(12) United States Patent
`Kuratomi et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,791,566 B1
`Sep. 14, 2004
`
`US006791566B1
`
`(54) IMAGE DISPLAY DEVICE
`
`(75) Inventors: Yasunori Kuratomi, Sanda (JP); Junko
`Asayama, Suita (JP); Akifumi
`Ogiwara, Hirakata (JP); Tsuyoshi
`Uemura, Kadoma (JP); Kazunori
`Komori, Sanda (JP)
`
`(73) Assignee: Matsushita Electric Industrial Co.,
`Ltd. (JP)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 64 days.
`10/088,418
`Sep. 18, 2000
`PCT/JP00/06341
`
`(21)
`(22)
`(86)
`
`Appl. No.:
`
`PCT Filed:
`
`PCT No.:
`
`§ 371 (6X1),
`(2), (4) Date: Mar. 18, 2002
`
`(87)
`
`PCT Pub. No.: WO01/22391
`
`PCT Pub. Date: Mar. 29, 2001
`Foreign Application Priority Data
`
`(30)
`Sep.17,1999
`May 29, 2000
`
`(JP) ......................................... .. 11-262968
`(JP) ..................................... .. 2000157757
`
`(51) Int. Cl.7 ................................................ .. G09G 5/00
`(52) US. Cl. ....................... .. 345/589; 345/204; 345/44;
`345/690; 348/678
`(58) Field of Search ....................... .. 345/102, 204—207,
`345/87, 10—11, 210, 19—21, 697, 699, 44,
`47, 49, 51, 62, 75.2, 82—83, 589, 690; 348/68—69,
`7071, 377, 582, 609, 615, 678, 677
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,659,183 A
`5,517,333 A *
`5,745,201 A *
`5,757,343 A *
`
`4/1987 SuZaWa .................... .. 350/345
`5/1996 Tamura et al. ..
`358/518
`4/1998 Kawai et al.
`349/110
`
`5/1998 Nagakubo . . . . . . .
`
`. . . . .. 345/63
`
`6,208,327 B1 * 3/2001 Ho et al. .................. .. 345/690
`6,271,825 B1 * 8/2001 Greene et al. ...... ..
`345/694
`6,445,365 B1 * 9/2002 Taniguchi et al. ........... .. 345/9
`
`FOREIGN PATENT DOCUMENTS
`
`......... .. G01R/13/20
`
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`JP
`
`12/1975
`50-157013
`1/1982
`57-13478
`6/1986
`61-120581
`2/1988
`63-20680
`401318966 A * 12/1989
`03-005725
`1/1991
`03-051889
`3/1991
`03-062796
`3/1991
`06-161371
`6/1994
`06-282241
`10/1994
`08-015701
`1/1996
`08-101660
`4/1996
`09-101761
`4/1997
`10-207416
`8/1998
`11-095735
`4/1999
`11-109927
`4/1999
`
`* cited by examiner
`
`Primary Examiner—MattheW C. Bella
`Assistant Examiner—Wesner Saj ous
`(74) Attorney, Agent, or Firm—Parkhurst & Wendel, L.L.P.
`(57)
`ABSTRACT
`
`In a display device according to a ?rst aspect of the
`invention, luminance is not made substantially uniform over
`the entire display screen but rather imparted a luminance
`gradient such that luminance substantially monotonously
`decreases from the center of the screen toWards the periph
`eral portion using a luminance gradient forming means,
`Whereby poWer consumption is reduced compared to cases
`in Which the entire screen shoWs substantially uniform
`luminance. In a second aspect of the invention, by optimiz
`ing gray level characteristics and thereby improving a vieW
`er’s impression of brightness, luminance gradient is made
`even less perceivable Without increasing poWer consump
`tion. These con?gurations provide large screen, loW poWer
`consumption display devices.
