I'm actually really disappointed this wasn't addressed with a fairly obvious "checksum" mechanism - Including the ":" in the center, and giving that it's own lenticular pattern, such that only when viewed from the right angle do you see the : and otherwise you just see numbers.
(I'm not sure if this is really plausible - Are the numbers close enough that that would actually work? If you're off to the right or left, or too high or too low, could you potentially see the : with a different set of numbers than intended?)
Seems completely plausible to me, and elegant to boot. To minimize design change, the product name could be the alignment check.
{You being too high/low} and {the angle of the numbers/check being wrong} are two ways of saying the exact same thing when ignoring the world beyond you and it, like using a handheld mirror.
Hm, could that be fixed by covering it with a "privacy screen" (used to cover computer screens, basically a bunch of tiny parallel tubes (all-angle) or grooves (one-axis, probably best for this))? Then maybe a blur on top of that to increase the viewing angle again (though that might cut the light too much since it's not an active emitter)?
