WO2007060565A2

WO2007060565A2 - Backlight of the scanning illumination type for flat panel displays

Info

Publication number: WO2007060565A2
Application number: PCT/IB2006/054147
Authority: WO
Inventors: Erik Boonekamp; Rachid Kherrazi; Albertus A. S. Sluijterman
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2005-11-22
Filing date: 2006-11-07
Publication date: 2007-05-31
Anticipated expiration: 2008-05-22
Also published as: WO2007060565A3

Abstract

A scanning illumination (backlight) system for a display, comprising a window (2) for emitting light in the direction of the display (3), a reflector (8), arranged parallel and opposite the window, a plurality of elongated light sources (6, 6', 6',...) arranged between to the window and the reflector (8), wherein each light source (6, 6', 6',...) is provided with a single reflective layer (7, 7', 7') at the side facing the window (2) which reflects back part of the light from the source to the reflector (8). The reflective layer (7, 7', 7') forms an elongated concave reflecting surface relative to the light source (6, 6', 6',.) and may or may not be in direct contact with the source. A (partly) reflective light barrier (9,9', 9', is arranged between a pair of neighbouring sources. The light barrier (partition) has a substantially rectangular shape and is substantially free from the absorption of light generated by the sources. The partitions reduce blur in video images by limiting the cross- talk between two neighbouring lamps.

Description

Scanning illumination system

FIELD OF THE INVENTION

The invention relates to a scanning illumination system for illuminating a display device.

The invention further relates to a display device comprising said scanning illumination system.

BACKGROUND OF THE INVENTION

Such a scanning illumination system is referred to as a so-called "direct-lit" backlight or "direct-under" type of backlight illumination system. The illumination systems are used, inter alia, as backlighting of (image) display devices, for example for television receivers and monitors. Such illumination systems can particularly suitably be used as a backlight 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. The illumination system is particularly suitable for application in large-screen LCD display devices for television and professional applications.

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 employs a control circuit. In particular, 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, hot-cathode fluorescent lamps, or external electrode fluorescent lamps are used as a light source, or alternatively light-emitting diodes (LEDs) may be employed as a light source in the illumination system. In mercury-vapor discharge lamps, mercury constitutes the primary component for the (efficient) generation of ultraviolet (UV) light. A luminescent layer comprising a luminescent material (for example, a fluorescent powder) may be present on the inner wall of the lamp envelope, also known as the discharge vessel, to convert UV to other wavelengths, for example, to UV-B and UV-A for tanning purposes (sun panel lamps) or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. In its simplest form, backlights for display devices comprise a number of tubular fluorescent lamps in a rectangular box. On the inside of the box, the walls are covered with a highly reflective (white) coating. The light-emission window is a diffuser or is covered with a diffuser through which light can escape from the box. In case of a relatively high lamp density (number of lamps per cm), the uniformity of the light output is normally sufficient. However, when the lamp density decreases, the uniformity of the backlight also decreases. In such cases the lamp tubes are readily "visible" through the light-emission window.

A known problem with an LCD-TV module is that moving images tend to exhibit motion artifacts as a result of blurring by the fact that the light modulators cannot instantly respond to changes in transmission levels. It has been shown that the use of a so- called scanning illumination system reduces the blurring problem. Scanning illumination systems typically comprise a plurality of light sources, arranged in a panel- like fashion. The scanning of the scanning illumination system is typically performed by switching the light sources on and off in accordance with video signals applied to the display device. The video signals include image data to be displayed and synchronization data which allow synchronous scanning of the display device to form an image. The light sources are addressed sequentially in accordance with the synchronization data, illuminating a group of light modulators typically after the required transmission levels in the light modulators have been reached. The degree of motion blur reduction is characterized by the effective duty cycle of the scanning illumination system. The effective duty cycle is a function of the electrical duty cycle and a part related to the optical cross-talk between two neighboring lamps. A lower effective duty cycle of the scanning illumination system results in an improved motion blur reduction.

