For the DARWIN mission the extremely low planet signal levels require an optical instrument design with utmost efficiency to guarantee the required science performance. By shaping the transverse amplitude and phase distributions of the receive beams, the singlemode fibre coupling efficiency can be increased to almost 100%, thus allowing for a gain of more than 20% compared to conventional designs. We show that the use of "tailored freeform surfaces" for purpose of beam shaping dramatically reduces the coupling degradations, which otherwise result from mode mismatch between the Airy pattern of the image and the fibre mode, and therefore allows for achieving a performance close to the physical limitations. We present an application of tailored surfaces for building a beam shaping optics that shall enhance fibre coupling performance as core part of a space based interferometer in the future DARWIN mission and present performance predictions by wave-optical simulations. We assess the feasibility of manufacturing the corresponding tailored surfaces and describe the proof of concept demonstrator we use for experimental performance verification.
The optical design of freeform surfaces is particularly demanding due to the inherently large number
of degrees of freedom. Therefore the technique of tailoring optical freeform surfaces (mirrors or
lenses), which is based on solving the underlying differential equations, is routinely used in nonimaging
optics to efficiently achieve predefined light distributions.
Tailoring can also be employed for imaging optics: an optical freeform surface can be tailored such
that one condition is exactly fulfilled - i.e. spherical aberration at a specific point is corrected.
Recently, we extended this method such that two freeform surfaces can be tailored at the same time
(double tailoring). This gives the freedom to impose a second condition, which is also exactly
fulfilled. Thereby, this method allows for instance to simultaneously correct spherical aberration and
satisfy additional conditions, i.e. the sine condition.
As an example for a tailored off-axis aplanatic system we show a head-up display (HUD) consisting
of two simultaneously tailored freeform mirrors
Tailoring allows designing a freeform optical surface which precisely produces a desired irradiance distribution on a
desired target from a known small source by solving a partial differential equation. Existence and uniqueness of the
solution depends on the boundary conditions. We present two alternative types of boundary conditions which ensure a
unique exact solution. We have implemented and tested these boundary conditions in our freeform tailored optics
package ffTOP®.
Applications requiring high lumen packages are traditional the domain of light sources like discharge lamps. Currently,
LEDs make their way into such applications. LED street lighting projects, which are regularly covered in the press,
provide a case of point.
Life time and luminous efficacy are considered as being the main advantages of LEDs. Nonetheless, other current light
sources for street lighting have similar performance. Analysing a street-lamp as a complete system, we can show that
LED solutions have significant advantages if highly efficient optics are used. We present an example with tailored free-form
optics. These make efficient use of the valuable LED light by exactly redistributing it into the desired illuminance
pattern.
The Carambola is an optical device designed to allow the deterministic and multiple recycling of light rays. The rays transit through the source a defined number of times before exiting in the same phase space as light directly emitted and not recycled. The brightness enhancement by light recycling (the optical light recycling factor) with the Carambola depends on the reflectivity of the reflecting walls of the Carambola, as well as on the size of the source and on the optical thickness of the source. The results of a ray-tracing simulation and an analytical model are promising an optical light recycling factor up to three for a Xenon high-pressure arc discharge lamp.
The laws of Planck and Kirchhoff are fundamental to a physical model yielding the radiance distribution emitted from a filament light source. The filament is made of a wire coil, or a coiled coil, or even a coiled coiled coil. Some parts of the filaments face other parts. Some light is reflected or absorbed between parts of the filament. This effect is termed light recycling. Light recycling depends on the geometry of the filament, and its material properties. Our model is based on the thermodynamics, geometry, material and electrical properties of a filament lamp. Light recycling is integrated into the model. The model calculates the one-dimensional temperature distribution in the filament by solving the time dependent heat transfer equation. The results of the model are verified with absolute radiance measurements. Parameters are identified in order to increase the accuracy of the values used for material characterization. The source model may be integrated into optical software packages.
