Sang-Hee Nam

Analysis of Optimal Phase Retardation of a Fringe Field-Driven Homogeneously Aligned Nematic Liquid Crystal Cell

We studied the optimal cell retardation value that shows the maximal transmittance in a fringe-field-driven homogeneously aligned (HA) nematic liquid crystal (LC) cell. When the LC with positive dielectric anisotropy is used, the transmittance is much higher than that of an in-plane-field-driven HA cell. The unexpected electrooptic behaviors are caused because the transmittance of light from a deformed LC in a white state cannot be explained using only the uniaxial medium model that describes an in-plane-field-driven HA cell.

Dynamic Stability of the Fringe-Field Switching Liquid Crystal Cell Depending on Dielectric Anisotropy of a Liquid Crystal

A voltage-dependent transmittance curve of the fringe-field switching (FFS) mode was studied with varying dielectric anisotropy of a liquid crystal (LC). The results show that for the LC with positive dielectric anisotropy the disclination lines existing between domains extend into the active region with increasing voltage over the operating voltage, whereas their width becomes smaller with increasing voltage for the LC with negative dielectric anisotropy. This indicates that the LC with negative dielectric anisotropy shows strong dynamic stability in the FFS mode, which can be a great advantage when fabricating microdisplays.

Wide-Viewing-Angle Hybrid Aligned Nematic Liquid Crystal Cell Controlled by Complex Electric Field

We have developed a hybrid aligned nematic liquid crystal (LC) cell driven by a complex electric field. In the device, the pixel electrode exists on the bottom substrate and the counter electrodes exist on the top and bottom substrates such that with a bias voltage both vertical and horizontal fields are generated. The LC molecules are hybrid aligned with homogeneous alignment on the bottom substrate where the alignment direction is coincident with one of the transmission axes of the crossed polarizers. Therefore, the cell appears to be black in the absence of an electric field. When a voltage is applied to obtain a white state, both vertical and horizontal fields enable the LC molecules to rotate with lowered tilt angles than those in the dark state. The device shows a much wider viewing angle than that of the twisted nematic mode, high light efficiency and low driving voltage in electro-optic characteristics.

Investigation of PbI2 Film Fabricated by a New Sedimentation Method as an X-ray Conversion Material

Radiation detectors are currently fabricated by a screen-print method at room temperature. However, this method has many disadvantages in terms of thickness control and electron trapping. We fabricated polycrystalline PbI2 films by a new sedimentation method and compared the results with those obtained by the existing screen-print (SP) method. We investigated the electrical and structural properties of the films. We fabricated 2 ×2 cm2 sample films with a thickness of about 200 µm. A field emission scanning electron microscopy (FE-SEM) analysis showed that these films were of higher density than those fabricated by a conventional SP method. We also measured the photosensitivity and dark current of the films. The photosensitivity of the films fabricated by the new sedimentation method was 4.8 pC/(mR·mm2), and the dark current was 2.2 pA/mm2 at 1.0 V/µm. The linearity of the film ranged from 3 to 12 mAs, which is promising for diagnostic radiography.

Quantitative evaluation of mercuric iodide thick film for x-ray imaging device

In this paper, we investigated electrical characteristics of the X-ray detector of mercuric iodide (HgI2) film fabricated by PIB(Particle-In-Binder) Method with thicknesses ranging from approximately 200μm to 240μm. In the present study, using I-V measurements, their electrical properties such as leakage current, X-ray sensitivity, and signal-to-noise ratio (SNR),were investigated. The results of our study can be useful in the future design and optimization of direct active-matrix flat-panel detectors (AMFPD) for various digital X-ray imaging modalities.

Effect of Annealing on the X-ray Detection Properties of Nano-sized Polycrystalline Lead Oxide Films

Polycrystalline lead oxide (PbO) film is an excellent candidate material for a direct conversion X-ray detector. However, the thick-bulky film tends to significantly reduce the charge collection efficiency for recombination process, and the effective number of electron-hole pairs is lower than that of thin film, because it is difficult to fabricate high-dense and thick PbO films. In this paper, we first synthesized nano-sized PbO particles that could be used in a novel high-efficiency flat panel X-ray detector using a simple solution/combustion method. Energy dispersive X-ray spectrometry, X-ray diffraction, and field emission scanning electron microscopy were used to analyze the component ratio and morphology of the PbO particles as a function of annealing temperature. Then, 150-µm-thick PbO films were deposited on glass substrates using a particle-in-binder method at room temperature. The influences of annealing before deposition on the X-ray detection characteristics of the PbO films were investigated in detail. The key parameters–the dark current, X-ray sensitivity, signal-to-noise ratio, and signal decay–were measured. The annealing conditions strongly affected the electrical properties of the PbO films. The X-ray sensitivity of films annealed in oxygen gas increased dramatically with increasing annealing temperatures up to 500 °C.