`
`43 Claims, 34 Drawing Sheets
`
`/1O
`
`Light Source
`
`I Luminance Gradient Forming Means
`
`I
`
`/13
`Image Display Means
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 1 0f 34
`
`US 6,791,566 B1
`
`
`
`2822 $33G mmmEH
`
`
`
`282 minnow E235 mucm?Esq
`
`
`
`
`
`H JOE
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 2 0f 34
`Sheet 2 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 2
`FIG. 2
`
`T NWT/21
`
`--—>
`
`—-—>
`
`-—->
`
`-->
`
`—'-——>
`
`16
`16
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 3 0f 34
`Sheet 3 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 3
`FIG. 3
`
`b nE
`
`!n
`i‘I
`
`i
`
`@5269
`9
`$
`69
`$
`69
`9
`$
`9% Q
`6 e QQQ
`$ $$
`6
`
`8
`Q
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 4 0f 34
`Sheet 4 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 4
`FIG. 4
`
`1
`
`r=(x2+Y2) /2
`
`(0)
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 5 0f 34
`Sheet 5 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 5
`FIG. 5
`
`Fax/a? +<Y/b>Z )"2
`r=((X/a)2 +<yx192 >“2
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 6 6f 34
`Sheet 6 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 6
`FIG. 6
`
`r=rect<x/(a - h).V/(b - v) >
`
`
`
`[:1
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 7 0f 34
`Sheet 7 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 7
`FIG. 7
`
`
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 8 0f 34
`
`US 6,791,566 B1
`
`FIG. 8
`
`1=rect(1/ (h - a(xcos 6 —-ysin 6 )),1/(n ' b<XSiI1 6 17008 9 )) )
`
`<<<<>>>
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 9 0f 34
`
`US 6,791,566 B1
`
`FIG. 9
`
`A Relative Luminance
`1.27
`
`~0.9
`
`0.9 >Distance
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 10 0f 34
`
`US 6,791,566 B1
`
`Hw
`
`SUBGW Xwdspq 332ml
`
`m?x mam aoupqwl 1118111913 KBICISFCI
`
`/33
`
`Fine AdJUS-UIIBDI Means
`
`Gray Scale
`
`A
`
`Od'LD'CQ
`
`
`
`Video Signal
`
`FIG. 10
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 11 0f 34
`
`US 6,791,566 B1
`
`FIG. 11
`
`()utput A
`
`F(0,0)
`
`>
`1.0 Input Signal
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 12 0f 34
`
`US 6,791,566 B1
`
`FIG. 12
`
`Output
`A
`
`.
`7’ Curve at the Center of Dlsplay Screen
`
`
`
`__________ _____ Fbw) --------
`
`------ ~
`
`7’ Curve at (x,y)
`
`>
`1.0 Input Signal
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 13 0f 34
`
`US 6,791,566 B1
`
`FIG. 13
`
`)
`
`Transmissivity
`
`Input Signal
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 14 0f 34
`Sheet 14 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 14
`FIG. 14
`
`
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep.14,2004
`
`Sheet 15 0f 34
`SheetlS 0f34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`TMMM
`
`
`
`3.03
`2 .05
`
`
`Rama82>
`Rama 82>
`
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 2004
`Sep. 14, 2004
`
`Sheet 16 0f 34
`Sheet 16 0f 34
`
`US 6,791,566 B1
`US 6,791,566 B1
`
`FIG. 16
`FIG. 16
`
`L7
`L6
`L5
`
`L4 1.3
`
`
`
`L2 L1
`
`' 1
`//
`
`//@@
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`
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`
`1% ‘ I
`
`l
`
`l
`
`31 S2
`
`S4 S5
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 17 0f 34
`
`US 6,791,566 B1
`
`FIG. 17
`
`250
`
`200 150
`
`(1) i=3 .= /
`
`§ 100 I
`D
`A
`
`50
`
`\
`
`\
`
`O
`-1
`
`-0.5
`
`0
`Distance r
`
`l
`0.5
`
`1
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 18 0f 34
`
`US 6,791,566 B1
`
`FIG. 18
`
`0.7
`
`
`
`Brightness Index
`
`312 /
`
`0.3 ‘
`
`v /
`
`0.2
`0.1
`
`\
`
`\
`
`0
`-1
`
`.