The brightness theorem states that it is impossible to increase the spectral radiance of light by passive optical devices, which seems intuitive because spectral radiance is connected to temperature; increasing it seems to violate the second law of thermodynamics. However, consider a gray thermal source, that is a source that emits thermal radiation but with less than unit emissivity. Kirchhoff's law states that absorptivity is equal to emissivity. Thus if we redirect part of the emitted radiation back onto the source, some of it would not be absorbed but either transmitted or reflected instead. Consequently, this radiation would be superposed to the thermal radiation from the source in the same phase space and thereby the spectral radiance would be increased. In fact, thermal sources such as high pressure discharge lamps feature absorptivity-emissivity values far below unity. In this contribution with spectroscopic measurement we show that for such sources the spectral brightness can be increased. This does not contradict the second law because light recycling reduces the irreversible entropy production inherent to the radiation process. It is equivalent to increasing the optical thickness of sources for the price of reducing the total phase space of emitted radiation by the same factor.
The idea of light recycling is rather simple. Assume that part of the light emitted by a light source is returned to the light source itself. If the light source does not completely absorb this light then the part which is not absorbed, is still available for further use. The hidden virtue of light recycling is that the recycled light is superposed in the same phase space (etendue) as the original radiation. Thus the average radiance of the source is increased albeit at the price of a reduction of total radiant power. This seems to violate the Second Law of Thermodynamics because the temperature of the radiation is related to the spectral radiance. Increasing the radiance amounts to reducing the entropy. However, radiating into free space is an irreversible process in which entropy is created. Light recycling reduces the entropy carried by the radiation by reducing the entropy production rate in the emission process itself. We show that the maximum radiance which can be achieved by light recycling is marked by the equilibrium radiance. The equilibrium luminescent spectrum diverges as photon energies approach the splitting of the quasi Fermi levels. The familiar spectrum of LEDs clearly does not diverge because the absorptivity/emissivity approaches zero in this regime. These features render light recycling particularly attractive. We report on preliminary laboratory measurements which show encouraging results.
To this day the standard method of imaging design is optimization. Recently we have introduced tailoring as a radically different paradigm of optical design. Tailoring determines the shape of optical surface, a priory free, by solving one or more differential equations. This method has proved successful in illumination design where a high level of detail needs to be accommodated and indeed a perfect solution is possible often with only one optical surface.
The weakness of tailoring is first that it cannot adequately deal
with weak requirement which need to be optimized, because they can
not be precisely met or at least not met simultaneously. Examples
range from manufacturability, to size, sensitivity to tolerances,
but also includes imaging errors. Wassermann and Wolf showed in a classical paper how tailoring can be used in an imaging system in order to achieve aplanatism with the addition of two aspheres. In our contribution we present a synthesis which combines the virtues of optimization with those of tailoring for imaging design. It encompasses freeform surfaces and thus a huge number of effective parameters, however, only a few of these are subject to optimization. On the other hand our method can adequately use optimality criteria such as conflicting features in a figure of merit, which need to be compromised upon. Finally the result is a mathematically rigorous optimum with respect to whatever figure of merit is specified.
In several previous publications we have outlined how a freeform optical surface can be tailored to achieve a desired irradiance on a reference surface. The shape of the surface is determined by numerically solving a differential equation. This approach has the advantage of being able to accommodate a huge amount of detail,
equivalent to several thousand to millions of parameters for which
classical optimisation techniques are not feasible. Laser beam shaping requires controlling the phase of the wave front as well. One surface is not sufficient, but two surfaces can be calculated by simultaneously solving a system of coupled differential equations. Justin L. Kreuzer patented this idea in 1965 (US Patent No. 3476463). Wassermann and Wolf outlined already in 1949 a procedure by which two aspheric surfaces are determined by solving differential equations such as to render an imaging optical system aplanatic. The work of Kreuzer as well as that of Wassermann and Wolf refers to rotational systems, where the cross section curve suffices to determine the surface and the equations are less demanding. We show how to extend the method of three-dimensional tailoring to simultaneously tailor two surfaces which are not necessarily rotationally symmetric, such as to achieve non-rotational irradiance distributions or off-axis
devices. The desired irradiance translates into an equation which combines the first and the second derivatives of the surfaces.
