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.
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.