1. An apparatus for an orientation dependent radiation source comprising: (a) at least two radiation patterns, wherein each radiation pattern has a longitudinal axis; (b) a detectable property within each radiation pattern that is measured at a point of measurement located on the corresponding longitudinal axis; and, (c) a landmark located at the point of measurement, wherein the longitudinal axis of each radiation pattern is arranged to intersect at a point of intersection corresponding to the point of measurement of the detectable property for each of the at least two radiation patterns.