Large Aperture Size Research Articles

Tunable Fresnel lens using nanoscale polymer-dispersed liquid crystals

An electrically tunable Fresnel zone plate lens is demonstrated using nanoscale polymer-dispersed liquid crystal droplets. In the voltage-off state, the zone plate behaves like a positive lens. The measured diffraction efficiency is close to the theoretical limit and is controllable by voltage. The major advantages of such a Fresnel lens are simple fabrication, large aperture size, polarization independent, and fast switching speed, although the required operating voltage is relatively high.

Tunable electronic lens using a gradient polymer network liquid crystal

Tunable electronic lenses using gradient polymer network liquid crystal (PNLC) cells were demonstrated. By changing the photomask pattern, both positive and negative lenses were fabricated. The advantages of such a PNLC lens are low operation voltage, large aperture size, and simple electrode design. To overcome the polarization dependence, stacking two orthogonal homogeneous PNLC cells is considered.

Influence of the antenna diagram on a stellar interferometer that is suffering from telescope-pointing errors.

We report our experimental investigations of the influence of differential telescope-pointing errors on data corruption in an optical stellar interferometer. This effect was investigated theoretically as a function of the telescope antenna diagram, which depends on the aperture diameter. Using a laboratory breadboard consisting of a three-telescope array, we carried out the experiments with various aperture diameters and complex objects. The results matched the simulation and demonstrate that, when there is no error in pointing, a large aperture size induces correctible error but that, with a pointing error, data corruption becomes critical. In both cases, the larger the apertures, the more corrupt the data.

Pursuit affects precision of perceived heading for small viewing apertures

We investigated the interaction between extra-retinal rotation signals and retinal motion signals in heading perception during pursuit eye movement. For limited viewing aperture, the variability in perceived heading strongly depends on the pattern of motion directions. Heading towards a point outside the aperture generates nearly parallel aperture flow. This results in lower precision of perceived heading than heading that renders the radial pattern of flow visible. We ask if the precision is limited by the pattern of flow visible on the retina or that on the screen. During fixation, the two patterns are identical. They are decoupled during pursuit, since pursuit changes radial flow within the aperture on the screen into nearly parallel flow on the retina, and vice versa. The extra-retinal signal is known to reduce systematic errors in the direction of pursuit, thus compensating for the rotational flow during pursuit. We now ask if the extra-retinal signal also affects the precision of heading percepts. It might if at the spatial integration stage the rotational flow has been subtracted out already. A compensation beyond the integration stage, however, cannot undo the change in retinal motion directions so that an effect of pursuit on precision cannot be avoided. We measured the variable and systematic errors in perceived heading during fixation and pursuit for a frontal plane approach, while varying duration, dot lifetime and aperture size. We found precision is effected by pursuit as much as predicted from the pattern of retinal flow, while compensation is significantly greater than zero. This means that the interaction between the extra-retinal signal and visual motion signals takes place after spatial integration of local motion signals. Furthermore, compensation increased significantly with longer duration (0.5–3.0 s), but not with larger aperture size (10–50°). A larger aperture size did increase the eccentricity of perceived heading.

Physical properties of new collimator cone system for stereotactic radiation therapy developed in samsung medical center

Purpose: A new collimator cone system has been developed at the Samsung Medical Center that overcomes some of the limitations of present commercially supplied collimator cones. The physical properties of the newly developed cone system are described in this report. Methods and Materials: The new cones have relatively larger aperture sizes (3.0–7.0 cm in diameter) and are 16 cm in length. Each new cone is fabricated with cerrobend alloy melted and poured into a stainless steel housing that is permanently fixed to a mounting plate. The mounting plate of the new cone is designed to insert into the wedge mount slot of the gantry head. The mechanical accuracy of the central axis of the cone pointing to the isocenter was tested using film, a steel ball positioned at the isocenter by the mechanical isocenter device. For the evaluation of beam flatness and penumbra, off-axis ratios at 5 cm depth were measured by film dosimetry using polystyrene phantom. Results: The average error of the mechanical isocenter was 0.27 mm (± 0.16 mm). The beam flatness was excellent in the central region of the beam, and the average penumbra width was 3.35 mm (± 0.25 mm). The new cone design has more clearance between the patient’s head and the gantry, and can more easily be removed from the gantry head because it slides in and out of the wedge slot. This facilitates changing cone sizes during one treatment session, and makes the process of double exposure port films easier. Conclusions: A new collimator cone system for stereotactic radiation therapy has been developed. The mechanical accuracy and physical properties are satisfactory for clinical use, and the new design permits a wider range of clinical applications for stereotactic radiation therapy.

