The optical transfer function calibration
The optical transfer function describes the relations between the normalized phase difference and amplitude ratio of the first and second harmonics in the modulated signal. The amplitude ratio is given by β4, the phase difference by β5. The expectation value for β4 is 1 and for β5 is zero. The calibration model describes the corrections with respect to these expectation values.
The calibration model describes the corrections to the expectation values as a third order polynomial in position coordinates (w along scan, z across scan), up to second order in colour-index dependence, and the linear combinations between colour index and position coordinates (see the tabs in the figure above).