`-0.5
`
`0
`
`1
`0.5
`
`1
`
`Distance r
`
`

`

`U.S. Patent
`
`Sep. 14, 2004
`
`Sheet 19 0f 34
`
`US 6,791,566 B1
`
`FIG. 19
`
`0.0035
`0.0030
`0.0025
`2 0.0020
`*‘ 0.0015
`0.0010
`0.0005
`0.0000
`
`................ / \
`/ \
`
`\
`
`/
`
`0.5
`
`o- 7
`Distance r
`
`0.5
`
`1
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 20 0f 34
`
`US 6,791,566 B1
`
`FIG. 20
`
`Improved Brightness Index
`
`Index
`
`Brightness
`
`Distance r
`
`

`

`US. Patent
`
`2M,
`
`M
`
`US 6,791,566 B1
`
`
`
`382\mmamfiowns:0%
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`om/mm
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`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 22 0f 34
`
`US 6,791,566 B1
`
`FIG. 22
`
`First Lookup Table
`
`
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 23 0f 34
`
`US 6,791,566 B1
`
`FIG. 23
`
`Second Lookup Table
`
`L
`
`inan
`
`.
`
`
`
`S
`
`Position
`
`0.1
`
`0.2
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 24 0f 34
`
`US 6,791,566 B1
`
`'3
`
`(D
`
`3;
`:>
`‘5'
`
`3 8
`
`5,
`
`0)
`
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`
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`
`FIG. 24A
`
`FIG. 24B
`
`FIG. 24C
`
`
`O
`
`2.2)
`
`1.8)
`(Tr—1.5)
`
`T 2
`
`Y
`
`(Video Signal Level)
`
`T o (7:22)
`
`(7’=2.4)
`“r 1
`T 2 (7:2.8)
`
`V
`
`(Video Signal Level)
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 25 0f 34
`
`US 6,791,566 B1
`
`Signal Level 100% across Entire Screen
`______________.____..__.._—
`
`FIG. 25A
`
`100%
`
`‘
`
`Luminance
`
`Signal
`
`FIG. 25B
`
`100%
`
`'
`
`Center
`
`Horizontal Distance
`
`Center
`
`Horizontal Distance
`
`FIG. 25C
`
`100%
`
`ResultofMultiplying_
`
`'
`
`Center
`
`Horizontal Distance
`
`FIG. 25D
`
`100%
`
`'
`
`I
`
`Center
`
`Horizontal Distance
`
`FIG. 25E
`
`100%
`
`Final Output
`
`Center
`
`Horizontal Distance
`
`Prior Art can exhibit the same result
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 26 0f 34
`
`US 6,791,566 B1
`
`Siglal Level 50% across Entire Screen
`
`FIG. 26A
`
`Luminance
`
`50%
`
`Center
`
`Horizontal Distance
`
`FIG. 26B
`
`100% —
`
`'
`
`Gain 2
`
`Center
`
`Horizontal Distance
`
`FIG. 26C .
`RCSUIt Of
`Multiplying
`
`50% _
`
`;
`:
`
`i
`
`Center
`
`Horizontal Distance
`
`FIG. 26D
`
`100%
`
`'
`
`Gain 1
`
`Center
`
`Horizontal Distance
`
`FIG. 26E
`
`Final Output
`
`50V
`0
`
`5
`"
`
`
`
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 27 0f 34
`
`US 6,791,566 B1
`
`
`
`_S__g______inal Level 50% + 100%
`
`FIG. 27A
`
`100%
`
`Luminance Signal
`
`FIG.27B
`
`100% -
`
`GainZ
`
`Center
`
`Horizontal Distance
`
`Center
`
`Horizontal Distance
`
`FIG. 27C
`
`100%
`
`50% Multiplying
`
`Result of
`
`.
