This PDF file contains the front matter associated with SPIE Proceedings Volume 11495, including the Title Page, Copyright Information, and Table of Contents.

Welcome to Nonimaging OPTICS 2020 at San Diego, California It is a pleasure to welcome you to the 2020 Nonimaging Optics conference in San Diego California. I have always looked forward to this conference as an opportunity to reconnect with friends and colleagues, catch up with new developments and enjoy beautiful San Diego where the weather is always perfect. In fact, no visit to San Diego would be complete without a visit to Coronado Island. But, this year the conference is virtual, so we have to make the best of it. I will start by acknowledging our co-chair, Eli Yablonovich of University of California, Berkeley. followed by our program committee; Pablo Benitez, University Polytechnic of Madrid, William Casserly, Synopsys, Angel Garcia-Botella, University Polytechnic of Madrid, Michael Haney, ARPA-E, Washington, D.C.Lun Jiang, University of California, Merced, Sarah Kurtz, University of California,Merced, Juan Carlos Minano, University Polytechnic of Madrid, Neri Shatz, Surefire. There is an overall motif that characterizes this conference which (personal opinion) makes it distinctive. It can be summarized by quoting from one of the presentations.Masoumi, Bernard, Winston and Jiang. "Indirect approaches can lead to solutions of problems impossible to solve directly. For example, thermodynamics can be used to solve maximum-concentration problems in nonimaging optics". There is also a new book, Nonimaging Optics, Taylor and Francis, which should be available later this year. The authors, Winston, Jiang, Oliker as we speak. are finalizing the proofs. Something to look forward to for the next conference.

String theory was developed to cope with problems not otherwise amenable to conventional methods. It was found that making certain correspondences enabled a solution, one well known correspondence is the ADS/CFT that enables the problem of one representation that can be carried out in another. In optics, certain problems such as reaching the concentration limit allowed by thermodynamics was not amenable to conventional optics techniques, one has to change the paradigm to use the language of radiative heat transfer to find the solution.

The flow line has been an interesting way of explaining the formation of nonimaging optics. As a field, the flow line implies that the ideal nonimaging optics is deeply related to the laws of thermodynamics. However, it has been a mystery since the topic was founded how such a connection can be formed. We present vector potential field of the flow line field of the new perspective on this hard question.

The flowline optical design method, based on the definition of the geometrical flux vector J, is one of the techniques to nonimaging optics design. The main property of flowline method is, its ability to estimate how radiant energy is transferred by the optical systems using the concepts of vector field theory, like field line, flux tube, vector potential overcoming traditional raytrace methods. In this paper we postulate the existence of virtual sources which can help us in the computation of J vector produced by reflective and refractive optical systems. This new computation method provides a new optical design method based on flowline.

We propose a computational optimization method for designing segmented nonimaging concentrators with planar or tubular absorbers. Concentrators are represented as concatenated linear segments, forming a faceted reflector. Ray tracing is then employed to compute the optical concentration ratio equivalent for a given concentrator configuration. The user may chose to systematically optimize the locations of the concentrator vertices or the interior angles between adjacent segments. Since gradient-based solvers become easily trapped in local minima for this problem, generalized pattern search is used to obtain optimized concentrator configurations. For a small and fixed number of segments, we show that optimized segmented concentrators preform better than an equivalent number of uniformly spaced segments of the collocated ideal shape. We also show that concentrators with a lower number of segments are more robust to manufacturing errors.

Conventional parabolic trough solar concentrators have the benefit of only requiring 1-axis tracking and having a line-focus suitable for heat extraction using long receiver pipes. However, by being 1-axis concentrators, their fundamental limit of concentration is 212x, compared to the 45 000x limit for 2-axis concentrators. We propose to use two recent developments from nonimaging optics to develop practical high-concentration line-focus concentrators. The first is the use of beam-steering lens arrays to redirect sunlight, allowing a concentrator to benefit from 2-axis tracking without being aimed directly towards the sun. The second is the use of étendue squeezing to increase concentration across one axis, at the cost of reduced concentration across the second axis. We show how these two developments may be used to create line-focus concentrators not limited by the 212x concentration limit, and present our work towards designing a practical system implementing these concepts.

We study the formation of caustic surfaces produced by simple lenses, considering a point source placed at arbitrary position along the optical axis. It is well known that the shape of the caustic is directly related to the image errors introduced by an optical system, and it can be modified by changing the parameters involved in the process of refraction. Implementing an exact ray tracing, we have obtained an analytic equation that describes a family of rays that are propagated radially from the point source placed at arbitrary distances along the optical axis as a function of an angle of emission of the source. Once obtained the parametric equation that describes the family of all the rays refracted through the singlet lens, we provide an analytical equation for the caustic surface as a function of all the parameters involved in the process of refraction. Additionally, assuming a paraxial approximation, we obain Gauss equation of image forming and some examples are presented.

