82 research outputs found
Imaging with parallel ray-rotation sheets
A ray-rotation sheet consists of miniaturized optical components that
function - ray optically - as a homogeneous medium that rotates the local
direction of transmitted light rays around the sheet normal by an arbitrary
angle [A. C. Hamilton et al., arXiv:0809.2646 (2008)]. Here we show that two or
more parallel ray-rotation sheets perform imaging between two planes. The image
is unscaled and un-rotated. No other planes are imaged. When seen through
parallel ray-rotation sheets, planes that are not imaged appear rotated,
whereby the rotation angle changes with the ratio between the observer's and
the object plane's distance from the sheets.Comment: 8 pages, 6 figure
Metarefraction
Imagine a thin sheet that performs optical illusions on the scene behind it. For example, a window that appears to reverse depth and to image objects in front of the sheet, or alternatively swimming goggles that cancel the refraction of surrounding water. This thesis will explore how such sheets may be realized.
With the refinement of optical fabrication technologies, it is now possible to mass-produce miniaturized optical components. Repeating them over the surface of a sheet, their combined effect may realize optical effects from the structure, rather than the substance, of the sheet. Specifically, such components may realize arbitrary ray-direction mappings at each point on the
sheet. Here such mappings, metarefractions, are explored from a range
of perspectives.
This thesis will explore the inception, theoretical development and ultimately the experimental realization of metarefraction. At its core, this work is primarily mathematical in nature but draws upon both experimental and computational techniques in order to test and visualize the concepts that will be discussed. Examples of such ray-direction mappings will be explored as will
their ray- and wave-optical implications.
This thesis is structured as follows: Initially, the definition of metarefraction, along with some existing examples, is presented. Then, ray mappings are related to negative refraction, a subject that metarefraction has a surprising number of parallels to. New forms of metarefraction are then introduced, before being incorporated into imaging systems. Later, ray-optical transformations, such as metarefraction, are shown to be limited by implicit wave-optical restrictions. In some cases, these vastly reduce the number of light fields that may be exactly transformed. After this, the most general possible metarefraction is sought, and a simple case is realized experimentally. Further restrictions are then determined, before finishing with a discussion and summary, and by considering possible directions that future work could develop in
Fermat's principle with complex refractive indices and local light-ray rotation
We describe local light-ray rotation in terms of complex refractive indices.
We show that Fermat's principle holds, and we derive an extended Snell's law.
The change in the angle of a light ray with respect to the normal to a
refractive-index interface is described by the modulus of the refractive-index
ratio, the rotation around the interface normal is described by the argument of
the refractive-index ratio.Comment: 3 pages, 2 figure
Superoscillation in speckle patterns
Waves are superoscillatory where their local phase gradient exceeds the
maximum wavenumber in their Fourier spectrum. We consider the superoscillatory
area fraction of random optical speckle patterns. This follows from the joint
probability density function of intensity and phase gradient for isotropic
gaussian random wave superpositions. Strikingly, this fraction is 1/3 when all
the waves in the two-dimensional superposition have the same wavenumber. The
fraction is 1/5 for a disk spectrum. Although these superoscillations are weak
compared with optical fields with designed superoscillations, they are more
stable on paraxial propagation.Comment: 3 pages, two figures, Optics Letters styl
Metamaterials for light rays: ray optics without wave-optical analog in the ray-optics limit
Volumes of sub-wavelength electromagnetic elements can act like homogeneous
materials: metamaterials. In analogy, sheets of optical elements such as prisms
can act ray-optically like homogeneous sheet materials. In this sense, such
sheets can be considered to be metamaterials for light rays (METATOYs).
METATOYs realize new and unusual transformations of the directions of
transmitted light rays. We study here, in the ray-optics and scalar-wave
limits, the wave-optical analog of such transformations, and we show that such
an analog does not always exist. Perhaps, this is the reason why many of the
ray-optical possibilities offered by METATOYs have never before been
considered.Comment: 10 pages, 3 figures, references update
Experimental demonstration of ray-rotation sheets
We have built microstructured sheets that rotate, on transmission, the direction of light rays by an arbitrary, but fixed, angle around the sheet normal. These ray-rotation sheets comprise two pairs of confocal lenticular arrays. In addition to rotating the direction of transmitted light rays, our sheets also offset ray position sideways on the scale of the diameter of the lenticules. If this ray offset is sufficiently small so that it cannot be resolved, our ray-rotation sheets appear to perform generalized refraction
Local light-ray rotation
We present a sheet structure that rotates the local ray direction through an
arbitrary angle around the sheet normal. The sheet structure consists of two
parallel Dove-prism sheets, each of which flips one component of the local
direction of transmitted light rays. Together, the two sheets rotate
transmitted light rays around the sheet normal. We show that the direction
under which a point light source is seen is given by a Mobius transform. We
illustrate some of the properties with movies calculated by ray-tracing
software.Comment: 9 pages, 6 figure
Community Impact of Public Processions
Research into the impact of public processions on community life in Scoltand. The research paid particular attention to the impact of problematic processions, and how these processions could be better policed and managed
TIM, ray-tracing program for forbidden optics
TIM (The Interactive METATOY) is a ray-tracing program specifically tailored
towards our research in METATOYs, which are optical components that appear to
be able to create wave-optically forbidden light-ray fields. For this reason,
TIM possesses features not found in other ray-tracing programs. TIM can either
be used interactively or by modifying the openly available source code; in both
cases, it can easily be run as an applet embedded in a web page. Here we
describe the basic structure of TIM's source code and how to extend it, and we
give examples of how we have used TIM in our own research.Comment: 19 pages, 15 figure
Relationship Between Venules and Perivascular Spaces in Sporadic Small Vessel Diseases
Background and Purposeâ
Perivascular spaces (PVS) around venules may help drain interstitial fluid from the brain. We examined relationships between suspected venules and PVS visible on brain magnetic resonance imaging.
Methodsâ
We developed a visual venular quantification method to examine the spatial relationship between venules and PVS. We recruited patients with lacunar stroke or minor nondisabling ischemic stroke and performed brain magnetic resonance imaging and retinal imaging. We quantified venules on gradient echo or susceptibility-weighted imaging and PVS on T2-weighted magnetic resonance imaging in the centrum semiovale and then determined overlap between venules and PVS. We assessed associations between venular count and patient demographic characteristics, vascular risk factors, small vessel disease features, retinal vessels, and venous sinus pulsatility.
Resultsâ
Among 67 patients (69% men, 69.0±9.8 years), only 4.6% (range, 0%â18%) of venules overlapped with PVS. Total venular count increased with total centrum semiovale PVS count in 55 patients after accounting for venule-PVS overlap (ÎČ=0.468 [95% CI, 0.187â0.750]) and transverse sinus pulsatility (ÎČ=0.547 [95% CI, 0.309â0.786]) and adjusting for age, sex, and systolic blood pressure.
Conclusionsâ
Despite increases in both visible PVS and suspected venules, we found minimal spatial overlap between them in patients with sporadic small vessel disease, suggesting that most magnetic resonance imaging-visible centrum semiovale PVS are periarteriolar rather than perivenular
- âŠ