Recently, LED-based sources with high luminance (109 cd/m2 ) have been developed. These sources can be applied in projection systems, as well as in other applications requiring high luminance. The technology makes use of a transparent phosphor rod that is pumped by a multitude of blue LEDs. Most of the converted light is guided in the rod towards one of its small sides, where it is extracted using suitable extraction optics. The radiant conversion efficiency (blue flux to converted flux) is presently approaching 0.3. The causes for this limitation are discussed. The available phosphor materials emit light in various wavelength regions, ranging from green to yellow and red. These can be used in various light sources, e.g. for DLP and LCD projection.
Although the maximum brightness of LEDs has been increasing continuously during the past decade, their luminance is still far from what is required for multiple applications that still rely on the high brightness of discharge lamps. In particular for high brightness applications with limited étendue, e.g. front projection, only very modest luminance values in the beam can be achieved with LEDs compared to systems based on discharge lamps or lasers. With dedicated architectures, phosphor-converted green LEDs for projection may achieve luminance values up to 200-300 Mnit. In this paper we report on the progress made in the development of light engines based on an elongated luminescent concentrator pumped by blue LEDs. This concept has recently been introduced to the market as ColorSpark High Lumen Density LED technology. These sources outperform the maximum brightness of LEDs by multiple factors. In LED front projection, green LEDs are the main limiting factor. With our green modules, we now have achieved peak luminance values of 2 Gnit, enabling LED-based projection systems with over 4000 ANSI lm. Extension of this concept to yellow and red light sources is presented. The light source efficiency has been increased considerably, reaching 45-60 lm/W for green under practical application conditions. The module architecture, beam shaping, and performance characteristics are reviewed, as well as system aspects. The performance increase, spectral range extensions, beam-shaping flexibility, and cost reductions realized with the new module architecture enable a breakthrough in LED-based projection systems and in a wide variety of other high brightness applications.
The concept of an LED-based source with high lumen density is described. It contains a luminescent rod in which LED light is converted to light with a longer wavelength that is extracted from a small face of the rod. The fundamental limitations and possibilities are discussed, as well as the constituents needed. Results are shown for two realized high lumen density sources. A source with YAG:Ce as phosphor is extensively characterized and the results are compared to modeling results. A source with an optimized green emitting phosphor is used for projection. With 64 pump LEDs at 490 W peak electrical input and 50% duty cycle, a peak luminous flux of 18000 lm and a peak luminance of over 1000 cd/mm2 is obtained, with an efficacy of 37 peak lm/W.
Although LEDs have been introduced successfully in many general lighting applications during the past decade, high brightness light source applications are still suffering from the limited luminance of LEDs. High power LEDs are generally limited in luminance to ca 100 Mnit (108 lm/m2sr) or less, while dedicated devices for projection may achieve luminance values up to ca 300 Mnit with phosphor converted green. In particular for high luminous flux applications with limited étendue, like in front projection systems, only very modest luminous flux values in the beam can be achieved with LEDs compared to systems based on discharge lamps. In this paper we introduce a light engine concept based on a light converter rod pumped with blue LEDs that breaks through the étendue and brightness limits of LEDs, enabling LED light source luminance values that are more than 4 times higher than what can be achieved with LEDs so far. In LED front projection systems, green LEDs are the main limiting factor. With our green light emitting modules, peak luminance values well above 1.2 Gnit have been achieved, enabling doubling of the screen brightness of LED based DLP projection systems, and even more when this technology is applied to other colors as well. This light source concept, introduced as the ColorSpark High Lumen Density (HLD) LED technology, enables a breakthrough in the performance of LED-based light engines not only for projection, where >2700 ANSI lm was demonstrated, but for a wide variety of high brightness applications.
Secondary optics that allow for the integration of a light-emitting diode (LED)-based luminescent light source into various étendue-limited applications—such as projection systems—are investigated. Using both simulations and experiments, we have shown that the optical efficacy of the luminescent light source can be increased using a collimator. A thorough analysis of the influence of the collimator’s refractive index on the optical outcoupling and luminance is investigated and it is shown that it is most optimal to use a refractive index of 1.5. The optimal shape of the collimator is equal to that of a compound parabolic concentrator. Experimental results show that by using a collimator, we can improve the amount of outcoupled light with a factor of 1.8 up to 2.1 depending on the used optical configuration of the LED-based luminescent light source.
