(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property Organization
`International Bureau
`
`(43) International Publication Date
`21 August 2008 (21.08.2008)
`
`(51) International Patent Classification:
`G09G 3/34 (2006.01)
`
`(21) International Application Number:
`PCT/IB2008/050427
`
`(22) International Filing Date: 6 February 2008 (06.02.2008)
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(30) Priority Data:
`071021984
`
`13 February 2007 (13022007)
`
`EP
`
`(71) Applicant (for all designated States except US): KONIN-
`KLIJKE PHILIPS ELECTRONICS N.V.
`[NLfNL];
`Groenewoudseweg 1, NL—5621 BA Eindhoven (NL).
`(72) Inventor; and
`(75) Inventor/Applicant (for US only): SLUYTERMAN, Al-
`bertus, A., S. [NL/NL]; c/o High Tech Campus Building
`44, NL—5656 AE Eindhoven (NL).
`
`(74) Agents: BEKKERS, Joost, J., J. et al.; High Tech Cam—
`pus Building 44, NL—5656 AE Eindhoven (NL).
`(81) Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`
` (10) International Publication Number
`
`WO 2008/099299 A1
`
`A0, AT, AU, AZ, BA, BB, BG, BH, BR, BW, BY, BZ, CA,
`CH, CN, CO, CR, CU, CZ, DE, DK, DM, DO, DZ, EC, EE,
`EG, ES, FI, GB, GD, GE, GII, GM, GT, IIN, IIR, IIU, ID,
`IL, IN, IS, JP, KE, KG, KM, KN, KP, KR, KZ, LA, LC,
`LK, LR, LS, LT, LU, LY, MA, MD, ME, MG, MK, MN,
`MW, MX, MY, MZ, NA, NG, NI, NO, NZ, OM, PG, PH,
`PL, PT, RO, RS, RU, SC, SD, SE, SG, SK, SL, SM, SV,
`SY, TJ, TM, TN, TR, TT, TZ, UA, UG, US, UZ, VC, VN,
`ZA, ZM, ZW.
`
`(84)
`
`Designated States (unless otherwise indicated, for ever
`kind of regional protection available): ARIPO (BW, G1 I,
`GM, KE, LS, MW, MZ, NA, SD, SL, SZ, TZ, UG, ZM,
`ZW), Eurasian (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM),
`European (AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, FI,
`FR, GB, GR, HR, HU, IE, IS, IT, LT, LU, LV, MC, MT, NL,
`NO, PL, PT, RO, SE, SI, SK, TR), OAPI (BF, BJ, CF, CG,
`CI, CM, GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG).
`Declaration under Rule 4.17:
`
`as to applicant’s entitlement to apply for and be granted a
`patent (Rule 4.17(ii))
`Published:
`
`with international search report
`
`[Continued on next page]
`
`(54) Title: DISPLAY DEVICE AND METHOD OF DISPLAYING AN IMAGE
`
`
`
`for reducing power consumption.
`
`(57) Abstract: The invention relates
`to adisplay device (10) for displaying
`an image (I) within a frame period (T).
`The display device comprises a plurality
`of pixels (20) and a backlighting unit
`(30) for illuminating the pixels (20).
`The display device further comprises a
`sub—image generator (42) for generating
`from the image (I) a first sub—image
`(11) displayed in a first
`sub—frame
`period (T1) of the frame period (T),
`and for generating a subsequent second
`sub—image (I2) being displayed in a
`second sub—frame period (T2). The first
`sub—image
`(11) defines
`transmission
`values of the plurality of pixels (20)
`in conjunction with a first backlight
`luminance (L1). The second sub—image
`(I2) defines transmission values of the
`plurality of pixels (20) in conjunction
`with a
`second backlight
`luminance
`(L2).
`The first backlight
`luminance
`is different from the second backlight
`luminance. Byusing different backlight
`luminances
`for displaying the
`first
`and second sub—images,
`the display
`device
`applies
`greyrfield
`insertion
`for improving motion fidelity and in
`addition dims
`the backlighting unit
`during the first or second subrimage
`
`
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`— before the expiration of the time limit for amending the
`claims and to be republished in the event of receipt of
`amendments
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`WO 2008/099299
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`PCT/IB2008/050427
`
`DISPLAY DEVICE AND METHOD OF DISPLAYING AN IMAGE
`
`l
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`FIELD OF THE INVENTION:
`
`The invention relates to a method of displaying an image on a display device
`
`within a frame period.
