Aliasing mitigation in optical microscopy of dynamic biological samples by use of temporally modulated color illumination and a standard RGB camera

Significance: Despite recent developments in microscopy, temporal aliasing can arise when imaging dynamic samples. Modern sampling frameworks, such as generalized sampling, mitigate aliasing but require measurement of temporally overlapping and potentially negative-valued inner products. Conventional cameras cannot collect these directly as they operate sequentially and are only sensitive to light intensity. Aim: We aim to mitigate aliasing in microscopy of dynamic monochrome samples by implementing generalized sampling via the use of a color camera and modulated color illumination. Approach: We solve the overlap problem by spectrally multiplexing the acquisitions and using (positive) B-spline segments as projection kernels. Reconstruction involves spectral unmixing and inverse filtering. We implemented this method using a color LED illuminator. We evaluated its performance by imaging a rotating grid and its applicability by imaging the beating zebrafish embryo heart in transmission and light-sheet microscopes. Results: Compared to stroboscopic imaging, our method mitigates aliasing with performance improving as the projection order increases. The approach can be implemented in conventional microscopes but is limited by the number of available LED colors and camera channels. Conclusions: Generalized sampling can be implemented via color modulation in microscopy to mitigate temporal aliasing. The simple hardware requirements could make it applicable to other optical imaging modalities. (C) The Authors.

Temporally aliased video microscopy: an undersampling method for in-plane modal analysis of microelectromechanical systems

Martinus Gijs, Marc Stranczl, Christophe Yamahata

A simple optical method is proposed for performing in-plane experimental modal analysis of micromachined structures with a conventional charge-coupled device (CCD) camera. The motion of a micromechanical device actuated by high frequency sinusoidal forces (kHz range) is recorded at the fixed sampling rate of a camera (typically 28 frames/sec.) which is configured with a short shutter aperture time (1/5000 s). Provided a CCD sensor with sufficient sensitivity, lots of information are contained in the video on the dynamics of the vibrating system despite the limited frame rate. Taking advantage of the theory of undersampling, we show that the dynamics of systems with several kHz bandwidth can be retrieved very easily. For demonstration purposes, we first study a push-pull electrostatic comb- drive actuator, which is a well-known damped harmonic oscillator system. Then, we show that our measurement method also provides useful information on the behavior of nonlinear systems. In particular, we can characterize systems' superharmonic and subharmonic resonances in a straightforward way.

Institute of Electrical and Electronics Engineers2012

Broadband Digital Holographic Camera for Microscopy

Zahra Monemhaghdoust

In this thesis, we explore a combination of analog and digital holographic techniques to demonstrate high speed quantitative phase imaging (QPI) using low coherence (broad bandwidth) light sources. Off-axis digital holographic microscopy is inherently a coherent method of phase quantification and thus suffers from coherent noise. However, only one measurement is required to retrieve field information which gives an advantage in term of image acquisition speed. While incoherent methods of phase quantification remove the problem of coherent noise, they require acquiring several measurements which is sometimes not acceptable in the study of dynamical systems. Therefore, this thesis is an attempt to extend off-axis digital holographic microscopy from the coherent to the incoherent regime to maintain the benefit of high speed measurement capability of off-axis holography while reducing the coherent noise. First, we demonstrate that by using a suitable combination of holographic gratings, off-axis digital holographic microscopy can work with spectrally broadband illumination sources. Holographic gratings introduce a tilt in the coherence plane of one of the reference or object arms of an off-axis digital holographic microscope to expand the interference area to the whole field of view of the camera. Although QPI provides useful information in addition to state-of-the-art intensity-based microscopic imaging, its use has not permeated yet far beyond research laboratories. One potential reason is the need for a stand-alone dedicated microscope system, which is expensive and not back-compatible to the existing wide-field microscopes. In this Regard, the second part of this thesis explores an add-on camera module which can be attached to the camera port of any standard intensity microscope. The techniques developed to extend the off-axis DHM in the first part of the thesis will be used to demonstrate a proof-of-concept camera module which operates with broadband light sources. Overall, through the combination of analog and digital holography, the goal of this work is to extend full-field off-axis holography to incoherent regime to benefit from real-time speckle-free measurements, optical sectioning and realization of a digital holographic camera which can be placed in the image plane of a standard optical microscope.

EPFL2015

Broadband Digital Holographic Camera for Microscopy

Zahra Monemhaghdoust

EPFL2015