The edge ray theorem has become a tenet for illumination design. For idealized configurations this theorem has proved extremely powerful and has led to new methods of design. However, many real life sources have no clean delimitation and thus the boundary of the phase space is not easy to specify. Optical efficiency expressed by the fraction of the light captured by the optical system and transferred to the target on one hand and average radiance at the target on the other hand become two conflicting objectives in illumination design. Optimal design generally requires a single scalar merit function. Two different objectives require a metric which specifies the relative valuation before optimality may even be defined. In practice the relative valuation depends on the application and seldom may be specified in advance. It is, however, possible to refer to a slightly weaker criterion, called pareto-optimality which allows to postpone the decision on the relative valuation. In this contribution we show how to derive the upper limits of efficiency and average radiance for all paretooptimal illumination designs using a given source. If an optical system observes the edge ray principle such that the boundary of the source follows a iso-radiance hyper-surface in phase space, then this system is pareto-optimal. To calculate the characteristic curve one needs to sort phase space according to radiance. We show an example of an illumination lens which was inspired by phase space sorting. We have found this tool extremely useful because it allows to outline the range of options before actually embarking on the optical design.
Given a set of available LEDs or other light sources with known specifications including spectrum, total luminous flux (in lumens), and efficacy (in lumens per watts), we show how to select that combination which yields light of the desired (photometric) color and, in addition, maximizes various objectives such as efficacy, luminous flux, and color rendering index.
3-D tailoring is a constructive method for the design of free-form optical elements for illumination. The light of a point source is redirected in a controlled manner to cast a prescribed irradiation pattern on a target surface. Free parameters can be used to control the shape of the surface resulting from the tailoring process. Every change in the parameters may lead to an entirely different design. Hence the choice of parameters is crucial for the technical feasibility and the visual appearance of the luminaire. Examples of free parameters are the chosen caustics, trimming of the surface, the choice between mirror and lens optics, and the mutual orientation of source and optical elements.
Due to antiquated technologies (calculation methods, regulations, lighting and luminaire concepts, production techniques) current outdoor lighting causes a lot of problems like light pollution, glare, energy waste etc.
New types of luminaires, and in consequence new outdoor lighting concepts, can be created by combining advanced calculation methods for optical surfaces with recent production technologies and novel light sources such as short arc metal halide lamps. Light emitted from this small Etendue light sources can precisely be redirected by 3D-curved surfaces manufactured with injection molding, milling and aluminium metallization. The required optical design may use techniques like complex surface calculations and 3D-Tailoring.
An innovative concept based on the latest findings in visual perception research is to focus the light of such short arc light sources onto a facetted secondary mirror which provides the desired illuminance distribution on a facade or a public place. These systems are designed to fulfill lighting requirements as well as providing visual comfort. Thus lamps with improved color rendering, luminous efficacy and increased lifetime are used and glare is minimized by splitting the reflector into many facets (light spot decomposition).
A few examples of realized projects will be presented where such complex facetted surfaces are used to reach a special quality of light. Using novel techniques like 3D-Tailoring, each facet can be designed to individually create the desired (e.g. uniform) illuminance distribution on the target surface - in this case, a large facade. For this particular application, we chose to impose a square boundary for each facet, in order to tile the rectangular aperture of the secondary mirror without compromising efficiency.
Proc. SPIE. 5186, Design of Efficient Illumination Systems
KEYWORDS: Reflectors, Prisms, Solar concentrators, Waveguides, Silver, Reflectivity, Monte Carlo methods, Solids, Performance modeling, 3D microstructuring
Our paper treats the conversion of light beams with circular cross-section into light beams of square cross-section as well as the conversion of light beams with rectangular cross-sections of different aspect ratios. We calculate the possible concentration ratio,and introduce symmetry-breaking microstructures in order to mix high with low skewness without affecting the axial component of the k -vector. A typical example for its application is as secondary concentrator-homogenizer in 3D-photovoltaic (PV)concentrator systems,for square PV-cells. For the performance of modern multi-junction cells it is crucial to homogenize the incoming radiation in the secondary, both in location and color. We compare the performance of homogenizers with and without microstructures and show the advantage of adding this feature. Several designs are modeled and the performance is compared by Monte-Carlo ray-tracing. With wall microstructures,light uniformity and concentration is significantly better.