`
`The invention also relates to the display device, a backlighting unit for use in
`
`the display device and a computer program product.
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`BACKGROUND OF THE INVENTION:
`
`Pixelated display devices such as Liquid Crystal Display devices (further also
`
`referred to as LCD devices) are increasingly used for displaying motion pictures and TV
`
`signals. Fast moving objects within a motion picture displayed on an LCD device are a
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`challenge to an LCD device and often result in a perceived blurred image of the moved
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`object. This is caused by the sample-and-hold nature of the liquid crystal cells: during a
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`frame time (which is a time during which a single image of the motion picture is shown on
`
`the display device) the liquid crystal cells remain in the same state after being addressed. Due
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`to this sample-and-hold effect a smooth continuous motion is transformed into a step-wise
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`motion which is perceived by a viewer as a blurred image.
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`Several methods of displaying an image on a display device have been
`
`proposed to improve the blurred image of a moving object in the motion picture. One of the
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`known methods of improving the display of moving objects in the motion picture is the so-
`
`called grey-field insertion method. In this method the image is split into two subsequent sub-
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`images. Each of the sub-images is displayed within subsequent sub-frame periods of the
`
`frame period. Typically, a first sub-image shows the image at a maximum light intensity of
`
`the display device. In a subsequent second sub-image, typically, only the pixels are displayed
`
`of the image of which the intensity in the first sub-image was not sufficient to fully obtain the
`
`image. When the first sub-image and the second sub-image are overlaid, the original image is
`
`obtained. Due to the splitting of the image in two consecutive sub-images in which a major
`
`part of the image intensity is displayed in the first sub—image during part of the frame time,
`
`the sample—and—hold time of the display device is reduced and as a result, motion artifacts are
`
`also reduced.
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`A drawback of the known grey-field insertion method is that the power
`
`consumption of the display device is relatively high.
`
`SUMMARY OF THE INVENTION:
`
`It is an object of the invention to provide an improved grey—field insertion
`
`method to achieve reduced power consumption in the display device.
`
`According to a first aspect of the invention the object is achieved with a
`
`display device for displaying an image within a frame period, the display device comprising a
`
`plurality of pixels and a backlighting unit for illuminating the pixels, each pixel comprising a
`
`light transmission valve for controlling a transmission of the light emitted by the backlighting
`
`unit, the display device further comprising:
`
`-
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`a sub-image generator for generating from the image a first sub-image being
`
`displayed in a first sub-frame period of the frame period, and for generating a subsequent
`
`second sub-image being displayed in a second sub-frame period of the frame period, the first
`
`sub-image defining transmission values of the plurality of pixels in conjunction with a first
`
`backlight luminance, the second sub-image defining transmission values of the plurality of
`
`pixels in conjunction with a second backlight luminance, the first backlight luminance being
`
`different from the second backlight luminance, the display device being arranged for
`
`10
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`15
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`overlaying the first sub-image and the second sub-image, and
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`20
`
`—
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`a backlight signal generator for generating a first backlight signal for driving
`
`the backlighting unit to emit the first backlight luminance for displaying the first sub—image,
`
`and for generating a second backlight signal for driving the backlighting unit to emit the
`
`second backlight luminance for displaying the second sub—image.
`
`The effect of the display device according to the invention is that the first and
`
`25
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`30
`
`second sub-images are generated taking into account the first and second backlight
`
`luminance. The first and second backlight luminance are luminance values emitted from the
`
`backlighting unit during the displaying of the first and second sub-images, respectively. The
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`first and second backlight luminances are different. When using grey-field insertion for
`
`improving motion blur, a major part of the average pixel intensity of the image must be
`
`displayed in one of the sub-images which are displayed during a sub-frame period. In the
`
`display device according to the invention, the difference in average pixel intensity between
`
`the first and second sub-images is generated by both splitting the image into the first and
`
`second sub-image, and by using the first and second backlight luminance which are different.
