No / PURPOSE. Potential vision testing attempts to predict the visual outcome that might be expected as a result of a cataract operation. This report details the clinical utility of critical flicker frequency (CFF) as a potential vision test (PVT). METHODS. CFF thresholds were determined in 31 subjects with age-related idiopathic cataract and no other eye disease, 19 subjects with macular disease (MD) and clear ocular media, and 24 age-matched control subjects. In addition, the CFF technique was administered before cataract surgery in 52 patients and compared with the information provided by presurgical case history and ocular examination alone (ophthalmological judgment [OJ]) and results from two commonly used PVTs (the retroilluminated pinhole and the potential acuity meter). RESULTS. CFF thresholds obtained in the nonsurgical cataract group were unrelated to cataract severity and were similar to those in the control group. In contrast, CFF scores were significantly related to visual acuity (VA) in the MD group. In the pre- and postsurgical studies, OJ predicted postoperative VA very well in patients with moderate cataract and normal fundi and better than all the PVTs. OJ performed less well in patients with comorbid eye disease and dense cataracts, when information from the PVTs would probably have been useful. CFF provided the most accurate predictions of postoperative VA in the small sample of patients with dense cataracts. CONCLUSIONS. CFF was unaffected by cataract, yet sensitive to MD, and provided useful information about the postoperative visual outcome beyond that obtained through history and ocular examination in patients with dense cataracts.

No description available.

Critical flicker fusion (CFF) functions are reported during the course of dark adaptation under various states of cone and rod adaptation. Also, measurements were made of the stimulus intensity necessary to just perceive flicker for a wide range of frequencies. The resulting data clearly demonstrate a rod-cone interaction causing a suppression of the threshold. Our results reproduce the familiar rod and cone branches of the CFF function, and confirm the assertion that CFF is dependent upon state of retinal adaptation. Intensities necessary for flicker detection were recorded at various frequencies during the cone plateau. However, the very same frequencies required as much as 1.5 Log units more intensity when recorded after complete rod recovery. We were able to detect the cone-rod interference only at the time when the rods begin to significantly contribute to the threshold. Our data suggest a channel whereby the rods have an accessible mechanism of directly influencing the cone signal as well as the total contribution to a threshold. The probable retinal area of this interaction is also discussed. Results are reported from a rod monochromat and six color normals. This study discusses other stimulus properties which could also reveal the rod-cone suppression. / Master of Science

Voltage (lighting) flicker is mainly caused by the electric arc furnaces (EAF) facility supplied by the medium and high voltage power network. In addition to that, because of the increase of wind power generation in both quantity and capacity, intermittent power output of wind turbines under wind speed variation could also cause voltage flickers that affect the performance of lighting and electronics devices in the neighboring feeder buses. Successful voltage flicker prediction and propagation estimation would help both utility and customers in dealing with the problem. This dissertation presents a nonlinear model for the short term prediction of voltage flicker due to EAF operations. In this study, synchronized voltage flicker measurement was conducted at several EAF facilities to understand the stochastic behavior of voltage flicker. The electric loading condition during EAF melting process shows a long term qualitative behavior of a dynamic system and illustrates a special structure of a fractal system. With the fractal structure identification, the behavior hidden behind the voltage flicker time series measurement could be grasped. Using a phase space reconstruction technique and Lyapunov exponent (LE) of state trajectory in the phase space, based on actual voltage flicker measurements, it is proved that the voltage flicker time series is chaos. By using LE, three formulations are adopted to build the prediction models and illustrate the feasibility of short term EAF voltage flicker prediction. Currently, some Asian countries are using the Japanese £GV10 flicker voltage standard. Due to the adoption of IEC standard by IEEE and European countries, a rational conversion of flicker planning limits between different standards would help utilities consider revising or changing their voltage flicker standards and planning limits. Statistical analyses of Pst and £GV10 measurement are conducted in this study. Under different EAF types and operation conditions, reasonable conversion factors between Pst and £GV10 standards are derived, and the flicker transfer factor between different voltage levels of the power supply system are presented.

yes / Purpose Flicker-defined form (FDF) stimuli have recently been adopted for visual field testing. A key difference between FDF and traditional perimetric stimuli is that the entire display background contains flickering dots. The purpose of this study was to determine whether the perception of FDF stimuli is influenced by lateral interactions involving regions beyond the stimulus border in young healthy observers. Methods Experiment 1 measured the effect of surround size and retinal eccentricity on the detection of the FDF contour. Psychometric functions were collected for surround diameters of 20°, 30° and 40°, and with stimuli centred at eccentricities of 0°, 10° and 20°. Experiment 2 measured the effect of target-surround temporal phase difference on apparent temporal contrast (flicker strength) of the target for both the FDF stimulus and a solid-field stimulus. Psychometric functions were collected for target-surround phase differences of 0°, 45°, 90°, 135° and 180°. Results Our results show a mild surround-suppression effect for FDF stimuli that is independent of surround size. Magnitudes of FDF surround suppression were consistent with the reduced temporal contrast energy of the stimulus compared to solid-field stimuli. Conclusion FDF stimuli necessarily have both flickering target and background. Our results suggest that visual field defects outside the target are unlikely to markedly influence the detection and perception of the FDF stimulus. Nevertheless, mild surround suppression of contrast arises for FDF stimuli, hence interactions between the background and the target area may influence FDF results in conditions that alter centre-surround perceptual effects.

