Humbat Nasibov

Performance Analysis of the CCD Pixel Binning Option in Particle-Image Velocimetry Measurements

This paper proposes convenient methods to increase the dynamic speed range in particle-image velocimetry (PIV) measurements, which employ a charge-coupled device (CCD) camera binning option. Although the binning procedure decreases spatial resolution along specified dimensions, its ability to increase the camera frame rate and preserve the field-of-view are important advantages, which need to be thoroughly investigated. In order to demonstrate the advantages of the CCD binning option, we have carried out experiments both on static images of in-plane particle displacements and on dynamic images of real-fluid flows. In the first experiment, we have analyzed static images of 0.5, 1, and 1.9 μm size fluorescent polystyrene microparticles placed on thick quartz glass plate and illuminated by high-power LED. The in-plane images were captured at various CCD binning options, the distance between particles was then calculated using cross-correlation analysis and a subpixel interpolation scheme based on a Gaussian filter. In the second experiment, the fluid flow in a 30 × 300 × 50 000 μm microchannel was recorded at various CCD binning modes, and then, evaluated using ensemble-averaged normalized cross-correlation analysis with Gaussian subpixel interpolation. Velocity profiles obtained at various CCD binning modes were compared with those obtained at the normal mode. Error analysis has shown that despite the loss of the spatial resolution, the advantages of pixel binning, specifically increased sensitivity and a total full-frame rate, can be useful in PIV measurements, especially in laminar-fluid flows.

The use of CCD pixel binning in PIV measurements

In this work, we study the effects of a CCD camera binning option, which increases camera frame rate throughput at the expense of loosing spatial resolution, on the Particle Image Velocimetry measurements. The camera frame rate defines upper bound on a speed of a flow a PIV system can measure. We have carried our experiments on the real images of the 1 µm size fluorescence polystyrene microparticles placed onto a thick quarts glass plate and illuminated with the high-power LED. The in-plane images were captured at various CCD binning factors and the distances between particles were calculated using cross-correlation analysis with a sub-pixel interpolation scheme based on the Gaussian regression. As a result of this study, we show that despite the decrease in the spatial resolution, the advantages of the pixel binning, such as increase in sensitivity and the total full frame rate, can be useful for LED illuminated PIV measurements, especially in laminar fluid flows.

Subpixel centroid position error analysis in particle tracking velocimetry induced by the CCD pixel binning

Particle tracking velocimetry (PTV) is a non-invasive, full field optical measuring technique that has become one of the dominant tools for obtaining velocity information in fluid motion. In PTV experiments, the fluid of interest is seeded with fluorescent tracer particles, where measurement of individual particle displacements, recorded by means of digital camera at two instances of time, is further used to ascertain overall flow motion. Upper limit of a flow speed a PTV system can measure is bound by the frame rate of a camera used, and the systems accuracy is limited by the accuracy of the particle centroid estimation. In order to increase the upper limit, we investigated the use of CCD binning option, which doubles camera frame rate, preserves effective field of view, suppresses photon and readout noise of CCD at the expense of loss in spatial resolution. This study provides quantitative assessment of tradeoff between aforementioned advantages of binning over the loss in spatial resolution, which can increase uncertainty in particle centroid estimation. We carried our experiments using scientific grade PixeFly camera and analyzed 1μm and 1.9μm size red fluorescence polystyrene microspheres, placed on a quartz glass plate, by the Hirox variable zoom lens (1-10x) conjugated with a OL- 700II objective. For each binning mode (horizontal, vertical and composite), we investigated and reported estimation errors of various cross-correlation and center of mass based centroid localization methods, using more than 100.000 particle image pairs. We found that, performing vertical binning in the context of laminar flows doubled measurable flow rate, while caused only a negligible estimation error in the order of hundredth of a pixel.

Low-cost pulsed solid state illumination for microPIV measurements

Particle Image Velocimetry (PIV) is a non-invasive, full-field optical measurement technique that has become a dominant tool for velocity measurement of fluids and gases at both macro (traditional PIV) and micro (microPIV) scales. In PIV experiments, the fluid under the investigation is seeded with tracer particles, which are shining under an excitation by a properly tuned light source. The idea behind the method is to precisely register the position of corresponding particles in two shifted instances of time and then using these records calculating particle displacements, i.e. flow velocity. In most PIV experimental setups, illumination is performed using dual cavity pulse lasers, whose outputs reach several hundreds mJ at short pulse lengths (tens of nano-seconds). Unfortunately, such laser systems are very expensive and bulky. In this work, we investigate a possibility to replace the laser illumination with a high power LED illumination, aiming towards the development of the cost effective and portable microPIV systems. We have developed an electronic circuit, which drives LEDs with a high current over short time duration. The driver circuit is triggered by an internal electronics of the CCD camera, and is able to produce single or double current pulses per camera trigger. Besides, the circuit also allows i) flexible adjustment of the pulse duration (from 1 μs up to tens of msec), ii) the time delay within pulse pairs, which is crucial for double-frame mode, and iii) time delay between the trigger signal and current pulses. We present experimental results of flow velocity measurements obtained using the microPIV system and the developed illumination setup. We have investigated the flow of water, which was seeded with the spherical-polystyrene-fluorescent particles, inside rectangular microchannels. For illumination, a LumiLED LED with a peak wavelength at 470 nm was used at the double-illumination mode, where current pulses of up to 10 A at duration of 5 μs were achieved.

