One of the most interesting problems in the illumination research community is the design of optics able to generate prescribed intensity patterns with extended input sources. Such optics would be ideally applied to the current generation of extended, high-brightness, high-CRI LEDs used in general illumination, allowing reduced size of luminaires and improved efficiency. But in 3D, for non-symmetric configurations, how to design optics for prescribed intensity using extended sources remains an open question. We present an alternative approach to this problem, for the case of extended Lambertian sources, in which the design strategy is based on the definition of selected “edge wavefronts” of an illumination system. The extended emitter is represented by input wavefronts originating from selected points belonging to its edge; the prescribed intensity pattern, instead, is put in relationship with specific output edge wavefronts. The optic is calculated by requiring that it transforms the input edge wavefronts exactly into the output ones. This wavefront-matching procedure can be achieved, for example, with the Simultaneous Multiple Surfaces method (SMS). We show examples of freeform optics calculated according to the above procedure, which create non-rotationally symmetric irradiance patterns out of extended sources. A fine tuning of the output design wavefronts allows accurate control over the uniformity and extension of the output patterns, as well as on the definition of cut-offs and intensity gradients.
The Freeform RXI collimator is a remarkable example of advanced nonimaging device designed with the 3D Simultaneous Multiple Surface (SMS) Method. In the original design, two (the front refracting surface and the back mirror) of the three optical surfaces of the RXI are calculated simultaneously and one (the cavity surrounding the source) is fixed by the designer. As a result, the RXI perfectly couples two input wavefronts (coming from the edges of the extended LED source) with two output wavefronts (defining the output beam). This allows for LED lamps able to produce controlled intensity distributions, which can and have been successfully applied to demanding applications like high- and low-beams for Automotive Lighting.
Nevertheless, current trends in this field are moving towards smaller headlamps with more shape constraints driven by car design. We present an improved version of the 3D RXI in which also the cavity surface is computed during the design, so that there are three freeform surfaces calculated simultaneously and an additional degree of freedom for controlling the light emission: now the RXI can perfectly couple three input wavefronts with three output wavefronts. The enhanced control over ray beams allows for improved light homogeneity and better pattern definition.
In this work, an optical design approach is presented to design an ultrashort throw distance projection system by combination of an off-the-shelf refractive lens and two off-axis freeform mirrors. These two freeform mirrors are used to greatly shorten the projection distance by more than three times compared to conventional (rotationally symmetric) systems, while still maintaining a good imaging quality. Firstly, a direct design method that enables the simultaneous calculation of two off-axis freeform-profile mirrors by partially coupling more than three fields is introduced. The specifications of the conventional refractive lens are taken into account during this procedure. The pupil matching principle is applied to ensure good performance between the two sub-systems. The calculated mirrors then serve as a good starting point for optimization using commercial optical design software. To step from freeform profiles to freeform surfaces, the calculated two profiles are fitted into odd polynomials to evaluate the image quality and then re-fitted into XY polynomials for further optimization. Finally, the polynomial coefficients of the two freeform mirrors are imported into the optical design program. The merit function is built from RMS spot radii over the full field, and additional constraints are made for correcting distortion. After optimization, the calculated initial design quickly converges to a well performing imaging system. As an example, an ultrashort throw distance projection lens with a large 80-inch diagonal image at 400mm throw distance is designed, analyzed and compared with literature data. The values of MTF are over 0.6 at 0.5 lp/mm and the distortion is less than 1.5%: showing a very good and well balanced imaging performance over the entire field of view.
There is currently a desire to produce thinner LED backlights and frontlights so that the devices which use these components can be as thin and lightweight as possible. This is particularly true for smartphones and tablets both of which make extensive use of such components. The push for thinner devices may lead to situations in which the backlights are thinner than the height of the LED emitting area. This paper deals with the coupling of LEDs and thin light guides, describing some possible ways to efficiently inject light from a relatively large LED into a thinner backlight. These solutions use étendue-squeezing optics, and linear edges which allow high-efficiency light injection.
Axisymmetric aplanatic systems have been used in the past for solar concentrators and condensers (Gordon et. al, 2010).
It is well know that such a system must be stigmatic and satisfy the Abbe sine condition. This problem is well known
(Schwarzschild, 1905) to be solvable with two aspherics when the system has rotational symmetry.
However, some of those axisymmetric solutions have intrinsically shading losses when using mirrors, which can be
prevented if freeform optical surfaces are used (Benitez, 2007).
In this paper, we explore the design of freeform surfaces to obtain full aplanatic systems. Here we prove that a rigorous
solution to the general non-symmetric problem needs at least three free form surfaces, which are solutions of a system of
partial differential equations (PDE). We also present the PDEs for a three surface full aplanat. The examples considered
have one plane of symmetry, where a consistent 2D solution is used as boundary condition for the 3D problem. We have
used the x-y polynomial representations for all the surfaces, and the iterative algorithm formulated for solving the above
said PDE has shown very fast convergence.
