Henri Jagt

Linearly polarized light-emitting backlight

A new polarized backlight system for liquid-crystal displays (LCDs) is presented in which one linear polarization is preferentially coupled out by anisotropic scattering. The lightguide consists of a polymeric polarization-dependent scattering film adhered to a transparent polymeric substrate. By changing the scattering power of the film, the polarized light outcoupling angles can be influenced and optimized to achieve a maximum outcoupling centered along the normal direction. The other linear polarization is mainly trapped in the lightguide and is shown to be recycled to enhance the overall light and/or energy efficiency. With a proper substrate choice, the achieved local contrast exceeds 14 over a 50-mm range. A collimated light input further enhances the polarized contrast to well over 17.

45.3: Micro-structured Polymeric Linearly Polarized Light Emitting Lightguide for LCD Illumination

A new linearly polarized light emitting lightguide system is presented, consisting of a micro-structured anisotropic polymer film which is coated with an isotropic layer and adhered to a transparent polymeric substrate. With conventional edge-lighting of the lightguide very high polarized contrasts are realized, exceeding 100. A gain in efficiency can be achieved by recycling of the trapped light with the orthogonal polarization.

Efficient and cost-effective polarized-light backlights for LCDs

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.

16.2: A Fully Flexible Colour Display

A flexible version is shown of a state of the art passive matrix LCD; a full colour STN display. By using polymer substrates, “ultra-thin” optical layers (like retarders, colour filters and polarisers) and a flexible backlight a fully flexible CSTN demonstrator is obtained, that can be bend to a radius of 40 mm during operation.

STN-gels in fast passive matrix displays

Liquid crystalline physical gels have been prepared in a super twisted nematic configuration. These gels show a remarkable increase in switching speed from the in field to out of field relaxation. We explore whether these gels might be used in fast passive matrix displays.

High-brightness source based on luminescent concentration.

The concept of a high-luminance light source based on luminescent conversion of LED light and optical concentration in a transparent phosphor is explained. Experiments on a realized light source show that a luminous flux of 8500 lm and a luminance of 500 cd/mm2 can be attained using 56 pump LEDs at 330 W electrical input power. The measurement results are compared to optical simulations, showing that the experimental optical efficiency is slightly lower than expected. The present status enables applications like mid-segment digital projection using LED technology, whereas the concept is scalable to higher fluxes.

LED-based projection source based on luminescent concentration

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.

High lumen density sources based on LED-pumped phosphor rods: opportunities for performance improvement

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.

Polarized back- and frontlights for LCDs

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.