The bar code uses two pigments which have different optical characteristics in the infrared range. (newscientist.com)
The designs included two different arrangements of the same set of particles, where each configuration had different optical characteristics. (sciencedaily.com)
This produces spots with the same optical characteristics as the pits that are pressed in mass - produced CDs, so the discs it produces will work in ordinary CD players. (newscientist.com)