A reflectance volume holographic imaging (VHI) endoscope has been designed for simultaneous in vivo imaging of surface and subsurface tissue structures. Prior utilization of VHI systems has been limited to ex vivo tissue imaging. The VHI system presented in this work is designed for laparoscopic use. It consists of a probe section that relays light from the tissue sample to a handheld unit that contains the VHI microscope. The probe section is constructed from gradient index (GRIN) lenses that form a 1:1 relay for image collection. The probe has an outer diameter of 3.8 mm and is capable of achieving 228.1 lp/mm resolution with 660-nm Kohler illumination. The handheld optical section operates with a magnification of 13.9 and a field of view of 390 μm×244 μm. System performance is assessed through imaging of 1951 USAF resolution targets and soft tissue samples. The system has also passed sterilization procedures required for surgical use and has been used in two laparoscopic surgical procedures.
Ovarian cancer is the most deadly gynecologic cancer, a fact which is attributable to poor early detection and survival once the disease has reached advanced stages. Intraoperative laparoscopic volume holographic imaging has the potential to provide simultaneous visualization of surface and subsurface structures in ovarian tissues for improved assessment of developing ovarian cancer. In this ex vivo ovarian tissue study, we assembled a benchtop volume holographic imaging system (VHIS) to characterize the microarchitecture of 78 normal and 40 abnormal tissue specimens derived from ovarian, fallopian tube, uterine, and peritoneal tissues, collected from 26 patients aged 22 to 73 undergoing bilateral salpingo-oophorectomy, hysterectomy with bilateral salpingo-oophorectomy, or abdominal cytoreductive surgery. All tissues were successfully imaged with the VHIS in both reflectance- and fluorescence-modes revealing morphological features which can be used to distinguish between normal, benign abnormalities, and cancerous tissues. We present the development and successful application of VHIS for imaging human ovarian tissue. Comparison of VHIS images with corresponding histopathology allowed for qualitatively distinguishing microstructural features unique to the studied tissue type and disease state. These results motivate the development of a laparoscopic VHIS for evaluating the surface and subsurface morphological alterations in ovarian cancer pathogenesis.
We present the design for an endoscopic system capable of imaging tissues of the ovary at two selected imaging depths
simultaneously. The method utilizes a multiplexed volume hologram to select wavefronts from different depths within
the tissue. It is the first demonstration of an endoscopic volume holographic imaging system. The endoscope uses both
gradient index (GRIN) optical components and off the shelf singlet lenses to relay an image from the distal tip to the
proximal end. The endoscope has a minimum diameter of 3.75 mm. The system length is 30 cm which is connected to a
handle that includes the holographic components and optics that relay the image to a camera. Preliminary evaluation of
the endoscope was performed with tissue phantoms and calibrated targets, which shows lateral resolution ≈ 4 μm at an
operating wavelength of 660 nm. The hologram is recorded in phenanthraquinone doped poly methacrylate and is
designed to produce images from two tissue depths. One image is obtained at the tissue surface and the second 70 μm
below the surface. This method requires no mechanical scanning and acquires an image at the camera frame rate. The
preliminary ex-vivo results show good correlation with histology sections of the same tissue sections.
In grating-over-lens spectrum splitting designs, a planar transmission grating is placed at the entrance of a plano-convex lens. Part of the incident solar spectrum is diffracted at 15-30° from normal incidence to the lens. The diffracted spectral range comes to a focus at an off-axis point and the undiffracted spectrum comes to a focus on the optical axis of the lens. Since the diffracted wave is planar and off-axis, the off-axis focal points suffer from aberrations that increase system loss. Field curvature, chromatic and spherical aberrations are compensated using defocusing and a curved focal plane (approximated with each photovoltaic receiver). Coma is corrected by modifying the off-axis wavefront used in constructing the hologram. In this paper, we analyze the use of non-planar transmission gratings recorded using a conjugate object beam to modify the off-axis wavefront. Diverging sources are used as conjugate object and reference beams. The spherical waves are incident at the lens and the grating is recorded at the entrance aperture of the solar concentrator. The on-axis source is adjusted to produce an on-axis planar wavefront at the hologram plane. The off-axis source is approximated to a diffraction limited spot producing a non-planar off-axis wavefront on the hologram plane. Illumination with a planar AM1.5 spectrum reproduces an off-axis diffraction-limited spot on the focal plane. This paper presents ray trace and coupled wave theory simulations used to quantify the reduction in losses achieved with aberration correction.
