Mathematical Treatment of Determination of Straightness and Flatness of Surfaces (Metrology)

7.5.
For determining straightness, autocollimator is placed on a reference surface and a block
containing reflector is traversed along a line on the surface whose straightness is to be
determined and readings noted at regular intervals. Such observations can also be taken using
precision level. These readings are first converted to give cumulative height difference from
the starting and the best line through the points determined by above method can be found by
applying in method of least squares, according to which the sum of the deviations of the points
from the best line should be zero, and the sum of the square of the deviationsjrom the best
line should be minimum. For the first condition the centrqid_of the points (X, Y) must lie on
this line. All the points are then retabulated taking points X, Y as the zero reference. Then