Figure 1.

(a) when an incident illumination field E₀ passes through an essentially transparent sample such as a cell, the phase of the wavefront at any given point in space is delayed by a factor that is directly proportional to the refractive index of the sample, n, and the optical path length, h. The interferogram obtained by interfering the incidence field E0 with the transmitted field ET is mathematically transformed to obtained a physical image of the refractive index variations in the x-y plane. (b) As shown in this NBS-1963A resolution target, the phase distribution is then expressed as surface height distribution with nanometer sensitivity in the z-direction. (c) In biological samples of uniform refractive index, the phase image, Δϕ(x,y), is directly proportion to the height profile, h(x,y), where the proportion constant, k, is a function of wavelength and refractive index. The scale bar on the right shows cell thickness in microns.