Measurement instrument with interferometer and method

A ball bar gage for obtaining the accuracy of numerical control machines and the like including telescoping means integrated with an interferometer and the method of measuring the distances between two locations including an initialization process that ascertains the absolute length of the distance being measured from the differential changes in length measured by the interferometer and a computer that calculates by a trilateration computation 2 or 3 dimensional coordinates by computing the measurements obtained by the ball bar gage. A laser beam or other light source are contemplated to energize the interferometer. Similar calculations are performed with the ball bar gage and computer over a number of spatial locations commensurate with determining the accuracy of the machine being evaluated.