Transmissive optical components tend to be classified as either diffractive or refractive, based on which phenomenon more accurately describes the way they function. Although their optical functions are governed by distinct physical phenomena, Fresnel lenses are often confused or conflated with Diffractive Optical Elements (DOEs) like Fresnel zone plates and kinoforms; this is due to their similarities in shape, the method of defining their surfaces, and even the name. This research was conducted as an opportunity to describe the distinguishing characteristics of DOEs and Fresnel lenses, as well as the acceptable terminology to use with both classes of optics. First, background on diffraction and geometrical optics will be discussed, followed by methods used to design and fabricate both types of elements. Then, back focal distance metrology is performed to demonstrate the difference in function. The experiment performed supports the fact that DOEs and Fresnel lenses are not the same. This work is intended as a clarification of nomenclature and functionality, as well as an illumination of what makes DOEs and Fresnel lenses distinct.
Virtual reality and augmented reality devices require increasingly demanding optical components. Head mounted displays for VR systems often use molded Fresnel lenses, which can be affordably mass produced, maintain low weight, and still achieve high optical performance. Here, we describe an optical system designed for a wide field-of-view, consumer VR headset. Custom tooling was fabricated via diamond turning in order to injection mold the acrylic lenses. Each optical channel is composed of two lenses. The lenses have a spherical-convex surface and an aspheric-convex Fresnel on a spherical-concave surface; the radii of the spherical surfaces differ between the two lenses. Each lens pair relays the image from a compatible smartphone to the eye. To assess the quality of the lenses, the surface finish and surface profiles were measured using a white light interferometer and a contact profilometer, respectively. The lenses were assembled into a custom headset, and their performance was demonstrated via commercial VR software.
Intraoperative margin assessment to evaluate resected tissue margins for neoplastic tissue is performed to prevent reoperations following breast-conserving surgery. High resolution microendoscopy (HRME) can rapidly acquire images of fresh tissue specimens, but is limited by low image contrast in tissues with high optical scattering. In this study we evaluated two techniques to reduce out-of-focus light: HRME image acquisition with structured illumination (SI-HRME) and topical application of Lugol’s Iodine. Fresh breast tissue specimens from 19 patients were stained with proflavine alone or Lugol’s Iodine and proflavine. Images of tissue specimens were acquired using a confocal microscope and an HRME system with and without structured illumination. Images were evaluated based on visual and quantitative assessment of image contrast. The highest mean contrast was measured in confocal images stained with proflavine. Contrast was significantly lower in HRME images stained with proflavine; however, incorporation of structured illumination significantly increased contrast in HRME images to levels comparable to that in confocal images. The addition of Lugol’s Iodine did not increase mean contrast significantly for HRME or SI-HRME images. These findings suggest that structured illumination could potentially be used to increase contrast in HRME images of breast tissue for rapid image acquisition.
Electro-Chemical Polishing is routinely used in the anodizing industry to achieve specular surface finishes
of various metals products prior to anodizing. Electro-Chemical polishing functions by leveling the
microscopic peaks and valleys of the substrate, thereby increasing specularity and reducing light scattering.
The rate of attack is dependent of the physical characteristics (height, depth, and width) of the microscopic
structures that constitute the surface finish. To prepare the sample, mechanical polishing such as buffing or
grinding is typically required before etching. This type of mechanical polishing produces random
microscopic structures at varying depths and widths, thus the electropolishing parameters are determined in
an ad hoc basis. Alternatively, single point diamond turning offers excellent repeatability and highly
specific control of substrate polishing parameters. While polishing, the diamond tool leaves behind an
associated tool mark, which is related to the diamond tool geometry and machining parameters. Machine
parameters such as tool cutting depth, speed and step over can be changed in situ, thus providing control of
the spatial frequency of the microscopic structures characteristic of the surface topography of the substrate.
By combining single point diamond turning with subsequent electro-chemical etching, ultra smooth
polishing of both rotationally symmetric and free form mirrors and molds is possible. Additionally,
machining parameters can be set to optimize post polishing for increased surface quality and reduced
processing times. In this work, we present a study of substrate surface finish based on diamond turning tool
mark spatial frequency with subsequent electro-chemical polishing.
In order to diagnose cancer, a sample must be removed, prepared, and examined under a microscope, which is expensive, invasive, and time consuming. Fiber optic fluorescence endomicroscopy, where an image guide is used to obtain high-resolution images of tissue in vivo, has shown promise as an alternative to conventional biopsies. However, the resolution of standard endomicroscopy is limited by the fiber bundle sampling frequency and out-of-focus light. A system is presented which incorporates a plastic, achromatic objective to increase the sampling and which provides optical sectioning via structured illumination to reject background light. An image is relayed from the sample by a fiber bundle with the custom 2.1-mm outer diameter objective lens integrated to the distal tip. The objective is corrected for the excitation and the emission wavelengths of proflavine (452 and 515 nm). It magnifies the object onto the fiber bundle to improve the system’s lateral resolution by increasing the sampling. The plastic lenses were fabricated via single-point diamond turning and assembled using a zero alignment technique. Ex vivo images of normal and neoplastic murine mammary tissues stained with proflavine are captured. The system achieves higher contrast and resolves smaller features than standard fluorescence endomicroscopy.
In order to diagnose cancer in breast tissue, a sample must be removed, prepared, and examined under a microscope. To provide an alternative to conventional biopsies, an endomicroscope intended to perform optical biopsies is demonstrated. The system provides high resolution, high contrast images in real-time which could allow a diagnosis to be made during surgery without the need for tissue removal. Optical sectioning is achieved via structured illumination to reject out of focus light. An image is relayed between the sample plane and the imaging system by a coherent fiber bundle with an achromatized objective lens at the distal tip of the fiber bundle which is the diameter of a biopsy needle. The custom, plastic objective provides correction for both the excitation and emission wavelengths of proflavine (452 nm and 515 nm, respectively). It also magnifies the object onto the distal tip of the fiber bundle to increase lateral resolution. The lenses are composed of the optical plastics Zeonex E48R, PMMA, and polystyrene. The lenses are fabricated via single point diamond turning and assembled using a zero alignment technique. The lateral resolution and chromatic focal shift were measured and in vitro images of breast carcinoma cells stained with proflavine were captured. The optical biopsy system is able to achieve optical sectioning and to resolve smaller features than the current high resolution microendoscope.
To reduce the number of invasive tissue biopsies and needle aspirations performed during cancer screenings, endomicroscopes
can be used to image tissue in vivo. However, when optical fiber bundles are used to transmit the image,
the resolution of such systems is limited by undersampling due to the spacing of the bundle's individual fibers for a
given field of view. We propose a method to increase the sampling of an optical biopsy system and thereby improve the
system's resolution. The method involves taking several images, shifting the object and fiber bundle slightly relative to
each other from one image to the next. Multiple shifting patterns were evaluated to determine which provided the
greatest increase in resolution. The shifted images are later realigned and recombined by a custom algorithm. By
combining four shifted images of a USAF resolution target, we were able to measure an improvement in the resolution of
the system from approximately 3.9 μm to 2.2 μm. When tested on cultured cells, a visible increase in detail was
detectable. This technique can provide the basis for improving the diagnostic abilities of optical biopsy systems.
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