The influence of CCD pixel binning option to its modulation transfer function

In recent years CCD manufacturers have been supplying their devices with multi-purpose abilities to manipulate the CCDs readout pattern, where one of these versatile options is a flexible pixel binning option. The pixel binning is a process of combining multiple pixel charges in horizontal, vertical or in both directions simultaneously, into a single charge. The binning process positively influences to the signal-to-noise ratio, sensitivity and frame rate at the cost of decreasing spatial resolution, which, in its turn, negatively influences to the spatial frequency response of the imaging system (i.e. to the output image quality). The modulation transfer function (MTF) is an essential measure for characterizing the spatial-frequency response of the array imaging system. In this work we have performed a theoretical and experimental investigation of the MTF of CCD array in the context of the pixel binning option. We have derived a generalized equation of the geometrical MTF for the v x h binning mode, where v and h denote the numbers of binned pixels in vertical and horizontal directions, respectively. The MTF measurements were performed using a method, based on the generation of laser speckle and utilizing the high resolution (1360×1024) monochrome CCD array. The MTF of normal mode, 2×1-horizontal, 1×2 - vertical, and 2x2 quadratic binning modes were measured by employing single-slit aperture method. CCD binning is widely used in spectroscopy, astronomy, in many image processing applications, such as autofocus, object tracking, etc. The results of this work can be useful for designing optical systems, involving CCD pixel binning option.

Autocorrelation analysis of spectral dependency of surface roughness speckle patterns

When a surface is illuminated with a highly coherent light such as a laser beam, the speckle pattern of bright and dark regions is observed. It depends on the surface parameters and carries important information about the roughness of the surface. Various methods and techniques are employed for the determination of surface roughness parameters from speckle pattern properties. In this paper, an experimental approach for surface roughness evaluation based on the autocorrelation analysis of the spectral properties of speckle patterns caused by milled metal surfaces is reported. The speckles at three 633, 604 and 543 nm wavelengths of He-Ne laser were analyzed. It was found that autocorrelation analysis is very sensitive to small variations in speckle sizes, caused by spectral properties of speckle patterns such as increasing the wavelength lead to increased speckle sizes. The results are in good agreement with the results obtained from the mechanical stylus profilometer for the milled metal surfaces with roughness values Ra; 0.36µm (low roughness) and 1.98µm (high roughness). The technique reported here has a great potential for precise and non-contact optical measurements of rough surfaces.

Hyperspectral image processing and display techniques in the context of forensic document investigation

Nowadays, deployment of forensic document investigation systems that employ hyperspectral image processing is being widespread. The reason such systems are being preferred is mainly due to their non-destructive nature of document investigation, fast processing time, and the ability to reveal erased and covered writings, and details. On the other hand, analyzing hyperspectrum, obtained after a document scan, is a nontrivial task, which requires special training. Even trained document investigators may spend a plenty of effort to reveal covert evidence. In this work, we discuss hyperspectral image processing and display techniques we have developed to ease the job of forensic document investigators. Besides, our techniques will help to the investigators performing more constructive guesses during their investigation process. Additionally, this work aims to provide general knowledge about the hyperspectral image processing based document investigation to the interesting reader.

Seamless image stitching algorithm using radiometric lens calibration for high resolution optical microscopy

Image mosaicing method is an image-based rendering and visualization method which is a common procedure in the generation of panoramic (composite) images and applications, such as creating virtual reality, super resolution, object insertion and object removal, etc. Specifically, the image stitching approach becomes common in super resolution digital imaging microscopy, where it is required to achieve both high magnification and a large field of view. This work presents the algorithm for the stitching of different overlapped high resolution views of the whole scene obtained from high magnification optical microscopes into a large radiometric balanced image through maximization of a cross-correlation function. To seamlessly blend and reduce the possibility of intensity and color mismatching of multiple source images in mosaicing, a radiometric calibration of the microscope optical system is employed. The algorithm was tested with grayscale and color images, and showed convincing and robust results in image stitching and blending.