`FIG. 27D
`
`100%
`
`Gain 1
`
`FIG. 27B
`
`100%
`
`Final Output
`
`Center
`
`Horizontal Distance
`
`Center
`
`Horizontal Distance
`
`;
`
`.
`
`------
`
`Horizontal Distance
`
`Prior Art
`
`

`

`US. Patent
`
`4,
`
`43pm
`
`US 6,791,566 B1
`
`Mmm.m
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`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 29 0f 34
`
`US 6,791,566 B1
`
`FIG. 29
`
`Ambient Light Level (Cd/m2)
`
`0~1
`
`l~10
`
`10~100
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 30 0f 34
`
`US 6,791,566 B1
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`US. Patent
`
`Sep. 14, 2004
`
`Sheet 31 0f 34
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`US 6,791,566 B1
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`US. Patent
`
`Sep. 14, 2004
`
`Sheet 32 0f 34
`
`US 6,791,566 B1
`
`FIG. 32
`
`
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 33 0f 34
`
`US 6,791,566 B1
`
`FIG. 33
`
`
`
`Bottom End
`
`Center
`
`Top End
`
`

`

`US. Patent
`
`Sep. 14, 2004
`
`Sheet 34 0f 34
`
`US 6,791,566 B1
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`

`

`US 6,791,566 B1
`
`1
`IMAGE DISPLAY DEVICE
`
`TECHNICAL FIELD
`
`The present invention relates to an image display device.
`
`BACKGROUND ART
`
`Image display devices are generally categorized into two
`types—non-emissive type and emissive type. Non-emissive
`type devices have an external
`light source means and
`employ a display element which modulates the light from
`the light source means in order to display images. Examples
`include liquid crystal monitors, liquid crystal projectors, and
`the like. Emissive type devices do not have an external light
`source means but the display element itself emits light to
`display images. For example, CRTs, PDPs, FEDs, organic
`ELs, and the like fall under this type.
`For these conventional
`image display devices, high
`luminance, high contrast, high resolution, and lower power
`consumption have been desired to improve the picture
`quality. Of these, luminance has the greatest influence on the
`viewer’s perception of images and therefore is the most
`important parameter.
`In view of this, various attempts have been made in the
`past to make luminance uniform over the display screen.
`Such attempts include, for example,
`in the case of non-
`emissive type devices, using the characteristics of the light
`source means, and in the case of the emissive type devices,
`varying video signal in an appropriate manner, in order to
`achieve more uniform display screen luminance.
`Taking a liquid crystal display device as an example, a
`conventional technique for making the luminance uniform
`within the screen is discussed below. The liquid crystal
`display device comprises, as shown in FIG. 32, a liquid
`crystal display element 1901, a backlight 1902, and a driving
`means 1903 for the liquid crystal display element 1901. The
`backlight 1902 comprises at least a light source 1904, a
`transparent light guiding plate 1905 for supplying light from
`the light source to the liquid crystal display element 1901,
`and a reflective cover 1906 for covering the light source. ON
`the back surface of the light guiding plate, a plurality of
`scattering microdots 1907 are formed numerously so that the
`luminance within the plane is controlled by the shapes and
`positions of the formed scattering microdots 1907.
`The light discharged from the light source 1904 enters
`from an end face of the light guiding plate 1905 and is
`transmitted inside the light guiding plate, repeating the total
`reflection. All or part of the light which has entered the
`scattering microdots 1907 changes traveling direction, and
`the light which is incident on the upper surface of the light
`guiding plate at an angel smaller than a critical angle is
`discharged as output light, entering the liquid crystal display
`element 1901.
`
`Accordingly, the distribution of scattering microdots on
`the back surface determines the luminance distribution in the
`screen; the conventional backlight 1902 has such a configu-
`ration that the areas of the dots are made greater from the
`peripheral portion of the screen towards the center portion
`and thereby the distribution of screen luminance is made
`80% or greater such that uniform brightness is achieved.
