Illustration of the Hartmann sensing principle. Top: phase of the wavefront incident on the entrance surface of the mask. The phase has been scaled by λ/2π so that the normal to the tangent is parallel to the local direction of propagation of the wavefront. The arrows indicate the direction of propagation at the centre of each mask hole. Middle: intensity of the wavefront as it propagates from the mask (top) to the detector (bottom). The colour scale is shown on the right. Bottom: the one-dimensional intensity profile of the wavefront as measured by the detector.