Mismatch of the coherence plane and focal plane in LC-OCT. The plain red lines represent the beam in the plane of the figure, whereas the dotted red lines represent the beam in the direction orthogonal to the plane of the figure. (a) The interferometer is adjusted so that the coherence plane and the focal plane coincide at the surface of the sample. (b) When the interferometer is lowered by a distance z, the coherence plane is at a distance z_coh below the sample surface and the focal planes at a distance z_foc (here n > n_im).