Application and Optimisation of the Spatial Phase Shifting ...

84 Electronic or Digital Speckle Pattern Interferometry
restriction that ϕ O (x k ,y)ϕ O (x k+n ,y) for all n, and the same applies to I b (x k ,y) and M I (x k ,y); this is, spatial
fluctuations of these quantities should be as small as possible. The time dependence of ϕ O can be
neglected unless ϕ O fluctuates substantially within the integration time for the camera frames; α n has no
time dependence at all, because the phase shift is determined by the stable geometry depicted in Fig. 3.24.
The I n are then processed as described in 3.2; however, when working spatially, it is reasonable to use
evaluation formulae with n ∈{–1, 0, 1} or n ∈{–1, 0, 1, 2} because, as Fig. 3.26 shows, the central pixel
of the cluster will be the one that has best spatial correlation with its neighbours and to which the resulting
ϕ O should be assigned. From this it follows that 1