Plasmonic nanostructures are known to influence the emission of near-by emitters. They can enhance the absorption and modify the external quantum efficiency of the coupled system. To evaluate the possibility of using plasmonics to enhance the light emission of a phosphor-converted LED device and create an efficient directional light source, regular arrays of aluminium nanoparticles covered with a red dye layer are investigated. In arrays of aluminum nanocylinders with a diameter of ca 140 nm combined with a thin (650 nm) layer of luminescent material, very narrow resonances have been observed, which lead to large enhancement factors of up to 70 and 20 for excitation with a directional blue laser source and a lambertian LED respectively, in a small spectral range for particular angles. The measured resonances agree very well with finite-difference time-domain numerical simulations. These changes in the angular emission profile of the red dye as well as the spectral shape of its emission can help to optimize the efficacy of phosphor-converted LED modules and increase the amount of useable light in a certain angular cone. Using Fourier microscopy, large modifications of the angular emission profile as well as spectral shaping are observed for these plasmonic LED devices if compared to reference samples without plasmonic nanostructures.
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.
Compact and inexpensive solar concentrators can be designed by using transmission gratings that diffract incident
sunlight into a light guide. To this end a grating should have a small period and maintain a high diffraction
efficiency over a wide range of incident angles. We numerically study the angular dependence of the diffraction
efficiency of surface-relief gratings using Rigorous Coupled-Wave Analysis. It is shown how one can control the
angular acceptance of gratings by tuning the refractive index or the grating topology. Gratings with a high
refractive index maintain a high diffraction efficiency over a wide range of incident angles. By adjusting the
topological symmetry one can design a grating with a high diffraction efficiency over a narrow range of incident
angles, or a grating with a more homogenous distribution of the diffraction efficiency.
Organic polymeric chiral nematic liquid crystalline (cholesteric) wavelength selective mirrors can increase the efficiency
of luminescent solar concentrators (LSCs) when they are illuminated with direct sunlight normal to the device.
However, due to the angular dependence of the reflection band, at larger incidence angles the cholesterics reflect away
some incoming sunlight that could have been absorbed by the luminophore. As a result, the increase in LSC efficiency
after application of a cholesteric reflector drops if the light incident to the device is at angles larger than 30 degrees. The
cholesteric reflectors still have a positive impact on device performance for light incident up to 45-50 degrees but at
larger angles efficiency decreases when a cholesteric reflector is added. This affects the performance of the LSC device
when illuminated with indirect incident light, especially when the incident light has a large contribution of photons above
Transmission gratings that combine a large diffraction angle with a high diffraction efficiency and low angular
and wavelength dispersion can be used to concentrate sunlight in a light guide and for lighting applications.
Surface-relief gratings with sub-wavelength grating periods can have these properties. In this paper we study
their diffraction efficiency for general conical angles of incidence. We show the presence of regions in the space
of incident angles where light is efficiently coupled into or out of total internal reflection. It is demonstrated how
this distribution of the diffraction efficiency over angular space can be adjusted by changing the grating geometry.
Finally, these properties are qualitatively verified using holographically produced surface relief gratings.
In a Luminescent Solar Concentrator (LSC), short-wavelength light is converted by a luminescent material into longwavelength
light, which is guided towards a photovoltaic cell. In principle, an LSC allows for high concentration, but in
practice this is prevented by loss mechanisms like limited sunlight absorption, limited quantum efficiency and high self
absorption. To tackle these problems, a suitable luminescent material is needed. Another important loss mechanism is the
escape of luminescent radiation into directions that do not stay inside the light guide. To reduce this amount, wavelengthselective
filters can be applied that reflect the luminescent radiation back into the light guide while transmitting the
incident sunlight. In this paper, we discuss experiments and simulations of new luminescent and filter materials. We will
introduce a phosphor with close-to-optimal luminescent properties. A problem for use in an LSC is the large scattering of
this material; we will discuss possible solutions for this. Furthermore, we will discuss the use of broad-band cholesteric
filters in combination with this phosphor.
