Proc. SPIE. 10940, Light-Emitting Devices, Materials, and Applications
KEYWORDS: Light sources, Light emitting diodes, Solar concentrators, Ceramics, Semiconductor lasers, Collimation, Projection systems, Light sources and illumination, High power diode lasers, Digital Light Processing, Laser systems engineering, Light
Although LEDs have penetrated successfully in many lighting domains, high brightness light source applications are still suffering from their limited luminance. High power LEDs are generally limited to less than 100 Mnit (10<sup>8</sup> lm/m<sup>2</sup>sr), while dedicated devices for projection may achieve pulsed peak luminance values up to 200 Mnit for phosphorconverted green. For high luminous flux applications with limited etendue, like in stage or architecture spot lighting or in front projection, in the beam only very modest luminance values can be achieved with LEDs compared to systems based on discharge lamps. In this paper we evaluate light engine concepts based on static luminescent converters pumped by blue laser diodes, and concepts based on luminescent concentrators pumped by blue LEDs. Both concepts break through the flux and brightness requirements for these applications by enabling luminance values that are a factor five to ten higher than what can be achieved with LEDs. With continuous wave irradiation of a 10 mm<sup>2</sup> static converter by multiple laser diodes, 47 klm yellow-green emission was achieved at 1.5 Gnit source luminance, or 40 klm @1.2 Gnit in a collimated beam. With yellow-green light concentrator modules, 16 klm yellow-green emission was achieved at 1.2 Gnit collimated beam luminance. Thermal conditions are much more relaxed in luminescent concentrator modules than for static laser diode (LD) pumped converter systems. The High Lumen Density (HLD) LED-based luminescent concentrator, with its advantage of scalability in both flux and luminance, enables breakthrough performance in projection systems and in a wide variety of other applications. Laser-pumped converters, on the other hand, easily scale in flux proportional to their source size at constant luminance. They show very high flux capability and comparable brightness, enabling scope extension with extremely high flux solid state light sources.
Tens of blue diode lasers are focused onto a ceramic phosphor to create a high-brightness high-lumen output light engine. Illumination uniformity and scattering properties of the phosphor impact the efficiency of the system.
A novel responsive lighting system is presented capable of lowering the color temperature of emitted light on dimming. It is based on a single white light emitting LED and a thermo-responsive scattering coating. The coated LED automatically emits light of lower correlated color temperature (CCT) when the power is reduced, while maintaining a chromaticity close to the black body curve. Existing systems all use multiple color LEDs, additional control circuitry and mixing optics. An optical ray tracing model can explain the experimental results.
An innovative way of modeling dual-phosphors-converted white mid-power LEDs is established and demonstrated. We use near-field chromatic luminance measurement data to select ray tracing modeling parameters in LightTools and propose some key parameters to predict characteristics of LEDs accurately. Based on our model, we can precisely predict the change in efficacy and in correlated colour temperature of embedding LEDs in optical materials.
Sub-micron diffraction gratings have been used for two LED illumination applications. One is to create a transparent see through luminaire which can be used to illuminate and read a paper document or e-book. A second is a light sensor that can be used in a feedback loop to control a multicolor LED lamp. Optical design and experimental proof-of-principle are presented.
A novel method is presented to inject the light of millimeter-sized high-brightness blue LEDs into light guides of submillimeter
thickness. Use is made of an interference filter that is designed to pass only those modes that will propagate in
the light guide by total internal reflection. Other modes are reflected back to the LED cavity and recycled, leading to an
With this method a collimator has been designed and made that is only 1mm thick, with a diameter of 6.5mm. It creates a
beam of 26deg Full Width at Half Maximum. Presently, collimators with these characteristics have a thickness of 10-20mm and a diameter of 20-30mm and require careful mounting and alignment. The new collimator contains a
4.5micron thick interference filter made of 54 layers of Nb<sub>2</sub>O<sub>5</sub> and SiO<sub>2</sub> layers. The filter is optically coupled to the LED
with Silicone adhesive which makes the configuration very robust. A cylindrical lightguide, tapered from 6.5mm to
2.5mm diameter and 1mm thick captures the light that passes the filter, folds the light path and redirects the beam.
