1. An optical data storage medium, suitable for use for in the retrieval of data by means of incident illumination of wavelength .lambda. and an optical objective having a numerical aperture of NA, said storage medium comprising: an optical differentiation layer disposed to receive the illumination, said optical layer comprising a plurality of first micro-optical differentiation elements, each said first micro-optical differentiation element configured to have a first optical characteristic, and each said first micro-optical differentiation element disposed in said optical differentiation layer a distance of at least .lambda./.sub.2 NA from adjacent said first micro-optical differentiation elements, and a plurality of second micro-optical differentiation elements, each said second micro-optical differentiation element configured to have a second optical characteristic and each said second micro-optical differentiation element disposed in said optical layer a distance of at least .lambda./.sub.2 NA from adjacent said second micro-optical differentiation elements; a second layer, comprising a plurality of data sites, in optical communication with said optical differentiation layer such that illumination incident upon said first micro-optical differentiation element produces illumination reflected from a first one of said plurality of data sites disposed on said second layer having a first optical characteristic and illumination incident upon said second micro-optical differentiation element produces illumination reflected from a second one of said plurality of data sites disposed on said second layer having a second optical characteristic.