Ashkan Arianpour

Switchable telescopic contact lens

We present design and first demonstration of optics for a telescopic contact lens with independent optical paths for switching between normal and magnified vision. The magnified optical path incorporates a telescopic arrangement of positive and negative annular concentric reflectors to achieve 2.8 x magnification on the eye, while light passing through a central clear aperture provides unmagnified vision. We present an experimental demonstration of the contact lens mounted on a life-sized optomechanical model eye and, using a pair of modified commercial 3D television glasses, demonstrate electrically operated polarization switching between normal and magnified vision.

Fluidic lens laparoscopic zoom camera for minimally invasive surgery.

We developed a miniaturized laparoscopic zoom camera that is 17 mm long, has >4X optical zoom, and works under 300 lux. This camera is suitable for advancing minimally invasive surgery. Demonstration surgery (cholecystectomy) was performed.

An optical-coding method to measure particle distribution in microfluidic devices

We demonstrated an optical coding method to measure the position of each particle in a microfluidic channel. The technique utilizes a specially designed pattern as a spatial mask to encode the forward scattering signal of each particle. From the waveform of the forward scattering signal, one can obtain the information about the particle position and velocity. The technique enables us to experimentally investigate the complex relations between particle positions within the microfluidic channel and flow conditions and particle sizes. The method also produces insight for important phenomenon in microfluidic and lab-on-a-chip devices such as inertial focusing, Dean flow, flow confinement, etc.

An Optomechanical Model Eye for Ophthalmological Refractive Studies

PURPOSE To create an accurate, low-cost optomechanical model eye for investigation of refractive errors in clinical and basic research studies. METHODS An optomechanical fluid-filled eye model with dimensions consistent with the human eye was designed and fabricated. Optical simulations were performed on the optomechanical eye model, and the quantified resolution and refractive errors were compared with the widely used Navarro eye model using the ray-tracing software ZEMAX (Radiant Zemax, Redmond, WA). The resolution of the physical optomechanical eye model was then quantified with a complementary metal-oxide semiconductor imager using the image resolution software SFR Plus (Imatest, Boulder, CO). Refractive, manufacturing, and assembling errors were also assessed. A refractive intraocular lens (IOL) and a diffractive IOL were added to the optomechanical eye model for tests and analyses of a 1951 U.S. Air Force target chart. RESULTS Resolution and aberrations of the optomechanical eye model and the Navarro eye model were qualitatively similar in ZEMAX simulations. Experimental testing found that the optomechanical eye model reproduced properties pertinent to human eyes, including resolution better than 20/20 visual acuity and a decrease in resolution as the field of view increased in size. The IOLs were also integrated into the optomechanical eye model to image objects at distances of 15, 10, and 3 feet, and they indicated a resolution of 22.8 cycles per degree at 15 feet. CONCLUSIONS A life-sized optomechanical eye model with the flexibility to be patient-specific was designed and constructed. The model had the resolution of a healthy human eye and recreated normal refractive errors. This model may be useful in the evaluation of IOLs for cataract surgery.

Image processing for cameras with fiber bundle image relay

Some high-performance imaging systems generate a curved focal surface and so are incompatible with focal plane arrays fabricated by conventional silicon processing. One example is a monocentric lens, which forms a wide field-of-view high-resolution spherical image with a radius equal to the focal length. Optical fiber bundles have been used to couple between this focal surface and planar image sensors. However, such fiber-coupled imaging systems suffer from artifacts due to image sampling and incoherent light transfer by the fiber bundle as well as resampling by the focal plane, resulting in a fixed obscuration pattern. Here, we describe digital image processing techniques to improve image quality in a compact 126° field-of-view, 30 megapixel panoramic imager, where a 12 mm focal length F/1.35 lens made of concentric glass surfaces forms a spherical image surface, which is fiber-coupled to six discrete CMOS focal planes. We characterize the locally space-variant system impulse response at various stages: monocentric lens image formation onto the 2.5 μm pitch fiber bundle, image transfer by the fiber bundle, and sensing by a 1.75 μm pitch backside illuminated color focal plane. We demonstrate methods to mitigate moiré artifacts and local obscuration, correct for sphere to plane mapping distortion and vignetting, and stitch together the image data from discrete sensors into a single panorama. We compare processed images from the prototype to those taken with a 10× larger commercial camera with comparable field-of-view and light collection.

