for you.
POV clock based on PIC16F84: [Link]