An apparatus for modulating an incident beam includes a body that is non-transmissive and rotatable about an axis perpendicular to a surface of the body. The body has a first set of features including transmissive features with respect to the incident beam along a first radial path at a first radial distance from the axis and a second set of features including data storage features along a second radial path at a second radial distance from the axis. The apparatus also includes a reference sensor disposed over a first position along the second radial path. In the apparatus. the radial distances are different and the numbers of transmissive features and data storage features are relatively prime. When the body is rotating, the first set of features modulate the incident beam and the reference sensor generates a reference signal based on the data storage features traversing the first position.