Abstract: Disclosed is a machine readable code of a set of distinguishable symbols including at least a first symbol for encoding zeroes and a second symbol for encoding ones, wherein the first symbol exhibits a first color during machine reading and the second symbol exhibits a second color during machine reading, wherein the first color and the second color are detectably different under machine reading conditions.

Abstract: An reimageable medium composed of: a substrate; and a photochromic material, wherein the medium is capable of exhibiting a color contrast and an absence of the color contrast, wherein the medium has a characteristic that when the medium exhibits the absence of the color contrast and is then exposed to an imaging light corresponding to a predetermined image to result in an exposed region and a non-exposed region, the color contrast is present between the exposed region and the non-exposed region to form a temporary image corresponding to the predetermined image that is visible for a visible time, wherein the medium has a characteristic that when the temporary image is exposed to an indoor ambient condition for an image erasing time, the color contrast changes to the absence of the color contrast to erase the temporary image in all of the following: (i) when the indoor ambient condition includes darkness at ambient temperature, (ii) when the indoor ambient condition includes indoor ambient light at ambient temp