US20070115685A1

US20070115685A1 - Flat lighting source, luminance correcting circuit, luminance correcting method and liquid crystal display

Info

Publication number: US20070115685A1
Application number: US11/592,289
Authority: US
Inventors: Yuji Kondo; Nobuaki Honbo
Original assignee: NEC LCD Technologies Ltd
Current assignee: Tianma Japan Ltd
Priority date: 2005-11-22
Filing date: 2006-11-03
Publication date: 2007-05-24
Also published as: KR20070054101A; CN1971367A; JP2007141799A; KR100854192B1; TW200727047A

Abstract

A flat lighting source is provided which is capable of applying illuminating light to an entire display region of a display panel. A current or voltage is supplied from each LED (Light Emitting Diode) driving/correcting circuit to each LED. Part of light emitted from each of the LEDs is converted into electrical signals and a resistance value of each photodiode decreases in proportion to luminance. If luminance of light emitted from each of the LEDs increases, a resistance value of each of the photodiodes decreases while the luminance of light emitted from each of the LEDs decreases. The resistance value is fed back to each of the LED driving/correcting circuits which changes a driving current or a driving voltage so that luminance of light emitted from each of the LEDs corresponds to a luminance setting voltage. The photodiode is mounted in a one-to-one relationship for every LED.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a flat lighting source, luminance correcting circuit, and luminance correcting method to be used in the flat lighting source and liquid crystal display and particularly to the flat lighting source suitably used when illuminating light with uniform luminance has to be provided to an entire displaying region of a display panel, for example, in the case of a liquid crystal display device where illuminating light is provided by a backlight, and to the luminance correcting circuit and the luminance correcting method to be used in the flat lighting source.
The present application claims priority of Japanese Patent Application No. 2005-337871 filed on Nov. 22, 2005, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, as a backlight for a liquid crystal display device, a CCFL (Cold Cathode Fluorescent Lamp) has been used in many cases. However, in recent years, an LED (Light Emitting Diode) is also employed increasingly. In the LED backlight, ordinarily, a plurality of LEDs is connected in series and is driven at a constant current. Therefore, variations in current-luminance character-istic of each LED are reflected straightly as variations in luminance in a display region of a liquid crystal display device. Here, the “variations” represent secular changes in luminance characteristic to currents and temperatures. Conventional technologies to correct variations of these types are disclosed, for example, in the following references.
FIG. 12 is a diagram showing electrical configurations of main components of an illuminating device disclosed in Patent Reference 1 [(Japanese Patent Application Laid-open No. 2004-221158, Abstract, FIGS. 1 and 4)]. The disclosed illuminating device, as shown in FIG. 12, includes an LED printed circuit board (PCB) 10, a constant current source 20, and a temperature compensating circuit 30. Alarge number of LED 11, . . . , 11 are formed on the LED PCB 10. The constant current source 20 has a resistor 21, a transistor 22, an amplifying circuit 23, and a comparison voltage generating circuit 24. The temperature compensating circuit 30 includes an FET (Field Effect Transistor) 31, an LED 32, an amplifying circuit 33, and a photo-detecting device 34.
In the disclosed illuminating device, the LED 32 having the same temperature characteristic as each of the LED 11, . . . , 11 emits light which is converted by the photo-detecting device 34 into electrical signals. The electrical signals are input into the comparison voltage generating circuit 24. Signals output from the comparison voltage generating circuit 24 are input into the amplifying circuit 23 and currents determined by a specified reference value through the transistor 22 based on signals output from the amplifying circuit 23 are fed to each of the LEDs 11, . . . , 11 and changes in the temperature characteristic of each of the LEDs 11, . . . , 11 are compensated for. In this situation, if luminance of the LED 32 is changed by changes in the temperature (atmospheric temperature) as shown in the characteristic diagram G2 in FIG. 13, currents output from the constant current source 20 are changed by the comparison voltage generating circuit 24, amplifying circuit 23, transistor 22 as shown in the characteristic diagram H1 and the luminance of light emitted from the LED PCB 10 is corrected. As a result, in contrast to the state shown in the characteristic diagram G1 where the luminance changes with temperature, a characteristic appears that the luminance does not change with temperature as shown in the characteristic diagram G3.