Comparison of compound semiconductor radiation films deposited by screen printing method

Flat-panel direct conversion detectors used in compound substance of semiconductor are being studied for digital x-ray imaging. Recently, such detectors are deposited by physical vapor deposition(PVD) generally. But, most of semiconductors (HgI2, PbI2, TlBr, PbO) deposited by PVD method have shown difficult fabrication process and instability for large area x-ray imaging. Consequently, in this paper, we propose applicable potentialities for screen printing method that is coated on a substrate easily. It is compared to electrical properties among semiconductors such as HgI2, PbI2, PbO, CdTe under investigation for direct conversion detectors. Each film detector was coated onto the substrate and Substrates of 2cmx5cm had been used to evaluate performance of semiconductor radiation detectors. Fabricated films consisted of ~25 to 35 μm thick layer of semiconductor. Dark current, sensitivity, linearity, lag, and morphologic property were measured for evaluation of films performance. Dark current of PbO was acquired the lowest, and dark current of HgI2 at the operation voltage of ~1V/μm was observed 8pA/mm2. Sensitivity is observed higher about ten times than the others. And then HgI2 is observed the best SNR in four materials. In four semiconductors, it is shown in good linearity. Such a value is not better than PVD process, but it is easy to be fabricated in high quality for large area X-ray Imaging. Our future efforts will concentrate on optimization of growth of film thickness that is coated onto a-Si TFT array.

Multi-resolution wavelet-transformed image analysis of histological sections of breast carcinomas

Multi-resolution images of histological sections of breast cancer tissue were analyzed using texture features of Haar- and Daubechies transform wavelets. Tissue samples analyzed were from ductal regions of the breast and included benign ductal hyperplasia, ductal carcinoma in situ (DCIS), and invasive ductal carcinoma (CA). To assess the correlation between computerized image analysis and visual analysis by a pathologist, we created a two-step classification system based on feature extraction and classification. In the feature extraction step, we extracted texture features from wavelet-transformed images at 10× magnification. In the classification step, we applied two types of classifiers to the extracted features, namely a statistics-based multivariate (discriminant) analysis and a neural network. Using features from second-level Haar transform wavelet images in combination with discriminant analysis, we obtained classification accuracies of 96.67 and 87.78% for the training and testing set (90 images each), respectively. We conclude that the best classifier of carcinomas in histological sections of breast tissue are the texture features from the second-level Haar transform wavelet images used in a discriminant function.

Experimental evaluation of a-Se flat-panel X-ray detector for digital radiography

In this paper, the evaluation of a selenium-based flat-panel digital X-ray detector is described. The prototype detector has a pixel pitch of 139 mum and a total active imaging area of 14 inches times 8.5 inches, making up a total of 3.9 million pixels. Several quantitative parameters have been devised that correlate with the abilities of imaging devices to perform clinical tasks. The concepts of MTF, NPS, and DQE have been well described and are very useful descriptors of resolution, noise, and signal-to-noise ratio transfer ability. Such parameters (sensitivity, leakage current, MTF, NPS, DQE) were examined to evaluate the performance of this system. The sensitivity of 4.82 nC/mR/cm2 was measured at 10 V/mum, while leakage current was measured at 270 pA/cm2. The measured MTF at 2 lp/mm was 40%, and the DQE at 1.5 lp/mm was 16%

Quantitative evaluation of mercuric iodide and selenium for X-ray imaging device

Analog film/screen systems have been replaced with digital X-ray imaging devices using direct conversion materials. In this paper, mercuric iodide (HgI/sub 2/) and amorphous selenium (a-Se) films were deposited through the particle-in-binder (PIB) and physical vapor deposition (PVD) methods, respectively. Using the MCNP 4C code, the interaction of X-ray photons in HgI/sub 2/ and a-Se bulk, their transport, and transmitted energy spectrum of continuous X-ray, with total absorbed energy were simulated. Using I-V measurements, their electrical properties, such as leakage current, X-ray sensitivity, and signal-to-noise ratio (SNR), were investigated. The results of our study can be useful in the future design and optimization of direct active-matrix flat-panel detectors (AMFPD) for various digital X-ray imaging modalities.