Jérôme Dohet-Eraly

Refocus criterion for both phase and amplitude objects in digital holographic microscopy.

For digital holographic microscopy applications, we modify the focus criterion based on the integration of the amplitude modulus to make possible its use regardless of the phase or amplitude nature of the objects under test. When applied on holographic data, the original criterion gives, at the focus plane, a minimum or a maximum, for amplitude or phase objects. The criterion we propose here operates on high-pass filtered complex amplitudes. It is shown that the proposed criterion gives a minimum for both types of objects when the focus plane is reached. Experimental results on real samples and simulations are provided, illustrating the efficiency and the potential of the method.

Quantitative assessment of noise reduction with partial spatial coherence illumination in digital holographic microscopy.

Improving image quality in digital holographic microscopy is achievable by using partial spatial coherence (PSC) illumination instead of fully coherent illumination. This Letter presents simple theoretical models to quantitatively assess the reduction of noise as a function of both the spatial coherence of the illumination and the defocus distance of the noise source. The first developed model states that the effect of the PSC can be studied by discretizing the field of view in the plane of the noise source. The second model, following a continuous approach, corroborates the discrete model and extends it. Experimental results confirm theoretical expectations.

Refocusing based on amplitude analysis in color digital holographic microscopy

A refocusing criterion adapted to red-green-blue (RGB) digital holographic microscopy is established. It is applicable for both amplitude and phase objects. This color criterion is based on a monochromatic criterion, using the integrated modulus amplitude. Simulated RGB holograms show the value of having color information, even for colorless samples; in addition, the position of the focus plane along the optical axis is determined more accurately. Simulations take into account both the numerical apertures of lenses and noise during the holographic process. We also implement an algorithm exponentially reducing the computation time required for detecting the focus plane. The method is validated on experimental holograms.

Fast numerical autofocus of multispectral complex fields in digital holographic microscopy with a criterion based on the phase in the Fourier domain

The knowledge of the complex amplitude of optical fields, that is, both quantitative phase and intensity, enables numeric reconstruction along the optical axis. Nonetheless, a criterion is required for autofocusing. This Letter presents a robust and rapid refocusing criterion suitable for color interferometric digital holographic microscopy, and, more generally, for applications where complex amplitude is known for at least two different wavelengths. This criterion uses the phase in the Fourier domain, which is compared among wavelengths. It is applicable whatever the nature of the observed object: opaque, refractive, or both mixed. The method is validated with simulated and experimental holograms.

Color imaging-in-flow by digital holographic microscopy with permanent defect and aberration corrections

Color imaging-in-flow of particles is performed using red-green-blue (RGB) digital holographic microscopy (DHM), whose sources are partially coherent. RGB DHM provides intensity and quantitative phase images in the three color channels, which is valuable for observing small objects in numerous fields. In-flow investigation on a large depth of field is made possible by the refocusing capability of DHM and has many potential applications. A method is also developed to automatically correct the color balance and compensate both intensity and phase defects and aberrations, providing high-quality imaging. Experimental results show color in-flow analysis of microplankton and confirm the efficiency of the correction method.

Partial spatial coherence illumination in digital holographic microscopy: Quantitative analysis of the resulting noise reduction

Imaging applications needing illumination with sufficiently high power density often request coherent light such as provided by laser or super-luminescent diodes. However the very high spatial coherence of those sources can generate coherent speckle noise and multiple reflection effects that may degrade the resulting image quality. In order to overcome such issues, we have shown that using partial spatial coherence illumination in interferometric digital holographic microscopy (DHM) greatly improves the image quality. Two models are here proposed to quantitatively assess the noise reduction as a function of both the spatial coherence, and the distance between the noise source and the recorded plane. We emphasize that these approaches may be useful in numerous imaging situations not restricted to DHM systems. The first developed model uses the discretization of the field of view in the plane of the noise source. This model is more intuitive but encounters some limitations. The second model, based on a continuous approach, corroborates the discrete model and extends it when necessary. Experimental validation of both models has been performed with a DHM, whose illumination has an adjustable spatial coherence. The noise was generated using a microscope slide with de-agglomerated particles. The relative standard deviation of fluctuations due to noise is shown to be inversely proportional to the product D|d| when this quantity is high, where D is the diameter of a pupil leading the spatial coherence and d is the defocus distance of the noise source. The continuous model is applicable in any case.

Refocus Criterion Based on the Phase in the Fourier Domain for Automatically Refocusing in Multispectral Digital Holographic Microscopy: Accuracy and Dependency Study

© 2018 The Author(s). The fast autofocus criterion using the phase in the Fourier domain, suitable for digital holographic microscopy when the complex field is known for at least two distinct wavelengths, is deeply investigated, which allows finer adjustment.

Partial spatial coherence illumination in digital holographic microscopy: quantitative analysis of the resulting noise reduction

Imaging applications needing illumination with sufficiently high power density often request coherent light such as provided by laser or super-luminescent diodes. However the very high spatial coherence of those sources can generate coherent speckle noise and multiple reflection effects that may degrade the resulting image quality. In order to overcome such issues, we have shown that using partial spatial coherence illumination in interferometric digital holographic microscopy (DHM) greatly improves the image quality. Two models are here proposed to quantitatively assess the noise reduction as a function of both the spatial coherence, and the distance between the noise source and the recorded plane. We emphasize that these approaches may be useful in numerous imaging situations not restricted to DHM systems. The first developed model uses the discretization of the field of view in the plane of the noise source. This model is more intuitive but encounters some limitations. The second model, based on a continuous approach, corroborates the discrete model and extends it when necessary. Experimental validation of both models has been performed with a DHM, whose illumination has an adjustable spatial coherence. The noise was generated using a microscope slide with de-agglomerated particles. The relative standard deviation of fluctuations due to noise is shown to be inversely proportional to the product D|d| when this quantity is high, where D is the diameter of a pupil leading the spatial coherence and d is the defocus distance of the noise source. The continuous model is applicable in any case.

In-Flow Imaging by Multispectral Digital Holographic Microscopy with Partially Coherent Illumination

Color digital holographic microscopy is adapted for in-flow analysis of particles. Automatic methods for correcting aberrations and color balance, and for rapid refocusing, are presented. Experimental results on microorganisms show high quality refocused color images.