FIG. 14 is a diagram showing one example of configurations of a conventional LED backlight. Colors emitted from the LEDs 41, . . . , 41 making up the LED backlight are a combination of R (Red), G(Green), and B(Blue) or white. The LEDs 41, . . . , 41 are mounted on the PCB 42 and every specified number of the LEDs 41, . . . , 41 are connected serially. In this state, the LEDs emitting white color having the number corresponding to each level of supply power are connected serially and, when the LEDs emitting the white color can not be contained in a row, a plurality of rows each including the LEDs emitting the white color is arranged and connected in parallel among rows in some cases. Also, when the LEDs emitting each of the R, G, and B is used, a specified number of LEDs in every color are serially connected. In this LED backlight, part of light components having given luminance emitted from the entire LEDs 41, . . . , 41 is converted into electrical signals by the photo-detecting device 43. In this case, by arranging the photo-detecting device 43 and the color filter 44 for each of R, G, and B, or by controlling the photo-detecting device 43 so that its light sensing characteristic has dependence on a wavelength, light having given luminance for each of R, G, and B is converted into an electrical signal. A driving current and a driving voltage to be fed to the LEDs 41, . . . , 41 are adjusted based on the electrical signal generated in proportion to the luminance and are controlled so that luminance of light received by the photo-detecting device 43 becomes constant.
As a result, the conventional LED backlight shown in FIG. 14 has a problem. That is, by keeping smooth track of a change in luminance of the light emitted from the LEDs 41, . . . , 41, the current and voltage to be fed to the LEDs 41, . . . , 41 existing in the vicinity of the photo-detecting device 43 are well controlled, however, it is difficult to keep track of a change in luminance of the light emitted from the LEDs 41, . . . , 41 existing at a location some distance from the photo-detecting device 43. Another problem is that, since radiation is difficult in a central portion of the PCB 42, temperatures are likely to rise and, therefore, due to place-to-place variations of a temperature in areas surrounding the LEDs 41, . . . , 41 and due to dependence of the LEDs 41, . . . , 41 upon temperatures, it is made difficult to make uniform the distribution of luminance. Moreover, when variations occur in luminance of light emitted from the LEDs 41, . . . , 41 due to secular changes in the LEDs 41, . . . , 41, only the luminance characteristic of the LED 41 existing in the vicinity of the photo-detecting device 43 is adjusted and variations still occur in the characteristic of luminance distribution of the entire LEDs 41, . . . , 41. To solve these problems, an LED backlight is disclosed in Patent Reference 2 [(Japanese Patent Application Laid-open No. 2005-115372 (Abstract, FIG. 7A, 7B, 8B, FIG. 10)) in which photo-detecting devices are mounted not in one place but in a plurality of places.
FIG. 15 is a diagram showing configurations of LEDs used as the conventional backlight disclosed in the Patent Reference 2. In the disclosed backlight, as shown in FIG. 15, four rows of the LEDs 51 each row having 31 pieces are arranged on the PCB 50 and are connected in series.
FIG. 16 is a diagram showing another configurations of LEDs used as the conventional backlight disclosed in the Patent Reference 2. In the disclosed backlight, as shown in FIG. 16, a plurality of rows of LEDs each having three to seven LEDs connected serially to one another is arranged.
FIG. 17 is a cross-sectional view of the conventional backlight in which the LEDs 51 shown in FIG. 15 or FIG. 16 are mounted. In this backlight 52, as shown in FIG. 17, each photo-detecting device 53 is mounted between the LEDs 51 and a diffusing body 54 and an LCD (Liquid Crystal Display) panel 55 are mounted in a direction in which light is emitted from each of the LEDs 51.
