Physical significance of backscattering phase measurements

Quantitative phase imaging of transparent objects in transmission allows for a direct interpretation of the results: the phase shift measured is linear in the refractive index contrast and object thickness. However, the same measurement in a backscattering geometry yields fundamentally different results, because the incident field component is absent from the detected field. As a result, the relationship between the measured phase and object properties is obscure. We derived analytical expressions for the propagating fields under the first-order Born approximation and studied the interpretation of the measured phase shifts in backscattering versus transmission geometries. Our analysis shows that the backscattering phase shift is the result of the plane wave superposition originating at various depths in the object, which makes it impossible to infer quantitative morphology or topography information of 3D transparent samples from a reflection phase image alone.