Tomographic Diffraction Microscopy (TDM) is a technique that makes it possible to assess for 3D complex refractive index of the investigated sample without fluorescent labeling. Therefore, TDM is a method of choice for the characterization of biological samples or functionalized surfaces. TDM is a generalization of Digital Holographic Microscopy with a full control of the angle of illumination over the object. Angle can be modified either by sweeping the illumination on the object, or by rotating the object while maintaining the angle of illumination. Combining several hundreds of acquisition, it is possible to retrieve a full 3D information about both refraction and absorption of the object. Nevertheless, the time needed for data acquisition might become prohibitive for routine investigations, or dynamic sample imaging. Moreover, simultaneous reflection and transmission characterization of sample remains an experimental challenge. Recently a method called “Mirror-Assisted Tomographic Diffraction Microscopy” (MA-TDM) have been proposed [Opt. Lett. 35, 1857 (2010)], which theoretically allows to achieved isotropic 3D resolution by combining, in a simpler fashion, reflection and transmission modes. When transparent sample are considered, one can take benefits of this mirroring effect to limit the amount of acquired holograms, while maintaining the resolution of TDM. We propose to demonstrate this concept, using a specific preparation of the sample. It will be shown that, using an adequate data processing scheme, it is possible to reconstruct 3D objects using an annular illumination sweep, thus limiting the amount of acquisition. This study paves the way to a versatile TDM configuration allowing for both reflection and transmission acquisitions from a single image acquisition.
Form-factor and light efficiency are important issues Head-Mounted Displays face, since they both restrict their usage. Improving the form-factor means that for a defined visual stimulus, the system is smaller in volume. The light efficiency issue is linked to power consumption and time of use as well as the device’s ability to deliver, within a specific environment, enough luminance for the virtual image to be seen. This trade-off can also be found in imaging systems and Christophe Gaschet previously explored the optical design of onaxis imaging systems using curved sensors and particularly diopters number reduction thanks to Petzval shaped image plane. However, the behavior of an optical system changes dramatically when the design is off-axis. This paper focuses on demonstrating how using a curved microdisplay helps to improve the form-factor of a HMD system optimized using freeform optical design on a practical example. Curvature can also plays a great role in reducing the losses of light, but this imposes more constraints on the shapes to be given to the microdisplay. We discuss the trade-offs between these two advantages given by curved microdisplays. The mechanical feasibility of curved micro-displays will also be discussed, as well as the process to make a curved microdisplay, which is compatible with current mass-production CMOS displays. For OLED technology, the main resistance to curvature is the silicon substrate. The case for GaN technologies shows other mechanical limitations. We can predict the highest reachable curvature values, depending on microdisplay size and technology.
KEYWORDS: Digital holography, Diffraction, Time division multiplexing, Mirrors, Optical transfer functions, Tomography, Demodulation, Microscopy, Reflectivity, Refractive index
Tomographic diffractive microscopy (TDM) is an imaging technique, which allows for recording the refractive index of unlabelled transparent specimens. Based on diffraction theory, it can be implemented in transmission or in reflection. In this paper, a new TDM data acquisition and reconstruction method is proposed. The purpose is to use the mirror effect of a reflecting material to establish a double illumination system. Neglecting backward-diffracted fields, the setup reduces to a double-transmission TDM, which combined with an azimuthal rotation of the illumination, allows for faster and simplified acquisitions. We also point out a new demodulation method based only on Fourier transforms.
Tomographic diffractive microscopy (TDM) is an imaging technique which allows for recording the complex optical index of unlabelled specimens. It is based on diffraction theory with a spatially coherent illumination and interference demodulation. Different methods have been developped like illumination rotation with fixed sample or sample rotation with fixed illumination. However this last technique is difficult to set up. Hence, we propose a novel reconstruction technique applicable to axisymmetric unlabelled specimens. It consists in a numerical rotation of the Ewald cap of sphere generated by a zero-degree illumination on the sample. Due to the specimen symmetry, we show that the Fourier space can be filled in the direction perpendicular to the axis of symmetry.
KEYWORDS: Image resolution, Time division multiplexing, Spatial resolution, 3D image processing, Image resolution, 3D acquisition, Digital holography, Microscopes, 3D displays
Tomographic diffractive microscopy allows for imaging unlabeled specimens, with a better resolution than conventional microscopes, giving access to the index of refraction distribution within the specimen, and possibly at high speed. Principles of image formation and reconstruction are presented, and progresses towards realtime, three-dimensional acquisition, image reconstruction and final display, are discussed, as well as towards three-dimensional isotropic-resolution imaging.
The development of new nanomaterials, devices and systems is very much dependent on the availability of new techniques for nanometrology. There now exists many advanced optical imaging techniques capable of subwavelength resolution and detection, recently brought to the forefront through the 2014 Nobel Prize for chemistry for fluorescent STED and single molecule microscopy. Label-free nanoscopy techniques are particularly interesting for nanometrology since they have the advantages of being less intrusive and open to a wider number of structures that can be observed compared with fluorescent techniques. In view of the existence of many nanoscopy techniques, we present a practical classification scheme to help in their understanding. An important distinction is made between superresolution techniques that provide resolutions better than the classical λ/2 limit of diffraction and nanodetection techniques that are used to detect or characterize unresolved nanostructures or as nanoprobes to image sub-diffraction nanostructures. We then highlight some of the more important label-free techniques that can be used for nanometrology. Superresolution techniques displaying sub-100 nm resolution are demonstrated with tomographic diffractive microscopy (TDM) and submerged microsphere optical nanoscopy (SMON). Nanodetection techniques are separated into three categories depending on whether they use contrast, phase or deconvolution. The use of increased contrast is illustrated with ellipsometric contrast microscopy (SEEC) for measuring nanostructures. Very high sensitivity phase measurement using interference microscopy is then shown for characterizing nanometric surface roughness or internal structures. Finally, the use of through-focus scanning optical microscopy (TSOM) demonstrates the measurement and characterization of 60 nm linewidths in microelectronic devices.
