Dihedral Error and Its Effects on an SMR

Ideally, the three reflective surfaces or panels of a Spherically Mounted Retroreflector (SMR) are aligned perfectly perpendicular or at a 90 degree angle from each other. The Dihedral Error of an SMR is the difference between the actual angular alignment of these surfaces and the nominal 90 degree angle. 

The dihedral error of an SMR results in a change in the shape, size and/or position of the reflected laser beam. This prevents the beam from properly entering the true center point or origin of the tracker and can increase the error of the SMR. If the dihedral errors are out of specification, this can result in an additional error during a runout check (comparison of measurements while rotating the SMR in a stable nest), create additional backsight error or result in poor tracking / inability to track as the distance from the tracker increases.
 

Keywords: dihedral error, SMR, SMR error, SMR specs, SMR specifications