Trust region versus line search for computing the optical flow

We consider the numerical treatment of the optical flow problem by evaluating the performance of the trust region method versus the line search method. To the best of our knowledge, the trust region method is studied here for the first time for variational optical flow computation. Four different optical flow models are used to test the performance of the proposed algorithm combining linear and nonlinear data terms with quadratic and TV regularization. We show that trust region often performs better than line search; especially in the presence of non-linearity and non-convexity in the model.

Positional stability of holographic optical traps

The potential of digital holography for complex manipulation of micron-sized particles with optical tweezers has been clearly demonstrated. By contrast, its use in quantitative experiments has been rather limited, partly due to fluctuations introduced by the spatial light modulator (SLM) that displays the kinoforms. This is an important issue when high temporal or spatial stability is a concern. We have investigated the performance of both an analog-addressed and a digitally-addressed SLM, measuring the phase fluctuations of the modulated beam and evaluating the resulting positional stability of a holographic trap. We show that, despite imparting a more unstable modulation to the wavefront, our digitally-addressed SLM generates optical traps in the sample plane stable enough for most applications. We further show that traps produced by the analog-addressed SLM exhibit a superior pointing stability, better than 1 nm, which is comparable to that of non-holographic tweezers. These results suggest a means to implement precision force measurement experiments with holographic optical tweezers (HOTs).

Holographic optical tweezers combined with back-focal-plane displacement detection

A major problem with holographic optical tweezers (HOTs) is their incompatibility with laser-based position detection methods, such as back-focal-plane interferometry (BFPI). The alternatives generally used with HOTs, like high-speed video tracking, do not offer the same spatial and temporal bandwidths. This has limited the use of this technique in precise quantitative experiments. In this paper, we present an optical trap design that combines digital holography and back-focal-plane displacement detection. We show that, with a particularly simple setup, it is possible to generate a set of multiple holographic traps and an additional static non-holographic trap with orthogonal polarizations and that they can be, therefore, easily separated for measuring positions and forces with the high positional and temporal resolutions of laser-based detection. We prove that measurements from both polarizations contain less than 1% crosstalk and that traps in our setup are harmonic within the typical range. We further tested the instrument in a DNA stretching experiment and we discuss an interesting property of this configuration: the small drift of the differential signal between traps.

Le siège de Paris, histoire du blocus de 1870-1871 : avec carte

Collection : Bibliothèque populaire

Paris pendant le siège, 1870-1871 / par Arnold Henryot

Collection : Bibliothèque historique

A novel optical intracellular imaging approach for potassium dynamics in astrocytes.

Astrocytes fulfill a central role in regulating K+ and glutamate, both released by neurons into the extracellular space during activity. Glial glutamate uptake is a secondary active process that involves the influx of three Na+ ions and one proton and the efflux of one K+ ion. Thus, intracellular K+ concentration ([K+]i) is potentially influenced both by extracellular K+ concentration ([K+]o) fluctuations and glutamate transport in astrocytes. We evaluated the impact of these K+ ion movements on [K+]i in primary mouse astrocytes by microspectrofluorimetry. We established a new noninvasive and reliable approach to monitor and quantify [K+]i using the recently developed K+ sensitive fluorescent indicator Asante Potassium Green-1 (APG-1). An in situ calibration procedure enabled us to estimate the resting [K+]i at 133±1 mM. We first investigated the dependency of [K+]i levels on [K+]o. We found that [K+]i followed [K+]o changes nearly proportionally in the range 3-10 mM, which is consistent with previously reported microelectrode measurements of intracellular K+ concentration changes in astrocytes. We then found that glutamate superfusion caused a reversible drop of [K+]i that depended on the glutamate concentration with an apparent EC50 of 11.1±1.4 µM, corresponding to the affinity of astrocyte glutamate transporters. The amplitude of the [K+]i drop was found to be 2.3±0.1 mM for 200 µM glutamate applications. Overall, this study shows that the fluorescent K+ indicator APG-1 is a powerful new tool for addressing important questions regarding fine [K+]i regulation with excellent spatial resolution.

Membrane insertion and antibody recognition of a glycosylphosphatidylinositol-anchored protein: an optical study.

The kinetics of binding of a glycolipid-anchored protein (the promastigote surface protease, PSP) to planar lecithin bilayers is studied by an integrated optics technique, in which the bilayer membrane is supported on an optical wave guide and the phase velocities of guided light modes in the wave guide are measured. From these velocities, the optical parameters of the membrane and PSP layers deposited on the waveguide are determined, yielding in particular the mass of PSP bound to the membrane, which is followed in real time. From a comparison of the binding rates of PSP and PSP from which the lipid moiety has been removed, it is shown that the lipid moiety plays a key role in anchoring the protein to the membrane. Specific and nonspecific binding of antibodies to membrane-anchored PSP is also investigated. As little as a fifth of a monolayer of PSP is sufficient to suppress the appreciable nonspecific binding of antibodies to the membrane.