In a Luminescent Solar Concentrator, short-wavelength light is converted by a luminescent material into longwavelength
light, which is light guided towards a photovoltaic cell. In principle, a Luminescent Solar Concentrator
allows for high concentration, since the heat generated by the conversion process can be used to lower the entropy of
light. However, less controlled loss mechanisms prevent high concentration factors in practice. One important loss
mechanism is the escape of luminescent radiation into directions that do not stay inside the light guide. To reduce this
amount, wavelength-selective filters can be applied that reflect the luminescent radiation back into the light guide while
transmitting the incident sunlight. However, a filter optimized for reflecting as much as possible luminescent radiation
will reflect part of the incident sunlight at high angles. Depending on the luminophore properties, it may be possible to
design a suitable filter. In this paper, the interdependence of the luminophore and filter properties will be clarified and
quantified using simulations. Optimal luminophore-filter combinations will be discussed, as well as the feasibility to
realize them in practice.
We have developed a general ray-tracing method in the
geometrical-optics approach which enables the modeling of in general
inhomogeneous liquid crystal configurations. In this manuscript, we
discuss two prominent examples in which we calculate the optical
properties of two liquid crystal configurations. We first present
simulations of a liquid crystal-based electro-optical device that
enables a switching effect due to a back reflection phenomenon. In
these simulations, we exploit the optical properties of a liquid
crystal with a special Freedericksz alignment. Secondly we
present preliminary results of the optical properties of a liquid
crystal-based optical element that can actively control guiding and
extraction of light. A promising application of such a device can be
found in for example beam control devices for lighting applications
or applications that require local dimming and highlighting.
The use of an LCD equipped with lenticular lenses is an attractive route to achieve an autostereoscopic multi-view 3D
display without losing brightness. However, such a display suffers from a low spatial resolution since the pixels are
divided over various views. To overcome this problem we developed switchable displays, using LC-filled switchable
lenticulars. In this way it is possible to have a high-brightness 3D display capable to regain the full native 2D resolution
of the underlying LCD. We showed the feasibility of LC-filled switchable lenticulars in several applications. For
applications in which it is advantageous to be able to display 3D and 2D on the same screen, we made a prototype
having a matrix electrode structure. A problem with LC-filled lenses is that in the 2D state there is a residual lens effect
at oblique angles. This effect and a possible solution are discussed as well.
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.
We discuss residual lens effects in multi-view switchable auto-stereoscopic lenticular-based 2D/3D displays. With the introduction of a switchable lenticular, it is possible to switch between a 2D mode and a 3D mode. The 2D mode displays conventional content, whereas the 3D mode provides the sensation of depth to the viewer. The uniformity of a display in the 2D mode is quantified by the
quality parameter modulation depth. In order to reduce the modulation depth in the 2D mode, birefringent lens plates are investigated analytically and numerically, by ray tracing. We can conclude
that the modulation depth in the 2D mode can be substantially decreased by using birefringent lens plates with a perfect index match between lens material and lens plate. Birefringent lens plates do not disturb the 3D performance of a switchable 2D/3D display.
In many applications, the presence of domain walls limits the performance of liquid crystal displays (LCDs) in terms of brightness, contrast and response speed. Examples are found in wide-viewing-angle LCDs in which each pixel contains domains with different director orientations. Microscope measurements of various types of LCDs are presented and compared with the results of advanced two- and three-dimensional simulations. The background of the modeling programs is reviewed, especially if new methods are used. One example is that of a double-domain twisted-nematic (TN) LCD configuration that was made using photo-alignment. The shape of the domain wall and its effect on the transmitted intensity are described correctly by simulations. In another example it is shown that the experimental results for in-plane switching (IPS) structures can be understood with the help of advanced optical simulation methods that take into account diffraction effects. In a final example, the occurrence of domain walls in liquid crystal on silicon (LCoS) is discussed.