Today’s SSL illumination market shows a clear trend towards high flux packages with higher efficiency and higher CRI,
realized by means of multiple color chips and phosphors. Such light sources require the optics to provide both near- and
far-field color mixing. This design problem is particularly challenging for collimated luminaries, since traditional
diffusers cannot be employed without enlarging the exit aperture and reducing brightness (so increasing étendue).
Furthermore, diffusers compromise the light output ratio (efficiency) of the lamps to which they are applied.
A solution, based on Köhler integration, consisting of a spherical cap comprising spherical microlenses on both its
interior and exterior sides was presented in 2012. When placed on top of an inhomogeneous multichip Lambertian LED,
this so-called Shell-Mixer creates a homogeneous (both spatially and angularly) virtual source, also Lambertian, where
the images of the chips merge. The virtual source is located at the same position with essentially the same size of the
original source. The diameter of this optics was 3 times that of the chip-array footprint.
In this work, we present a new version of the Shell-Mixer, based on the Edge Ray Principle, where neither the overall
shape of the cap nor the surfaces of the lenses are constrained to spheres or rotational Cartesian ovals. This new Shell-
Mixer is freeform, only twice as large as the original chip-array and equals the original model in terms of brightness,
color uniformity and efficiency.
Freeform optical surfaces have been in much demand recently due to improved techniques in their manufacturability and design methodology, and the degrees of freedom it gives the designers. Specifically in the case of off-axis mirror systems, freeform surfaces can considerably reduce the number of surfaces and compensate for some of the higher order aberrations as well, which improves the overall system performance. In this paper, we explore the design of freeform surfaces to obtain full aplanatic mirror systems, i.e., free of spherical aberration and circular coma of all orders. It is well know that such a system must be stigmatic and satisfy the Abbe sine condition. This problem is well known (Schwarzschild, 1905) to be solvable with two aspheric when the system has rotational symmetry. Here we prove that a rigorous solution to the general non-symmetric problem needs at least three free form surfaces, which are solutions of a system of partial differential equations. The examples considered have one plane of symmetry, where a consistent 2D solution is used as boundary condition for the 3D problem. We have used the x-y polynomial representations for all the surfaces used, and the iterative algorithm formulated for solving the above mentioned partial differential equations has shown very fast convergence.
Today’s SSL illumination market shows a clear trend to high flux packages with higher efficiency and higher CRI, realized by means of multiple color chips and phosphors. Such light sources require the optics to provide both near- and far-field color mixing. This design problem is particularly challenging for collimated luminaries, since traditional diffusers cannot be employed without enlarging the exit aperture and reducing brightness. Furthermore, diffusers compromise the light output ratio (efficiency) of the lamps to which they are applied. A solution, based on Köhler integration, consisting of a spherical cap comprising spherical microlenses on both its interior and exterior sides was presented in 2012. The diameter of this so-called Shell-Mixer was 3 times that of the chip array footprint. A new version of the Shell-Mixer, based on the Edge Ray Principle and conservation of etendue, where neither the outer shape of the cap nor the surfaces of the lenses are constrained to spheres or 2D Cartesian ovals will be shown in this work. The new shell is freeform, only twice as large as the original chip-array and equals the original model in terms of color uniformity, brightness and efficiency.
Axisymmetric aplanatic concentrators have been used in the past for solar concentrators and condensers (Gordon et. al, 2010). It is well know that such a system must be stigmatic and satisfy the Abbe sine condition. This problem is well known (Schwarzschild, 1905) to be solvable with two aspherics when the system has rotational symmetry. However, some of those axisymmetric solutions have intrinsically shading losses when using mirrors, which can be prevented if freeform optical surfaces are used (Benitez, 2007). In this paper, we explore the design of freeform surfaces to obtain full aplanatic systems. Here we prove that a rigorous solution to the general non-symmetric problem needs at least three free form surfaces, which are solutions of a system of partial differential equations (PDE). We also present the PDEs for a three surface full aplanat. The examples considered have one plane of symmetry, where a consistent 2D solution is used as boundary condition for the 3D problem. We have used the x-y polynomial representations for all the surfaces, and the iterative algorithm formulated for solving the above said PDE has shown very fast convergence.
High flux and high CRI may be achieved by combining different chips and/or phosphors. This, however, results in
inhomogeneous sources that, when combined with collimating optics, typically produce patterns with undesired artifacts.
These may be a combination of spatial, angular or color non-uniformities. In order to avoid these effects, there is a need
to mix the light source, both spatially and angularly. Diffusers can achieve this effect, but they also increase the etendue
(and reduce the brightness) of the resulting source, leading to optical systems of increased size and wider emission
angles.
The shell mixer is an optic comprised of many lenses on a shell covering the source. These lenses perform Kohler
integration to mix the emitted light, both spatially and angularly. Placing it on top of a multi-chip Lambertian light
source, the result is a highly homogeneous virtual source (i.e, spatially and angularly mixed), also Lambertian, which is
located in the same position with essentially the same size (so the average brightness is not increased). This virtual light
source can then be collimated using another optic, resulting in a homogeneous pattern without color separation.
Experimental measurements have shown optical efficiency of the shell of 94%, and highly homogeneous angular
intensity distribution of collimated beams, in good agreement with the ray-tracing simulations.
