Near-eye display performance is usually summarized with a few simple metrics such as field of view, resolution, brightness, size, and weight, which are derived from the display industry. In practice, near-eye displays often suffer from image artifacts not captured in traditional display metrics. This work defines several immersive near-eye display metrics such as gaze resolution, pupil swim, image contrast, and stray light. We will discuss these metrics and their trade-offs through review of a few families of viewing optics. Fresnel lenses are used in most commercial virtual reality near-eye displays in part due to their light weight, low volume and acceptable pupil swim performance. However, Fresnel lenses can suffer from significant stray light artifacts. We will share our measurements of several lenses and demonstrate ways to improve performance. Smooth refractive lens systems offer the option for lower stray-light viewing but usually at the cost of a much larger size and weight in order to get to the same pupil swim performance. This can be addressed by using a curved image plane but requires new display technology. Polarization-based pancake optics is promising and can provide excellent image resolution and pupil swim performance within an attractive form-factor. This approach, however, generally results in low light efficiency and poor image contrast due to severe ghosting. We will discuss some of the main limitations of that technology.
Near-eye display users universally request larger fields of view for enhanced immersion, presence, and device utility. Unlike frame rate or device weight, field of view cannot be represented precisely as a single number. Quoting field of view as a diagonal, a carry-over from the display industry, could refer to either the monocular or stereo field of view and gives no indication of the field of view boundary shape. This work defines an unambiguous metric evaluation of field of view based on solid angle, accounting for eye relief, interpupillary distance, eye rotation, and device alignment. The approach allows optical system designers to identify weak points in the optics/display/rendering pipeline. To accompany modeling, a measurement scheme was developed to metrically compare field of view over various real-world user conditions. Best practices for visualizing and communicating field of view are also presented. This work reviews the methods used to increase field of view, with discussion of the monocular and binocular artifacts that arise in large field of view systems. The limitations and advantages of optical tiling, canting, and extreme distortion are described, using relevant examples in the commercial VR space. The fundamental tradeoffs between resolution, field of view, and optical quality over field are discussed, including a review of methods to maximize field of view without sacrificing on-axis resolution. Until display and optics technology can fully match the human visual system, the intermediate objective is to find the best experience match in field of view, resolution, and optical quality given existing hardware limitations. Qualitative assessments of the relative value of different regions of the human visual field will be provided.
Conventional concentrating photovoltaic (CPV) systems track the sun with high precision dual-axis trackers. The emergent field of tracking-integrated optics has the potential to simplify the mechanics of CPV systems by loosening or eliminating the need for dual-axis tracking. In a tracking-integrated scheme, external module tracking is complemented or entirely replaced by miniature tracking within the module. This internal tracking-integration may take the form of active small-motion translation, rotation of arrayed optics, or by passive material property changes induced by the concentrated light. These methods are briefly reviewed. An insolation weighting model is presented which will aid in the design of tracking-integrated optics by quantifying the tradeoff between angular operation range and annual sunlight collection. We demonstrate that when tracking-integrated optics are used to complement external module tracking about a horizontal, North-South oriented axis, truncating the operational range may be advantageous. At Tucson AZ latitude (32.2°N), 15.6% of the angular range may be truncated while only sacrificing 3.6% of the annual insolation. We show that modules tracked about a polar-aligned axis are poorly-suited for truncation.
Line-focus parabolic trough mirrors for solar thermal generation cannot produce the high concentration required for concentrating photovoltaic (CPV) systems. We describe a freeform lens array with toroidal symmetry which intercepts the low-concentration line focus to produce a series of elongated, high-concentration foci. The design employs 2D Kӧhler illumination to improve the acceptance angle in one direction. The two-stage concentrator has 1000X average geometric concentration with an acceptance angle of +/-1.49° in the azimuthal direction and +/-0.29° in the elevation direction. Preliminary results of a prototype roll-forming process are shown in thermoplastics and B270 glass.
Here we report a newly developed method for gravity sag molding of large glass solar reflectors, 1.65 m x 1.65 m square, with either line or point focus, and short focal length. The method is designed for high volume manufacture when incorporated into a production line with separate pre-heating and cooling. The tests reported here have been made in a custom batch furnace, with high power radiative heating to soften the glass for slumping. The mold surface is machined to the required shape as grooves which intersect the glass at cusps, reducing the mold contact area to <1%. Optical metrology of replicas made with the system has been carried out with a novel test using a linear array of coaligned lasers translated in a perpendicular direction across the reflector while the deviation of each beam from perfect focus is measured. Slopes measured over an array of 4000 points show an absolute accuracy of <0.3 mrad rms in sx and sy. The most accurate replicas we have made are from a 2.6 m<sup>2</sup> point focus mold, showing slope errors in x and y of 1.0 mrad rms. The slump cycle, starting with rigid flat glass at 500C, uses a 350 kW burst of radiative heating for 200 seconds, followed by radiative and convective cooling.