Dynamic illumination can improve functionality for multiple application areas, including lighting, AR/VR, automotive, medicine, and security. Some applications require a uniform illumination pattern of continuously variable divergence or size for improved functionality. Such dynamic functionality has previously been achieved, for example, by longitudinally moving a source relative to a curved reflector, which can result poor uniformity, or through zoom configurations in which the longitudinal distances between lenses in the system are dynamically adjusted. Advances in high precision manufacturing methods such as diamond machining have facilitated the practical implementation of freeform optical components, enabling new design approaches and concepts for illumination systems. In this paper, we explore the use of arrays of transmissive pairs of freeform surfaces to enable efficient and uniform dynamic illumination in a compact package. This work builds on the Alvarez lens concept, in which a pair of transmissive XY-polynomial freeform surfaces generates variable optical power through lateral relative shifts. Design approaches and simulation results are presented.

Direct design of illumination optics for extended light sources is challenging but rewarding. Most of the current direct methods, which are developed specifically for the treatment of prescribed intensity designs, usually cannot yield acceptable illumination in the near field where the influence of lens size on the irradiance distribution cannot be ignored. Here, for the first time, to the best of our knowledge, we develop a direct method for designing aspherical illumination lenses with prescribed irradiance properties for extended sources in 3D geometry. The proposed method is valid in both near field and far field. The proposed method is numerically and experimentally evaluated. The results obtained show the effectiveness of the proposed method.

We present a new approach to relate the re ector's shape and the direction in which light gets reflected. Using a simple conformal map the reflector profile can be transformed into a curve that describes light's direction after reflection, as well as the amount of it. The method, currently limited to 2D (i.e. re ectors with a certain symmetry), can be used for both segmented and smooth reflectors, as well as with extended sources. Applications are discussed.

When photoluminescence photons propagate in a planar luminescent waveguide, some are lost due to self-absorption and leakage from the top and bottom surfaces. The probability of the photon reaching a specified edge of the waveguide depends on the distance from the emission point. It is formulated by expressing the spectral intensities of the waveguided photons in cylindrical geometry first. Scattering and re-emission events are neglected for simplicity. Then, by specifying the perimeter of a waveguide and its photon-collecting region, the resultant radial distribution is converted to an Edge Collection Function. This is defined as the probability of collecting photons at a specified edge in case of single-spot excitation. It allows one to calculate optical efficiency for an arbitrary incident light pattern. This function is calculated for square and hexagonal waveguides which are suited for tiling. A figure of merit is defined as the product of the following factors: the probability of absorbing the incident light, Edge Collection Function averaged over the whole incident surface, and the geometric concentration factor. Its value for a hexagonal waveguide is slightly higher than that for a square one.

The postulation of Flowline design method, in the frame of nonimaging optics, was a fundamental step in the development of Field Theory of Geometrical Optics. During lasts years there have been new advances in the application of Field Theory elements to Flowline analysis, vector potential, interface factors, contour integrals. In this paper we starts a new natural step in the development of Flowline theory, we study Hamiltonian formulation of Flowline, as a first step for Wigner formalism analysis. We study two different perspectives in our approach to the problem, first the classical ray optics approach and second a new approach from Radiative force point of view. Radiative force approach seems to be more advantageous than ray optics, but further studies will be needed to confirm that hypothesis.

Compound Parabolic Concentrators (CPCs) were introduced from the non-imaging optics into Visible Light Communications (VLCs) in 2012. They currently offer the only streamlined way to solve the light amplification problem when small active-area photodetectors are used. VLC use in airborne communications implies severe challenges when optical fibers deliver data. Here, we will describe optical design strategies permitting to meet these stringent requirements. The concentrator's modeling uses recently introduced concepts for a freeform design.

We propose a simple and novel approach to calculate of the solution of the radiative transport equation (RTE) without scattering and absorption in a solar cell that consists in an aperture, a receiver, and two reflective boundaries. We only get the solution at the boundaries, as inside the cell the intensity is constant along the rays.

Nonimaging optics is described in the context of radiative transfer theory. The radiative transport equation reduces to a local equation, whose solution can be expressed using the Green’s function formalism. This yields a surface integral equation for the radiance, which can be useful for analytical and numerical calculations.