The published international patent application WO2005/024502 Al discloses an illumination system for a display device. The illumination system comprises a light emission window, a reflector and a plurality of elongated fluorescent lamps positioned between the light emission window and the reflector. Each lamp is provided with a reflective layer between the light source and the light emission window, the reflective layer forming an elongated concave reflecting surface. The reflecting surface covering the light source has a covering angle within a certain range. The illumination system has a uniform light distribution at its light-emission window. A disadvantage of the known illumination system is that the optical cross-talk between the lamps is relatively large, resulting in a relatively large effective duty cycle and therefore relatively small motion blur reduction.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a scanning illumination system with an improved motion blur reduction while maintaining a uniform light distribution of the illumination system as well as a good optical efficiency of the illumination system. According to the invention, a scanning illumination system of the kind mentioned in the opening paragraph for this purpose comprises: a light-emission window for emitting light in the direction of the display device, a reflector for reflecting light, the reflector being arranged substantially parallel to and opposite the light emission window, - a plurality of elongated light sources arranged between the light-emission window and the reflector, each light source being provided with a reflective layer at the side facing the light emission window for reflecting part of the emitted light by the light source in the direction of the reflector, - the reflective layer forming an elongated concave reflecting surface relative to the light source, an elongated, at least partly reflective light barrier positioned between a pair of neighboring light sources, the light barrier having a substantially rectangular shape and being substantially free from adsorption of light generated by the light sources. Due to the light barriers of the scanning illumination system according to the invention, a more discrete stroke of light is generated by the light source, reducing the optical cross-talk between two neighboring light sources. The latter allows to reduce the effective duty cycle, improving the motion blur reduction. Direct light emitted by the light sources in the direction of the light-emission window is partly reflected towards the rear wall. The rear wall in combination with the light barriers acts as a mixing chamber for this partly reflected light. A uniform illumination at the light-emission window of the illumination system is obtained by properly tuning the reflectivity of the reflective layer of the light source for a given height of the light barrier. Compared to the known illumination system, a lower reflectivity of the reflective layer is required in order to have a uniform illumination at the light-emission window. Hence, more light generated by the light sources is used for direct lighting the light-emission window. Light that is not reflected at a side wall of the light barrier, is passed on through the light barrier, and exits the light barrier at its other side wall, allowing it to contribute to the total light output of the illumination system. As a result, a good optical efficiency of the illumination system is obtained.

It is noted that the unpublished international patent application

PCT/IB2005/052417 [attorney's docket PHNL040861], describes an illumination system for illuminating a display device. The illumination system comprises a light emission window, a reflector and a plurality of elongated fluorescent lamps positioned between the light emission window and the reflector. Each lamp is provided with two elongated light emission apertures for emitting light in the direction of the reflector. Between every pair of neighboring lamps a light barrier is positioned that is attached to the backside of the illumination system. However, it is not desccribed that the light barrier is substantially free from adsorption of light generated by the fluorescent lamps. It is noted that published patent application JP2004-22352 discloses a "direct- under" backlight device. The backlight device comprises a plurality of lamps arranged in parallel and a reflecting plate arranged on the back surface of the backlight device. A triangular-shaped reflector is positioned between two neighboring lamps. A disadvantage of the known illumination system is that light that is not reflected by the triangular-shaped reflector will enter this reflector element and will be absorbed to some extent by the reflector element as multiple reflections will give larger losses. The absorbed light is lost for illumination of the display device.

An embodiment of the scanning illumination system according to the invention is characterized in that that the illumination system has a height H being the distance between the light-emission window and the reflector, the light sources have a diameter d and are arranged at a pitch/? with respect to each other, the reflecting surface covers the light source over a covering angle φ, the covering angle φ being in the range of:

180° - 2 ■ arctan ²⁽-^{H~ d}^ < φ < 180°

P

wherein di is the distance between the center of the light source and the reflector. By properly selecting the number of light sources in the illumination system, represented by the pitch/?, the placement of the light sources with respect to the reflector, represented by the distance di between the center of the light source and the reflector, and by carefully constructing the shape and the size of the reflective layer relative to the light source, expressed by the range for the covering angle φ, the distribution of light over the light- emission window can be influenced such that a relatively high uniform illumination of the display device is achieved. A uniformity parameter can be defined, as disclosed in published international patent application WO2005/024502 Al, which, given the above-mentioned design parameters, shows a minimum. A minimal uniformity parameter is indicative of a relatively high uniformity of the light emitted by the illumination system according to the invention. A computer program, e.g. employing ray-tracing simulations, can be employed to find out what the best configuration is. Such a computer program can be given certain boundaries for certain parameters, for instance with respect to the height H of the illumination system.