A solar concentrator with a highly asymmetric acceptance cone is investigated. Concentrating photovoltaic systems require dual-axis sun tracking to maintain nominal concentration throughout the day. In addition to collecting direct rays from the solar disk, which subtends ~0.53 degrees, concentrating optics must allow for in-field tracking errors due to mechanical misalignment of the module, wind loading, and control loop biases. The angular range over which the concentrator maintains <90% of on-axis throughput is defined as the optical acceptance angle. Concentrators with substantial rotational symmetry likewise exhibit rotationally symmetric acceptance angles. In the field, this is sometimes a poor match with azimuth-elevation trackers, which have inherently asymmetric tracking performance. Pedestal-mounted trackers with low torsional stiffness about the vertical axis have better elevation tracking than azimuthal tracking. Conversely, trackers which rotate on large-footprint circular tracks are often limited by elevation tracking performance. We show that a line-focus concentrator, composed of a parabolic trough primary reflector and freeform refractive secondary, can be tailored to have a highly asymmetric acceptance angle. The design is suitable for a tracker with excellent tracking accuracy in the elevation direction, and poor accuracy in the azimuthal direction. In the 1000X design given, when trough optical errors (2mrad rms slope deviation) are accounted for, the azimuthal acceptance angle is +/- 1.65°, while the elevation acceptance angle is only +/-0.29°. This acceptance angle does not include the angular width of the sun, which consumes nearly all of the elevation tolerance at this concentration level. By decreasing the average concentration, the elevation acceptance angle can be increased. This is well-suited for a pedestal alt-azimuth tracker with a low cost slew bearing (without anti-backlash features).
Using illumination modeling, we provide a comparison of glass Total Internal Reflection Concentrators (TIRC) and
metal Hollow Reflective Concentrators (HRC) used as secondary concentrator elements in a dish-based highconcentration photovoltaic (CPV) system. Comparisons of optical efficiency, flux uniformity, off-axis acceptance angle, and cost are vital to choosing an ideal secondary concentrator element for a CPV system employing multi-junction (MJ) cells. In many CPV systems, a free-form optic or sharp-cornered rectangular TIRC composed of glass is used to increase the geometrical flux concentration at the surface of the MJ cell, and may also serve as homogenizers to mix the light to increase flux uniformity. We have demonstrated in on-sun testing that an electroformed metal HRC can be used in place of a glass TIRC of the same geometry, eliminating the need for polymeric bonding to the MJ cell surface, and providing a side-contact surface pathway for active cooling. Although geometrically equivalent, we show that glass TIRC’s achieve superior off-axis performance (higher etendue from surface refraction) and are generally acknowledged to have less degradation than optics with over-coated silver, yet metal HRC’s employing over-coated silver are superior in spectral absorption characteristics under high solar flux (no losses from glass absorption or Fresnel surface reflections) and don't require accurate glass pressing into many shapes. To better understand the trade-offs between optical efficiency, off-axis performance, mechanical tolerances, cost and reliability, metal (HRC) and glass (TIRC) tapered funnels are analyzed at the surface of equal irradiance in a Kohler-Illumination concentrator system, and a trade study is presented.
The University of Arizona has developed a new dish-based High Concentration Photovoltaic (HCPV) system which is in
the process of being commercialized by REhnu, Inc. The basic unit uses a paraboloidal glass reflector 3.1 m x 3.1 m
square to bring sunlight to a high power point focus at a concentration of ~20,000x. A unique optical system at the focus reformats the concentrated sunlight so as to uniformly illuminate 36 triple junction cells at 1200x geometric
concentration<sup>1</sup>. The relay optics and cells are integrated with an active cooling system in a self-contained Power Conversion Unit (PCU) suspended above the dish reflector. Only electrical connections are made to the PCU as the active cooling system within is completely sealed. Eight of these reflector/PCU units can be mounted on a single two axis tracking structure<sup>2</sup>. Our 1st generation prototype reflector/PCU unit consistently generated 2.2 kW of power normalized to 1kW/m<sup>2</sup> DNI in over 200 hours of on-sun testing in 2011<sup>3</sup>. Here, we present on-sun performance results for our 2<sup>nd</sup> generation prototype reflector/PCU unit, which has been in operation since June 2012. This improved system consistently generates 2.7 kW of power normalized to 1kW/m<sup>2</sup> DNI and has logged over 100 hours of on-sun testing. This system is currently operating at28% DC net system efficiency with an operating cell temperature of only 20°C above ambient. Having proven this system concept, work on our 3<sup>rd</sup> generation prototype is underway with a focus on manufacturability, lower cost, and DC efficiency target of 32% or better.