`
`The luminance emitted by the backlighting unit in at least one of the sub-frame periods is
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`adapted, for example, by dimming the backlighting unit in accordance with the luminance
`
`required to display the sub-images. As a result, both motion blur and power consumption of
`
`the display device are reduced.
`
`In the known grey-field insertion method, the backlight luminance emitted by
`
`the backlighting unit is maintained constant for each sub—image to be displayed. The first
`
`sub—image shows the image at maximum average pixel intensity. The subsequent second sub—
`
`image only displays the pixels for which the transmitted pixel intensities in the first sub—
`
`image were not sufficient to obtain the image. So the average pixel intensity of the second
`
`sub-image is relatively low. This rclativcly low average pixel intensity in the second sub-
`
`image is achieved by tuning the light transmission of the individual pixels of the display
`
`device. As a result, much power is wasted when displaying the second sub-image in the
`
`known grey-field insertion method. In the display device according to the invention, the
`
`luminance of the backlighting unit is adapted for each of the sub-images to be displayed. The
`
`backlighting system is, for example, dimmed when displaying a sub-image having a
`
`relatively low average pixel intensity.
`
`The dimming of backlighting units to reduce power consumption is known per
`
`se. However, to optimally reduce power consumption, both sub-fields must substantially have
`
`identical average pixel intensities and must thus be substantially equally dimmed. This,
`
`however, would result in fast moving objects to be blurred. In the display device according to
`
`the invention, the backlight luminances of the first and second sub—images are different, for
`
`example, by dimming the backlighting unit when displaying the first or the second sub—
`
`image. Due to the difference in backlight luminance when displaying the first and second
`
`sub—images in the method according to the invention, motion blur is reduced while the
`
`dimming of the backlighting unit during the display of the first or the second sub-image
`
`results in a reduction of the power consumption.
`
`The reduction of the backlight luminance emitted during the first and/or
`
`second sub-image has the further benefit that the display device produces less stray light.
`
`Pixels in a liquid crystal display device typically leak some of the light through the pixel
`
`when a relatively high luminance must be blocked. This leakage of light causes stray light to
`
`be emitted from the display device. This stray light reduces the contrast of the image
`
`displayed on the display device. By reducing the luminance of the backlighting unit to a level
`
`required to display the first and/or second sub-images, the amount of stray light is reduced,
`
`which results in improved contrast.
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`4
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`An even filrther benefit when reducing the backlight luminance during the first
`
`and/or second sub-image is that the viewing angle of the display device is improved. The
`
`viewing angle of a liquid crystal display device is determined by a solid angle at which the
`
`light transmitted by the liquid crystal cells is emitted from the display device. Typically in
`
`liquid crystal cells, the solid angle depends on the level of transmission of the liquid crystal
`
`cell: when the transmission is relatively high, the viewing angle is larger than when the
`
`transmission of the liquid crystal cell is relatively low. When dimming the backlighting unit
`
`during displaying an image having a reduced average pixel intensity, the transmission of the
`
`liquid crystal cells have to be set at a higher transmission level to omit the same amount of
`
`light. Due to this increased transmission of the liquid crystal cells to compensate for the
`
`dimming of the backlighting unit, the viewing angle of the display device is increased.
`
`In an embodiment of the display device, the display device fithher comprises a
`
`pixel driver for driving the pixels of the display device to obtain the required transmission of
`
`the pixels for displaying the first sub-image in conjunction with the first backlight luminance,
`
`and for displaying the second sub-image in conjunction with the second backlight luminance.
`
`In an embodiment of the display device, the display device is arranged for
`
`displaying the first sub-image at a higher backlight luminance than the second sub-image.
`
`Alternatively, the second sub-image is displayed at a higher backlight luminance than the
`
`first sub-image. Which of the two sub-images is displayed at a higher backlight luminance
`
`depends, for example, on the type of display device or, for example, on the sequence of
`
`images to be displayed. The first sub—image of the current image may, for example, comprise
`
`the higher backlight luminance when the previous image of the current image also had a
`
`relatively high average pixel intensity. The first sub—image of the current image may, for
`
`example, comprise the lower backlight luminance when the previous image of the current
`
`image had a relatively low average pixel intensity.
