What is the reference measurement mode of FrontSurfer?

In this mode, the wavefront is reconstructed from two intensity patterns captured by the wavefront sensor. The first one (main pattern) is measured in presence of the the aberration to be reconstructed, and the second one (reference pattern) is measured for calibration purposes, i.e., with the aberration removed. In this mode, the microlens array (Hartmann mask) can be neither complete nor regular; one can even use an array with random positioning of sub-apertures.