Given a set of available LEDs or other light sources with known spectrum, total luminous flux (lumen) and efficacy (lumen/Watt), price etc. we show how to select that combination which yields light of desired (photometric) color and, in addition, maximizes efficacy, luminous flux , color rendering index or other objectives.
The key idea of Fresnel optics is to decouple the global slope from the local slope by breaking up the optical surface into small facets. The size of the facets is irrelevant as long as they are larger than the wavelength of light, so that the system behaves according to geometrical optics, and at the same time small compared the
overall size of the optical surface. From the point of view of phase-space conservation, Fresnel optics suffer from a basic shortcoming. The phase-spaces of incoming and outgoing radiation beams need not automatically be equal. This results in either a dilution of radiation or losses or both. On the other hand, decoupling local from
global slope allows to tailor the overall shape of the Fresnel lens independently from designing the individual facets. We show that it is possible to closely match incoming and outgoing radiation beams with a particular choice of the global shape of the Fresnel surface. This shape imultaneously minimizes dilution and blocking.
Snell's law allows to find the slope of an optical surface needed
to redirect a given incoming ray into a given outgoing ray. Since
a prism comprises two surfaces the problem of redirecting one ray
with a prism is underdetermined. In a range of situations it is
possible to determine a prism such as to simultaneously match two
given incoming rays into two given output rays. This allows to
tailor 2D Fresnel optics for finite sources and targets. If source
and target subtend equal angles as seen from the Fresnel lens,
then the facets are symmetric resembling the minimum deviation
configuration, which also minimizes chromatic aberration based an
the dispersion in the material of the lens.
The general problem to find the shape of a refractive surface such as to produce a desired brightness distribution on a given target surface from a known point source leads to a boundary value problem with an elliptic partial differential equation of the Monge-Ampere type. This equation has been described and analyzed in the literature. The purpose of our contribution is to present a venue for a numerical solution as well as several solved examples. The essence of our algorithm for a numerical solution appears to be the explicit incorporation of the condition for the existence of a pseudopotential for a normal vector field.
Compound parabolic concentrators (CPCs) are good candidates for secondary concentrators in solar applications. Practical considerations, however, sometimes dictate the use of planar as opposed to curved reflectors. In addition polygonal apertures may be desired in order to tile a large area with several smaller secondary concentrators. We analyze in our contribution secondary concentrators which approximate a CPC but consist of plane facets with various numbers of subdivisions in axial and circumferential direction. We found that an `intuitive' axial profile with a constant angle between neighboring facets does not lead to optimal performance. We optimized by ray-tracing the size and orientation of the facets and found concentrators with significantly higher performance as compared to the `intuitively' facetted CPCs. The resulting shapes are usually significantly longer than classical CPCs. The higher the reflectivity of the surfaces, the longer the optimized concentrators get, approaching the infinite cone as an ideal concentrator for perfect reflectivity.
We present a new approach in optical design whereby two- stage axisymmetric reflectors are tailored with a completely imaging strategy, and can closely approach the thermodynamic limit to radiation concentration at near-maximum collection efficiency. Practical virtues include: (1) an inherent large gap between the receiver and the second-stage mirror; (2) an upward-facing receiver; (3) the possibility of compact units (large rim angles), i.e., low ratios of total depth to total width; and (4) no chromatic aberration. We describe how one can tailor both the primary and secondary mirrors so as to insure that spherical aberration is eliminated in all orders, and circular coma is canceled up to first order in the angle subtended by the radiation source. An illustrative solution that attains about 93% of the thermodynamic limit to concentration is presented for a far-field source, as is common in solar energy and infrared detection applications. Double-tailored imaging concentrators are similar in principle to complementary Cassegrain concentrators that comprise a paraboloidal primary mirror and the inner concave surface of a hyperboloid secondary reflector, but have monotonic contours that are substantially different with far superior flux concentration.
