JEOS:RP - Journal of the European Optical Society Rapid publications
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MAP entropy estimation: applications in robust image filtering
Get PDFWe introduce a new approach for image filtering in a Bayesian framework. In this case the probability density function (pdf) of thelikelihood function is approximated using the concept of non-parametric or kernel estimation. The method is based on the generalizedGaussian Markov random fields (GGMRF), a class of Markov random fields which are used as prior information into the Bayesian rule, whichprincipal objective is to eliminate those effects caused by the excessive smoothness on the reconstruction process of images which arerich in contours or edges. Accordingly to the hypothesis made for the present work, it is assumed a limited knowledge of the noise pdf,so the idea is to use a non-parametric estimator to estimate such a pdf and then apply the entropy to construct the cost function for thelikelihood term. The previous idea leads to the construction of Maximum a posteriori (MAP) robust estimators, since the real systems arealways exposed to continuous perturbations of unknown nature. Some promising results of three new MAP entropy estimators (MAPEE) forimage filtering are presented, together with some concluding remarks
Dual wavelength continuous wave laser using a birefringent filter
Get PDFWe report simultaneous dual wavelength continuous laser emission with minimum cavity elements. Tunable dual wavelength emission between 805 nm and 840 nm was observed with controlled peak separation around two nanometers, which corresponds to approximately one terahertz. Dual wavelength laser operation is possible using a novel intracavity two plate birefringent filtering element
Refractive index determination of SiO2 layer in the UV/Vis/NIR range: spectrophotometric reverse engineering on single and bi-layer designs
Get PDFIn this paper we use spectrophotometric measurements and a Clustering Global Optimization procedure to determine the complex refractive index of SiO2 layer from 250 nm to 1250 nm. A special commercial optical module allows the reflection and transmission measurements to be made under exactly the same illumination and measurement conditions. We compare the index determination results obtained from two different single layer SiO2 samples, with high and low index glass substrates, respectively. We then determine the refractive index of SiO2 for a bi-layer design in which the first deposited layer is Ta2O5. The corresponding solutions are discussed and we show that the real part of the complex refractive index obtained for a bi-layer is slightly different to that found for a single layer investigation. When SiO2 is included inside a thin film stack, we propose the use of an index determination method in which a bi-layer is used for the real part of the complex refractive index, and single layer determination is used for the imaginary part of the refractive index in the UV range
Passive scene imaging of absorbing gases by narrowband dielectric filter modulation
Get PDFThis paper examines a method for locating within a scene a distribution of an absorbing gas using a passive imaging technique. An oscillatory modulation of the angle of a narrowband dielectric filter located in front of a camera imaging a scene, gives rise to an intensity modulation that differs in regions occupied by the absorbing gas. A preliminary low cost system has been constructed from readily available components which demonstrates how the location of gas within a scene can be implemented. Modelling of the system has been carried out, especially highlighting the transmission effects of the dielectric filter upon different regions of the image
Label-free discrimination of cells undergoing apoptosis by hyperspectral confocual reflectance imaging
Get PDFAmong the optical techniques used for exploring the properties of cells and tissues, those based on hyperspectral label-free analysis are particularly interesting due to their non-invasive character and their ability to fast collect a huge number of information on the different sample constituents and their spatial distribution. Here we present results obtained with a novel hyperspectral reflectance confocal microscope of label-free discrimination of cells undergoing apoptosis. Our data, analyzed by means of a powerful statistical method, enable to obtain information on the biological status at a single cell level through the local measurement of reflectivity. Furthermore, an optical model of the local dielectric response gives an additional insight of the parameters linking the optical responsivity to the biological status
The effect of nanoparticle size on thermal diffusivity of gold nano-fluid measured using thermal lens technique
