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Julia W. Evans

Bio: Julia W. Evans is an academic researcher from University of California, Davis. The author has contributed to research in topics: Adaptive optics & Deformable mirror. The author has an hindex of 13, co-authored 32 publications receiving 765 citations. Previous affiliations of Julia W. Evans include Lawrence Livermore National Laboratory & National Science Foundation.

Papers
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Proceedings ArticleDOI
TL;DR: The Gemini Planet Imager (GPI) as discussed by the authors is a system for direct imaging of the planets themselves, which can achieve contrast levels of 10-7 to 10-8, sufficient to detect warm self-luminous Jovian planets.
Abstract: The next major frontier in the study of extrasolar planets is direct imaging detection of the planets themselves. With high-order adaptive optics, careful system design, and advanced coronagraphy, it is possible for an AO system on a 8-m class telescope to achieve contrast levels of 10-7 to 10-8, sufficient to detect warm self-luminous Jovian planets in the solar neighborhood. Such direct detection is sensitive to planets inaccessible to current radial-velocity surveys and allows spectral characterization of the planets, shedding light on planet formation and the structure of other solar systems. We have begun the construction of such a system for the Gemini Observatory. Dubbed the Gemini Planet Imager (GPI), this instrument should be deployed in 2010 on the Gemini South telescope. It combines a 2000-actuator MEMS-based AO system, an apodized-pupil Lyot coronagraph, a precision infrared interferometer for real-time wavefront calibration at the nanometer level, and a infrared integral field spectrograph for detection and characterization of the target planets. GPI will be able to achieve Strehl ratios > 0.9 at 1.65 microns and to observe a broad sample of science targets with I band magnitudes less than 8. In addition to planet detection, GPI will also be capable of polarimetric imaging of circumstellar dust disks, studies of evolved stars, and high-Strehl imaging spectroscopy of bright targets. We present here an overview of the GPI instrument design, an error budget highlighting key technological challenges, and models of the system performance.

193 citations

Journal ArticleDOI
TL;DR: Novel visualization methods that simplify AO-OCT data viewing are presented, and include co-registration of AO+OCT volumes with fundus photography and stitching of multiple AO/OCT sub-volumes to create a large field of view (FOV) high-resolution volume.
Abstract: Ultrahigh-resolution adaptive optics-optical coherence tomography (UHR-AO-OCT) instrumentation allowing monochromatic and chromatic aberration correction was used for volumetric in vivo retinal imaging of various retinal structures including the macula and optic nerve head (ONH). Novel visualization methods that simplify AO-OCT data viewing are presented, and include co-registration of AO-OCT volumes with fundus photography and stitching of multiple AO-OCT sub-volumes to create a large field of view (FOV) high-resolution volume. Additionally, we explored the utility of Interactive Science Publishing by linking all presented AO-OCT datasets with the OSA ISP software.

99 citations

Journal ArticleDOI
TL;DR: A novel instrument capable of acquiring, simultaneously, adaptive optics enhanced scanning laser ophthalmoscopy and optical coherence tomography images of the human cone mosaic in vivo is described and the capability of this instrument is demonstrated with the measurement of thehuman cone spacing in perifoveal retina.
Abstract: We describe a novel instrument capable of acquiring, simultaneously, adaptive optics enhanced scanning laser ophthalmoscopy and optical coherence tomography (OCT) images of the human cone mosaic in vivo. The OCT system is based on transversal scanning of the sample with a line scan rate of 14 kHz, approximately 20 times faster than a previously reported instrument. We demonstrate the capability of this instrument with the measurement of the human cone spacing in perifoveal retina.

95 citations

Journal ArticleDOI
TL;DR: Ground based high-contrast imaging (e.g. extrasolar giant planet detection) has demanding wavefront control requirements two orders of magnitude more precise than standard adaptive optics systems can be achieved with a 1024-Micro-Electrical-Mechanical-Systems deformable mirror.
Abstract: Ground based high-contrast imaging (e.g. extrasolar giant planet detection) has demanding wavefront control requirements two orders of magnitude more precise than standard adaptive optics systems. We demonstrate that these requirements can be achieved with a 1024-Micro-Electrical-Mechanical-Systems (MEMS) deformable mirror having an actuator spacing of 340 µm and a stroke of approximately 1 µm, over an active aperture 27 actuators across. We have flattened the mirror to a residual wavefront error of 0.54 nm rms within the range of controllable spatial frequencies. Individual contributors to final wavefront quality, such as voltage response and uniformity, have been identified and characterized.

71 citations

Journal ArticleDOI
TL;DR: Close-loop adaptive optics is used to test the visual benefit of correcting monochromatic high-order aberrations on spatial vision for observers ranging in age from 18 to 81 years, indicating a larger role of additional optical factors when the impact of HOAs is removed and lending support for the importance of neural factors in age-related changes in spatial vision.
Abstract: The contributions of optical and neural factors to age-related losses in spatial vision are not fully understood. We used closed-loop adaptive optics to test the visual benefit of correcting monochromatic high-order aberrations (HOAs) on spatial vision for observers ranging in age from 18 to 81 years. Contrast sensitivity was measured monocularly using a two-alternative forced-choice (2AFC) procedure for sinusoidal gratings over 6 mm and 3 mm pupil diameters. Visual acuity was measured using a spatial 4AFC procedure. Over a 6 mm pupil, young observers showed a large benefit of AO at high spatial frequencies, whereas older observers exhibited the greatest benefit at middle spatial frequencies, plus a significantly larger increase in visual acuity. When age-related miosis is controlled, young and old observers exhibited a similar benefit of AO for spatial vision. An increase in HOAs cannot account for the complete senescent decline in spatial vision. These results may indicate a larger role of additional optical factors when the impact of HOAs is removed, but also lend support for the importance of neural factors in age-related changes in spatial vision.