`
`In an embodiment of the display device, the display device is arranged for
`
`displaying the first sub-image at a maximum backlight luminance. Alternatively, the display
`
`device is arranged for displaying the second sub-image at a maximum backlight luminance.
`
`A benefit of this embodiment is that, when displaying one of the first or second sub-images at
`
`the maximum backlight luminance, the difference between the first and second backlight
`
`luminances is also a maximum achievable difference. This maximum difference results in an
`
`optimal reduction of motion blur.
`
`In an embodiment of the display device, the backlighting unit comprises a
`
`plurality of light sources arranged for operating in a scanning mode of operation, and the first
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`WO 2008/099299
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`PCT/132008/050427
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`5
`
`backlight luminance comprises a plurality of first light-source luminances for illuminating
`
`the first sub-image in a scanning mode of operation, and the second backlight luminance
`
`comprises a plurality of second light-source luminances for illuminating the second sub-
`
`image in a scanning mode of operation, the plurality of first light-source luminances being
`
`different from the corresponding plurality of second light—source luminances.
`
`A scanning mode of operation is typically already used to improve the motion
`
`fidelity of the image displayed on the display device. In the current embodiment the scanning
`
`mode of operation is combined with the improved grey—field insertion according to the
`
`invention, which results in a further reduction of motion blur in the display device together
`
`with a reduction of power consumption.
`
`In an embodiment of the display device, each particular first light-source
`
`luminance in the plurality of first light-source luminances is adapted to a luminance required
`
`by a corresponding particular part of the first sub-image illuminated with the particular first
`
`light-source luminance. Alternatively, each particular second light-source luminance in the
`
`plurality of second light-source luminances is adapted to a luminance required by a
`
`corresponding particular part of the second sub-image illuminated with the particular second
`
`light-source luminance. A benefit of this embodiment is that the illumination of a particular
`
`part of the image is adapted to what is required by, for example, the content of the particular
`
`part of the image. This results in a substantially optimal illumination of the different parts of
`
`the display device. Each first light—source luminance within the first backlight luminance may
`
`be different depending on the content of the first sub—image. In addition, each second light—
`
`source luminance within the second backlight luminance may be different depending on the
`
`content of the second sub—image.
`
`In an embodiment of the display device, the display device is arranged for
`
`adapting the luminance emitted by the backlighting unit in the sub-frame period by adapting
`
`a length of a light pulse emitted by the backlighting unit. This results in the first backlight
`
`signal and/or the second backlight signal being a pulse width-modulated signal.
`
`In an embodiment of the display device, the display device is arranged for
`
`adapting the luminance emitted by the backlighting unit in the sub-frame period by adapting
`
`an intensity of a light pulse emitted by the backlighting unit.
`
`According to a second aspect of the invention, the object is achieved with a
`
`backlighting unit for use in the display device according to the invention, the backlighting
`
`unit being a dimmable backlighting unit arranged for adapting the luminance of the light
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`WO 2008/099299
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`6
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`emitted by the backlighting unit for every sub-frame period in response to the first and/or
`
`second backlight signal.
`
`In an embodiment of the backlighting unit, the backlighting unit comprises an
`
`array of light sources arranged for operating in a scanning mode of operation, the
`
`backlighting unit being arranged for adapting the luminance of each particular light source in
`
`the array of light sources. A benefit of this embodiment is that the backlighting unit enables a
`
`combination of scanning mode of operation and improved grey—field insertion. This results in
`
`a further improvement of the motion fidelity of the display device.
`
`According to a third aspcct of thc invcntion thc obj cct is achicvcd with a
`
`method of displaying an image on a display device Within a frame period, the display device
`
`comprising a plurality of pixels and a backlighting unit for illuminating the pixels, each pixel
`
`comprising a light transmission valve for controlling a transmission of the light emitted by
`
`the backlighting unit, the method comprising the steps of:
`
`-
`
`generating from the image a first sub-image being displayed in a first sub-frame
`
`period of the frame period, and a subsequent second sub-image being displayed in a second
`
`sub-frame period of the frame period, the first sub-image defining transmission values of the
`
`plurality of pixels in conjunction With a first backlight luminance, the second sub-image
`
`defining transmission values of the plurality of pixels in conjunction with a second backlight
`
`luminance, the first backlight luminance being different from the second backlight
`
`luminance, the display device being arranged for overlaying the first sub—image and the
`
`second sub—image, and
`
`—
`
`generating a first backlight signal for driving the backlighting unit to emit the
`
`first backlight luminance for displaying the first sub—image, and generating a second
`
`backlight signal for driving the backlighting unit to emit the second backlight luminance for
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`displaying the second sub-image.