Analysis and compensation of moiré effects in fiber-coupled image sensors

Imaging fiber bundles can relay a curved image surface to a conventional at focal plane, effectively providing the curved image sensor needed for some high performance lenses. If the fiber bundle period or image sensor pitch are very different, the system resolution is determined by the oversampled fiber or sensor feature. But crosstalk imposes an approximately 2µm minimum waveguide pitch, and light collection and fabrication constraints impose a lower limit of 1-2µm for the sensor pitch. Maximizing image information leads to some degree of aliasing, which appears in the form of moiré pattern on the raw image sensed. For example, a 30 Mpixel 120° field of view imager using a 1.75µm Bayer filtered CMOS focal plane with 2.5µm pitch fiber bundle yielded images with visible moiré. Here we present a study of moiré effects in fiber-coupled image sensors, including a method for quantitative analysis of moiré, and experimental characterization of the sensors with 1.1µm pixel pitch, the highest spatial resolution in commercially available focal plane arrays. We investigate the effect of exposure time of the sensor, angle of incidence of collimated light, and imaging lens F/# on the raw moiré pattern strength. This study provides guidelines for optimization and operation of high resolution fiber-coupled imagers.

Quantitative analysis and temperature-induced variations of moiré pattern in fiber-coupled imaging sensors.

Imaging fiber bundles can map the curved image surface formed by some high-performance lenses onto flat focal plane detectors. The relative alignment between the focal plane array pixels and the quasi-periodic fiber-bundle cores can impose an undesirable space variant moiré pattern, but this effect may be greatly reduced by flat-field calibration, provided that the local responsivity is known. Here we demonstrate a stable metric for spatial analysis of the moiré pattern strength, and use it to quantify the effect of relative sensor and fiber-bundle pitch, and that of the Bayer color filter. We measure the thermal dependence of the moiré pattern, and the achievable improvement by flat-field calibration at different operating temperatures. We show that a flat-field calibration image at a desired operating temperature can be generated using linear interpolation between white images at several fixed temperatures, comparing the final image quality with an experimentally acquired image at the same temperature.

Digital image processing for wide-angle highly spatially variant imagers

High resolution, wide field-of-view and large depth-of-focus imaging systems are greatly desired and have received much attention from researchers who seek to extend the capabilities of cameras. Monocentric lenses are superior in performance over other wide field-of-view lenses with the drawback that they form a hemispheric image plane which is incompatible with current sensor technology. Fiber optic bundles can be used to relay the image the lens produces to the sensors planar surface. This requires image processing to correct for artifacts inherent to fiber bundle image transfer. Using a prototype fiber coupled monocentric lens imager we capture single exposure focal swept images from which we seek to produce extended depth-of-focus images. Point spread functions (PSF) were measured in lab and found to be both angle and depth dependent. This spatial variance enforces the requirement that the inverse problem be treated as such. This synthesis of information allowed us to establish a framework upon which to mitigate fiber bundle artifacts and extend the depth-of-focus of the imaging system.

Enhanced Field of View Fiber Coupled Image Sensing

Fiber-coupled sensors allow detection of spherical image surfaces. We investigate the field of view possible with a single straight fiber bundle, demonstrating how the field can be extended by annular microprisms near the image surface.

Curved fiber bundles for monocentric lens imaging

Monocentric lenses allow high resolution panoramic cameras, where imaging fiber bundles transport the hemispherical image surface to conventional focal planes. Refraction at the curved image surface limits the field of view coupled through a single bundle of straight fibers to less than ±34°, even for NA 1 fibers. Previously we have demonstrated a nearly continuous 128° field of view using a single lens and multiple adjacent straight fiber-coupled image sensors, but this imposes mechanical complexity of fiber bundle shaping and integration. However, a 3D waveguide structure with internally curved optical fiber pathways can couple the full continuous field of view onto a single focal plane. Here, we demonstrate wide-field imaging using a monocentric lens and a single curved fiber bundle, showing that the 3D bundle formed from a tapered fiber bundle can be used for relaying a 128° field of view from a curved input to the planar output face. We numerically show the coupling efficiency of light to the tapered bundle for different field of views depends on the taper ratio of the bundle as well as center of the curvature chosen for polishing of the fiber bundle facet. We characterize a tapered fiber bundle by measuring the angle dependent impulse response, transmission efficiency and the divergence angle of the light propagating from the output end of the fiber.