In 1824 William Rowan Hamilton presented a memoir to the Royal Irish Academy on Optics(Trans. R. Irish. Acacamy, XV, 1828), which was the foundation for transformational optics, classical mechanics, nonimaging optics and thermodynamical foundation of nonimaging optics,etc. It is useful for us even in 2013 to revisit the Hamilton resolution.
The field of illumination optics has a number of applications where using free-form reflective surfaces to create a required light distribution can be beneficial. Oliker’s concept of combining elliptical surfaces is the foundation of forming a reflector for an arbitrary illuminance distribution. The algorithm for fast implementation of this concept is discussed in detail. It is based on an analytical computation of a 3D cloud of points in order to map the reflector shape with the required flux distribution. Flux delivered to chosen zones across the target can be calculated based on the number of associated cloud points and its locations. This allows optimized ellipse parameters to achieve the required flux distribution without raytracing through the reflector geometry. Such a strictly analytical optimization is much faster than building reflector geometry and raytracing each step of the optimization. A generated 3D cloud of points can be used with a standard SolidWorks feature to build the loft surface. This surface consists of adjacent elliptical facets and should be smooth to maintain continuous irradiance across the target. A secondary operation to smooth the surface profile between elliptical facets is discussed. Examples of proposed algorithm implementations are presented.
The Cherenkov Telescope Array (CTA) will be the largest cosmic gamma ray detector ever built in the world. It will be installed at two different sites in the North and South hemispheres and should be operational for about 30 years. In order to cover the desired energy range, the CTA is composed of typically 50-100 collecting telescopes of various sizes (from 6 to 24-m diameters). Most of them are equipped with a focal plane camera consisting of 1500 to 2000 Photomultipliers (PM) equipped with light concentrating optics, whose double function is to maximize the amount of Cherenkov light detected by the photo-sensors, and to block any stray light originating from the terrestrial environment. Two different optical solutions have been designed, respectively based on a Compound Parabolic Concentrator (CPC), and on a purely dioptric concentrating lens. In this communication are described the technical specifications, optical designs and performance of the different solutions envisioned for all these light concentrators. The current status of their prototyping activities is also given.
We present experimental evidence for improving the open-circuit voltage – and thereby efficiency - of photovoltaics via the external recycling of photon emission. This strategy is equivalent to limiting the angular extent of photon emission - effective only in photovoltaics with high external luminescent efficiency. This is why the effect has not been observed in current solar cell technologies. It is attainable with the latest generation of ultra-efficient single-junction non-concentrator thin-film GaAs cells. The findings are explained in terms of basic photovoltaic thermodynamics.
Luminescent solar concentrators (LSCs) convert sunlight to electricity. We study light propagation in LSCs that contain semiconductor nanoparticles. We use radiative transport theory to predict the LSCs’ performance and their optimal design parameters. In particular, a luminescent radiative transport theory is proposed that can take the reabsorption effects into account accurately. The computational results based on radiative transport theory are studied in detail and compared with Monte Carlo simulations for photon transport. The results of this study will aid the development of highly-efficient LSCs.
Non-uniform irradiance patterns created by Concentrated Photovoltaics (CPV) concentrators over Multi-Junction Cells (MJC) can originate significant power losses, especially when there are different spectral irradiance distributions over the different MJC junctions. This fact has an increased importance considering the recent advances in 4 and 5 junction cells. This work presents a new CPV optical design, the 9-fold Fresnel Köhler concentrator, prepared to overcome these effects at high concentrations while maintaining a large acceptance angle, paving the way for a future generation of high efficiency CPV systems of 4 and 5 junction cells.
High Concentration photovoltaics systems (HCPV) allow for improved efficiency but, due to Etandue conservation, have low optical acceptance. Mechanical tracking is normally employed to maintain the necessary alignment of the system axis with the sun. This, however, prevents HCPV from integration in urban and residential environments. We propose here optofluidic based approaches to achieve a stationary tracking optical concentrator by internal modifications of the system optics based on the manipulation of liquid interfaces or multiphase systems. Transparency induced by phase transitions and electrophoretic driven mechanisms will be discussed. Theoretical framework, multiphysics modeling and preliminary experimental results will be presented.
Spectrally selective coatings are common in low and medium temperature solar applications from solar water heating collectors to parabolic trough absorber tubes. They are also an essential element for high efficiency in higher temperature Concentrating Solar Power (CSP) systems. Selective coatings for CSP are usually prepared using advanced expensive methods such as sputtering and vapor deposition. In this work, coatings were prepared using low-cost wet-chemistry methods. Solutions based on Alumina and Silica sol gel were prepared and then dispersed with black spinel pigments. The black dispersions were applied by spray/roll coating methods on stainless steel plates. The spectral emissivity of sample coatings was measured in the temperature range between 200 and 500°C, while the spectral absorptivity was measured at room temperature and 500°C. Emissivity at wavelengths of 0.4–1.7 μm was evaluated indirectly using multiple measurements of directional reflectivity. Emissivity at wavelengths 2–14 μm was measured directly using a broadband IR camera that acquires the radiation emitted from the sample, and a range of spectral filters. Emissivity measurement results for a range of coated samples will be presented, and the impact of coating thickness, pigment loading, and surface preparation will be discussed.