Measurements on collimator prototypes show good agreement with the designed characteristics. This promising
approach enables much more compact collimators optics that offer material cost savings and design freedom.
A new approach is described to couple light from high-power blue LEDs (1x1mm<sup>2</sup>) into a thin large area light guide
using a dielectric interference multilayer as an angular filter. The goal is to achieve large area luminaires such as
backlight systems. The method overcomes the drawback of structuring holes or recessions in light guides when using a
system with side-emitting LEDs. Several new LED-masking filters have been designed to improve the polarization
dependency and coupling efficiency from a previous design that used a stack of inorganic bi-layers. Firstly, a polymeric
multilayer manages to increase the coupling efficiency from 52% to 69%. Secondly, a birefringent multilayer filter
enables to fully suppress the Brewster's angle effect and to realize identical behaviour for both s- and p- polarized light
at large angles. Thirdly, by adding layers of a third inorganic material to the original bi-layer stack with refractive index
in between the other two, the transmittance equality is improved and the coupling efficiency can reach up to 63%.
A novel angular filter is proposed for a large area planar collimating luminaire. A planar collimator of 2x18o cone angle
with scalable and simplified structure is demonstrated. It has a thickness of only 5mm. Compact collimators and planar
collimators have been analyzed with the ray tracing software LightTools. Measurements on this collimator filter and
optical designs match well with the predicted performance.
A new approach to thin film filter design, the so called saddle-point method of global optimization was explored and
showed promising initial results. A 40-layer collimator filter with a 30% lower merit function was obtained and is
A new method using a thin-film multilayer filter is described to couple light from high-power LEDs into a thin light
guide such as an LCD backlight. Light emitted below the critical angle is reflected back to the LED and recycled. Largeangle
emitted light passes the filter and is transported by total internal reflection in the light guide. The light guide can be
as thin as 0.3mm for an LED of 1x1mm<sup>2</sup>, and the best coupling efficiency is estimated to be around 80%. With this
approach, a backlight system can be greatly simplified but also compact collimators can be realized. In this paper the
optical design and testing of the filter is described, and a 1mm thick, 6.5mm diameter collimator is presented that emits
in a cone of 2×13°. Measurements on prototypes show good agreement with the designed characteristics.
To improve the optical efficiency and to reduce the number of optical components of LCD backlighting systems, two
types of polarized-light backlights have been made from micro-structured birefringent polymeric layers. One type uses
uniaxially oriented PEN and PET foils that have been structured by diamond-tool machining or by hot-embossing, and
subsequently laminated onto a flat PMMA light guide. The second type uses a liquid crystalline polymeric layer
laminated onto a micro-structured light guide. S-polarized light is preferentially extracted from the light guides. The
efficiency has been measured to be 1.6-1.7 times higher than for a conventional backlight. Costs, thickness and
complexity are decreased since no micro-prismatic brightness enhancement foils or reflective polarizer foils are needed.
A colour-separating backlight can be made by using a surface-relief grating as an outcoupling structure on top of a lightguide. By combining such a structure with a birefringent layer, a polarised colour-separating backlight can be realised. We discuss experiments and simulations on a prototype of such backlight structures, as well as directions how to optimise them. First optimised samples of gratings made by laser-interference lithography show promising results.
New designs are presented of backlight systems for transmissive and transflective LCD’s based on stretched PET films with a well-defined micro-structure, which emit highly collimated or diffuse and linearly polarized light with a high efficiency. Moreover, edge-lit waveguide systems are discussed equipped slanted phase gratings which combine a range of desirable features such as a high transparency in direct view, a direct emission of light at normal angles to the plane of the waveguide and a purely unidirectional out-coupling of light towards the LCD-side. Moreover, these illumination systems emit colored, linearly polarized light which should contribute significantly to the energy efficiency of transmissive, transflective and reflective LCD displays.