Voltage fluctuations under 30 Hz in the electricity grid, leads to oscillations in the light intensity that can be perceived by human eye, which is called flicker. In this thesis, the sources of the flicker at a point of common coupling is investigated. When there are more than one flicker sources connected to a PCC, individual effects of each flicker source is determined by using a new method which depends on the reactive current components of the sources. This method is mainly based on the flickermeter design defined by the International Electrotechnical Commission (IEC), but uses the current variations in addition to the voltage variations to compute flicker. The proposed method is applied to several different types of loads supplied from a PCC and their flicker contributions on the busbar are investigated. Experiments are performed on field data obtained by the power quality analyzers (PQ+) developed by the National Power Quality Project and the method has been found to provide accurate results for flicker contributions of various loads. The PQ+ analyzers with the proposed flicker contribution detection algorithm are called Flicker Contribution Meters (FCM) and they will be installed at the points of the Turkish Electricity Transmission Network when required.

Minimizing electrical noise is an increasingly important topic. New systems and modulation techniques require a lower noise threshold. Therefore, the design of RF and microwave systems using low noise devices is a consideration that the circuit design engineer must take into account. Properly measuring noise for a given device is also vital for proper characterization and modeling of device noise. In the case of an oscillator, a vital part of a wireless receiver, the phase noise that it produces affects the overall noise of the system. Factors such as biasing, selectivity of the input and output networks, and selectivity of the active device (e.g. a transistor) affect the phase noise performance of the oscillator. Thus, properly selecting a device that produces low noise is vital to low noise design. In an oscillator, 1/f noise that is present in transistors at low frequencies is upconverted and added to the phase noise around the carrier signal. Hence, proper characterization of 1/f noise and its effects on phase noise is an important topic of research. This thesis focuses on the design of a microwave transistor 1/f noise (flicker noise) measurement system. Ultra-low noise operational amplifier circuits are constructed and used as part of a system designed to measure 1/f noise over a broad frequency range. The system directly measures the 1/f noise current sources generated by transistors with the use of a transimpedance (current) amplifier. Voltage amplifiers are used to provide the additional gain. The system was designed to provide a wide frequency response in order to determine corner frequencies for various devices. Problems such as biasing filter networks, and load resistances are examined as they have an effect on the measured data; and, solutions to these problems are provided. Proper representation of measured 1/f noise data is also presented. Measured and modeled data are compared in order to validate the accuracy of the measurements. As a result, 1/f noise modeling parameters extracted from the measured 1/f noise data are used to provide improved prediction of oscillator phase noise.

This investigates the voltage flicker problem of a large steel plant and presents the mitigation strategy by applying the static var compensator (SVC) and cogenerator. The fluctuation of real power and reactive power consumption by an arc furnace has been measured and recorded during the steel production process. The dynamic load model of the A/C arc furnace is derived based on the actual field data and has been included in the computer simulation by the CYME software package for load flow analysis. The block diagrams of SVC controller and the excitation system of cogenerators are considered to solve the response of reactive power compensation according to the voltage fluctuation of the control bus. To maintain the electric service reliability of arc furnace when an external utility fault occurs, the tie line tripping and load shedding is implemented to prevent the tripping of cogenerator after system disturbance. It is found that the dynamic load behavior of arc furnace in the isolated industrial power system can be well compensated by the cogenerator with adaptive control of exciter and governor to generate proper reactive power and real power according to the fluctuation of bus voltage and system frequency respectively.

Voltage flicker limitation of electric utilities has been discussed in the past three decades. Arc furnace is one of the most disturbing loads that cause flicker problems in the power network. If displeasing flicker levels are predictable, then corrective solution such as static var compensation or furnace controls could be developed in cooperation between the utility and the customer. In the past, the electric fluctuations in the arc furnace voltage have been proven to be chaotic in nature. This thesis proposes a phase space approach based on nonlinearity chaotic techniques to analyze and predict voltage flicker. The determination of the phase space dimension and the application of Lyapunov exponent for flicker prediction are described. Test results have shown that accurate prediction results are obtainable for short term flicker prediction based on chaos theory.

Thesis (M.S.)--University of Alabama at Birmingham, 2009. / Title from PDF title page (viewed on Sept. 3, 2009). Includes bibliographical references (p. 45-52).