Known designs of reflectors for cylindrical absorbers with gaps either suffer from radiation losses or from dilution which is equivalent to losing maximum concentration. In order to avoid both kinds of losses the global shape of a micro-structured reflector must be such that the gap and the real absorber occupy equal projected angles as seen from all locations. This condition is used to tailor the global shape. We show ideal reflectors for different gap positions. There is no upper limit for the gap size imposed by physical laws.
Microstructures can be viewed as nonimaging devices where all edge rays involved do not change direction over the aperture(s) of the device. This is equivalent to saying that the distance to source and target is much larger than the dimension of the device, which justifies the name microstructure for these devices. Consequently the shape of a microstructure device is independent of its size in this limit. The condition that the etendue of source and target must be equal, generally implies that the projected angle and not the angle itself under which source and target are seen must be equal. This requires at least two surfaces (reflective or refractive). We show how the shape of these two surfaces can be simultaneously tailored to the desired 4 directions delimiting a finite source and target.
We generalize a previous derivation of theoretical upper limits on measures of flux-transfer performance of nonimaging optical systems to include the case of sources and targets having inhomogeneous distributions of radiance and importance weighting. This generalization is of practical importance in understanding the limitations of optimally designed projectors which collect light from complex 3D sources such as filament or HID sources. The performance limits are derived from the conservation of etendue and--for the case of rotationally symmetric optics-- from skewness conservation. The limits on performance are calculated for examples involving the use of rotationally symmetric optics to transfer flux from homogeneous and inhomogeneous spherical sources to a homogeneous disk shaped target having a phase-space volume equal to that of the source. It is shown that an optical system which is optimal for the case of a homogeneous source and target will not necessarily provide the best achievable performance when used in conjunction with an inhomogeneous source and/or target occupying the same regions of phase space as the homogeneous source and target. The theoretical upper limits are shown to be consistent with the performance of three numerically optimized reflector designs.
Theoretical upper limits on measures of flux-transfer performance due to skewness conservation in rotationally symmetric nonimaging optical systems have recently been discovered and quantified. These limits can have an adverse impact on the performance of projection or coupling optics which collect light from 3D sources. In this contribution we show that these limits can be exceeded by employing nonrotationally symmetric configurations. We consider the problems of maximizing flux transfer from both a homogeneous spherical source and a homogeneous cylindrical source to a homogeneous disk-shaped target of equal etendue. We find that the performance limits due to skewness conservation for these problems can be overcome by numerically optimized reflectors possessing a nonrotationally symmetric star-like cross-section.
In solar tower plants, where a rotationally symmetric field of heliostats surrounds the tower, an axisymmetric secondary concentrator such as a Compound Parabolic Concentrator (CPC) or a tailored concentrator or a cone is the obvious choice. For locations at higher latitudes, however, the reflecting area of the heliostats may be used more efficiently if the field of heliostats is located opposite to the sun as seen from the tower. Then the field is asymmetric with regard to the tower. In the case of an asymmetric field, an axisymmetric concentrator necessarily has a concentration significantly lower than the upper limit. Furthermore, the area on the ground from which a tilted axisymmetric concentrator accepts radiation is an ellipse, including also heliostats very distant to the tower producing a large image of the sun. Therefore we investigate asymmetric secondaries. From the shape of the edge ray reflectors constructed for rays in the central south-north plane we conclude that a skew cone reflector might be appropriate for the field and optimized its free parameters by means of raytracing. Asymmetric concentrators may increase the concentration by up to 25% at the same efficiency compared to optimized axisymmetric CPC or cone reflectors.