`
`According to a fourth aspect of the invention the object is achieved with a
`
`computer program product comprising instructions for causing a processor to execute the
`
`method of displaying an image on a display device according to the invention.
`
`According to a fifth aspect of the invention the object is achieved With a video
`
`30
`
`stream being adapted according to the method of displaying an image on a display device
`
`according to the invention. The video stream comprises the first and second sub-images and
`
`the first and second backlight luminances.
`
`BRIEF DESCRIPTION OF THE DRAWINGS:
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`WO 2008/099299
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`PCT/132008/050427
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`7
`
`These and other aspects of the invention are apparent from and will be
`
`elucidated with reference to the embodiments described hereinafter.
`
`In the drawings:
`
`Fig. 1A shows a schematic front View of a display device according to the
`
`invention, and Fig. 1B shows a cross—sectional view of a display device according to the
`
`invention,
`
`Fig. 2 schematically shows the splitting of an image into a first and second
`
`sub—image, and
`
`Figs. 3A and 3B show a luminous output of thc backlighting unit during the
`
`sub-frame periods of the period, Fig. 3A showing a variation of the luminous output, using a
`
`length of the light pulse, and Fig. 3B showing a variation of the luminous output, using an
`
`intensity ofthe light pulse.
`
`The Figures are purely diagrammatic and not drawn to scale. Particularly for
`
`clarity, some dimensions are exaggerated strongly. Similar components in the Figures are
`
`denoted by the same reference numerals as much as possible.
`
`DETAILED DESCRIPTION OF EMBODIMENTS:
`
`Fig. 1A shows a schematic front View of a display device 10 according to the
`
`invention, and Fig. 1B shows a schematic cross-sectional view of a display device 10
`
`according to the invention. The display device 10 comprises a plurality of pixels 20 and a
`
`backlighting unit 30. The plurality of pixels 20 are arranged substantially parallel to the
`
`backlighting unit 30 and substantially cover the backlighting unit 30. In Fig. 1A only a few
`
`pixels 20 of the plurality of pixels 20 are shown for clarity reasons. The pixels 20 each
`
`comprise a light transmission valve (not shown) which controls a transmission of the light
`
`emitted by the backlighting unit 30 by each of the pixels 20. An image I (see Fig. 2) can be
`
`displayed on the display device 10 by adapting the transmission of each of the pixels 20 of
`
`the plurality ofpixels 20. Typically, each pixel 20 comprises a plurality of sub-pixels (not
`
`shown), and each sub-pixel of the pixel 20 comprises a color filter (not shown) such that
`
`color images can be displayed on the display device 10. The display device 10 further
`
`comprises a processor 40 for receiving and processing a video signal V. The video signal V
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`typically comprises a sequence of images. Each image I in the sequence of images must be
`
`displayed on the display device 10 in one frame period T (see Fig. 3). The processor 40
`
`extracts the individual images I from the video signal V and subsequently processes each
`
`image I such that the image I can be displayed on the display device 10. The processor 40 of
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`the display device 10 according to the invention comprises a sub-image generator 42 for
`
`splitting each image I into a first sub-image 11 (see also Fig. 2) and a second sub-image I2
`
`(see also Fig. 2). The first sub-image 11 is displayed in a first sub-frame period T1 (see Fig.
`
`3) and the second sub-image 12 is displayed in a second sub-frame period T2 (see Fig. 3).