A hybrid photovoltaic/illumination rooftop module is proposed for greenhouse environmental control. The design counterbalances seasonal irradiance and temperature extremes while simultaneously generating renewable energy. Oneaxis tracking is used to increase photovoltaic collection. The tracking system also provides a seasonal variation of shading and ventilation to control temperature in the greenhouse. A faceted illumination structure is added to the module, which modifies irradiance by changing the solar angle. Irradiance in the greenhouse is simulated throughout the year for a particular site latitude and facet angle. Adjustments in the design can be made to customize the rooftop for a specific site location, illumination range, and irradiance uniformity.
Transformation Optics (TO) has enabled new methodologies for the design and specification of gradient-index (GRIN) lenses for radio-frequency and optical applications by linking refractive index gradients to a mathematically equivalent change in geometry in another dimension. With the new mathematical design tools, there have been many interesting devices introduced in the literature, such as optical collimators and absorbers (optical “black holes”), GRIN couplers and bends for optical waveguides, and compressed or flattened collimating lenses for imaging and non-imaging applications. Many of the most interesting TO designs are not feasible for implementation, however, due to the complex anisotropic, inhomogeneous material parameters required by the full TO formulation. Instead, restricting the geometric transformations to be mathematically conformal or quasi-conformal (qTO) eliminates the anisotropic material requirements and allows implementation with an isotropic 2D or 3D GRIN profile, for which multiple fabrication methods exist in the RF and IR wavelength ranges and are under development for the complete optical spectrum. We present an overview of the usefulness of combining TO, qTO, and GRIN optics for energy concentration along with the associated design and analysis techniques. Moving away from traditional lenses to GRIN and TO optics for which, in general, no analytical geometric optics or full-wave solution exists, involves the development of new design strategies for individual lenses and systems of lenses. We demonstrate results obtained using advanced, multivariate optimizations that are tightly coupled to a fast, advanced inhomogeneous ray tracing engine for electrically-large lenses, and to an efficient body-of-revolution solver for electrically-small cylindrically-symmetric lenses.
We consider the problem of designing imaging lenses consisting of thin, concentric, spherical annuli, each having a constant refractive index at a specified wavelength. For single-wavelength designs, we describe a recursive algorithm capable of generating the annular layer structure required to achieve arbitrarily low aberration levels, based on a specified structure of the outer layers. For the design of color-corrected lenses, we show how the single-wavelength design method can provide a parsimonious parametrization scheme that can be used in conjunction with a global optimization process to search for designs exhibiting minimal aberrations over the desired waveband of operation and that also satisfy design constraints.
High-speed card-to-card optical interconnects are highly demanded in high-performance computing and data centers. Compared with other solutions, free-space optical interconnects have the capability of providing both reconfigurability and flexibility. In this paper we propose and experimentally demonstrate a free-space based reconfigurable optical interconnect architecture and it is capable of connecting cards located both inside the same rack as well as in different racks. Results show that 3×10 Gb/s data transmission is achieved with a worst-case receiver sensitivity better than -9.38 dBm.
Angular color variation in white, phosphor-converted LEDs causes undesirable yellow rings in the beams of spotlights. We developed an inverse method to design TIR collimators that remove the angular color variation for point light sources and significantly reduce color variation for extended light sources, without the need for facets, holographic foils or scattering surfaces. We performed several numerical simulations to evaluate the performance of this point source method for extended light sources.
The supporting ellipsoids and linear programming algorithms build upon the property of conics to address the inverse problem of finding the freeform surface that directs light from a point source to produce a prescribed target distribution. We review the properties and main computational limitations of the two methods, and show that a fast flux estimation method based on contour detection can be used in combination with the supporting ellipsoid algorithm. Once the intersections between neighboring conic patches on the reflector are known, it is possible to estimate the collected flux by using the vertices of the intersection boundary. The advantage of using the intersection method to estimate the flux instead of the more common approach – Monte Carlo ray tracing – is that there is no tradeoff between speed and accuracy. Examples of flux estimation with the intersection method for different target configurations are shown.
We report on an extension of the previously published two-step freeform optics tailoring algorithm using a Monge-Kantorovich mass transportation framework. The algorithm's ability to design multiple freeform surfaces allows for the inclusion of multiple distinct light paths and hence the implementation of multiple lighting functions in a single optical element. We demonstrate the procedure in the context of automotive lighting, in which a fog lamp and a daytime running lamp are integrated in a single optical element illuminated by two distinct groups of LEDs.
As LEDs move into general lighting developing high efficiency, optically uniform linear light sources is critical. Presented is a new linear light emitting optic designed to work with blue LEDs (450-460nm). This white appearance remote phosphor optic combines a high efficiency phosphor down-converter layer with an integrated secondary optic into a single component. The optic is produced using co-extrusion of two different optical materials into a single component.
