In an ideal system, the input-out relationship is linear (i.e., straight line). Any departure from straight line is expressed as non-linearity. Two methods are used for measurement. The “best straight line” approach compromises the end points and situates the line to give the most optimistic answer. Rustrak non-linearity measurement is based on end point calibration which gives a truer value and a standard, more convenient method for calibrator and recalibration.