`For example, let us assume a case where light sources are
`horizontally disposed at end faces of the light guiding plate,
`the end faces being at the top and bottom of the screen. It has
`been known that, supposing the areas of the scattering
`microdots on the back surface are uniform over the entire
`screen, the luminance distribution is mostly formed in a
`vertical direction (along the y axis), resulting in a distribu-
`tion in which a region including the center is dark, as shown
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`2
`in FIG. 33. This is due to the fact that a large portion of the
`light is scattered in portions of the light guiding plate which
`are near the light source and is discharged therefrom.
`In order to compensate such a luminance distribution, it
`has been suggested that the areas of the scattering microdots
`be varied such as to be proportional to the reciprocal of the
`luminance distribution obtained in the case when the areas
`of the scattering microdots are uniform over the entire area
`of the display screen. That is, the areas of the scattering
`microdots are varied in the vertical direction in the screen so
`that the scattering microdots nearer to the center have larger
`areas. Thereby, uniformity of the luminance within the
`screen can be increased to as high as 80% or higher.
`Next, a case of an emissive type display is described as an
`example. In a emissive type,
`it has been suggested that
`non-uniformity in the display element itself be corrected.
`Specifically, in order to compensate the luminance variation
`between the pixels, luminance variation is compensated in
`each pixel by only the varied value.
`Generally, a driving means of a display device comprises
`a video signal decoding means 2101, a signal correcting
`means 2102, and a display element interface means 2103.
`The video signal decoding means 2102 serves the purpose of
`producing RGB color signals and horizontal and vertical
`synchronizing signals from ordinary NTSC signals.
`The signal correcting means 2102 corrects each signal of
`R, G, and B and, essentially, corrects gray level character-
`istics in consideration of input-output characteristics of a
`display element 2104. The display element interface means
`2103 serves the purpose of adjusting the corrected signal to
`match a signal level of the display element.
`The signal correcting means 2102 is provided primarily
`for the purpose of achieving good gray level characteristics;
`however, when the image display means 2104 has some
`factors leading to luminance variation, it additionally com-
`prises a means for correcting luminance variation. For
`example, there are cases where luminance variation between
`pixels, for example, is caused by inaccuracy in the produc-
`tion of display elements. In such cases, to make luminance
`in the screen uniform, gray level characteristics is varied for
`each pixel so that the luminance is made uniform at a certain
`level. More specifically,
`the signal correcting means
`comprises, in the form of an integrated memory, a lookup
`table which defines a gray level characteristic for each of the
`pixels, and a table lookup is performed synchronized with
`the synchronizing signals, so that luminance is appropriately
`modulated.
`
`As described above, in prior art display devices, various
`attempts have been made to make brightness uniform in the
`display screen.
`In recent years, display screen sizes have been increased,
`and even for home use TVs, 20-inch or larger screens have
`been desired. However, conventional display devices have
`drawbacks in that power consumption considerably
`increases as screen sizes increase. When a given luminance
`is required,
`the amount of power consumed increases in
`proportion to the area. Moreover, when a higher resolution
`is required as display size increases, the area per each pixel
`becomes smaller and therefore efficiency generally reduces.
`For this reason, when larger screen sizes and higher reso-
`lutions are desired without changing the luminance,
`the
`amount of power consumed increases even further.
`Thus,
`if the luminance in the display screen is kept
`uniform, an increase in power consumption is inevitable. In
`addition, simply reducing luminance may make it possible
`to suppress an increase in power consumption, but the image
`will appear dark to the viewer of the image.
`DISCLOSURE OF THE INVENTION
`
`It is therefore an object of a first aspect of the present
`invention to provide a display device which is capable of
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`US 6,791,566 B1
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`3
`reducing power consumption while displaying images that
`can create the viewer impression of bright images. More
`specifically,
`in order to solve the foregoing and other
`problems,
`there is provided in accordance with the first
`aspect of the present invention a display device in which the
`luminance is gradually decreased from the center of the
`display screen towards the peripheral portion, and by mak-
`ing the luminance gradient
`less perceivable utilizing a
`certain type of optical illusion, a reduction in power con-
`sumption is achieved without impairing viewer perception
`of a bright image.