KEYWORDS: Visualization, Holograms, 3D image processing, 3D image reconstruction, Microscopy, 3D acquisition, Tomography, Time division multiplexing, Computer architecture, Image restoration
Phase microscopy techniques regained interest in allowing for the observation of unprepared specimens
with excellent temporal resolution. Tomographic diffractive microscopy is an extension of
holographic microscopy which permits 3D observations with a finer resolution than incoherent light
microscopes. Specimens are imaged by a series of 2D holograms: their accumulation progressively
fills the range of frequencies of the specimen in Fourier space. A 3D inverse FFT eventually provides
a spatial image of the specimen.
Consequently, acquisition then reconstruction are mandatory to produce an image that could prelude
real-time control of the observed specimen. The MIPS Laboratory has built a tomographic
diffractive microscope with an unsurpassed 130nm resolution but a low imaging speed - no less than
one minute. Afterwards, a high-end PC reconstructs the 3D image in 20 seconds. We now expect
an interactive system providing preview images during the acquisition for monitoring purposes.
We first present a prototype implementing this solution on CPU: acquisition and reconstruction are
tied in a producer-consumer scheme, sharing common data into CPU memory. Then we present
a prototype dispatching some reconstruction tasks to GPU in order to take advantage of SIMDparallelization
for FFT and higher bandwidth for filtering operations. The CPU scheme takes 6
seconds for a 3D image update while the GPU scheme can go down to 2 or > 1 seconds depending
on the GPU class. This opens opportunities for 4D imaging of living organisms or crystallization
processes. We also consider the relevance of GPU for 3D image interaction in our specific conditions.
Many biological researches require observation of the sample at a sub-micrometer scale. However, most biological
samples are transparent, and thus are barely visible in conventional transmission microscopy. Techniques like
interference contrast or oblique illumination permit to record an improved contrast, but are useful for morphological
studies only, because the interaction of incoherent, non-polarized and polychromatic illumination with matter is very
complex, so that the recorded contrast cannot be linked to local physical properties of the sample, as for example the
index of refraction. We have developed a diffractive tomographic microscope, which permits the observation of
unstained-, transparent samples in 3-D. This device is based on a combination of microholography, which records the
field diffracted by the specimen in both amplitude and phase using a high numerical aperture objective and a phase
stepping interferometer, with a variable illumination of the sample (tomography) using a high numerical aperture
condenser. The successive holograms are numerically recombined in the Fourier space, and the reconstruction of the
specimen index of refraction distribution is based on the first Born approximation for weakly diffractive samples.
Examples of biological specimens observed with this technique are given, and possible evolutions of the instrument are
discussed.
3-D optical fluorescent microscopy becomes now an efficient tool for volume investigation of living biological samples.
Developments in instrumentation have permit to beat off the conventional Abbe limit, in any case the recorded image
can be described by the convolution equation between the original object and the Point Spread Function (PSF) of the
acquisition system. If the goal is 3-D quantitative analysis, whether you improve the instrument capabilities, or (and)
you restore the data. These last is until now the main task in our laboratory. Based on the knowledge of the optical
Transfer Function of the microscope, deconvolution algorithms were adapted to automatic determine the regularisation
threshold in order to give less subjective and more reproducible results. The PSF represents the properties of the image
acquisition system; we have proposed the use of statistical tools and Zernike moments to describe a 3-D system PSF and
to quantify the variation of the PSF. This first step toward standardization is helpful to define an acquisition protocol
optimizing exploitation of the microscope depending on the studied biological sample.
We have pointed out that automating the choice of the regularization level; if it facilitates the use, it also greatly
improves the reliability of the measurements. Furthermore, to increase the quality and the repeatability of quantitative
measurements a pre-filtering of images improves the stability of deconvolution process. In the same way, the PSF pre-filtering
stabilizes the deconvolution process. We have shown that Zernike polynomials can be used to reconstruct
experimental PSF, preserving system characteristics and removing the noise contained in the PSF.
Fluorescent microscopes suffer from limitations; photobleaching and phototoxicity effects, or influence of the sample
optical properties to 3-D observation. Amplitude and phase of the object can be reached with optical tomography based
on a combination of microholography with a tomographic illumination. So indices cartography of the specimen can be
obtained, and combined with fluorescence information it will open new possibilities in 3-D optical microscopy.
The optical microscope has proven to be an invaluable tool in biology, because of its unique capabilities of 3-D imaging of living specimens. However, compared to other techniques, the achievable resolution is limited. Several techniques have been proposed to improve the resolution of the fluorescence microscope. The confocal set-up is the first of them. Interference effects can also be used to sharpen up the point spread function (PSF) in 4Pi microscopy. Another approach is the so-called STimulated Emission Depletion microscopy, which has permitted to decrease the resolution down to about 100 nm in three dimensions, and below 50 nm in either the x-y plane, or along the z-axis.
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