The effectiveness of Light Emitting Diode (LED) depends on several inherent factors.which contribute.to their overall efficiency. Here we explore the Light Extraction Efficiency (LEE) ηextraction, in the LED semiconductor substrate, which can be defined as the number of photons emitted into free space per second divided by the number of photons emitted from the active semiconductor region per second. The generally large difference in index of refraction nsc between the semiconductor substrate and the surrounding media(air) nair creates the total internal reflection (TIR) that is responsible for the limiting factor in light extraction of the material. LEEs differ depending on the semiconductor material used for the pn junction as well as the surface treatment of the material. It has been shown that rough surface treatments create more favorable conditions for light extraction. To better understand what takes place at the surface of the semiconductor, we adress the derivation of blackbody radiation and how it is related to our model. We then introduce a triangle-like geometry into the rough surface treatment and demonstrate using monte-carlo ray-trace (Light Tools Software), that close to 100% LEE can be obtained for surface treatments of this design.
We present a numerical analysis of different fiber termination shapes in order to study the maximum numerical aperture that can be obtained in end emitting plastic optical fibers with diameters around 10 mm. Our analysis includes the modeling of polished fibers with parabolic shape, conical lensed fibers, and wedged fibers with different lengths, angles and curvatures respectively. The optimization of these parameters allows us to obtain a maximum possible angle which the light can be emitted at the plastic fiber end. These results contribute to minimize the use of fiber components in luminaire systems which can be based in solar concentrators coupled to plastic optical fibers, and consequently it allows us to reduce their installation cost. We also analyze the light distribution of the emitted light and the optical tolerances of the parameters above mentioned to evaluate the performance of the optimized fiber lens. These results are of great interest for the improvement and design of compact luminaire systems based in optimized plastic fiber lens for indoor illumination.
Organic light emitting diode (OLED) is a typical surface source with continuous luminous area. It is important to know its far-field condition since most of lighting designs are based on the far-field characteristics. In this paper, the relative far-field distances (RFFD) for OLEDs with round, rectangular, annular, hemispherical and semi-cylindrical shapes are calculated. The RFFDs for LED arrays with the same shapes are also given for comparison. Results show that the RFFDs for OLEDs are smaller than that of LED arrays with the same shapes and sizes for most situations.
If a surface light source is in far-field working condition, the error from using the inverse-square law about intensity and distance should be less than 1%. However, the results of where the far-field begin may be quite different with different error definition. In this paper, by comparing several formulas commonly used for error analysis, the root mean square (RMS) weighted by the sum of intensity at infinity is proposed as the preferred error formula for far-field distances calculation. The relative far-field distances (RFFDs) for LED arrays with different radiation pattern are calculated based on this error definition.
In order to study the influence of road lighting standard on the photometric quantities and energy consumption, road lighting standards of CIE (International Commission on Illumination) and IESNA (Illuminating Engineering Society of North America) were compared in methodology, evaluation criteria and threshold. With the calculation of 20 typical assembly of roads and luminaries, it is found that the luminance is higher in IESNA than that in CIE. On the other hand, energy consumption is much less in IESNA than that in CIE under the same conditions.
In many photovoltaic (PV) or sunlight-illumination systems, solar trackers are always essential to obtain high energy/flux concentration efficiency, and that would lead to increase cost and extra power consumption due to the complex structure and heavy weight of the trackers. To decrease the cost while without sacrificing efficiency, a Fresnellens concentrator incorporated with a simple and cheap shutter, which consists of high reflective mirrors instead of conventional trackers, is proposed in this paper to provide solar tracking during the daytime. Thus, the time-variant and slant-incident sunlight rays can be redirected to vertically incident upon the surface of the Fresnel lens by appropriately arranging mirrors and swinging them to the proper slant angles with respect to the orientation of sunlight. The computer simulation results show that power concentration efficiency over 90%, as compared with the efficiency of directly normal incident sunlight, can be achieved with the mirror reflectance of 0.97 and for any solar incident angle within ±75 degrees to the normal of the Fresnel lens. To verify the feasibility and performance of the concentrator with the proposed shutter, a sunlight illumination system based on this novel structure is demonstrated. Both computer simulation and practical measurement results for the prototype of the sunlight illumination system are also given to compare with. The results prove the simple and high efficient shutter applicable to general PV or sunlight-illumination systems for solar tracking.
In many applications, the emitting light from LEDs with different colors need to be mixed together on a large scale target plane and this illumination mode is usually generated with the help of a diffuser. Abandoning the traditional method, we proposed a LED color mixing method which can produce an illumination pattern with both high color uniformity and irradiance uniformity. This method is composed of the two main aspects: arrangement of irradiance array and design of LED lens. With the method, an independent rectangular irradiance distribution is generated by each lens unit, and the large scale color uniform illumination is obtained by arraying the irradiance distribution. A 3×3 array of LED module units consisting of 36 LED lens units with 4 different colors are designed, and a desired result with high color uniformity is obtained. This color mixing method is feasible and practical, and is superior to the existing methods.