`It is an object of a second aspect of the present invention
`to make brightness distribution in the display screen less
`perceivable by utilizing a certain type of psychological
`illusion, even when the luminance in the peripheral portion
`is decreased.
`
`In order to solve the foregoing and other problems, the
`present invention provides, according to the first aspect of
`the invention, a display device comprising at least an image
`display means and a luminance gradient forming means for
`forming a luminance gradient, wherein the luminance gra-
`dient forming means is such that, when displaying a full-
`white signal, a luminance of the image display means
`substantially monotonously decreases from substantially the
`center of a display screen of the image display means
`towards a peripheral portion thereof. The term “monoto-
`nously” is understood to mean “continuously.”
`Specifically, a display device according to the first aspect
`of the invention does not have substantially a uniform
`luminance over the entire display screen but is imparted with
`a luminance gradient which substantially monotonously
`decreases from the center of the screen toward the peripheral
`portion, and thereby, the power consumption is reduced in
`comparison with cases where the luminance is made sub-
`stantially uniform over the entire screen.
`In this configuration, it is preferable that, in order to make
`the luminance gradient less perceivable for a viewer, the
`luminance of a display image be substantially monotonously
`decreased. When this is the case, it is more preferable that
`the luminance is monotonously decreased from the center of
`the display screen in a horizontal or vertical direction since
`luminance gradients having symmetry are even less notice-
`able. For similar reasons, it is preferable that the luminance
`gradient be substantially symmetrical with respect
`to a
`vertical axis through substantially the center of the display
`screen or a horizontal axis through substantially the center of
`the display screen.
`From these viewpoints, it is considered that there are
`several preferable examples of distribution profile of lumi-
`nance gradient. For example, when the luminance gradient
`is distributed so as to be a concentric circle-like profile, the
`luminance gradient is very difficult to perceive. The term “a
`concentric circle-like luminance distribution” is intended to
`mean a distribution such that the lines connecting the points
`having approximately the same luminance form substan-
`tially circular shapes centered on substantially the center of
`the display screen. Luminance distribution profiles are
`defined in a like manner throughout herein.
`The above-described configurations makes the luminance
`distribution less noticeable for the following reasons. Since
`human pupils are circular, the region that a human is capable
`of perceiving at one time has a near circle shape.
`Consequently, when a luminance gradient distribution pro-
`file in the display screen is circular, the region that a human
`is capable of perceiving at one time and the distribution
`profile of the luminance gradient are approximately similar
`in shape, and for this reason, psychological effect of the
`luminance gradient becomes small due to a kind of optical
`illusion. The luminance distribution profiles need not be
`substantially circle-like profiles and similar effects are
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`obtained with ellipse-like profiles and rhombus-like profiles,
`so it is also preferable to form these luminance distribution
`profiles.
`In the cases of ellipse-like luminance gradients, the ratio
`of the major axis and the minor axis may be made substan-
`tially equal to the ratio of the horizontal length and the
`vertical length of the image display screen to make the
`luminance gradients less noticeable. As a consequence, the
`shape of appearance of the display image and the distribu-
`tion profile of the luminance gradient are nearly similar in
`shape, and therefore, psychological effect of the luminance
`gradient becomes small due to a kind of optical illusion. For
`similar reasons, a rectangle-like luminance distribution is
`also one of the examples of preferable luminance distribu-
`tions.
`
`Such luminance gradient may be defined by mathematical
`functions. Now, let us consider a luminance gradient func-
`tion f(x,y) which is substantially equal to a luminance B at
`a point in the display screen which has a distance x from the
`origin in the horizontal direction and a distance y in the
`vertical direction, the origin being substantially the center of
`the display screen. Here, when the luminance gradient
`function f(x,y) is expressed with a luminance distribution
`profile function r(x,y) as fx,y)=f(r), luminance gradient can
`be expressed in a simpler manner.