An upper limit on concentration for any optical device has previously been derived from the conservation of etendue. In this contribution we derive more stringent upper limits for the efficiency and the concentration of rotationally symmetric optical devices which are a consequence of skewness conservation. If the desired source and target have different skewness distributions, then losses or dilution or both will limit the performance of the optical system. For all skewness values, for which the source contains more radiation than the target, the difference is lost. Conversely, for all skewness values, for which the target contains a large etendue than the source, the difference remains empty and results in dilution. We calculate the limiting curve of optical transfer efficiency versus concentration relative to the maximum concentration possible and provide a design example that is practically at this limit. We also provide another design example that addresses the challenge posed at the last SPIE meeting, namely to transfer the maximum radiation from a Lambertian spherical source to a disk target of equal etendue under a reflector- to-source minimum distance constraint. We conjecture that even rotationally symmetric problems may benefit from asymmetric optical systems.
The goal of the optical design of luminaires and other radiation distributors is to attain the desired illumination on the target with a given source. Usually there are constraints that should be satisfied, such as avoiding glare, maximizing the optical efficiency and respecting practical size limitations (not to mention considerations of fabrication costs, availability of materials and esthetics). While the required design procedure is well known for situations where the source can be approximated as a point or as a line, the development of an explicit analytical design method (as opposed to numerical search) for extended sources has begun only a few years ago. A solution for extended isotropic sources can be obtained by establishing a one-to-one correspondence between target points and edge rays, using the tools of nonimaging optics. The designs are called TED (tailored edge-ray designs). Particular solutions have been found in separate papers by Ries and Winston and by Gordon, Rabl and Ong. The present paper present a topological classification of all possible solutions in two dimensions and discusses their general characteristics. We show that any illumination distribution can be obtained exactly in the central region of the target, but in general there will be a certain amount of spillover outside this region. Some flexibility for tailoring designs to specific requirements (size, glare control, etc.) can be gained by the choice of the solution type, the choice of the boundary conditions, and by the use of hybrid configurations that combine several types of solution. The design method is illustrated with specific examples.
Often the direction of radiation is important for technological reasons. Melting substances may drip down, or for air receivers, convection may cause instabilities. One obvious solution is to reorient the radiation before concentration with a planar mirror. This is only practical, if the angular spread is considerably less than 45 degrees. Here we propose a section of a torus with reflecting walls to reorient the radiation. The torus, by virtue of its rotational symmetry will not reject any radiation, even if the incoming radiation is close to the thermodynamic limit and thus completely diffuse. A toroid reflector can be easily manufactured from massive material and cooled. It seams a compact and practical device. We calculate the number of reflections and discuss applications of such a device in solar furnaces.
The caustic of a set of edge rays is defined as the set of intersection points of adjacent edge rays. For a body having a smooth differentiable contour, the caustic of its edge rays coincides with the contour of the body. Therefore one would assume that by calculating the caustic of the edge rays as they are produced by a 2D concentrator such as a trough, the optimal shape for the absorber, e.g. the minimal surface absorber capable of intercepting all rays, should also coincide with the shape of the caustic. We show that this conjecture is not valid in general, but only if the caustic indeed forms a closed smooth curve. For parabolic trough systems, the caustic intersects and forms closed domains for half rim angles of around 60 degrees and 120 degrees. In both cases the contour is not smooth. Therefore the optimal shape is not given by the domain enclosed by the caustic. We present a general recipe of how to construct minimum surface absorbers for given caustics in 2D and apply this to the case of trough parabolic concentrators. We show practical absorber shapes for parabolic troughs with various rim angles. The optimal contour depends discontinuously on the rim angle. The area of the optimum shape for a rim angle of 90 degrees is 0.72 of the area of the smallest cylindric absorber capable of intersecting all rays.
The use of two stage optical designs, with reasonably compact devices, is required for being close to the thermodynamic limit to optical concentration of solar collectors. In this work we will present the design and the first test results of a second stage concentrator to be added to the existing primary of the solar furnace of Plataforma Solar de Almeria (PSA), designed to improve the final concentration and to increase its working temperature. Different options have been considered, including CPC, CEC, Trumpet, Cassegrainian, and tailored edge-ray devices. For the geometry of the PSA solar furnace, the tailored edge ray comes closest to the thermodynamic limits. It also is the most suitable from a practical point of view.