A High Performance Dichroic Glass Polarizer with A Thickness of 15–35 µm

A dichroic glass polarizer having good optical and thermal properties is fabricated with a thickness of 15–35 µm. This polarizer has a contrast ratio greater than 40 dB with 30 µm thickness at 1.54 µm wavelength. Insertion loss was measured to be less than 0.06 dB. No difference in contrast ratio was confirmed against incident angle shift from normal to 7°. Large aperture size as large as 10×10 mm square and good thermal durability were realized. This polarizer is considered to be useful for lens-free and tilted configurations of optical devices.

Influence analysis of dwell time on focused ion beam micromachining in silicon

Influence of dwell time on focused ion beam (FIB) micromachining process by means of single pixel writing mode sputtering silicon is discussed in this paper. It affects line broadening and sputtering depth during the milling process in silicon material. It is proved by the experimental results that long dwell time leads to deep milling depth due to reducing scanning numbers of pixel. The broadening effect by the wing of Gaussian distribution profile is different from the variation of beam limiting aperture size. In the case of the aperture size 150 μm and below, the effect is little with changing of dwell time. It is obviously serious for large aperture size with the short dwell times of less than 3 μs.

Eigenmode confinement in the dielectrically apertured Fabry-Perot microcavity [VCSEL

The three-dimensional (3-D) optical confinement of the dielectrically apertured Fabry-Perot microcavity is analyzed by considering electromagnetic coupling between the confined region and parasitic waveguide modes that exist outside the aperture. If the cavity length is close to one-half the emission wavelength and for large enough aperture sizes, all parasitic waveguide modes can be evanescent except one which couples only weakly to the emitting region. If the aperture diameter is too small, loss of optical confinement is predicted, and the optical mode size at which this occurs can be estimated from the cavity length and the relative shifts in the vertical resonances inside and outside the apertured region.

Design issues related to the control of continuous crystallizers

This paper addresses design and control issues related to a continuous crystallization process. Design addresses the configuration of actuators for this process. Results from both model studies and from experimental analysis show how different operation and design choices, related to the actuators, influence the process behaviour. The main conclusion is that classified removal of small crystals (fines) and large product crystals improves the shape of the crystal size distribution (CSD) at steady-state, however, at the expense of open-loop cycling of the CSD. A simple SISO feedback control strategy that acts on the fines removal rate can be used to stabilize these cycles effectively. Fine particles are classified with an annular zone. The size of the annular zone can be decreased without affecting the dynamic controllability, significantly. Classified removal of product crystals from the crystallizer is achieved with a vibrating screen. The aperture size of the screen and the flow feeding the screen are considered important design parameters. Good product quality is obtained with a low feed flow and a large aperture size. Controllability analysis further reveals that for both designs only two degrees of freedom exist, i.e. a quantity related to the small particles in the CSD and the total mass production rate per unit of time can be controlled independently.

Design Issues Related to the Control of Continuous Crystallizers

In this paper design and control aspects for a continuous crystallizer are described. Design is limited to the configuration of actuators for this process. Results from both model studies and from experimental analysis show how different operation and design choices related to the actuators influence the process dynamics. The main conclusion is that classified removal of fine and product particles strongly affects the dynamics of the crystal size distribution (CSD). Large fines flows induce limit cycles in the CSD. A simple SISO feedback control strategy that acts on the fines removal rate effectively stabilizes these cycles. Classified removal of product crystals from the crystallizer volume with a vibrating screen enhances the stationary process performance. In this case, the aperture size of the screen and the flow feeding the screen axe considered as important design parameters. Good product quality is obtained with a low feed flow and a large aperture size, however, at the expense of open-loop cycling, which necessitates the implementation of a feedback controller. Also in this case, a feedback controller which uses fines removal as its actuator stabilizes the process dynamics.

DESTRUCTION OF PEAK SWITCHING IN STRONG-FIELD AUTOIONIZATION PHOTOELECTRON SPECTRA

In this paper, the effect of the spatial inhomogeneity of the laser field and the size of detector acceptance angle or aperture on strong-field autoionization photoelectron spectra is investigated. It is shown that the photoelectron spectra are drastically affected by the above factors , for small detector acceptance angle or aperture size, peak switching appears in the photoelectron spectra, but for large detector acceptance angle or aperture size, peak switching is destroyed completely.