`Since luminance gradient is less noticeable as the slope of
`luminance gradient becomes more gentle, it is preferable
`that the luminance gradient function f(r) be monotonously
`decreased with respect to the variable r of the luminance
`distribution profile function. One example is a case of a
`linearly decreasing luminance gradient. Another example is
`that of a luminance gradient that decreases in an exponential
`function-like manner with respect to r.
`This configuration is even more preferable because the
`slope of the luminance gradient is small in the vicinity of the
`center of the display screen but becomes larger towards the
`outward portion, whereby luminance gradient is even less
`perceivable, compared to the case where a luminance lin-
`early decreases. This is due to the fact that when a human
`observes an image, he or she usually has a tendency to gaze
`mainly at
`the center of the display screen. That
`is,
`the
`luminance gradient in the portion at which a viewer focuses
`is small while the luminance gradient
`in the peripheral
`portion of the display screen, which is not intensely viewed,
`is large and therefore, the luminance gradient in the display
`screen is made less perceivable. For similar reasons, a
`luminance gradient
`in which the luminance decreases
`according to powers of r and a luminance gradient that
`sinusoidally and monotonously decreases towards the
`peripheral portion are also preferable.
`It is preferable that the degree of luminance gradient
`image viewers can tolerate (how low the luminance in the
`peripheral portion can be made) be determined so as to
`conform to the results of human-engineering-based subjec-
`tive assessments. In other words,
`it is preferable that a
`luminance gradient in a display screen match with a variety
`of threshold values defined according to the results of
`subjective assessments. These results are generally referred
`to as detection limit, permissible limit, and tolerable limit
`(limit for practical use).
`According to the results of the subjective assessment
`experiments carried out by the present inventors, the thresh-
`old luminance values which are defined by the ratio of the
`luminance at
`the boundary of the display screen region
`having a screen diagonal ratio of 0.9 to the luminance of the
`center portion when a full-white signal is displayed are 55%
`115% for the detection limit, 30%:10% for the permissible
`limit, and 15%=5% for the tolerable limit (limit for practical
`use). Thus,
`it
`is preferable that
`luminance gradients be
`determined so as to satisfy the above conditions.
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`

`

`US 6,791,566 B1
`
`5
`When a luminance gradient satisfies the condition of the
`detection limit, (the luminance at the boundary of the display
`screen region having a screen diagonal ratio of 0.9 is about
`55% of that of the center of the display screen), 50% of the
`viewers do not perceive the luminance gradient. When a
`luminance gradient satisfies the condition of the permissible
`limit, the luminance gradient is permissible for 50% of the
`viewers and power consumption is further reduced com-
`pared to the case where the condition of the detection limit
`is satisfied. Likewise, when the condition of the tolerable
`limit is satisfied, power consumption is reduced even further.
`This technique of reducing power consumption by form-
`ing a luminance gradient
`in the display screen using a
`luminance gradient forming means can be applied to any
`display devices that comprise a non-emissive or emissive
`type image display means. In the cases of display devices
`having a non-emissive image display means, it is possible
`that a light source means that provides light to the non-
`emissive image display means be provided with a luminance
`gradient forming means. For example, a display device
`using a transmissive liquid crystal panel as the non-emissive
`image display means has a light source and a light guiding
`plate as the light source means. By using a distribution of
`scattering dots formed on the back surface of the light
`guiding plate, a desired luminance distribution can be
`formed.
`
`Either of non-emissive display means and emissive dis-
`play means is capable of forming a desired luminance
`gradient by modulating input signals with a luminance
`gradient forming means. In this configuration, the luminance
`gradient forming means may comprise a lookup table that
`determines a gray level characteristic for each pixel,
`whereby the luminance distribution in the display screen is
`formed into a desired profile.
`Likewise, it is possible to employ a configuration in which
`the luminance gradient forming means for varying the gains
`of the level shifter may be provided in the interface portion
`with the display element.
`When the image display means is an FED, a desired
`luminance gradient can be formed in the display screen by
`providing an extraction voltage varying means as the lumi-
`nance gradient forming means.
