Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – Plural light emitting devices
Reexamination Certificate
2002-05-06
2003-05-20
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
Plural light emitting devices
C257S079000, C257S080000, C257S081000, C250S227110
Reexamination Certificate
active
06566689
ABSTRACT:
The invention relates to an illumination system for illuminating a display device, which illumination system is provided with a light-emitting panel comprising
a front wall, a rear wall situated opposite said front wall, and edge areas between the front wall and the rear wall,
at least one of the edge areas of the panel being light-transmitting,
at least one light source being associated with the light-transmitting edge area, and
in operation, light originating from the light source being incident on the light-transmitting edge area and spreading in the panel.
The invention also relates to a display device comprising said illumination system.
Such illumination systems are known per se and are alternatively referred to as edge lighting systems. They are used, inter alia, as a backlighting of (image) display devices, for example for television receivers and monitors. Such illumination systems can particularly suitably be used as backlights for non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are used in (portable) computers or (cordless) telephones.
Said display devices generally include a substrate provided with a regular pattern of pixels, which are each driven by at least one electrode. In order to reproduce an image or a datagraphic representation in a relevant area of a (display) screen of the (image) display device, the display device uses a control circuit. In an LCD device, the light originating from the backlight is modulated by means of a switch or a modulator, while applying various types of liquid crystal effects. In addition, the display may be based on electrophoretic or electromechanical effects.
In the illumination systems mentioned in the opening paragraph, customarily a tubular low-pressure mercury vapor discharge lamp, for example one or more cold cathode fluorescent lamps (CCFL) are used as the light source, the light which is emitted, in operation, by the light source being coupled into the light-emitting panel which serves as an optical waveguide. This optical waveguide generally forms a comparatively thin and flat panel which is made, for example, of a synthetic resin material or glass, light being transported through the optical waveguide under the influence of (total) internal reflection.
Such an illumination system may also be provided with an alternative light source in the form of a plurality of optoelectronic elements, also referred to as electro-optic elements, such as electroluminescent elements, for example light-emitting diodes (LEDs). These light sources are generally provided in the proximity of, or are contiguous with, a light-transmitting edge area of the light-emitting panel, so that, in operation, light originating from the light source is incident on the light-transmitting edge area and spreads in the panel.
WO-A 99 53 236 discloses an illumination system which causes an LCD panel to be illuminated at different types of ambient light. For the light source use is made of incandescent lamps which couple light into a light-emitting panel, also referred to as light pipe. In the light-emitting panel, multiple reflections of light bring about a light distribution to illuminate the LCD panel.
An illumination system of the above-mentioned type has the disadvantage that the light distribution in the light-emitting panel, particularly at the edges of the light-emitting panel, is insufficiently uniform. As a result, the illumination uniformity of the display device is insufficient.
It is an object of the invention to completely or partly overcome the above-mentioned drawback.
In accordance with the invention, this object is achieved in that
the light source associated with the light-transmitting edge area of the light-emitting panel comprises clusters of at least three light-emitting diodes having a first, a second and a third light emission wavelength, which light emission wavelengths are different,
an imaginary mirror surface, which relates to the cluster, being situated in the center of each cluster,
the light-emitting diodes in the cluster being arranged in a direction perpendicular to the imaginary mirror surface, and
the light-emitting diodes being distributed over the cluster in such a manner that the spectral emissions of the light emitted by the light-emitting diodes are mirror symmetrical with respect to the imaginary mirror surface.
The LEDs of different colors cause undesirable color effects near the edges of the light-emitting panel. In such illumination systems, generally, clusters of LEDs are applied in the ratio R (red):G (green):B (blue)=1:1:1 or R:G:B=1:2:1, dependent upon the relative luminous flux emitted, in operation, by the LEDs. The use of a larger number of LEDs per cluster is possible, however, this has the disadvantage that homogeneously mixing the colors becomes gradually more difficult. In the case of a linear array of said LEDs, the pitch between the clusters of LEDs has a strong influence on the amount of space that is necessary before the light is sufficiently mixed to achieve the desired color uniformity. Given the above-mentioned known R:G:B ratios, the length necessary to sufficiently mix the light amounts to three to four times the pitch between the clusters of LEDs.
However, at said known ratios R:G:B=1:1:1 or 1:2:1, angular effects occur at the edges of the light-emitting panel. Particularly, undesirable color effects occur near the edges of the light-emitting panel, and the color point depends on the position on the light-emitting panel. The radiation pattern of the LEDs influences said angular effects.
The inventors have recognized that these angular effects are reduced substantially if the clusters of LEDs used are symmetrical as regards their light emission wavelength. The edge areas of the light-emitting panel, which extend transversely to the light-transmitting edge area, act as a (specular and/or diffuse) mirror for the light emitted by the LEDs. If the LEDs are arranged in the clusters in such a manner that they are symmetrical in terms of light emission wavelength, then the mirror effect of the edge areas of the light-emitting panel, which edge areas extend transversely to the light-transmitting edge area, is reduced substantially.
Arranging the LEDs in a cluster in such a manner that they are symmetrical in terms of light emission wavelength, implies that LEDs of the same light emission wavelength are situated on both sides of the imaginary mirror surface. If there is an odd number of LEDs per cluster, then the imaginary mirror surface intersects the middle LED in the cluster.
By virtue of the measure in accordance with the invention, the uniformity of the distribution of the light emitted by the illumination system is improved. As a result, a more uniform illumination of the (image) display device is obtained.
Preferably, the light source comprises symmetric clusters of blue, green and red light-emitting diodes. Blue, green and red are the primary colors used in (image) display devices. Preferably, LEDs are used having a high light output and a comparatively broad radiation pattern. Furthermore, in order to improve the mixing of light, preferably LEDs emitting light in pronounced lobes are used.
Preferably, each one of the light-emitting diodes has a luminous flux of at least 5 lm. LEDs having such a high output are alternatively referred to as LED power packages.
The smaller the number of LEDs per cluster, the more compact the illumination system can be designed.
A preferred embodiment of the illumination system is characterized in that
each one of the clusters comprises five light-emitting diodes,
one light-emitting diode having the first light emission wavelength being situated in the center of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the second light emission wavelength, and
each of said two light-emitting diodes being flanked, on the side facing away from the center of the cluster, by a light-emitting diode having the third light emission wavelength.
It is particularly pr
Harbers Gerard
Hoelen Christoph Gerard August
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