The correspondence between axial spatial frequency and wavelength in the reflection configuration. (a) Ewald’s sphere cap at a normal incidence in the reflection configuration. (b) The axial spatial frequency (k
) is only weakly dependent on the backscattering angles or lateral spatial frequency (k
). (c) Each axial spatial frequency (k
) can be encoded by each wavelength with a small uncertainty as indicated by two red dashed lines on Figure (b).