`
`Said first and second sub—frame period T1, T2 are part of the frame period T. The first and
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`second sub—images Il , 12 are split such that the first sub—image 11 is displayed using a first
`
`backlight luminance L1 (see also Fig. 2) in the first sub—frame period T1, and the second sub—
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`image I2 is displayed using a second backlight luminance L2 (see also Fig. 2). The first and
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`second backlight luminance L1, L2 are luminances emitted by the backlighting unit 30 during
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`the displaying of the first and second sub-images 11, 12, respectively. A pixel driver 44
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`receives the first and second sub-images 11, I2 and converts the first and second sub-images
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`I1, I2 into transmission values of the plurality ofpixels 20 of the display device 10. A
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`backlight signal generator 46 receives the first and second backlight luminances L1, L2 and
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`generates a first and second backlight signal for driving the backlighting unit 30 to emit the
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`first and second backlight luminances L1, L2. The sub-image generator 42 splits the image I
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`into the first and second sub-images Il , I2 such that the first and second sub-images 11, I2 are
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`illuminated by different backlight luminances L1, L2.
`
`The effect of the splitting of the image I into the first and second sub-images
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`11, I2, which first and second sub-images Il , 12 comprise a different average pixel intensity,
`
`improves the motion fidelity of the display device and is known as the grey—field insertion
`
`method. In the display device according to the invention the splitting of the image I is done
`
`such that different first and second backlight luminances L1, L2 are required for displaying
`
`the first and second sub—images 11, I2. The backlight luminance L1, L2 of the sub—image Il ,
`
`12 having the reduced average pixel intensity may be dimmed compared to the backlight
`
`luminance L1, L2 of the sub-image 11, 12 having the higher average pixel intensity. As a
`
`result the backlighting unit 30 may be dimmed during at least part of the frame period T,
`
`being the part ofthe frame period T in which the sub-image I1, I2 which has the lower
`
`average pixel intensity is displayed. This dimming of the backlighting unit 30 during part of
`
`the frame period T results in a reduction of the power consumption of the display device 10
`
`while improving the motion fidelity of the display device 10.
`
`In the cross-sectional view of the display device 10 as shown in Fig. 1B, the
`
`backlighting unit 30 comprises an array of light sources S1 , S2, ..., Sn for illuminating the
`
`pixels 20. The array of light sources S1, S2, ..., Sn may, in operation, be driven in a
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`continuous mode of operation, or may, in operation, be driven in a scanning mode of
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`operation. In the scanning mode of operation, different parts of the display device 10 are
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`illuminated by the different light sources S1, S2, ..., Sn sequentially. The backlighting unit
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`30 according to the invention is arranged to adapt the luminance of each individual light
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`source S1; S2; ...; Sn in the array of light sources S1, S2, ..., Sn during each of the sub-frame
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`periods T1, T2. As a result, the backlight luminance of the different parts during the first sub—
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`frame period T1 is set to be different from the corresponding backlight luminance of the
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`different parts during the second sub—frame period T2. This arrangement allows the different
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`parts of the image I to be illuminated at a different luminance, for example, depending on the
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`content of thc diffcrcnt parts of the image I. Furthcrmorc, this arrangement allows a particular
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`part of the image to be illuminated with a different luminance during the first sub-period T1
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`and during the second sub-period T2, resulting in applying the improved grey-field insertion
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`method to the image I displayed on the display device, and hence in improved motion fidelity
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`of the display device 10 while reducing the power consumption of the display device 10.
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`Alternatively, the array of light sources S1, S2, ..., Sn may be a two-
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`dimensional array of dimrnable light sources (not shown).
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`The backlighting unit 30 may comprise substantially any type of light source
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`S 1, S2, ..., Sn for illuminating the pixels 20 of the display device 10, such as Cold Cathode
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`Fluorescent Lamps (also known as CCFL), Light Emitting Diodes (also known as LEDs), or
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`laser light sources. The variation of the backlight luminance L1, L2 may be obtained by
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`changing a width of a light pulse generated by the backlighting unit 30 in the sub—frame
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`period T1, T2 (also known as pulse width modulation of the light output, or PWM
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`modulation). Alternatively, the variation of the backlight luminance L1, L2 may be obtained
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`by changing an intensity of the light pulse emitted by the backlighting unit 30 in a sub—frame
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`period T1, T2.