An embodiment of the scanning illumination system according to the invention is characterized in that the reflectance of the side walls of the light barrier is larger than 90 %. For values of the reflectance within this range a good optical separation between two neighboring light sources is obtained.

An embodiment of the scanning illumination system according to the invention is characterized in that the height h of the light barrier, being the dimension of the light barrier along an axis perpendicular to the light emission window and the reflector, is within the range of:

wherein d is the diameter of the light source and di is the distance between the center of the light source and the reflector. For values of the height of the light barrier within this range it is possible to achieve both a relatively uniform illumination of a display device and a reduction of blurring effects during the display of moving images.

An embodiment of the scanning illumination system according to the invention is characterized in that the ratio of the pitch/? between two neighboring light sources and the diameter d of the light sources is within the range of:

l ≤ p / d ≤ 4,

further improving the uniformity of the light distribution of the light-emission window. The position of the light sources in the illumination system with respect to the light-emission window and the reflector plays an important role in obtaining a uniform light distribution at the light-emission window. To this end, an embodiment of the illumination system according to the invention is characterized in that the ratio of the distance di from the center of the light source to the reflector and the diameter d of the light source is within the range of:

0.5 < U₁ I d < 1.5.

The upper and lower boundaries are determined by geometrical constraints of the illumination system.

According to the invention, a display device comprises a scanning illumination system according to claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a cross-sectional view of an assembly of a scanning illumination system and a display device comprising an embodiment of the scanning illumination system in accordance with the invention.

Figure 2 shows a cross-sectional view of an assembly of a scanning illumination system and a display device comprising an alternative embodiment of the scanning illumination system in accordance with the invention.

The figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. In the figures, like reference numerals refer to like parts whenever possible.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figures 1 and 2 are both a diagrammatic, cross-sectional view of an assembly of a scanning illumination system and a display device comprising a first and a second embodiment of the scanning illumination system in accordance with the invention. The illumination system comprises a translucent light-emission window 2 for emitting light into the direction of the display device 3. To reduce direct visibility of the light sources in the illumination system, the light-emission window 2 is preferably manufactured from a glass or a synthetic resin that preferably scatters the light diffusively. Preferably, the light-emission window 2 comprises a diffusing layer for diffusing the light emitted by the illumination system. The diffusing layer homogenizes the light emitted by the light-emission window 2.

In Figures 1 and 2, reference numeral 3 diagrammatically denotes a liquid crystal display (LCD) panel positioned adjacent to the light-emission window 2. The assembly of the illumination system with the light sources 6, 6\ 6", ... and the LCD panel forms a display device for displaying, for example, (video) images.

The rear wall of the illumination system comprises a reflector 8 with a reflectance, preferably, higher than 97 %. The high reflectivity may also be obtained by coating the walls of the illumination system by suitable diffuse reflecting materials such as TiO₂ or AI2O3. Particularly suitable diffuse reflective materials are calcium halophosphate and/or calcium pyrophosphate. Such a reflective material is provided in the form of a paint in which a binder, for example a fluorine copolymer, for example THV (tetrafluoroethylene- hexafluoropropylene-vinylidene fluoride copolymer), is used, as well as a solvent (for example MIBK). Other additives may be added to the paint mixture, for example those which have improved flowing or mixing characteristics. In addition, the light absorption of visible light of the reflector 8 is very low, i.e. les than 3 %. In addition, a diffuse reflective material comprising calcium halophosphate and/or calcium pyrophosphate has substantially no color shift, i.e. such a material has a comparatively low wavelength dependence.

Preferably, the side walls of the illumination system are also provided with a similar, highly reflective coating. The rear wall with reflector 8 is arranged substantially parallel to and opposite to the light-emission window 2, the illumination system having a height H which is the distance between the light-emission window 2 and the reflector 8.