Get PDFA dual beam mode-mismatched thermal lens method has been employed to investigate the dependence of thermal diffusivity of gold nanofluid on nanoparticles sizes. The samples were prepared at various sizes by utilizing the gamma radiation method. In the dual beam mode-mismatched thermal lens a diode laser (532 nm) was used as an excitation beam and a He-Ne laser with the beam output at 632.8 nm was used as a probe beam. Thermal diffusivity of gold nano-fluid increased with the increasing particle sizes ranging from 10.4 to 29.6 nm
Method for distortion correction of multi-layered surface reconstruction using time-gated wavefront sensing approach
Get PDFIn order to estimate the multi-layer surface profile and to detect the inter-layer surfaces defects, gated wavefront sensing approach has been proposed in the previous works [1, 2]. However, the proposed methodology measures the wavefront that has been distorted by its prior surfaces (reflected wavefront) or post surfaces (transmitted wavefront). Analysis has to be performed to estimate the multi-layer wavefront sensing by taking into consideration the multi-layer surfaces condition. For reflected wavefront, the bottom layer(s) wavefront is (are) being distorted twice via separate interface points while traveling back to the lenslet arrays through our observation for the slope and phase measurement. The subsequent reconstructed surfaces are not accurate and corrected. Thus, a discrete layer correction technique for the surface reconstruction has been proposed to enhance the reconstruction accuracy by using the upper/top layer’s wavefront information. This paper discusses on the case of 2-layer system, where the reflected wavefront from the bottom layer has been distorted and its surface reconstruction has been corrected. The results show that the distortion is significant and the correction is deemed necessary for industrial application such as in wafer warpage inter-layer profile estimation
Fabrication of adhesive lenses using free surface shaping
Get PDFTwo approaches for fabricating polymer lenses are presented in this paper. Both are based on filling circular holes with UV curing adhesives. Initially, the viscous adhesive material creates a liquid and spherical free surface due to its own surface tension. This shape is then preserved by curing with UV-hardening light. For the first approach, the holes are generated in a 4 inch Si-wafer by deep reactive ion etching (DRIE) and for the second, a polydimethylsiloxane (PDMS) mould is manufactured. Three types of UV-curing adhesives are investigated (NOA 61, NOA 88 and NEA 121 by Norland Products). Preliminary to the determination of the lens curvature, a contact angle goniometer is used for taking side view images of the lenses. The radius of curvature is then extracted via image processing with the software MATLAB®. Furthermore, the surface roughness of the PDMS mould and the generated lenses is measured with a white light interferometer to characterize the casting process. The resolution power of the generated lenses is evaluated by measurement of their point spread functions (psf) and modulation transfer functions (mtf), respectively
Optical and THz Galois diffusers
Get PDFBinary surface reliefs with sub-wavelength features making up a pseudorandom pattern based on mathematical Galois fields GF(p^m) [1, 2] can scatter incoming waves into a large number of diffraction maxima within a huge solid angle. A one-dimensional (1D) Galois number sequence can be folded into a two-dimensional (2D) array by the sino-representation [2]. This concept was been verified for acoustic waves a long time ago [3, 4] and is investigated here for visible light and THz waves. Our Galois diffusers are designed as reflection reliefs and realised by electron beam lithography for the optical regime and UV photolithography for the THz regime. Our results show that optical and THz Galois surfaces are excellent diffusers for electromagnetic waves; they distribute the reflected intensity evenly over a large number of maxima nearly within the entire half solid angle in the backward direction
Laser speckle reduction based on angular diversity induced by Piezoelectric Benders
Get PDFSpeckle reduction techniques have been investigated in display systems with coherent light sources. We propose a simple but efficient method to reduce speckle via two fast vibrating piezoelectric benders. The concept is modulating laser beams to have angle diversity and reducing speckle by temporal averaging in the integration time of the detector. Experiments demonstrate that the speckle contrast is suppressed down to 0.06. Both free space and imaging geometry are considered. In order to fairly evaluate the speckle reduction, the optical configuration as well as the camera settings, which largely affect the speckle contrast, is discussed