FIG. 18 is a block diagram showing configurations of a correcting circuit to correct luminance characteristic of the LEDs 51 shown in FIG. 15 or FIG. 16. In the correcting circuit, a signal “a” output from the photo-detecting device 53 is detected by a detector 61 and, based on a detected signal “b”, a luminance characteristic of the LEDs 51 is corrected by the controlling unit 62.
In a backlight controlling unit of a liquid crystal display device disclosed in Patent Reference 3 (Japanese Patent Application Laid-open No. 2003-215534), an amount of light emitted from LEDs mounted as a backlight on a rear surface side of the liquid crystal display device is controlled according to brightness in a place surrounding the liquid crystal panel and even if a temperature which the LEDs use changes, the amount of emitted light is controlled so as to be a specified value.
However, the above conventional technologies have the following problems. In the illuminating device disclosed in the Patent Reference 1 and the backlight disclosed in the Patent Reference 2, it is impossible to detect and correct luminance of light emitted from an individual one out of a plurality of LEDs making up the LED light source. As a result, a problem occurs in that the occurrence of changes in luminance due to secular change in the LEDs causes a change in luminance distribution in the liquid crystal panel. Another problem is that, though total changes in luminance of light emitted from the LEDs caused by a change in temperatures can be corrected, a change in luminance of light emitted from an individual one of the LEDs cannot be corrected, as a result, causing a change in luminance distribution in the liquid crystal panel.
Moreover, the illuminating device disclosed in the Patent Reference 1 has still another problem. That is, in the disclosed conventional illuminating device, when a change in temperature of the LEDs or a secular change in the LEDs are detected, luminance of light emitted from LEDs different from the LEDs actually used as the light source is referenced, however, there occurs rare coincidence of temperature, temperature characteristic, secular change characteristic (life characteristic) between the LEDs actually used and the LEDs used as the reference, which makes it difficult to make exact correction to these characteristics. Also, in the backlight disclosed in the Patent Reference 2, a plurality of LEDs is connected to one another in series, however, it is impossible to correct the characteristic of luminance such as luminance of light emitted from an individual LED out of the plurality of LEDs. Actually, driving currents or a like for the entire group of a plurality of LEDs connected serially can be calibrated and luminance of light emitted from the group of the LEDs cannot be changed as a whole. Furthermore, in the backlight controlling device disclosed in the Patent Reference 3, luminance of light emitted from the LEDs is controlled according to brightness in a place surrounding the liquid crystal panel and, therefore, the object of the conventional invention is different from that of the present invention and thus the above problems cannot be solved by this conventional invention.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention to provide a flat lighting source which is capable of providing illuminating light with uniform luminance to an entire display region of a display panel of a liquid crystal display device or a like and a luminance correcting circuit to be used for the above.
According to a first aspect of the present invention, there is provided a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, including:
a plurality of luminance correcting circuits each to set luminance of light to be emitted from each of the plurality of light emitting devices at a targeted value for every light emitting device and to detect an amount of deviation in luminance of light from the targeted value occurring when each of the plurality of light emitting devices is turned ON, and to make the luminance of light coincide with the targeted value, based on the detected amount of deviation.
In the foregoing first aspect, a preferable mode is one wherein the luminance correcting circuit includes:
a plurality of photo-detecting devices so mounted as to correspond to each of the plurality of light emitting devices in a one-to-one relationship which receives light emitted from each of the plurality of light emitting device and generates a luminance detecting signal having a level corresponding to luminance of light emitted from each of the plurality of light emitting devices; and
a plurality of driving/correcting circuits each being mounted so as to correspond to each of the plurality of light emitting devices in a one-to-one relationship, each of which supplies driving power to each of the plurality of light emitting devices and detects deviation in luminance of light emitted from each of the plurality of light emitting devices from a targeted value, based on a level of the luminance detecting signal, and corrects the driving power so that the deviation is compensated for.
