Phase (in radian) of the cross-spectrum of images at successive levels in the FeI 630.1 nm, measured at μ = 0.776 as a function of the spatial frequency given in pixel units. The frequency zero is at pixel 65. Upper row, from left to right: cross-spectra of images at levels 1 and 2, 2 and 3, and 3 and 4. Middle row, from left to right: cross-spectra of images at levels 4 and 5, 5 and 6, 6 and 7 (note the effect of contrast inversion), lower row, from left to right: cross-spectra of images at levels 11 and 12, 12 and 13, 13 and 14.