KEYWORDS: Solar concentrators, Prototyping, Electrical efficiency, Solar cells, Solar energy, Dispersion, Fresnel lenses, Temperature metrology, Sun, Optics manufacturing
Most cost-effective concentrated photovoltaics (CPV) systems are based on an optical train comprising two stages, the first being a Fresnel lens. Among them, the Fresnel-Köhler (FK) concentrator stands out owing to both performance and practical reasons. We describe the experimental measurements procedure for FK concentrator modules. This procedure includes three main types of measurements: electrical efficiency, acceptance angle, and irradiance uniformity at the solar cell plane. We have collected here the performance features of two different FK prototypes (ranging different f -numbers, concentration ratios, and cell sizes). The electrical efficiencies measured in both prototypes are high and fit well with the models, achieving values up to 32.7% (temperature corrected, and with no antireflective coating on SOE or POE surfaces) in the best case. The measured angular transmission curves show large acceptance angles, again perfectly matching the expected values [measured concentration acceptance product (CAP) values over 0.56]. The irradiance pattern on the cell (obtained with a digital camera) shows an almost perfectly uniform distribution, as predicted by raytrace simulations. All these excellent on-sun results confirm the FK concentrator as a potentially cost-effective solution for the CPV market.
In SSL general illumination, there is a clear trend to high flux packages with higher efficiency and higher CRI addressed with the use of multiple color chips and phosphors. However, such light sources require the optics provide color mixing, both in the near-field and far-field. This design problem is specially challenging for collimated luminaries, in which diffusers (which dramatically reduce the brightness) cannot be applied without enlarging the exit aperture too much. In this work we present first injection molded prototypes of a novel primary shell-shaped optics that have microlenses on both sides to provide Köhler integration. This shell is design so when it is placed on top of an inhomogeneous multichip Lambertian LED, creates a highly homogeneous virtual source (i.e, spatially and angularly mixed), also Lambertian, which is located in the same position with only small increment of the size (about 10-20%, so the average brightness is similar to the brightness of the source). This shell-mixer device is very versatile and permits now to use a lens or a reflector secondary optics to collimate the light as desired, without color separation effects. Experimental measurements have shown optical efficiency of the shell of 95%, and highly homogeneous angular intensity distribution of collimated beams, in good agreement with the ray-tracing simulations.
A new design for a photovoltaic concentrator, the most recent advance based on the Kohler concept, is presented. The
system is mirror-based, and with geometry that guaranties a maximum sunlight collection area (without shadows, like
those caused by secondary stages or receivers and heat-sinks in other mirror-based systems). Designed for a concentration of 1000x, this off axis system combines both good acceptance angle and good irradiance uniformity on the solar cell. The advanced performance features (concentration-acceptance products –CAP- about 0.73 and affordable peak and average irradiances) are achieved through the combination of four reflective folds combined
with four refractive surfaces, all of them free-form, performing Köhler integration 2. In Köhler devices, the irradiance uniformity is not achieved through additional optical stages (TIR prisms), thus no complex/expensive elements to manufacture are required. The rim angle and geometry are such that the secondary stage and receivers are hidden below the primary mirrors, so maximum collection is assured. The entire system was designed to allow loose assembly/alignment tolerances (through high acceptance angle) and to be manufactured using already well-developed methods for mass production, with high potential for low cost. The optical surfaces for Köhler integration, although with a quite different optical behavior, have approximately the same dimensions and can be manufactured with the same techniques as the more traditional secondary optical elements used for concentration (typically plastic injection molding or glass molding). This paper will show the main design features, along with realistic performance simulations considering all spectral characteristics of the elements involved.
Development of a novel HCPV nonimaging concentrator with high concentration (>500x) and built-in spectrum splitting
concept is presented. It uses the combination of a commercial concentration GaInP/GaInAs/Ge 3J cell and a
concentration Back-Point-Contact (BPC) silicon cell for efficient spectral utilization, and external confinement
techniques for recovering the 3J cell's reflection. The primary optical element (POE) is a flat Fresnel lens and the
secondary optical element (SOE) is a free-form RXI-type concentrator with a band-pass filter embedded in it - Both the
POE and SOE performing Köhler integration to produce light homogenization on the receiver. The band-pass filter
transmits the IR photons in the 900-1200 nm band to the silicon cell. A design target of an "equivalent" cell efficiency
~46% is predicted using commercial 39% 3J and 26% Si cells. A projected CPV module efficiency of greater than 38%
is achievable at a concentration level larger than 500X with a wide acceptance angle of ±1°. A first proof-of concept
receiver prototype has been manufactured using a simpler optical architecture (with a lower concentration, ~100x and
lower simulated added efficiency), and experimental measurements have shown up to 39.8% 4J receiver efficiency using
a 3J cell with a peak efficiency of 36.9%.
