Effects of spatial coherence in diffraction phase microscopy

Abstract

Quantitative phase imaging systems using white light illumination can exhibit lower noise figures than laser-based systems. However, they can also suffer from object-dependent artifacts, such as halos, which prevent accurate reconstruction of the surface topography. In this work, we show that white light diffraction phase microscopy using a standard halogen lamp can produce accurate height maps of even the most challenging structures provided that there is proper spatial filtering at: 1) the condenser to ensure adequate spatial coherence and 2) the output Fourier plane to produce a uniform reference beam. We explain that these object-dependent artifacts are a high-pass filtering phenomenon, establish design guidelines to reduce the artifacts, and then apply these guidelines to eliminate the halo effect. Since a spatially incoherent source requires significant spatial filtering, the irradiance is lower and proportionally longer exposure times are needed. To circumvent this tradeoff, we demonstrate that a supercontinuum laser, due to its high radiance, can provide accurate measurements with reduced exposure times, allowing for fast dynamic measurements.

Compact lensless phase imager

“…This effect is also present in transport of intensity experiments that share similar experimental conditions (i.e. sample-camera distance < 900μm) [29,30] and other quantitative phase imaging techniques [31,32]. However, lensless phase imagers are mainly used to image biological samples like cells [4][5][6][7][8][9][10] which sizes are typically less than 20μm for which the phase can be correctly computed.…”

Diffraction phase microscopy: monitoring nanoscale dynamics in materials science [Invited]

“…QPI has been performed using a variety of sources including lasers in both the visible and infrared (IR) regimes [27]. Broadband techniques using light emitting diodes (LEDs), super-continuum lasers, and standard halogen lamp illumination have also been demonstrated [20,[28][29][30]. White light methods generally exhibit lower noise due to the lower coherence, both spatially and temporally, which reduces noise mechanisms such as laser speckle.…”

“…White light methods generally exhibit lower noise due to the lower coherence, both spatially and temporally, which reduces noise mechanisms such as laser speckle. However, when used with insufficient spatial coherence, white light may introduce object-dependent artifacts such as halos, which prevent proper reconstruction of the topography [20,29,30]. Both the spatial and temporal coherence of the source must be considered when performing any type of interferometry.…”

“…Both the spatial and temporal coherence of the source must be considered when performing any type of interferometry. For accurate QPI measurements, the mutual intensity function must be sufficiently flat over the area of the largest object being measured [29].…”

“…show the computed height map and corresponding cross section of the micropillar control sample. The small peaks at the edges are due to missing high-frequency components not captured by the finite numerical aperture of the objective, sometimes referred to as the Gibbs phenomenon [29]. The height is proportional to the phase and depends on the mean wavelength of the illumination as well as the refractive index.…”