A cartridge for a data storage disk drive has a thin retroreflective marker. Light from a source is reflected from the marker almost exactly on its incident path. A planar light shaping optical structure such as a holographic light shaping diffuser (LSD) ruled or blazed diffraction grating, holographic diffraction grating, binary optics or holographic diffraction grating directs light to the detector. The LSD has degrees of angular diffusion in different orthogonal axes. This property makes possible unique identification of different types of cartridges.