In this paper a method to characterize the anisotropy of diffuse illumination incident on photovoltaic systems is presented. PV systems are designed based on standard conditions in which only consider direct and isotropic diffuse illumination. Anisotropic illumination can cause the PV system output to step outside of the design specifications. A baffled multi-detector sensor system is described having a discrete set of azimuthal and declination angle combinations in order to constantly sample the irradiance and the incidence angle of the diffuse illumination in all zenith directions. The sensor was deployed in the Tucson Electric Power Solar Test Yard alongside with commercially available PV systems that are independently monitored. Constant and transient sources of anisotropic diffuse illumination, such as surface reflection and cloud edge effects respectively, are measured and modeled with ray tracing software. Results of the method are described for characterizing diffuse illumination at the TEP Solar Test Yard. Understanding the anisotropic diffuse illumination can potentially allow to more accurately predict PV system or to optimize energy harvesting of systems with non-standard mounting conditions as well as building integrated photovoltaic applications.
In this work, a concentrating photovoltaic (CPV) design methodology is proposed which aims to maximize system
efficiency for a given irradiance condition. In this technique, the acceptance angle of the system is radiometrically
matched to the angular spread of the site’s average irradiance conditions using a simple geometric ratio. The optical
efficiency of CPV systems from flat-plate to high-concentration is plotted at all irradiance conditions. Concentrator
systems are measured outdoors in various irradiance conditions to test the methodology. This modeling technique is valuable at the design stage to determine the ideal level of concentration for a CPV module. It requires only two inputs: the acceptance angle profile of the system and the site’s average direct and diffuse irradiance fractions. Acceptance angle can be determined by raytracing or testing a fabricated prototype in the lab with a solar simulator. The average irradiance conditions can be found in the Typical Metrological Year (TMY3) database. Additionally, the information gained from this technique can be used to determine tracking tolerance, quantify power loss during an isolated weather event, and do more sophisticated analysis such as I-V curve simulation.
A design is presented for a planar spectrum-splitting photovoltaic (PV) module using Holographic Optical Elements (HOEs). A repeating array of HOEs diffracts portions of the solar spectrum onto different PV materials arranged in alternating strips. Several combinations of candidate PV materials are explored, and theoretical power conversion efficiency is quantified and compared for each case. The holograms are recorded in dichromated gelatin (DCG) film, an inexpensive material which is easily encapsulated directly into the panel. If desired, the holograms can focus the light to achieve concentration. The side-by-side split spectrum layout has advantages compared to a stacked tandem cell approach: since the cells are electrically isolated, current matching constraints are eliminated. Combinations of dissimilar types of cells are also possible: including crystalline, thin film, and organic PV cells. Configurations which yield significant efficiency gain using relatively inexpensive PV materials are of particular interest. A method used to optimize HOE design to work with a different candidate cells and different package aspect ratios is developed and presented. (Aspect ratio is width of the cell strips vs. the thickness of the panel) The relationship between aspect ratio and HOE performance properties is demonstrated. These properties include diffraction efficiency, spectral selectivity, tracking alignment sensitivity, and uniformity of cell illumination.
In this paper we investigate the use of holographic filters in solar spectrum splitting applications. Photovoltaic (PV)
systems utilizing spectrum splitting have higher theoretical conversion efficiency than single bandgap cell modules.
Dichroic band-rejection filters have been used for spectrum splitting applications with some success however these
filters are limited to spectral control at fixed reflection angles. Reflection holographic filters are fabricated by recording
interference pattern of two coherent beams at arbitrary construction angles. This feature can be used to control the angles over which spectral selectivity is obtained. In addition focusing wavefronts can also be used to increase functionality in the filter. Holograms fabricated in dichromated gelatin (DCG) have the benefit of light weight, low scattering and absorption losses. In addition, reflection holograms recorded in the Lippmann configuration have been shown to produce strong chirping as a result of wet processing. Chirping broadens the filter rejection bandwidth both spectrally and angularly. It can be tuned to achieve spectral bandwidth suitable for spectrum splitting applications. We explore different DCG film fabrication and processing parameters to improve the optical performance of the filter. The diffraction efficiency bandwidth and scattering losses are optimized by changing the exposure energy, isopropanol dehydration bath temperature and hardening bath duration. A holographic spectrum-splitting PV module is proposed with Gallium Arsenide (GaAs) and silicon (Si) PV cells with efficiency of 25.1% and 19.7% respectively. The calculated conversion efficiency with a prototype hologram is 27.94% which is 93.94% compared to the ideal spectrum-splitting efficiency of 29.74%.