The illumination system comprises a plurality of elongated light sources 6, 6\ 6", .... arranged between the light-emission window 2 and the reflector 8, the light sources 6, 6\ 6", ... having a diameter d and being arranged with a pitch/? with respect to each other. The light sources 6, 6\ 6", ... are positioned at a distance di with respect to the rear wall with reflector 8. Preferably, the light sources 6, 6\ 6", ... comprise a low-pressure mercury vapor discharge light source or a plurality of parts of low-pressure mercury vapor discharge light sources. For example, the light sources 6, 6\ 6", .... comprise hot-cathode fluorescent lamps, cold-cathode fluorescent lamps or external electrode fluorescent lamps. Each light source 6, 6\ 6", ... in the illumination system is provided with a reflective layer 7, T, T\ ... for reflecting part of the light emitted by the light source 6, 6\ 6", ... in the direction of the reflector 8. The reflective layer 1, T , T\ ... forms an elongated concave reflecting surface relative to the light source 6, 6\ 6", ... Referring to Figure 1, the reflective layer 7, T , T\ ... is formed by applying a suitable reflective foil that is laminated directly on part of the light source 6, 6\ 6", ... In another embodiment the reflective layer 7, T, T\ ... is spray- coated or sputter-coated directly on the light source 6, 6\ 6", ... Referring to Figure 2, the reflective layers 7, T, T\ ... are separate entities from the light sources 6, 6\ 6", ... , shaped like "caps" and are preferably made of glass or Plexiglass. Alternatively, a non- absorbing perforated material can be used as reflective layer 1, T , T\ ... Referring again to Figure 1 and 2, preferably, the reflective layer 1, T, T\ ... comprises a specular reflective or diffuse reflective layer. Preferably, the reflective layer 7, T , T\ ... is substantially free from adsorption. The reflective layer 1, T , T\ ... partly covers the light source 6, 6\ 6", ... at a covering angle φ. The reflectivity of the reflective layer 1, T , T\ ... can be adapted. In a still further advantageous embodiment the reflective layer 7, T , T\ ... is provided with brightness enhancement means. In an embodiment of this brightness enhancement means, grooves are applied which are preferably oriented in the length direction of the light sources 6, 6\ 6", ... It may be advantageous for obtaining a further improved light distribution at the light-emission window 2 to provide the reflective layer 1, T , T\ ... with openings for emitting part of the light emitted by the light source 6, 6\ 6", ... in the direction of the light- emission window 2. This may be done by scraping, or by removal by means of a laser. The illumination system further comprises a plurality of elongated light barriers 9, 9\ 9" , .... which are arranged with a pitch p with respect to each other. Each elongated light barrier 9, 9\ 9", ... is positioned substantially in the middle between a pair of neighboring light sources 6, 6\ 6", ... The elongated light barriers 9, 9\ 9", ... have a substantially rectangular shape and are positioned substantially perpendicular to the rear wall with reflector 8. Preferably, the side walls of the light barriers 9, 9\ 9", ... have a reflectance of at least 90 %. Preferably, the light barriers 9, 9\ 9", ... comprise a polymer material, and have a thickness sufficient to ensure the required mechanical strength and to avoid thermal deformation during operation of the illumination system. The light barriers 9, 9\ 9", ... are substantially free from adsorption of light generated by the light sources 6, 6\ 6", ... In an advantageous embodiment, the light barriers 9, 9\ 9", ... consist of a thin plate of micro cellular polyethylene terephthalate (MCPET, Furukawa Electric Co., LTD, Japan), having a high reflectance (~ 98%) and substantially zero absorbance over a broad wavelength range in the visible light, with a thickness of approximately 1 mm. The height h of the light barriers 9, 9\ 9", ... is preferable in the range of:

h ≤ [d/2 + di]. Referring to Figures 1 and 2, the illumination system typically has a height H of 26 mm, a pitch/? of the light sources of 50 mm, a position di of the light sources relative to the reflector 8 of 11 mm, the diameter d of the lamps amounts to 16 mm, and the height h of the light barriers 9, 9\ 9", ... amounts to 19 mm. The thickness of the light barriers 9, 9\ 9", ... typically varies between 0.5 and 2 mm. However, different dimensions are possible for alternative embodiments.

In an alternative embodiment, the height h of the light barriers is comparable to the height H of the illumination system. For example, for an illumination system with a height Hof 26 mm, a pitch/? of the light sources of 50 mm, a position di of the light sources relative to the reflector 8 of 11 mm, the height h of the light barriers 9, 9\ 9", ... amounts to approximately 24 mm. In case the height of the light barrier is comparable to that of the illumination system, the optical cross-talk between two neighboring light sources is further decreased, compared to light barriers having a lower height, further improving motion blur reduction.

