Optical tunneling of collimated monochromatic light of wavelength lambda through an air gap of thickness d(x, y) between two stress-free transparent prisms of the same refractive index N at an angle of incidence phi > the critical angle is accompanied by spatially variable transmission differential phase shift Delta(t)(x, y) that can be measured by a straight-through imaging ellipsometer with suitable polarizing and analyzing optics and two-dimensional array detector. Operation under the condition of equal tunneling for the p and s polarizations at phi(e) = arcsin root 2/(N-2 +1) leads to an analytical solution for d(x, y) in terms of Delta(t)(x, y). (C) 2014 Elsevier B.V. All rights reserved.