>I2of5 is a standard barcode format that is very easy to read. It was
>developed by Intermec for encoding the digits 0 to 9. The name
>Interleaved two of five comes from the format of the code. Each
symbol
>contains a pair of digits the first is encoded in the bars the second
is
>encoded in the interleaved spaces. Each character has three narrow
>elements and two wide elements. The full specification is found in
>MHI/AIM USD-1.


It's also very easy to get substitution errors, which would be pure
hell in this application.

I like the eight-colored ball idea best.
Could it be read by illuminating with red, green, and blue leds and
measuring the reflectance?
That would also be visually cool, another plus point with this type of
thing.