It takes both a machine and an observer to make this magical lighting trick work, since the effective ‘frame rate’ of the human eye effectively puts together a series of independent flashes to reconstruct the image in play.
Skip to around 1 minute and 30 seconds into the above video to get a sense of not only how this works but of other potential applications – a clock is just one kind of display, but anything is possible. You can write static (well, moving) messages, or scroll text or images or patterns, or even send remote commands if the device is wireless.
This particular model USB powered fan LED will spin really fast providing a good chill for those hot days of summer, and on the face of the fan a clock appears on the spinning blades. The blades are soft for safe use, and keeps the time even after removing it from the USB because of internal rechargable batteries. Doesn’t require any installation and the flexible neck lets you point it anywhere – great to stick into the side of a computer as it is quite portable and can be charged by USB.