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.
Ultra-precision angle measurement sensors have been developed for a large number of high-performance commercial, industrial and military applications. The sensors are designed to answer the general questions “where am I?” or “where is that?” in indoor and outdoor environments. These angle measurement sensors are composed of ultra-precision optical modules, specialized coding masks, analog optical detectors and digital processing. We call the general technology “Angle Coding” and the particular products Accu-Arc sensors. In comparison to all previous systems, Accu-Arc sensors are composed of small, lightweight, monolithic optical components that have a wide field of view and an intrinsically rugged package, have no moving parts, use low complexity electronics enabling low power and high speed, and use modern ultra-precision optical fabrication and assembly techniques for the lowest possible production costs combined with the highest possible performance.
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.
Fresnel lenses have been found by some optical systems designers to be useful in combination with a main lens to provide quality telecentric images. Aspheric Fresnel lenses are an ideal choice for this application because they achieve a high degree of telecentricity across the entire field of view and introduce very little distortion. In a telecentric system consisting of an aspheric Fresnel lens and an off the shelf non-telecentric main lens, the design parameters are few. Aberration theory, constraints on the visibility of the grooves, and physical constraints can effectively be used to quickly determine if a solution exists for a given application and identify the solution space if it does.
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.
We have investigated the light collection and collimation properties of both Fresnel lenses and the nonimaging (TIR)
“cones” typically used with LEDs. We have measured the integrated light output and its spatial distribution, and we have
also measured the sensitivity of these two parameters to misalignment between the optic and the LED. We find that for a
given distance from the LED to the front of the optic, a Fresnel lens can produce a narrower (better collimated) beam
than can a nonimaging “cone.” Various design and manufacturability factors must be weighed when determining which
solution to choose for a given illumination problem, and some of these are discussed.
A low-cost polymer infrared imaging lens well suited to military and security applications in the 8 to 14 μm region has been made. It has a focal length of 50 mm, and an f/number of 0.8. The design requires four aspheric or Fresnel surfaces. Improvements in molding have allowed significant improvements over a 25 mm focal length design previously discussed.
A low-cost polymer infrared imaging lens well suited to military and security applications in the 8 to 14 μm region has been made. It has a focal length of 25 mm, and an f/number of 0.8. The design requires four aspheric or Fresnel surfaces. Remarkable performance has been demonstrated.