An article comprising: a substrate that includes a physical surface; a retroreflective sheet on said physical surface; a layer on said retroreflective sheet with a human-readable message or image visible in the visible light spectrum; an optical code layer that includes an optical code (400, 530), wherein a material in the optical code layer that absorbs, scatters or otherwise inhibits retroreflection of said retroreflective sheet beneath the optical code layer in the infrared spectrum is used to create said optical code in said optical code layer, wherein the optical code (400,530) is not visible by the human eye in the visible light spectrum, said optical code (400, 530) includes a hierarchy of parent and child optical element sets, the parent optical element set including a plurality of parent optical elements each of a first size and the child optical element set including a plurality of child optical elements each of a second size that is smaller than the first size, wherein each parent optical element of the parent optical set represents a respective encoded bit, wherein the encoded bit represented by the respective parent optical element is based at least in part on a cumulative visual appearance of a corresponding subset of child optical elements of the child optical element set, wherein the cumulative visual appearance of each parent optical element is a respective visual gradient value indicating a degree of luminance in a range of gradient values having different degrees of luminance, the encoded bit represented by the respective parent optical element based on the visual gradient value for the respective parent optical element relative to a first threshold value, wherein each child optical element of the child optical element set represents a respective encoded bit, wherein the encoded bit represented by each respective child optical element of the child optical element set is based at least in part on a visual appearance of the respective child optical element, wherein a visual appearance of each child optical element is a respective visual gradient value in the range of gradient values, the encoded bit represented by the respective child optical element based on the visual gradient value for the respective child optical elements relative to a second threshold value that is different from the first threshold value, and wherein the encoded bits represented by the respective child optical elements are not decodable from a distance greater than a threshold distance, and wherein the encoded bits represented by the respective parent optical elements are decodable from the distance greater than the threshold distance. a substrate that includes a physical surface; a retroreflective sheet on said physical surface; a layer on said retroreflective sheet with a human-readable message or image visible in the visible light spectrum; an optical code layer that includes an optical code (400, 530), wherein a material in the optical code layer that absorbs, scatters or otherwise inhibits retroreflection of said retroreflective sheet beneath the optical code layer in the infrared spectrum is used to create said optical code in said optical code layer, wherein the optical code (400,530) is not visible by the human eye in the visible light spectrum, said optical code (400, 530) includes a hierarchy of parent and child optical element sets, the parent optical element set including a plurality of parent optical elements each of a first size and the child optical element set including a plurality of child optical elements each of a second size that is smaller than the first size, wherein each parent optical element of the parent optical set represents a respective encoded bit, wherein the encoded bit represented by the respective parent optical element is based at least in part on a cumulative visual appearance of a corresponding subset of child optical elements of the child optical element set, wherein the cumulative visual appearance of each parent optical element is a respective visual gradient value indicating a degree of luminance in a range of gradient values having different degrees of luminance, the encoded bit represented by the respective parent optical element based on the visual gradient value for the respective parent optical element relative to a first threshold value, wherein each child optical element of the child optical element set represents a respective encoded bit, wherein the encoded bit represented by each respective child optical element of the child optical element set is based at least in part on a visual appearance of the respective child optical element, wherein a visual appearance of each child optical element is a respective visual gradient value in the range of gradient values, the encoded bit represented by the respective child optical element based on the visual gradient value for the respective child optical elements relative to a second threshold value that is different from the first threshold value, and wherein the encoded bits represented by the respective child optical elements are not decodable from a distance greater than a threshold distance, and wherein the encoded bits represented by the respective parent optical elements are decodable from the distance greater than the threshold distance.