Using the recently-invented tailored edge-ray concentrator (TERC) approach for the design of compact two-stage high-flux solar collectors--a focusing primary reflector and a nonimaging TERC secondary reflector--we present: 1) a new primary reflector shape based on the TERC approach and a secondary TERC tailored to its particular flux map, such that more compact concentrators emerge at flux concentration levels in excess of 90% of the thermodynamic limit; and 2) calculations and raytrace simulations result which demonstrate the V-cone approximations to a wide variety of TERCs attain the concentration of the TERC to within a few percent, and hence represent practical secondary concentrators that may be superior to corresponding compound parabolic concentrator or trumpet secondaries.
Cone and trumpet are nonimaging concentrators which do not obey the traditional edge-ray principle. The latter states that edge rays from the source should be transferred to the edge of the target. These concentrators have traditionally been described in terms of the heuristic flow line principle. The edge-ray theorem has been generalized to include nonimaging reflectors with multiple reflections. One includes all multiply reflected rays as an auxiliary domain. The general edge-ray theorem then states that the edge rays to the union of source and auxiliary domain must be reflected to edge of the union of target and auxiliary domain by the first reflection. We show the setup for which cone and trumpet constitute perfect nonimaging concentrators in the light of the generalized edge-ray theorem. We discuss the cases where cones are very good approximations for the solutions of nonimaging problems.
For many tasks in illumination and collection the acceptance angle is required to vary along the reflector. If the acceptance angle function is known, then the reflector profile can be calculated as a function of it. The total flux seen by an observer from a source of uniform brightness (radiance) is proportional to the sum of the view factor of the source and its reflection. This allows one to calculate the acceptance angle function necessary to produce a certain flux distribution and thereby construct the reflector profile. We demonstrate the method for several examples, including finite size sources with reflectors directly joining the source.
We present a new type of ideal non-imaging secondary concentrator, the tailored edge-ray concentrator (TERC), that can closely approach the thermodynamic limit of concentration, and illustrate it for both linear and point-focus Fresnel reflectors. For large rim-angle heliostat fields, practical-sized secondaries with shapes that should be relatively easy to fabricate can achieve concentrations substantially above those of compound parabolic concentrators (CPCs). This superiority stems from designing so as to accommodate the particular flux from the heliostat field. The edge-ray principle used for generating the new secondary dictates a heliostat tracking strategy different from the conventional one, but equally easy to implement.
The transduction and conversion of radiant energy into work in a quantum process are dependant on the luminescent properties of the materials involved. Materials with photoluminescent efficiencies greater than 0.1% are likely candidates for solar cells and solar converters. The luminescent optical properties of a material are directly related to the output device parameters. The chemical potential of the incoming light is a function of the photon energy and incident radiance. The amount of work per particle, or voltage, that can be extracted by a solar converter is related to chemical potential of the excitation, which can be inferred from the photoluminescence efficiency at ambient temperature. A discussion is made as to the use and optical properties of materials such as Si and GaAs, FeS_{2}, and biological and organic dyes as efficient solar quantum converter materials. Proper choice of absorber thickness as to maximize the luminescent output observed is shown to optimize solar converter performance.
Concentrators based on geometrical optics increase the irradiance by increasing the projected solid angle, but conserve the radiance of radiation. The general principle for increasing the radiance, and thereby concentrating even diffuse radiation, resembles a light trap. Light, which enters the trap through a selective filter, is shifted in photon energy, for example, by a Stokes luminescent process. It is subsequently trapped because it is reflected by the filter. Concentration is limited, in the ideal case, by the reverse (anti-Stokes) process, which reaches equilibrium when incoming and concentrated radiation reach equal chemical potential. The laser is discussed as an example for a concentration not limited by thermodynamics. The limits imposed by quantum mechanics are derived. Real systems, with various losses, are discussed.
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