Niru K. Nahar

Asthenopia and Blink Rate Under Visual and Cognitive Loads

Purpose. Asthenopia has been associated with reading under visually stressful conditions. However, it is not known whether increased cognitive load contributes to asthenopic symptoms. The purpose of this study was to evaluate the contribution of increased cognitive load (with or without visual stress inducing conditions) to asthenopic symptoms associated with prolonged near work. Methods. Thirty-three visually normal subjects, aged 18 to 30 years, participated in the study. Subjects read texts or watched videos under different visual stress and cognitive loads. Visual stress conditions were good visual quality, low contrast, and induced refractive error. The cognitive load levels were watching video, reading fairy tales, and reading technical articles. As an additional task, subjects also listened to technical articles. At the end of each condition, subjects rated the magnitude of any asthenopic symptoms, visual discomfort, and cognitive discomfort they experienced during the task. Electromyography potentials recorded from the lower orbicularis oculi muscle were used to obtain blink rate. Results. Subjects reported greater internal symptoms for the refractive error condition coupled with higher cognitive load compared to good visual and low contrast conditions (p < 0.01). For the low contrast condition coupled with higher cognitive load, greater external symptoms were reported compared to good visual and refractive error conditions (p < 0.05). However, asthenopic symptoms were not reported for cognitively demanding tasks when the visual condition was good. Blink rates were not significantly different between the good visual and low contrast conditions within each cognitive load level. For the cognitively difficult reading conditions, blink rate was significantly decreased for the low contrast and good visual conditions compared to the refractive error condition. Conclusions. An interaction between cognitive and visual demands was observed. Greater cognitive loads accentuate the same differentiated symptoms normally caused by visual stressors.

Objective measurements of lower-level visual stress.

Purpose. To determine the sensitivity of the electromyography (EMG) response of the orbicularis oculi muscle to selected lower-level visually stressful conditions to establish the extent to which it can be used as a measure of visual discomfort. Methods. Thirty-one subjects (18 years or older) with 20/20 vision, without history of ocular pathology, oculomotor limitation, or cognitive deficits participated in the study. Subjects read on a computer display for 27 trials of 5 min duration under different low-level asthenopic conditions. The conditions were graded levels of font size, font type, contrast, refractive error, and glare. Orbicularis oculi activity was recorded using surface EMG. Blink-free epochs of EMG data were analyzed for power for all the conditions. Blink rate for all the trials was also measured. At the end of each trial, subjects rated the severity of visual discomfort experienced while reading. Results. Conditions that benefit from squint (refractive error and glare) showed increased EMG power (p < 0.001) from the orbicularis and increased blink rate (p = 0.002), whereas those that do not benefit from squint (small font and low contrast) showed no significant EMG response and a significant decrease in blink rate (p = 0.003 and p = 0.01). All conditions resulted in significant visual discomfort; the p value for font type was 0.039 and p < 0.001 for the other conditions. Conclusions. The results suggest that the squint-beneficial conditions are operated by a local mechanism involving contraction of the orbicularis and increase in reflex blinking, whereas those that do not benefit from squint do not engage the orbicularis and decrease blink, possibly through central inhibition of spontaneous blinking. The EMG response is a sensitive objective measure for the squint-beneficial conditions. However, for the non-squint-beneficial conditions, blink rate may be a more sensitive objective measure, although EMG with longer trial durations should be tested.

Design and demonstration of a switching engine for a binary true-time-delay device that uses a White cell

Optical true-time-delay devices based on the White cell can be divided into two general types: polynomial cells, in which the number of delays that can be obtained is related to the number of times m that a beam bounces in the cell raised to some power, and exponential cells, in which the number of delays is proportional to some number raised to the power of m. In exponential cells, the topic to be addressed, the spatial light modulator switches between a delay element and a null path on each bounce. We describe an improved design of this switching engine, which contains a liquid-crystal switch and a White cell. We examine astigmatism and corrections for it and present a specific design.

A Novel Slow-Wave Structure for High-Power -Band Backward Wave Oscillators With Mode Control

We present a novel slow-wave structure (SWS) to significantly enhance the performance of high-power backward wave oscillators (BWOs). The design features a periodic metallic ring insertion and a deeply corrugated cylindrical waveguide. Both serving to improve interaction impedance and flexibility in dispersion curve engineering. A new technique for mode control in waveguides is also introduced. In addition to demonstrating mode control in SWSs, the key aspects of the presented design are mode dominance reversal and a 100% improvement in interaction impedance that can be exploited to achieve greater power conversion efficiency and output mode purity. Performance comparisons on group velocity, phase velocity and interaction impedance of the new SWS versus the conventional corrugated waveguide are provided. We extend the concept of inhomogeneous SWSs by designing a three-section inhomogeneous SWS. Further simulations using a Particle in Cell code of a highly efficient three-section inhomogeneous Ka-band BWO generates a peak output power of a 5.92 MW at 27 GHz with a 58% peak efficiency.