A uniform light distribution at the light emission window 2 of the illumination system is attained by proper tuning of the reflectance of the reflective layer 7, T , T\ ... as a function of the position of the lamps 6, 6\ 6", ... In particular, by properly selecting the number of light sources 6, 6\ 6", .. in the backlight, represented by the pitch/?, the place of the light sources 6, 6\ 6", ... with respect to the reflector 8, represented by the distance di, and by carefully constructing the shape and size of the reflective layer 7, T , 7", ... adjacent to the light source 6, 6\ 6", ..., expressed by the range for covering angle φ, the distribution of light over the light-emission window 2 can be influenced such that a relatively high uniform illumination of the display device 3 is achieved. A uniformity parameter can be defined which, given the above mentioned design parameters, shows a minimum, as disclosed in the published patent application WO 2005/024502 Al. A minimal uniformity parameter is indicative of a relatively high uniformity of the light emitted by the illumination system according to the invention. Applying the above-mentioned method, a range for the covering angle φ is derived, for which the uniformity parameter shows a minimum and therefore a relatively high uniformity of the light emitted by the illumination system is obtained. Preferably, the reflecting surface of the reflective layer 1, T , T\ ... covering the light source 6, 6\ 6", ... has a covering angle φ in the range of: 180° - 2 ^■ arctan ²^ ^{dγ )} ≤ φ < 180°

Preferably, the ratio of the pitch/? of the light source and the diameter d of the light sources is in the range of:

1 < p / d < 4.

The position of the light sources 6, 6\ 6", ... in the illumination system with respect to the light-emission window 2 and the reflector 8 plays an important role in obtaining a uniform light distribution at the light-emission window 2. In practice, it was found out that the light sources 6, 6\ 6", ... are, preferably, placed relatively close to the rear wall, with reflector 8 of the illumination system. Preferably, the ratio of the distance di from the center of the light source to the reflector and the diameter d of the light source is in the range of:

Referring to Figure 2, in an alternative embodiment, the elongated light source 6, 6\ 6", ... comprises an array of LEDs. The array of LEDs may comprise white light LEDs that emit substantially white light or may comprise a mixture of LEDs emitting different primary colors, for example, three different types of LEDs emitting the primary colors red, green and blue.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Scanning illumination system for illuminating a display device, the illumination system comprising: a light-emission window (2) for emitting light in the direction of the display device (3), - a reflector (8) for reflecting light, the reflector being arranged substantially parallel to and opposite the light emission window, a plurality of elongated light sources (6, 6\ 6", ...) arranged between the light-emission window (2) and the reflector (8), each light source (6, 6\ 6", ...) being provided with a reflective layer at the side facing the light emission window (2) for reflecting part of the emitted light by the light source in the direction of the reflector (8), the reflective layer forming an elongated concave reflecting surface relative to the light source (6, 6\ 6", ...), an elongated, at least partly reflective light barrier (9, 9\ 9", ...) positioned between a pair of neighboring light sources, the light barrier having a substantially rectangular shape and being substantially free from adsorption of light generated by the light sources.

2. Scanning illumination system according to claim 1, characterized in that: - the illumination system has a height H being the distance between the light- emission window (2) and the reflector (8), the light sources (6, 6\ 6", ...) have a diameter d and are arranged at a pitch/? with respect to each other, the reflecting surface covering the light source at a covering angle φ, the covering angle φ being in the range of:

180° - 2 ^■ arctan ²^ ^{dγ )} ≤ φ < 180° wherein di is the distance between the center of the light source (6, 6\ 6", ...) and the reflector (8).

3. Scanning illumination system according to claim 1 or 2, characterized in that the reflectance of the side walls of the light barrier (9, 9\ 9", ...) is larger than 90 %.

4. Scanning illumination system according to claim 1 or 2, characterized in that the height h of the light barrier, being the dimension of the light barrier (9, 9\ 9", ...) along an axis perpendicular to the light emission window and the reflector, is within the range of:

wherein d is the diameter of the light source and di is the distance between the center of the light source (6, 6\ 6", ...) and the reflector (8).

5. Scanning illumination system according to claim 1 or 2, characterized in that the ratio of the pitch/? between two neighboring light sources (6, 6\ 6", ...) and the diameter d of the light sources is within the range of:

1 < p / d < 4.

6. Scanning illumination system according to claim 1 or 2, characterized in that the ratio of the distance di from the center of the light source (6, 6\ 6", ...) to the reflector (8) and the diameter d of the light source is within the range of:

7. A display device comprising a scanning illumination system as claimed in claims 1 to 6.