Here we present a novel optical design of the high concentration photovoltaics (HPCV) nonimaging concentrator
(>500x) with built-in spectrum splitting concept. The primary optical element (POE) is a flat Fresnel lens and the
secondary optical element (SOE) is a free-form RXI-type concentrator with a band-pass filter embedded in it, both POE
and SOE performing Köhler integration to produce light homogenization on the target. It uses the combination of a
commercial concentration GaInP/GaInAs/Ge 3J cell and a concentration Back-Point-Contact (BPC) silicon cell for
efficient spectral utilization, and external confinement techniques for recovering the 3J cell's reflection. Design targets
equivalent cell efficiency ~46% using commercial 39% 3J and 26% Si cells, and CPV module efficiency greater than
38%, achieved at a concentration level larger than 500X and wide acceptance angle (±1°). A first proof-of concept
receiver prototype has been manufactured using a simpler optical architecture (with a lower concentration, ~100x and
lower simulated added efficiency), and experimental measurements have shown up to 39.8% 4J receiver efficiency using
a 3J with peak efficiency of 36.9%.
While multichannel configurations are well established for non-imaging applications, they have not been used yet
for imaging applications. In this paper we present for the first time some of multichannel designs for imaging
systems. The multichannel comprises discontinuous optical sections which are called channels. The phase-space
representation of the bundle of rays going from the object to the image is discontinuous between channels. This
phase-space ray-bundle flow is divided in as many paths as channels there are but it is a single wavefront both at the
source and the target. Typically, these multichannel systems are at least formed by three optical surfaces: two of
them have discontinuities (either in the shape or in the shape derivative) while the last is a smooth one. Optical
surfaces discontinuities cause at the phase space the wave front split in separate paths. The number of discontinuities
is the same in the two first surfaces: Each channel is defined by the smooth surfaces in between discontinuities, so
the surfaces forming each separate channel are all smooth. Aplanatic multichannel designs are also shown and used
to explain the design procedure.
Concentration Photovoltaics (CPV) is one of the most promising areas for competitive solar electricity production. This
promise relies upon the use of high-efficiency triple-junction solar cells (which already have proven efficiencies over
41%) and upon advanced optics designs, which allow for high concentration concurrent with high manufacturing
tolerances, both key elements for low cost mass production.
In this paper we will present the progress in the development of the most advanced CPV optical designs at present. These
are based on free-form optics using Köhler homogenization. The degree of freedom of using free-forms allows the
introduction of multiple functionalities in a few optical elements, which provide the required concentration with high
tolerance and excellent light homogenization.
Different families are presented. The first group uses a Fresnel lens as a primary optic (called the FK concentrator and
the F-RXI concentrator) and a second group using mirrors as primaries (the XR and the XXR). How they compare
among them and also with classical designs will be discussed. The FK is in the process of being brought to market and
has already experimentally proven module electrical (DC) efficiencies over 30% (equivalent to over 32% with correction
to Tcell=25ºC) with no AR coatings at a concentration of 625x with high tolerance angle (over ±1.2º).
New ultra-thin optical designs are presented. They are formed by optical sections (called channels) working in parallel
(multichanneling) to provide the desired optical function. The phase-space representation of the bundle of rays going
from the source to the target is discontinuous between channels. This phase-space ray-bundle flow is divided in as many
branches as channels there are but it is a single trunk at the source and at the target. Typically, these multichannel
devices are at least formed by three optical surfaces: two of them have discontinuities (in the shape or in the shape
derivative) while the last one is smooth. The discontinuities of the optical surfaces are causing the separation of the flow
in branches (in the phase space). The number of discontinuities is the same in the two first surfaces: Each channel is
defined by the smooth surfaces in between discontinuities, so the surfaces forming each separate channel are all smooth.
No diffractive analysis is done.
The XR-Köhler concentrator1 is a design that has the possibility to work under high concentration, maintaining the high
acceptance angle and high irradiance uniformity on the solar cell. It is an on-axis free-form design that consists of a
reflective (X) and refractive (R) surface. For a geometrical concentration of about 800x the simulated results show an
acceptance angle of ±1.79deg with high irradiance uniformity on the solar cell. This article shows the design results of
the XR-Köhler and also a novel passive cooling system (LPI patented) that keeps the solar cell operation temperature
under 100°C at extreme conditions (wind speed = 0 m/s, module tilt angle = 45deg and Ta = 50°C). The results of using
the XR-Köhler device as a collimator when the light source has very high non-uniform luminance distribution, i.e.
multichip LEDs, are also here presented.
At module level (one single solar cell), the Fresnel-Köhler (FK) concentrator comprises a perfect irradiance uniformity
along with quite high concentration-acceptance angle product. At the same time, it maintains the efficiency/simplicity of
other Fresnel-based concentrators. In this work we will show the FK concentrator has loose manufacturing tolerances as
well. All these facts, along with the pill-box shape of its transmission curve, permit an enhanced performance of this
device, compared to its competitors, at array level, because the system is more insensitive to manufacturing errors, and
current mismatch is less likely to occur. Or the same performance can be achieved at a lower cost, exhausting the
tolerance budget by using inexpensive fabrication techniques. Depending on the concentrator, the actual power delivered
by an array might drop significantly with respect to the sum of the power delivered by single modules. Under certain
circumstances, the FK can reach a 1-10% electrical efficiency increase with regards to other concentrators sharing the
same technology.
