Frédéric Diaz

Wet-oxidation and automated colorimetry for simultaneous determination of organic carbon, nitrogen and phosphorus dissolved in seawater

Abstract We developed a simple and reliable method which allows simultaneous determination of organic forms of carbon (DOC), nitrogen (DON) and phosphorus (DOP) dissolved in seawater. Conversion of dissolved organic matter (DOM) to inorganic products (carbon dioxide, nitrate+nitrite and soluble reactive phosphate) is performed by a persulfate wet-oxidation in low alkaline condition. After oxidation, the concentration of the inorganic products dissolved in the sample was measured automatically by colorimetry using a 3-channel Technicon AutoAnalyzer system. A number of pure organic compounds were tested in the concentration range encountered in coastal and open ocean, indicating a high efficiency of the digestion procedure. The recovery range is similar to that obtained by other wet-oxidation procedures and by high-temperature catalytic oxidation techniques. Direct comparisons with usual methods used for separate determination of DOC, DON and DOP indicated a high efficiency of the procedure. Reproducibility tests demonstrated a very good precision (around 5%) for lagoonal and coastal waters, while precision was sometimes around 10–25% in oligotrophic oceanic waters, especially for DOP where values approached limits of detection for measuring phosphate. This method is highly suitable for routine analysis and especially appropriate for shipboard work.

Increase in depth of field taking into account deconvolution by optimization of pupil mask

We consider optimization of hybrid imaging systems including a phase mask for enhancing the depth of field and a digital deconvolution step. We propose an image quality criterion that takes into account the variability of the systems point-spread function along the expected defocus range and the noise enhancement induced by deconvolution. Considering the classical cubic phase mask as an example, we show that the optimization of this criterion may lead to filter parameters that are significantly different from those usually proposed to ensure the strict invariance of the PSF.

Superresolution along extended depth of focus with binary-phase filters for the Gaussian beam

In the paraxial Debye regime, simple and power-efficient pupil filters are designed to break the diffraction limit along a large depth of focus (DOF) for the Gaussian beam. Dependences of the superresolution factor, DOF gain, Strehl ratio, sidelobe strength, and axial intensity nonuniformity on the Gaussian profile in the pupil plane are characterized using the numerical method. Optimal filter designs are proposed for either high-resolution or ultra-large-DOF applications followed by experimental verifications.

Comparison of depth-of-focus-enhancing pupil masks based on a signal-to-noise-ratio criterion after deconvolution

We consider optimization of hybrid imaging systems including a pupil mask for enhancing the depth of field and a digital deconvolution step. In a previous paper [Opt. Lett. 34, 2970 (2009)] we proposed an optimization criterion based on the signal-to-noise ratio of the restored image. We use this criterion in order to optimize different families of phase or amplitude masks and to compare them, on an objective basis, for different desired defocus ranges. We show that increasing the number of parameters of the masks allows one to obtain better performance.

Design of a complex filter for depth of focus extension

Different methods such as axilens and binary-phase filter have been investigated to improve the depth of focus. A method is proposed to calculate an amplitude-phase pupil filter and obtain the desired distribution of intensity along the optical axis. It produces a narrow spot with a uniform intensity level over a large depth of focus, comparable to the performance obtainable with binary-phase filters. This filter is of particular interest for applications where very low intensity fluctuations along the focus range are required.

Optimization of Hybrid Imaging Systems Including Digital Deconvolution in the Presence of Noise

We address the depth of focus enhancement in hybrid imaging systems, including a phase mask and a deconvolution filter. A final image quality criterion is introduced to optimize and compare different masks.

Real-time increase in depth of field of an uncooled thermal camera using several phase-mask technologies.

Imaging systems that combine a phase mask in the pupil and digital postprocessing may have better performance than conventional ones. We have built such a system to enhance the depth of field of an uncooled thermal camera. The phase masks are binary, their structures are optimized thanks to an image quality criterion, and they have been realized with three different technologies that give equivalent results. The deconvolution postprocessing is performed in real time with a graphics processing unit. A significant increase of the depth of field of a factor 3 has been obtained.

Multi-sensor millimeter-wave system for hidden objects detection by non-collaborative screening

In this work, we present the development of a multi-sensor system for the detection of objects concealed under clothes using passive and active millimeter-wave (mmW) technologies. This study concerns both the optimization of a commercial passive mmW imager at 94 GHz using a phase mask and the development of an active mmW detector at 77 GHz based on synthetic aperture radar (SAR). A first wide-field inspection is done by the passive imager while the person is walking. If a suspicious area is detected, the active imager is switched-on and focused on this area in order to obtain more accurate data (shape of the object, nature of the material ...).

Comparison between a new holographically generated complex filter and the binary phase filter for depth of field extension

To improve the depth of field in imaging systems, we propose a new method for designing pupil filters with the classical approach used for computer-generated holograms. This method allows us to calculate a complex amplitude/phase filter in order to obtain a desired distribution of intensity along the optical axis, and thus the desired depth of field. We will compare our complex filter with binary-phase filters, which are one of the different methods already investigated to improve the depth of field in imaging applications. This study will reveal that the complex filter is an interesting alternative for applications where very low fluctuations of the amplitude distribution along the optical axis are required. It is indeed as energy-efficient as a pure phase filter even with a non negligible absorption. It also ensures to precisely tailor the shape of the focal line of an imaging lens, as the decrease of intensity is sharper outside the regions of interest than with the binary-phase filter. Moreover, it also benefits from lower sidelobes. With these characteristics, this new complex filter will be suitable particularly for 3D imaging applications.

Recent advances in joint optical-digital design for optronics applications

Increasing the capture volume of visible cameras while maintaining high image resolutions, low power consumption and standard video-frame rate operation is of utmost importance for hand-free night vision goggles or embedded surveillance systems. Since such imaging systems require to operate at high aperture, their optical design has become more complex and critical. Therefore new design alternatives have to be considered. Among them, wavefront coding changes and desensitizes the modulation transfer function (MTF) of the lens by inserting a phase mask in the vicinity of the aperture stop. This smart filter is combined with an efficient image processing that ensures optimal image quality over a larger depth of field. In this paper recent advances are discussed concerning design and integration of a compact imaging system based on wavefront coding. We address the design, the integration and the characterization of a High Definition (HD) camera of large aperture (F/1.2) operating in the visible and near infrared spectral ranges, endowed with wavefront coding. Two types of phase masks (pyramidal and polynomial) have been jointly optimized with their deconvolution algorithm in order to meet the best performance along an increased range of focus distances and manufactured. Real time deconvolution processing is implemented on a Field Programmable Gate Array. It is shown that despite the high data throughput of an HD imaging chain, the level of power consumption is far below the initial specifications. We have characterized the performances with and without wavefront coding through MTF measurements and image quality assessments. A depth-of- field increase up to x2.5 has been demonstrated in accordance with the theoretical predictions.