1. An apparatus comprising: a first phosphor layer on top of a substrate, the first phosphor layer having luminescent properties such that when it is illuminated by light having a first wavelength, it emits light having a second wavelength, the first phosphor layer is divided into a first two-dimensional grid of cells arranged in a first spatial pattern, wherein each cell of the first two-dimensional grid of cells corresponds to one bit of an identification code, a first portion of the first phosphor layer is removed from a first number of cells of the first two-dimensional grid of cells to expose the substrate for the first number of cells.