612 research outputs found
Simulation for Scanning Electron Microscopy
Simulations of images of surface steps obtained by high energy reflection electron microscopy are presented. It is shown that double images of simple steps, with no associated strain field, may occur when surface resonance conditions are established. Accurate calculation of image intensity requires large calculations and care is needed in relating the computed wave functions to those occurring for a semi-infinite incident wave. Estimates of the time to compute accurate wavefunctions are given and it is shown that they are reasonable for modem fast computers
Optical response of nanostructured metal/dielectric composites and multilayers
The homogeneous optical response in conducting nanostructured layers, and in insulating layers containing dense arrays of self assembled conducting nanoparticles separated by organic linkers, is examined experimentally through their effective complex indices (n*, k*). Classical effective medium models, modified to account for the 3-phase nanostructure, are shown to explain (n*, k*) in dense particulate systems but not inhomogeneous layers with macroscopic conductance for which a different approach to homogenisation is discussed, (n*, k*) data on thin granular metal films, thin mesoporous gold, and on thin metal layers containing ordered arrays of voids, is linked to properties of the surface plasmon states which span the nanostructured film. Coupling between evanescent waves at either surface counterbalanced by electron scattering losses must be considered. Virtual bound states for resonant photons result, with the associated transit delay leading to a large rise in n* in many nanostructures. Overcoating n-Ag with alumina is shown to alter (n*, k*) through its impact on the SP coupling. In contrast to classical optical homogenisation, effective indices depend on film thickness. Supporting high resolution SEM images are presented
Local biases drive, but do not determine, the perception of illusory trajectories
When a dot moves horizontally across a set of tilted lines of alternating orientations, the dot appears to be moving up and down along its trajectory. This perceptual phenomenon, known as the slalom illusion, reveals a mismatch between the veridical motion signals and the subjective percept of the motion trajectory, which has not been comprehensively explained. In the present study, we investigated the empirical boundaries of the slalom illusion using psychophysical methods. The phenomenon was found to occur both under conditions of smooth pursuit eye movements and constant fixation, and to be consistently amplified by intermittently occluding the dot trajectory. When the motion direction of the dot was not constant, however, the stimulus display did not elicit the expected illusory percept. These findings confirm that a local bias towards perpendicularity at the intersection points between the dot trajectory and the tilted lines cause the illusion, but also highlight that higher-level cortical processes are involved in interpreting and amplifying the biased local motion signals into a global illusion of trajectory perception
Local biases drive, but do not determine, the perception of illusory trajectories
When a dot moves horizontally across a set of tilted lines of alternating orientations, the dot appears to be moving up and down along its trajectory. This perceptual phenomenon, known as the slalom illusion, reveals a mismatch between the veridical motion signals and the subjective percept of the motion trajectory, which has not been comprehensively explained. In the present study, we investigated the empirical boundaries of the slalom illusion using psychophysical methods. The phenomenon was found to occur both under conditions of smooth pursuit eye movements and constant fixation, and to be consistently amplified by intermittently occluding the dot trajectory. When the motion direction of the dot was not constant, however, the stimulus display did not elicit the expected illusory percept. These findings confirm that a local bias towards perpendicularity at the intersection points between the dot trajectory and the tilted lines cause the illusion, but also highlight that higher-level cortical processes are involved in interpreting and amplifying the biased local motion signals into a global illusion of trajectory perception
Motion-induced distortion of shape
Motion, position, and form are intricately intertwined in perception. Motion distorts visual space, resulting in illusory position shifts such as flash-drag and flash-grab effects. The flash-grab displaces a test by up to several times its size. This lets us use it to investigate where the motion-induced shift operates in the processing stream from photoreceptor activation to feature activation to object recognition. We present several canonical, highly familiar forms and ask whether the motion-induced shift operates uniformly across the form. If it did, we could conclude that the effect occurred after the elements of the form are bound. However, we find that motion-induced distortion affects not only the position, but also the appearance of briefly presented, canonical shapes (square, circle, and letter T). Features of the flashed target that were closest to its center were shifted in the direction of motion more than those further from its center. Outline shapes were affected more than filled shapes, and the strength of the distortion increased with the contrast of the moving background. This not only supports a nonuniform spatial profile for the motion-induced shift but also indicates that the shift operates before the shape is established, even for highly familiar shapes like squares, circles, and letters
Diverted by dazzle: perceived movement direction is biased by target pattern orientation.
'Motion dazzle' is the hypothesis that predators may misjudge the speed or direction of moving prey which have high-contrast patterning, such as stripes. However, there is currently little experimental evidence that such patterns cause visual illusions. Here, observers binocularly tracked a Gabor target, moving with a linear trajectory randomly chosen within 18° of the horizontal. This target then became occluded, and observers were asked to judge where they thought it would later cross a vertical line to the side. We found that internal motion of the stripes within the Gabor biased judgements as expected: Gabors with upwards internal stripe motion relative to the overall direction of motion were perceived to be crossing above Gabors with downwards internal stripe movement. However, surprisingly, we found a much stronger effect of the rigid pattern orientation. Patches with oblique stripes pointing upwards relative to the direction of motion were perceived to cross above patches with downward-pointing stripes. This effect occurred only at high speeds, suggesting that it may reflect an orientation-dependent effect in which spatial signals are used in direction judgements. These findings have implications for our understanding of motion dazzle mechanisms and how human motion and form processing interact
A role for eyebrows in regulating the visibility of eye gaze direction
The human eye is unique amongst those of primates in having white sclera against which the dark iris is clearly visible. This high-contrast structure makes the gaze direction of a human potentially easily perceptible to others. For a social creature such as a human, the ability to perceive the direction of another’s gaze may be very useful, since gaze usually signals attention. We report data showing that the accuracy of gaze deviation detection is independent of viewing distance up to a certain critical distance, beyond which it collapses. This is, of itself, surprising since most visual tasks are performed better at closer viewing distances. Our data also show that the critical distance, but not accuracy, is affected by the position of the eyebrows so that lowering the eyebrows reduces the critical distance. These findings show that mechanisms exist by which humans could expand or restrict the availability of their gaze direction to others. A way to regulate the availability of the gaze-direction signal could be an advantage. We show that an interpretation of eyebrow function in these terms provides a novel explanation for several well-known eyebrow actions, including the eyebrow flash
A New SiC-Whisker-Reinforced Lithium Aluminosilicate Composite
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65747/1/j.1151-2916.1993.tb04016.x.pd
Low-Temperature Sintering of Alumina with Liquid-Forming Additives
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66046/1/j.1151-2916.1991.tb07825.x.pd
- …