It is essential to obtain high values of tolerance for CPV concentrators because manufacturing process always implies
some accuracy errors. This paper presents the Fresnel Köhler concentrator (FK), an advanced optical concentrator
comprising a Fresnel lens as a primary element and a refractive secondary element, both presenting free-form surfaces.
This optic produces both, the desired light concentration and high tolerance (i.e. high acceptance angle), as well as an
excellent light homogenization by Köhler integration simultaneously. A comparison between the FK and other current
conventional Fresnel-based CPV concentrators is also presented, being our concentrator superior to its competitors in
terms of tolerances, irradiance homogeneity and manufacturability.
New ultra-thin optical designs are presented that comprise discontinuous optical sections (called channels) working in
parallel (multichanneling) to provide the desired optical function. Aplanatic (a particular case of SMS-design)
multichannel designs are also shown and used to explain more easily the design procedure. Typically, these multichannel
devices are at least formed by three optical surfaces: one of them has discontinuities in the shape, a second one may have
discontinuities in its derivative while the third one is smooth. The number of discontinuities is the same in the two first
surfaces: Each channel is defined by the smooth surfaces in between the discontinuities, so that the surfaces forming
each separate channel are all smooth. No diffractive effects are considered.
LED light sources are finding ever increasing application in illumination. LEDs have many advantages, such as
high efficiency, long life, compactness, directional light emission, mechanical resistance, low-temperature
operation, light color control and low UV or IR emissions. These and other advantages make them very well
suited for general illumination applications as well as flashlights, car headlights, backlights, or frontlights. In most
applications, LEDs are combined with optics to direct their light output. Brighter LEDs have a smaller emission
area and, therefore, may be coupled to smaller optics. This is very important in many applications where
compactness is crucial, particularly automotive headlamps. When LED brightness is insufficient, it can be
augmented by recirculating part of the emitted light back to the LED's emitting surface. This increase in
brightness comes at the expense of a reduced flux-emission. As an example, the brightness of an LED with a
diffuse reflectivity of 70% may be increased by nearly that much if it is coupled to a high-efficiency recirculating
optic. Such augmentation, however, comes at the expense of a flux reduction, as much as 50%. Several optical
geometries are explored in this paper to achieve that recirculation together with raytracing results using on a
simple model of an LED. Also a number of optical architectures will be shown that escape the classical
nonimaging etendue limit associated with traditional optics.
Brightness enhancement of backlighting for displays is typically achieved via crossed micro prismatic films that are introduced between a backlight unit and a transmissive (LCD) display. Prismatic films let pass light only into a restricted angular range, while, in conjunction with other reflective elements below the backlight, all other light is recycled within the backlight unit, thereby increasing the backlight luminance. This design offers no free parameters to influence the resulting light distribution and suffers from insufficient stray light control. A novel strategy of light recycling is introduced, using a microlens array in conjunction with a hole array in a reflective surface, that can provide higher luminance, superior stray light control, and can be designed to meet almost any desired emission pattern. Similar strategies can be applied to mix light from different colored LEDs being mounted upside down to shine into a backlight unit.
A novel waveguide-optical integrator is introduced for applications to LEDs. The concept is based upon a Kohler illuminator made of Luneburg lenses. Typical Kohler illuminators are formed by pairs of thin lenses, and perform badly when the paraxial approximation is rough, i.e., when the angular span of the incoming rays is wide. In contrast, the new illuminator performs ideally for angular spans up to 90o (±45o), and has only a 3% loss for a 180o angular span. In general such an illuminator cannot be made in 3D, because adjacent Luneburg lenses overlap. It can, however, be implemented in planar optics, by using Rinehart geodesic lenses, which moreover do not use gradient index material. This waveguide device has application in illumination engineering as a light mixer, particularly for LEDs. Another light mixer using a combination of two kaleidoscopes with a geodesic lens is also presented. Irradiance at the exit of a kaleidoscope has good light mixing if the kaleidoscope is long enough, but the intensity is never well mixed, irrespective of the length. Inserting a Rinehart geodesic lens produces a 90-degree phase-space rotation of the rays, i.e., it exchanges irradiance and intensity. A further kaleidoscope assures complete mixing in both irradiance and intensity.
Light from several LEDs or other light sources may be combined using light guides shaped as manifolds. These manifolds are composed of smaller elements such as CPCs, angle transformers, angle rotators, light shifters, light guides or others. Although some components, such as CPCs or angle transformers, have all-optical surfaces, other devices may be designed with non-optical surfaces. These may be used to place the injection gate in the case of injection-molded optics, to attach handles or holders and other non-optical components to the manifold without affecting the optical performance. Also, in some of these devices, the geometry can be changed by simple changes in the position of the curves that compose the optic profile. These optics may be applied in efficiently combining light from several LEDs into one single large source, changing the aspect ratio of a light source or in distributing light from one (or more) sources onto several targets.