THz Transparent Metamaterials for Enhanced Spectroscopic and Imaging Measurements

We present a frequency-selective surfaces (FSS) transparent window exhibiting low losses for broadband spectroscopic and imaging measurements in terahertz (THz) frequencies. The intent is to replace existing samples holders (e.g., Z-cut crystal quartz) with transparent metamaterial windows and increase measurement sensitivity. To enable multiband and broadband responses, two designs of single-layer FSS with superstrate are demonstrated: circular slot and double square loop, and thus suppress losses in a given range of THz band.

Cold Test Validation of Novel Slow Wave Structure for High-Power Backward-Wave Oscillators

Previous work presented the concepts of mode dominance reversal using a novel slow wave structure (SWS). An example application of this mode reversal was a backward-wave oscillator operating in the Ka band. However, the concepts have not yet been experimentally validated. In this paper, we use a scaled version of the SWS to experimentally demonstrate mode dominance reversal in the S band. Using the detected resonances of the six SWS cells, a highly accurate synthetic technique is used to derive the dispersion curve. Simulations based on a commercial particle-in-cell code using this SWS design show peak output powers of 5.94 MW at 2.64 GHz with a 35% peak power electronic efficiency.

Novel Phaseless Gain Characterization for Circularly Polarized Antennas at mm-Wave and THz Frequencies

We propose a new method of gain characterization for circularly polarized antenna arrays especially effective for mm-wave/THz frequencies. The method does not require phase-measurement or rotation of the antenna under test (AUT). It is thus ideal for waveguide-based frequency bands. In contrast to conventional methods, we use reflection-only measurements, utilizing readily available geometries, such as a PEC-plate and a PEC-dihedral corner reflector, to estimate the co- and cross-polarized gain of the AUT. Predicted error using this approach is less than 0.07 dB for the co-pol and 0.26 dB for the cross-pol for an AUT with 17 dBi gain at 100 GHz. Experimental results for a radial line slot array antenna, operating in F-band, show good agreement between the conventional method, and that proposed using phase-less method.

Interactions of visual and cognitive stress

BACKGROUND The objective of this research is to assess the ocular and muscular response to long-duration reading under different visual and cognitive difficulty levels. METHODS Thirty-five subjects, with 20/20 vision and without history of ocular pathology or cognitive deficits, participated in the study. Subjects read under different visual and cognitive difficulty levels for 6 (30-minute) conditions. Upper and lower orbicularis oculi, frontalis, and trapezius muscle activities were recorded using surface electromyography (EMG). Aperture size, pupil diameter, and pulse rate of the subjects were recorded with a video camera, pulse meter, and ISCAN eye tracker (ISCAN Inc.), respectively. RESULTS The results show that the texts read with a refractive error caused increased orbicularis oculi EMG power and reduced aperture size (P < 0.001). There was no statistically significant difference between the conditions for pulse rate, pupil diameter, or EMG activity of the frontalis and trapezius muscles with either visual or cognitive stress presented in this experiment. CONCLUSION Visual stress experienced due to reading under an induced refractive error is potentially mediated by a local mechanism, different from the mechanism underlying reading under low contrast or high cognitive demand.

Coupling Loss From Free Space to Large Mode Area Photonic Crystal Fibers

Simulated and measured results are presented for the coupling loss from free space into a large mode area (LMA) photonic crystal fiber. The total measured loss (using bulk optics) for a signal (¿ = 1.55 ¿m) going in and out of a 1-m-long fiber was -2.15 dB. It was also observed that the fiber is not that sensitive to lateral misalignment compared that to the conventional single-mode fiber. Due to the low loss, this type of fiber could be used to create long delays in optical true-time-delay engines based on the White cell concept and others. The above loss can be further reduced by using field lenses or lensed LMA.

Bandwidth reconfigurable THz filter employing phase-change material

We propose a bandwidth reconfigurable spatial filter using frequency selective surfaces (FSS) combined with phase-change materials (PCMs), vanadium dioxide (VO2) for advanced terahertz (THz) applications. The FSS filter is simulated using sapphire as substrate and employs circular loops FSS with VO2 rings in between. Simulations of this filter show bandwidth difference in the frequency range of 0.4 - 0.75 THz. As a demonstration, we tested a reconfigurable FSS geometry and observed a 15% in bandwidth change as per design.