Wavefronts cannot be perceived because it is light intensity rather than phase that interacts directly with matter. Detectors such as our eye respond to brightness levels, not differences in optical path length. Detectors in wavefront sensors also respond to light level. However, this response is related to some kind of length measurement. This chapter will examine the basic parameters of a phasefront accessible to a wavefront sensor.
In the last chapter a wavefront was described by comparing it to an ideal spherical wavefront. A mathematical function, W(x,y), was developed which gave the optical path difference (OPD) between the actual wavefront and the reference wavefront at the pupil point (x,y). Let us now consider a specific case in which the actual wavefront is spherical but cocked or tilted slightly with respect to the reference spherical wavefront as illustrated in Fig. 2.1.
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