In LED projection displays, total lumen-output equals the source-luminance multiplied by the etendue of the spatial light modulator, the latter being a bottleneck that cannot be overcome. In addition, the luminance of existing LED sources is still too low for many projection-display applications. This has spurred research into finding ways to increase their brightness without a significant loss in efficacy. Current techniques to increase LED source-luminance include: (a) a photonic lattice atop the LED, which partially collimates the exiting light though lowering the efficacy (by Luminus), (b) recycling the LED light through the chip via TIR on a flat cover rather than a dome (several LED suppliers), and (c) a light-confining box with an exit aperture smaller than the chip (GoldenEye). All the above mentioned existing approaches achieve an increase in luminance for the LEDs at the expense of a considerable drop in efficacy. In this paper we present four novel and different ways (patent pending) to considerably enhance LED luminance and offering the possibility of having relatively high efficacy.
A novel waveguide-optical integrator is introduced for applications to LEDs. The concept is based upon a Kohler illuminator made of Luneburg lenses. Typical Kohler illuminators are formed by pairs of thin lenses, and perform badly when the paraxial approximation is rough, i.e., when the angular span of the incoming rays is wide. In contrast, the new illuminator performs ideally for angular spans up to 90° (±45°), and has only a 3% loss for a 180° angular span. In general such an illuminator cannot be made in 3D, because adjacent Luneburg lenses overlap. It can, however, be implemented in planar optics, by using Rinehart geodesic lenses. This waveguide device has application in illumination engineering as a light mixer, particularly for LEDs. Another light mixer using a combination of two kaleidoscopes with a geodesic lens is also presented. Irradiance at the exit of a kaleidoscope has good light mixing if the kaleidoscope is long enough, but the intensity is never well mixed, irrespective of the length. Inserting a Rinehart geodesic lens produces a 90-degree phase-space rotation of the rays, i.e., it exchanges irradiance and intensity. An further kaleidoscope assures complete mixing in both irradiance and intensity.
A novel LED light extraction and mixing optic and two free form SMS surfaces are employed in a high efficiency projection optic. By combining the light of several high brightness LEDs with a single optical element, an ultrabright light source can be formed, whose shape and emission characteristics can be adapted to almost many kinds of illumination problems. A LED combiner forms a virtual source that is tailored for application. The illuminance distribution of this virtual source facilitates the generation of the desired intensity pattern by projecting it into the far field. The projection is accomplished by one refractive and one reflective freeform surface calculated by the 3D SMS method. The method is demonstrated for an LED automotive headlamp. A high quality intensity pattern shape and a very sharp cutoff are created tolerant to LED to optics misalignment and illuminance variations across the LED surface. A low and high beam design with more than 75% total optical efficiency have been achieved.
Phosphor-conversion (PC) LEDs are the leading type of white solid-state lighting (SSL), due to the high efficacy of the yellow wavelengths of blue-stimulated photoluminescence. Conventional phosphor-conversion LEDs have the photoluminescent phosphor in immediate contact with the blue LED. Major types are the thin conformal phosphor and the thick or in-cup phosphor. The first trades away efficiency for increased luminance, while the latter gains efficiency at reduced luminance. In both cases the phosphor suffers from the elevated temperature of the blue chip, particularly the thermal quenching that reduces phosphor quantum efficiency. Also, the inevitable 15% Stokes heat of the phosphor conversion of blue light to longer-wave yellow light adds to the chip's heat load, as does much of a conformal phosphor's back-emission into the chip. It would be preferable to relocate the phosphor away from the chip illuminating it. Although remote phosphors have recently been showcased, their phosphor is much larger than the chip, greatly reducing luminance. A new design is presented of a Dual-Optic-based remote phosphor configuration with minimal increase in phosphor etendue over that of the source, as well as greatly improved spatial uniformity. Moreover, the yellow phosphor back-emission is recycled with a blue-pass mirror that re-illuminates the phosphor to increase its luminance. The result is a new white-light source with superior luminance, efficacy, and uniformity.
One of the most challenging applications for high brightness LEDs is in automotive headlights. Optical designs for a low or high beam headlights are plagued by the low flux and luminance of LEDs compared to HID or incandescent sources, by mechanical chip placement tolerances and by color and flux variations between different LEDs. Furthermore the creation of a sharp cutoff is very difficult without baffles or other lossy devices.
We present a novel LED headlight design that addresses all of the above problems by mixing the light of several LEDs first in a tailored light guide called LED combiner, thereby reducing color and flux variations between different LEDs and illuminance and color variations across the LED surfaces. The LED combiner forms a virtual source tailored to the application. The illuminance distribution of this virtual source facilitates the generation of the desired intensity pattern by projecting it into the far field. The projection is accomplished by one refractive and one reflective freeform surface calculated by the 3D SMS method. A high quality intensity pattern shape and a very sharp cutoff are created tolerant to LED to optics misalignment and illuminance variations across the LED surface.
A low and high beam design with more than 75% total optical efficiency (without cover lens) and performance as latest HID headlights have been achieved. Furthermore it is shown that the architecture has similar tolerance requirements as conventional mass produced headlights.