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`Fig. 2 schematically shows the splitting of an image 1 into a first and a second
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`sub-image Il , I2. The image I, for example, shows an image of a sunset near water. In such a
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`picture, the sun, a reflection of the sun in the water and the sky are relatively bright compared
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`to the remainder of the picture. In the example shown in Fig. 2, the image I is split into the
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`first sub-image 11 being the sub-image having the relatively high average pixel intensity, and
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`into the second sub-image I2, being the sub-image having the relatively low average pixel
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`intensity. Consequently, the second backlight luminance L2 corresponding to the second sub-
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`image 12 may be chosen to be lower than the first backlight luminance L1 corresponding to
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`the first sub-image 11. In the embodiment shown in Fig. 2, the first backlight luminance L1 is
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`constituted of a plurality of first light-source luminances LlSl, L1S2, ..., L2Sn and the
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`second backlight luminance L2 is constituted of a plurality of second light-source luminances
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`L281, L282,
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`L28n. The first light-source luminances L181, L182, ..., L28n are
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`substantially equal, while the second light-source luminance L281, L282, ..., L28n are
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`dimmed corresponding to the image requirements of the second sub-image 12. When
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`displaying the second sub—image 12, using a dimmed backlighting unit 30, the transmission
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`values of the pixel 20 in the second sub—image 12 must be adapted to the dimming level such
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`that still the required light intensity is emitted by each of the pixels 20. During the splitting of
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`the image I into the first and second sub—image II, I2, the dimming of the backlighting unit
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`30 must bc takcn into account whcn dctcrmining thc transmission values of thc individual
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`pixels 20 of the first and second sub-images 11, 12. This is done by the sub-image generator
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`42 of the display device 10. The subsequent overlaying of the first sub-image Il illuminated
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`with the first backlight luminance L1 and the second sub-image I2 illuminated with the
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`second backlight luminance L2, results in the image I.
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`For example, the first sub-image 11 is illuminated at a maximum backlight
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`luminance of the backlighting unit 30 and the second sub-image I2 is illuminated at a
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`dimmed backlight luminance of the backlighting unit 30. By illuminating one of the first or
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`second sub-images at the maximum backlight luminance of the backlighting unit 30, a
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`maximum difference between the first and second sub-image can be obtained, resulting in a
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`maximum improvement of the motion fidelity.
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`Alternatively, the image I may be split into more than two sub—images (not
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`shown) displayed by the display device in corresponding sub—frame periods. Each of the sub—
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`images defines transmission values of the plurality of pixels 20 in conjunction with a
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`corresponding backlight luminance L1, L2. At least one of the backlight luminance values is
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`different from the other backlight luminance values. The sequential overlaying of the sub-
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`images results in the image 1.
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`Figs. 3A and 3B show a luminous output of the backlighting unit 30 during the
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`sub-frame periods Tl , T2 ofthe frame period T. Fig. 3A shows a variation ofthe luminous
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`output, using a variation of a length w2 of the light pulse emitted by the backlighting unit 30.
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`In the embodiment shown in Fig. 3A the first backlight luminance L1 is the substantially
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`30
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`maximum luminance of the backlighting unit 30 while the second backlight luminance L2 is
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`a dimmed backlight luminance of the backlighting unit 30. The dimming of the backlighting
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`unit 30 in Fig. 3A is obtained by reducing the length of the light pulse emitted by the
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`backlighting unit 30 in the second sub-frame period T2. A sum of the backlight luminance
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`Ll, L2 displayed during the first and second sub-frame periods T1, T2 results in the required
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`luminance for displaying the image I.
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`Fig. 3B shows a variation of the luminous output, using a variation of an
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`intensity i1 of the light pulse emitted by the backlighting unit 30. In the embodiment shown
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`in Fig. 3B the second backlight luminance L2 is the substantially maximum luminance of the
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`backlighting unit 30 while the first backlight luminance L1 is a dimmed luminance of the
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`backlighting unit 30. The dimming of the backlighting unit 30 in Fig. 3B is obtained by
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`reducing the intensity of the light pulse emitted by the backlighting unit 30 in the first sub—
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`framc pcriod Tl. Thc sum of the backlight luminancc L1, L2 displayed during thc first and
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`second s