Rigid Contact Lens Design: Effects of Overall Diameter Changes on Tear Pump Efficiency

The effects of rigid contact lens overall diameter changes on tear pump efficiency were studied for lenses of 7 overall diameters (8.2 to 10.0 mm in 0.3-mm steps) while maintaining all other parameters (including a 7.4-mm optic zone diameter) constant. Oxygen uptake rates were measured in the normal, open eye and after static (without blinking) and dynamic (with blinking once every 5 s) wear of oxygen-impermeable (polymethyl methacrylate) contact lenses. Differences between uptake rates obtained under static vs. dynamic conditions served as an index of tear pump efficiency for each lens. Tear pump efficiency increased with the use of progressively smaller lens diameters and subjects with large palpebral aperture sizes were also found to have better tear exchange (r = 0.934, p = 0.006 for the 9.1-mm overall diameter reference lens).

Influence of Rigid Contact Lens Overall and Optic Zone Diameters on Tear Pump Efficiency

To determine the effects of contact lens overall diameter on tear pump efficiency, oxygen uptake rates were measured for the open-eye condition and following steady-state periods of static (without blinking) and dynamic (with blinking once every 5 s) contact lens wear of oxygen-impermeable (polymethylmethacrylate) lenses of 5 overall diameters (8.2, 8.5, 8.8, 9.1, and 9.4 mm). The optic zone diameters were 1.4 mm smaller than the overall diameters in each case, while all other parameters remained constant. Differences in corneal oxygen demand following the static and dynamic conditions are a quantitative index of the tear pump efficiency for each lens size on the same cornea, and the differences were found to increase with reduction in lens overall diameter. This indicated that a more efficient tear pump was associated with small diameter designs. In addition, subjects with large palpebral aperture size, relative to lens overall diameter, were found to demonstrate better tear exchange.

Three-dimensional reconstruction of laser-irradiated targets using URA coded aperture cameras

We present a new computed tomographic technique which uses two orthogonal uniformly redundant arrays (URA) coded aperture cameras with an iterative algorithm and its application to reconstruct a three-dimensional image of a compressed core of a laser-irradiated target. The advantage of this technique is that the URA coded aperture has a large total aperture size, so the system can view the object with a large solid angle, resulting in a high tomographic resolution even with a limited number of viewing directions. The technique has a large signal to noise ratio so that it can be applied to weak radiation sources such as hard X-rays from laser plasmas.

Generalized conic concentrators

Explicit forms for the six coefficients of a generalized quadric surface are derived in terms of the parameters of a tilted and displaced parent elliptical or hyperbolic surface. The parent conic parameters, along with the values for its tilt and displacement, are then related to the physical conic concentrators. In Sec. II, the parent conic parameters are related to the concentrator's large and small aperture sizes and the angle of the limiting ray.

A spectral-iteration approach for analyzing scattering from frequency selective surfaces

A novel technique, called the spectral-iteration approach, for analyzing the problem of scattering from periodically perforated screens which find useful applications as radomes, optical filters, artificial dielectrics, and so on is applied. The formulation is carried out in the spectral domain where a set of algebraic equations is obtained directly for the spectral coefficients of the aperture field distribution (or the induced current density) rather than via an integral equation formulation. These equations are then solved simultaneously using an iterative procedure developed in this paper that circumvents the need for matrix inversion. Because the matrix solution is avoided in the spectral approach, it is capable of handling large aperture sizes in a computationally efficient manner. The efficiency of computation results from the use of the fast Fourier transform (FFT) algorithm which is employed in the derivation of the algebraic equations and in the iteration procedure. A unique feature of the spectral-iteration approach is that it has a built-in boundary-condition check which provides a reliable indication of the accuracy of the solution. It is also shown that the spectral domain technique can be applied to even a wider class of geometries, e.g., the step discontinuity in a waveguide.

Near-field analysis by the plane-wave spectrum approach

Historically, most near-field analyses have relied upon aperture-integration techniques. The purpose of this paper is to point out the advantage of the plane-wave spectrum (PWS) approach for analysis of antenna near fields. The advantage of the PWS approach over aperture-integration techniques is quite substantial throughout the entire near-field region for circular apertures. Furthermore, the advantage increases in proportion to closeness to the aperture. The PWS approach also appears to offer advantage for noncircular apertures, e.g., rectangular, at least for large aperture sizes.

Performance Studies of a Quadrupole Mass Filter

The transmission efficiency and the resolving power of a quadrupole mass filter have been investigated as functions of entrance aperture size and frequency of excitation. The aperture size was varied from 10 to 40% of the rod diameter, and the frequency of excitation from 0.87 to 1.7 Mc. Larger transmitted ion currents at lower transmission efficiencies are observed with the larger aperture sizes. Higher resolution is obtained more readily with smaller apertures and higher frequencies.