Rotating cube corner reflectors - Another approach to the symmetry problem is the use of rotating corner cube retroreflectors. By spinning them off axis, the shear can be made to vary as a function of angle. Each shear value produces a different path length. A series of patent disclosures by Tank et al describe many variations on this theme (see for example, US patent 5,341,207 and 5,148,235). The advantages of this approach are intrinsic tilt- and shear-compensation, as well as fairly rapid scan rates. The disadvantages are the inherent imbalance of spinning corner cubes off axis, and above 6000 rpm distortion of the corner cube reflectors becomes significant. Large cube corner retroreflectors are somewhat expensive.