Figure 5. Principle of the Hartmann-Shack (HS) sensor. Upper diagram, A plane wave-front hits a single micro-lens and focuses at a point P located over the optical axis and at a distance f from the lens; for the same micro-lens an aberrated wave-front focuses at point A, also on the focal plane but shifted away from the optical axis. Lower diagram, For a set of micro-lenses, an aberrated wave-front will focus on unevenly spaced points (shifted away from the optical axis of each micro-lens) over the focal plane. It is the quantification of the individual shifts that allows one to determine the local slopes of the wave-front and then, from this information, retrieve the entire wave-front surface. The x,y arrows to the right of the upper diagram show the coordinate system directions (y is pointing upwards and x is coming outside from the page) used according to Equations 7 and 8.