26,394 research outputs found
Characterization of color cross-talk of CCD detectors and its influence in multispectral quantitative phase imaging
Multi-spectral quantitative phase imaging (QPI) is an emerging imaging
modality for wavelength dependent studies of several biological and industrial
specimens. Simultaneous multi-spectral QPI is generally performed with color
CCD cameras. However, color CCD cameras are suffered from the color crosstalk
issue, which needed to be explored. Here, we present a new approach for
accurately measuring the color crosstalk of 2D area detectors, without needing
prior information about camera specifications. Color crosstalk of two different
cameras commonly used in QPI, single chip CCD (1-CCD) and three chip CCD
(3-CCD), is systematically studied and compared using compact interference
microscopy. The influence of color crosstalk on the fringe width and the
visibility of the monochromatic constituents corresponding to three color
channels of white light interferogram are studied both through simulations and
experiments. It is observed that presence of color crosstalk changes the fringe
width and visibility over the imaging field of view. This leads to an unwanted
non-uniform background error in the multi-spectral phase imaging of the
specimens. It is demonstrated that the color crosstalk of the detector is the
key limiting factor for phase measurement accuracy of simultaneous
multi-spectral QPI systems.Comment: 16 pages, 8 figure
Measuring the flatness of focal plane for very large mosaic CCD camera
Large mosaic multiCCD camera is the key instrument for modern digital sky
survey. DECam is an extremely red sensitive 520 Megapixel camera designed for
the incoming Dark Energy Survey (DES). It is consist of sixty two 4k2k
and twelve 2k x 2k 250-micron thick fully-depleted CCDs, with a focal plane of
44 cm in diameter and a field of view of 2.2 square degree. It will be attached
to the Blanco 4-meter telescope at CTIO. The DES will cover 5000 square-degrees
of the southern galactic cap in 5 color bands (g, r, i, z, Y) in 5 years
starting from 2011.
To achieve the science goal of constraining the Dark Energy evolution,
stringent requirements are laid down for the design of DECam. Among them, the
flatness of the focal plane needs to be controlled within a 60-micron envelope
in order to achieve the specified PSF variation limit. It is very challenging
to measure the flatness of the focal plane to such precision when it is placed
in a high vacuum dewar at 173 K. We developed two image based techniques to
measure the flatness of the focal plane. By imaging a regular grid of dots on
the focal plane, the CCD offset along the optical axis is converted to the
variation the grid spacings at different positions on the focal plane. After
extracting the patterns and comparing the change in spacings, we can measure
the flatness to high precision. In method 1, the regular dots are kept in high
sub micron precision and cover the whole focal plane. In method 2, no high
precision for the grid is required. Instead, we use a precise XY stage moves
the pattern across the whole focal plane and comparing the variations of the
spacing when it is imaged by different CCDs. Simulation and real measurements
show that the two methods work very well for our purpose, and are in good
agreement with the direct optical measurements.Comment: Presented at SPIE Conference,Ground-based and Airborne
Instrumentation for Astronomy III, San Diego, 201
Soot volume fraction profiling of asymmetric diffusion flames through tomographic imaging
This paper presents the 3-D (three-dimensional) reconstruction of soot volume fraction of diffusion flames based on tomographic imaging and image processing techniques. Eight flexible imaging fiber bundles and two RGB (Red, Green and Blue) CCD (Charge-coupled Device) cameras are used to obtain concurrently the 2-D (two-dimensional) image projections of the flame from eight different angles of view around the burner. Algorithms which combine the tomographic and two-color pyrometric techniques are utilized to reconstruct the soot volume fraction distributions on both cross- and longitudinal-sections of the flame. A series of experiments were carried out on a gas-fired combustion rig for the determination of soot volume fraction using the algorithms proposed. Test results demonstrate the effectiveness of the developed algorithms
Characterization and Application of Hard X-Ray Betatron Radiation Generated by Relativistic Electrons from a Laser-Wakefield Accelerator
The necessity for compact table-top x-ray sources with higher brightness,
shorter wavelength and shorter pulse duration has led to the development of
complementary sources based on laser-plasma accelerators, in contrast to
conventional accelerators. Relativistic interaction of short-pulse lasers with
underdense plasmas results in acceleration of electrons and in consequence in
the emission of spatially coherent radiation, which is known in the literature
as betatron radiation. In this article we report on our recent results in the
rapidly developing field of secondary x-ray radiation generated by high-energy
electron pulses. The betatron radiation is characterized with a novel setup
allowing to measure the energy, the spatial energy distribution in the
far-field of the beam and the source size in a single laser shot. Furthermore,
the polarization state is measured for each laser shot. In this way the emitted
betatron x-rays can be used as a non-invasive diagnostic tool to retrieve very
subtle information of the electron dynamics within the plasma wave. Parallel to
the experimental work, 3D particle-in-cell simulations were performed, proved
to be in good agreement with the experimental results.Comment: 38 pages, 19 figures, submitted to the Journal of Plasma Physic
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