`A display device according to a second aspect of the
`present invention comprises at least a luminance gradient
`forming means and an image display means, wherein bright-
`ness index defined by Eq. (1) substantially monotonously
`decreases from substantially the center of the display screen
`towards the peripheral portion thereof.
`Eq. (1) has been established from the subjective assess-
`ment experiments carried out by the present inventors and is
`a criterion that shows a good correlation with psychological
`impression concerning brightness.
`As discussed in the section describing the first aspect of
`the invention,
`that a luminance monotonously decreases
`from the center of the display screen towards the peripheral
`portion means that a brightness index also monotonously
`decreases. This configuration is, as discussed in the section
`describing the first aspect of the invention, effective for
`reducing power consumption. However, because the bright-
`ness index simultaneously decreases, psychological impres-
`sion concerning brightness degrades according to the equa-
`tions. In view of this, in the first aspect of the invention, the
`profile of luminance gradient is controlled or the slope of
`luminance gradient is monotonously decreased in order to
`make luminance gradient
`less perceivable as luminance
`unevenness.
`
`On the other hand, in the second aspect of the invention,
`brightness index that decrease according to the equations are
`improved in order to improve viewer’s impression on bright-
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`6
`ness. Specifically, it is an object of the second aspect of the
`invention that degradation in the viewer’s impression of
`brightness due to decrease in luminance is improved by
`optimizing gray level characteristics, whereby luminance
`gradient is made even less perceivable. This configuration
`merely varies gray level characteristics, so basically, power
`consumption does not increase.
`In this configuration, brightness index may be compen-
`sated so as to be substantially uniform over the entire display
`screen area, whereby the luminance in the peripheral portion
`is reduced without degrading the viewer’s impression of
`brightness.
`As well as compensating brightness index so as to be
`substantially uniform over the entire display screen area, it
`is also possible that brightness index may be compensated
`within the region having a screen diagonal ratio of 0.5, for
`example. The reason is that when a human observes an
`image, the point at which he or she focuses on most is in the
`vicinity of the center portion of the display screen and the
`area that he or she can observe at one time is substantially
`within the region having a screen diagonal ratio of 0.5. Thus,
`by optimizing gray level characteristics in the display screen
`and thereby improving the distribution of brightness index,
`power consumption can be reduced without adversely
`affecting viewer’s impression of brightness.
`It is also preferable that the distribution profile of bright-
`ness index in the display screen be corrected. When the
`preferable luminance profile as described above is not
`attained due to individual differences of display elements,
`for example, it is possible to make luminance gradient less
`perceivable by making brightness index have a desirable
`distribution profile using a brightness index improving
`means, that is to say, by forming a distribution profile having
`high degree of symmetry as described above.
`In a similar manner, even when the actual luminance slope
`is shifted from a desired luminance slope, it is possible to
`make luminance gradient even less perceivable by matching
`brightness index with the desired luminance slope.
`Additionally, it is possible to make less perceivable lumi-
`nance unevenness in the display screen which is often a
`problem in a liquid crystal display element having a direct
`type backlight. As for a direct type backlight, which has a
`configuration such that several light sources are provided
`directly beneath a liquid crystal display element, the por-
`tions directly above the lamps are bright while the portions
`between the lamps are dark.
`to ameliorate picture quality
`It
`is possible, however,
`degradation due to such luminance distribution, because the
`brightness index improving means according to the second
`aspect of the invention is capable of making brightness
`index uniform in the display screen by appropriately adjust-
`ing gray level characteristics so that reduction in luminance
`is compensated.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of a display device constructed
`according to Embodiment 1 of the present invention.
`FIG. 2 shows the configuration of the display device
`constructed according to Embodiment 1 of the present
`invention.
`
`FIG. 3 is a plan view showing luminance gradient forming
`means in the display device constructed according to
`Embodiment 1 of the present invention.
`FIG. 4 is a diagram for illustrating a luminance distribu-
`tion profile of the display device construc