Nonimaging optics needs to address the interesting effects upon white-LED luminance of scattering within a photoluminescent phosphor, and how strong scattering leads to luminance recycling of TIR-trapped phosphor-emission. This paper analyzes LED optical systems that extract light by multiple internal reflections and varying degrees of bulk scattering. The luminance values of such devices can greatly exceed those predicted by the luminance-conservation law of etendue, formulated for non-scattering, non-recycling optical architectures. To illustrate this, the results of extensive modeling of LED architectures via a commercial raytracing package are described and analyzed. The analysis includes the effects of bulk scattering within the phosphor, and reveals the crucial role of diffuse reflectance, within the LED itself below its emitting layer. The study shows how it is possible to achieve an increase luminance in an LED via use of flat-windows over an LED as opposed to the traditional approach of dome-covers, albeit with some loss of overall luminosity extraction. The paper includes a discussion of luminance-luminosity tradeoffs and a summary of analytical and numerical methods for modeling optical systems involving bulk scattering.
A novel backlight concept suitable for LED's has been designed using the flow-line design method, which allows controlling both the illumination uniformity and light extraction without scattering the light. This contrasts with conventional LED backlight optical designs, which are based on the use of a light guide with Lambertian scattering features that break the guidance and extract the light. Since most of Lambertian scattered light is re-guided, the average ray path in conventional backlights is long and multiple bounces are needed, which may lead to low efficiency. On the other hand, the new design presented here is not only efficient but also provide a relatively high collimation of the output beam (an output beam within a 10 degrees half-angle cone, with total theoretical efficiency over 80% including Fresnel and absorption losses). Wider beams can be controlled by design or obtained by adding a holographic diffuser at the exit. The new design offers other very interesting practical features: it can be very thin, can be made transparent (which widens its applications, including front lighting), can mix the colors from several LED's or recover reflected polarization for LCD illumination.
An afocal system keeps parallel any two parallel rays emitted by a pair of point sources. If they have the same intensity at identical angles, this design will ensure that the far-field intensity will be higher than that of the bare sources as well as giving same the intensity to both, so that color balance is preserved. If these sources are of different color, it is possible to mix them in the far field while increasing the intensity in a prescribed way. If a third LED is placed at the midpoint between the other two, its intensity pattern will still be close to the one created for the other two souces. This enables the pixel to color-mix red, green and blue LEDs while increasing their apparent intensity and preserving the color-balance of the video signal across the far field. This enables large active-video screens to redirect and intensify their light towards the audience, instead of just spreading it out uselessly.
Conventional incandescent light bulbs have a wire filament acting as an extended light source with nearly constant intensity throughout its quasi-spherical emission pattern. Here we present a novel family of optical devices that make use of commercially available Lambertian or near-Lambertian LED light sources, in conjunction with tailored optical element bonded to the top surface of the LED. These hybrid devices can emulate the output of traditional incandescent filaments, or can be designed to produce a wide range of light output beam patterns. We call these new devices Virtual Filaments, as they can be designed to appear the same as an incandescent filament, with a similar light output pattern, and having a similar focal position above the base. These new lamps can then be used in the same applications as those they replace, thus eliminating the need to redesign or replace the original luminaire. We present several possible optical designs that can be used with a number of standard LEDs to replace standard incandescent bulbs. In one example we show a design that provides an output with near-uniform intensity across a full beam angle of 300 degrees, from a focal position 20 mm above an LED. Other major advantages of these new devices include their ability to be given sharp cutoffs, to homogenize non-uniform LED light sources and to color-mix the output of RGB LEDs.
The Simultaneous Multiple Surfaces design method (SMS), proprietary technology of Light Prescription Innovators (LPI), was developed in the early 1990's as a two dimensional method. The first embodiments had either linear or rotational symmetry and found applications in photovoltaic concentrators, illumination optics and optical communications. SMS designed devices perform close to the thermodynamic limit and are compact and simple; features that are especially beneficial in applications with today's high brightness LEDs. The method was extended to 3D "free form" geometries in 1999 that perfectly couple two incoming with two outgoing wavefronts. SMS 3D controls the light emitted by an extended light source much better than single free form surface designs, while reaching very high efficiencies. This has enabled the SMS method to be applied to automotive head lamps, one of the toughest lighting tasks in any application, where high efficiency and small size are required. This article will briefly review the characteristics of both the 2D and 3D methods and will present novel optical solutions that have been developed and manufactured to meet real world problems. These include various ultra compact LED collimators, solar concentrators and highly efficient LED low and high beam headlamp designs.
Simple optics composed of a spherical lens and a conic mirror are described and the relation between the radius of the lens and height of the cone on far field illuminance performance is analyzed for a fixed exit aperture dimension. Ray sets for real LEDs were used to simulate the performance of the hybrid optics and it is shown that there are combinations of values for the lens radius and cone height for which the optic produces an approximately constant illuminance pattern on a distant target. The effects of varying the lens radius while keeping the cone height constant, and of varying the cone height while keeping the lens radius constant, are also presented, as these variations result in beams of varying angular spread. It is shown that a relatively course two parameter optimization can find near optimum solutions, where the optimization is carried out using ray sets of commercially available LEDs and the merit function is constant illuminance.
