Dead-path error
Dead-path is the path between the laser interferometer and the null point of the measuring position. Dead-path error is the error caused by environmental changes on the dead-path during the measurement. The severity of the dead-path error is relative to the length of the dead-path. Ordinary environmental compensation systems can not compensate for dead-path error, so the only way to minimize the error is to minimize the dead-path length.
AcWaCo measures the refractive index of air along the whole laser beam path (including the dead-path) so the dead-path error is completely eliminated.