Tower difference, also called edge difference, is an important parameter to define whether the prism machining is qualified. The most commonly used definition is that the tower difference is equal to the parallel difference between the edge of a prism and its opposite working face.
We take the common right Angle prism as an example, the so-called edge difference refers to the Angle between the edge CC 'and the string plane AA' B 'B. During the measurement, the measured rectangular prism is placed on the precision Angle measuring instrument platform, and the three screws under the load are adjusted so that the optical axis of the autocollimator is perpendicular to the two right Angle planes AA 'C' C and B B 'C' C respectively (the Angle value of the autocollimator reflected back from the two right Angle planes is the same in the Y direction), that is, the optical axis of the autocollimator is perpendicular to the intersection edge CC 'of these two planes.
By rotating the stage, the autocollimation reflection image of the chord plane AA 'B' B of the prism enters the field of view of the autocollimator. If the Angle value of the autocollimation image of the chord plane AA 'B' B in the Y direction in the software is the same as the Angle value of the autocollimation image reflected back from the previous two right Angle planes in the Y direction, then it means that the edge difference of the right Angle prism is zero. Otherwise, the Angle difference of the two Y directions is the edge difference of the right Angle prism.