The simultaneous multiple surface (SMS) method in 3-D geometry is presented. Given two orthotomic input ray bundles and another two orthotomic output ray bundles, the method provides an optical system with two free-form surfaces that deflects the rays of the input bundles into the rays of the corresponding output bundles and vice versa. In nonimaging applications, the method enables controlling the light emitted by an extended light source much better than single free-form-surface designs, and also enables the optics contour to be shaped without efficiency losses. The method is also expected to find applications in imaging optics.
The Simultaneous Multiple Surface (SMS) method in 3D geometry is presented. Giving two orthotomic input ray bundles and other two orthotomic output ray bundles, the method provides an optical system with two free-form surfaces that deflects the rays of the input bundles into the rays of the corresponding output bundles and vice versa. In nonimaging applications, the method allows controlling the light emitted by an extended light source much better than single free-form surfaces designs, and also enables the optics contour to be shaped without efficiency losses. The method is expected to find also applications in imaging optics
For reasons both fluid-dynamic and stylistic, volumetric constraints on vehicular luminaires grow more exacting. For full design-freedom of luminaire placement and shape, new designs are needed that have shallow depth and are capable of emitting a beam that makes a net angle with the local surface normal. Automotive headlamps, fog-lamps, and daylight-running lamps may need to project their illumination patterns onto the road from a position on sloped front surfaces. A conventional paraboloid, however, must be recessed behind a sloped window, thus using up space inside the vehicle-skin. A conventional TIR lens, with its output beam centered on its axis of circular symmetry, will also have to intrude into the vehicle interior, and shine through a sloped window. Instead, the luminaire should be thin enough to mount on a vehicle’s skin without needing a hole to be cut into it, a luminaire also capable of emitting its beam substantially off the local normal. To this end, two new TIR lenses are introduced here that generate off-normal beams. In one, a circular TIR lens takes on an internal tilt of its symmetry axis to produce a collimated output beam with high tilt, nearly 45° from the surface normal of the lens exterior. In the other, an off-axis linear TIR lens can be made with an internal tilt to the reflected rays. When used with LEDs, this new linear lens can be combined with exterior transverse lenslets, tailored to meet an intensity prescription.
A new application of design techniques for nonimaging concentrators is presented. Variable Geometry Nonimaging Optics Devices are presented for 2D systems. These devices have a variable acceptance angle allowing for its continuous variation within a range of values. The relation between the sizes of entrance and exit apertures must then vary according to the laws of nonimaging optics. This implies a variation in the size of either the entrance or the exit aperture, resulting in two types of devices. Moveable mirrors are an integral part of the new devices discussed in this paper, with positions depending on the variable acceptance angle. These new combinations of optical and mechanical solutions, although not ideal, may represent approximation ideally.
The notion of transporting concentrated solar energy radiation by flexible optical fibers or fiber bundles has been developed for a variety of uses. With the aim of CW pumping a laser crystal outside the focusing area of a primary parabolic mirror, an optical fiber bundle with a frustum-type output end was used to transmit and concentrate solar energy to a flux level high enough to pump a solid state laser. The transmission properties of a fiber optic frustum-type concentrator was first analyzed with the help of a ray-tracing program, which revealed strong influences of both output diameter and length on the transmission efficiency of a frustum concentrator. The idea of achieving an ideal angular transformer with fiber optic technology in the area of nonimaging optics was also proposed. The output section of each optical fiber was polished to form a hexagonal frustum. When seven of these polished frusta from the optical fibers were joined together, a novel solar energy concentrator was obtained. The output power from the concentrator end was 67 W, corresponding to the solar flux of 23 W/mm2. The experimental results of transporting and concentrating the solar radiation by using four fiber bundle with a square frustum output end was also reported. The maximum solar flux of 28 W/mm2 was obtained with a single optical fiber of conical output end.
New ideas for the production of optical devices capable of concentrating solar radiation to a degree useful for an application like solar frying of food, but still retaining the ability to remain stationary for extended periods, are presented and discussed.
Flexible optical fibers and fiber bundles can be used to transfer concentrated sunlight to a desirable place where it could be used to pump a solid state laser. One flexible fiber bundle was built. It consisted of seven optical fibers. The output section of optical fibers were polished to an hexagonal form. The bundle was placed at the focus of a primary parabolic mirror to capture the solar energy in the core-region of the focal spot. The radiation exiting the fibers was concentrated with a DCPC or with a long conical concentrator. An optical adhesive was utilized to bond the fiber bundle and the concentrator together. For non-contact type concentration, a DCPC was utilized and no index compensating liquid was necessary. A moderate optical flux of 13 W/mm2 was measured, with a large angular divergence as expected together with a non-homogeneous light distribution from the output end of the DCPC concentrator; both were certainly responsible for the unsuccessful attempts at pumping the laser. Hence a long conical concentrator was designed and built. Experimental results shown that both the incident ray acceptance capability and the output light quality are better than the DCPC. A solar flux of about 20 W/mm2 was obtained. Success at pumping the crystal laser can now be expected and will be reported elsewhere.
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