In this research work, the performance of a novel secondary concentrator in the vacuum receiver of the parabolic trough collector was investigated. The secondary concentrator was designed, the optical and thermal performance analysis is conducted analytically and validated numerically by using Finite Element Analysis based computational software. From the initial optical and thermal modeling, it was found that the use of secondary reflector gives a concentration of 53X versus a concentration in 20’s X of conventional trough collectors to provide an optical efficiency of 75% and a thermal efficiency of 55% at 650^oC. The receiver with the novel secondary reflector was designed, assembled and tested indoors using resistance heaters. Heat loss measurement at 650^oC at the absorber demonstrated emissivity of less than 0.2.

The authors discuss the application potential of desalination for a novel low-cost solar thermal technology called the integrated compound parabolic concentrator (ICPC). This collector is designed using nonimaging optic principles, previously introduced in last year’s SPIE conference, 2019. Optical and thermal models are demonstrated, with preliminary experimental results. When coupled with a latent heat thermal storage system, the combined effort supplies sufficient heat demand for thermal desalination processes.

Solar photovoltaic / thermal (PVT) technologies are well suited to provide distributed and renewable electricity, space heating, and hot water in buildings. The high cost of current PVT technologies, however, remains a barrier to implementation as they compete for roof space with low cost PV. To reduce costs, a new kind of PVT collector has been developed which replaces traditional packaging materials with a low cost nonimaging optic and thermally efficient aluminum minichannel. A 20-tube (~2.4 m² aperture) prototype has been fabricated, demonstrating 57.4% thermal efficiency and 12.3% electric efficiency at 20 °C, with a stagnation temperature of ~80 °C. Based on this data we estimate the PVT collector will generate 226 kilowatt-hours (kWh) of electricity and 603 kWh of heat per square meter each year for a solar resource of 5.5 kWh/m²/day. In doing so, each installed square meter will replace 1280 kWh of natural gas consumption and 183.8 kg of CO₂ emissions. Technical performance is comparable with commercial PVT systems today, but with a much lower estimated module cost of $81/m² ($0.54/WDC). A side-by-side analysis indicates the PVT system would cost 15% less and use 30% less roof space than an equivalently sized PV+T system.

Hybrid solar system converts the incident solar radiation into electrical energy by utilizing complete solar spectrum. Such a system uses spectrum splitting solar concentrator for separation of solar spectrum into light and heat component and focusses on high-efficiency multi-junction solar cell and heat receiver. In this paper, chirped volume holographic grating (CVHG) is investigated as solar concentrator cum spectrum splitter. The grating is designed to separate the visible light of solar spectrum from the infrared wavelength band. The main advantage of CVHG over normal volume gratings are high diffraction efficiency, large bandwidth separation and lightweight. Chirp rate, period of grating, modulation index, thickness of grating, and grating profile are the critical parameters to be optimized. CVHG using photopolymer as a recording medium was designed with a dimension of 200x200x100µm³. The grating was simulated using rigorous coupled wave analysis for the incident solar spectrum wavelength ranging from 0.3 to 3.0 µm. CVHG designed with hyperbolic index profile showed concentration ratio of 15x and spectrum separation at 1.0µm.

This paper discusses the probability distribution functions (PDF) of monthly insolation for four locations in Libya based on analysis of 30 years of historical weather data calculated by National Renewable Energy Laboratory (NREL), in order to get appropriate probability distribution that best fits the data for a given month of the year. The frequency distributions used for solar irradiation data include Weibull, Normal, Lognormal and Gamma. The observed radiation at four locations in Libya on a monthly basis are analyzed to evaluate the suitability of the probability distribution functions based on the mean square errors. The analysis showed all the probability functions are appropriate for all the locations where weather conditions are relatively steady throughout the year. From the analysis it is concluded that Normal and Weibull distributions gives the best fit for observed solar radiation.

With mass production of solar panels and industrial learning bringing the cost of solar electricity below 2¢/kWh in many parts of the world, the question is increasingly asked: what research is left to do in solar energy? Cheap photovoltaics with large-scale battery storage on the horizon, so the argument goes, are about to solve all of our energy problems, so why should we bother ourselves with anything else? The invisible hand of the market is now in control, and will guide us to a sustainable solar future. This manuscript seeks to cast a bit of doubt onto this looming consensus and argue that there is still work to be done in ensuring a maximally sustainable solar future. Drawing on ongoing projects on building-integrated photovoltaics, agricultural photovoltatics, and photovoltaic-thermal systems, we will demonstrate how the emerging PV paradigm falls short of its greenest possible form, and illustrate potential technological solutions based on solar concentration to more fully realize this potential.

The external compound parabolic concentrator (XCPC) is an emerging solar thermal technology which combines nonimaging optics and metal-glass vacuum tube technology to provide high operating temperatures from a stationary collector. In this paper we describe the technology and detail the latest performance of a 50 m² solar field operating the following loads: space heating (70 °C), wastewater evaporation (150 °C), and double-effect absorption cooling (180 °C).