59 citations


Cited by
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Journal Article
TL;DR: In this article, the diffraction tomography theorem is adapted to one-dimensional length measurement and the resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.
Abstract: The diffraction tomography theorem is adapted to one-dimensional length measurement. The resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.

1,237 citations

Journal ArticleDOI
TL;DR: In this article, the authors summarize salient developments in research on aging and vision over the past 25 years, focusing on spatial contrast sensitivity, vision under low luminance, temporal sensitivity and motion perception, and visual processing speed.

545 citations

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate ultrahigh speed swept source/Fourier domain ophthalmic OCT imaging using a short cavity swept laser at 100,000 and 400,000 axial scan rates.
Abstract: We demonstrate ultrahigh speed swept source/Fourier domain ophthalmic OCT imaging using a short cavity swept laser at 100,000 – 400,000 axial scan rates. Several design configurations illustrate tradeoffs in imaging speed, sensitivity, axial resolution, and imaging depth. Variable rate A/D optical clocking is used to acquire linear-in-k OCT fringe data at 100kHz axial scan rate with 5.3um axial resolution in tissue. Fixed rate sampling at 1 GSPS achieves a 7.5mm imaging range in tissue with 6.0um axial resolution at 100kHz axial scan rate. A 200kHz axial scan rate with 5.3um axial resolution over 4mm imaging range is achieved by buffering the laser sweep. Dual spot OCT using two parallel interferometers achieves 400kHz axial scan rate, almost 2X faster than previous 1050nm ophthalmic results and 20X faster than current commercial instruments. Superior sensitivity roll-off performance is shown. Imaging is demonstrated in the human retina and anterior segment. Wide field 12×12mm data sets include the macula and optic nerve head. Small area, high density imaging shows individual cone photoreceptors. The 7.5mm imaging range configuration can show the cornea, iris, and anterior lens in a single image. These improvements in imaging speed and depth range provide important advantages for ophthalmic imaging. The ability to rapidly acquire 3D-OCT data over a wide field of view promises to simplify examination protocols. The ability to image fine structures can provide detailed information on focal pathologies. The large imaging range and improved image penetration at 1050nm wavelengths promises to improve performance for instrumentation which images both the retina and anterior eye. These advantages suggest that swept source OCT at 1050nm wavelengths will play an important role in future ophthalmic instrumentation.

470 citations

Journal ArticleDOI
TL;DR: In this article, a large set of models exploring the observational consequences of high entropy (hot) and low entropy (cold) initial conditions are provided, in the hope that this will ultimately help to distinguish between different physical mechanisms of planet formation.
Abstract: Gas-giant planets that form via core accretion might have very different characteristics from those that form via disk instability. Disk-instability objects are typically thought to have higher entropies, larger radii, and (generally) higher effective temperatures than core-accretion objects. In this paper, we provide a large set of models exploring the observational consequences of high-entropy (hot) and low-entropy (cold) initial conditions, in the hope that this will ultimately help to distinguish between different physical mechanisms of planet formation. However, the exact entropies and radii of newly formed planets due to these two modes of formation cannot, at present, be precisely predicted. It is possible that the distribution of properties of core-accretion-formed planets and the distribution of properties of disk-instability-formed planets overlap. We, therefore, introduce a broad range of 'warm-start' gas-giant planet models. Between the hottest and the coldest models that we consider, differences in radii, temperatures, luminosities, and spectra persist for only a few million to a few tens of millions of years for planets that are a few times Jupiter's mass or less. For planets that are {approx}five times Jupiter's mass or more, significant differences between hottest-start and coldest-start models persist for on the order of 100 Myr. We findmore » that out of the standard infrared bands (J, H, K, L', M, N) the K and H bands are the most diagnostic of the initial conditions. A hottest-start model can be from {approx}4.5 mag brighter (at Jupiter's mass) to {approx}9 mag brighter (at 10 times Jupiter's mass) than a coldest-start model in the first few million years. In more massive objects, these large differences in luminosity and spectrum persist for much longer than in less massive objects. Finally, we consider the influence of atmospheric conditions on spectra, and find that the presence or absence of clouds, and the metallicity of an atmosphere, can affect an object's apparent brightness in different bands by up to several magnitudes.« less

399 citations

Journal ArticleDOI
TL;DR: Various multimodal imaging modalities with OCT incorporated are reviewed, in that these multi-modal implementations can synergistically compensate for the fundamental limitations of OCT when it is used alone.
Abstract: In the last 25 years, optical coherence tomography (OCT) has advanced to be one of the most innovative and most successful translational optical imaging techniques, achieving substantial economic impact as well as clinical acceptance. This is largely owing to the resolution improvements by a factor of 10 to the submicron regime and to the imaging speed increase by more than half a million times to more than 5 million A-scans per second, with the latter one accomplished by the state-of-the-art swept source laser technologies that are reviewed in this article. In addition, parallelization of OCT detection, such as line-field and full-field OCT, has shortened the acquisition time even further by establishing quasi-akinetic scanning. Besides the technical improvements, several functional and contrast-enhancing OCT applications have been investigated, among which the label-free angiography shows great potential for future studies. Finally, various multimodal imaging modalities with OCT incorporated are reviewed, in that these multimodal implementations can synergistically compensate for the fundamental limitations of OCT when it is used alone.

383 citations