Errors in straightness of movement and in parallelity of axes can be measured by the LS 127ST set. The laser beam is split in the LS 127.1 differential beamsplitter into two parallel beams. After going through the optical wedge LS 127.2 the beams diverge at a small angle. The optical wedge functions as the straightness sensor. Reflector LS 127.3, formed by two inclined extremely precise plane mirrors, returns the beams back into the differential beamsplitter. The axis of the mirror reflector defines the straight line reference. This straightness measurement method is less sensitive to air turbulence disturbances than other methods. This straightness set also shows low sensitivity to possible inclinations of the straightness sensor during its movement at measurement.

The standard configuration for position measurement except the LS 20.1, LS 21.1, LS 70, LS 90.1, LS 90.3 is used for straightness measurement. The optical parts are fixed by the set of holders LS 51.