Also, a preferable mode is one wherein each of the plurality of light emitting devices is arranged, in a one-to-one relationship, in a vicinity of each of the plurality of photo-detecting devices.
Also, a preferable mode is one wherein each of the plurality of light emitting devices and each of the plurality of photo-detecting devices are mounted in a same package.
According to a second aspect of the present invention, there is provided a luminance correcting circuit to be used in a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, wherein each of the luminance correcting circuits sets luminance of light to be emitted from each of the plurality of light emitting devices at a targeted value for every light emitting device and detects an amount of deviation in luminance of light from the targeted value occurring when each of the plurality of light emitting devices is turned ON, and makes the luminance of light coincide with the targeted value based on the detected amount of deviation.
In the foregoing second aspect, a preferable mode is one that wherein includes:
a plurality of photo-detecting devices each being mounted so as to correspond to each of the plurality of light emitting devices in a one-to-one relationship and to receive light emitted from each of the plurality of light emitting devices; and
a plurality of driving/correcting circuits each being mounted so as to correspond to each of the plurality of light emitting devices in a one-to-one relationship and to supply driving power to each of the plurality of light emitting devices and to detect deviation in luminance of light emitted from each of the plurality of light emitting devices from a targeted value, based on a level of the luminance detecting signal, and to correct the driving power so that the deviation is compensated for.
According to a third aspect of the present invention, there is provided a luminance correcting method to be applied to a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, including;
setting luminance of light to be emitted from each of the plurality of light emitting devices at a targeted value for every light emitting device;
detecting an amount of deviation in luminance of light from the targeted value occurring when each of the plurality of light emitting devices is turned ON; and
making the luminance of light coincide with the targeted value based on the detected amount of deviation.
According to a fourth aspect of the present invention, there is provided a flat lighting source including:
a plurality of light emitting devices;
a plurality of luminance detecting devices, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to receive light emitted from the corresponding light emitting device; and
a plurality of luminance correcting circuits, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to correct luminance of light emitted from the corresponding light emitting devices, based on a difference between a predetermined desirable luminance value and a detected luminance value obtained from the corresponding luminance detecting device, so as to maintain uniformity of the luminance of each of the light emitting devices.
In the foregoing fourth aspect, a preferable mode is one wherein the light emitting devices each include a light emitting diode.
According to a fifth aspect of the present invention, there is provided a liquid crystal display provided with a flat lighting source for backlighting,
the flat lighting source including:
a plurality of light emitting devices;
a plurality of luminance detecting devices, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to receive light emitted from the corresponding light emitting device; and
a plurality of luminance correcting circuits, each of which is provided, in a one-to-one relationship with each of the plurality of the light emitting devices, to correct luminance of light emitted from the corresponding light emitting devices, based on a difference between a predetermined desirable luminance value and a detected luminance value obtained from the corresponding luminance detecting device, so as to maintain uniformity of the luminance of each of the light emitting devices.
In the foregoing fifth aspect, a preferable mode is one wherein the light emitting devices each include a light emitting diode.
With the above configurations, there is provided a plurality of luminance correcting circuits each of which sets luminance of light to be emitted from each of the light emitting devices at a specified targeted value and detects an amount of deviation of luminance of light emitted from each of the light emitting devices from the specified targeted value and makes the luminance of light emitted from each of the light emitting devices coincide with the specified targeted value based on the amount of deviation and, therefore, luminance of light emitted from each of the light emitting devices and brightness of an entire display region of a display panel can be made uniform. Light having given luminance emitted from each of the light emitting devices is converted into a luminance detecting signal by each of the photo-detecting devices in a one-to-one relationship and the luminance detecting signal is fed back to each of the driving/correcting circuits and, therefore, variations and changes in luminance of light emitted from each of the light emitting devices can be automatically compensated for. Light having given luminance emitted from each of the light emitting devices is fed back and a process of extracting only an amount of change in temperature for feeding back is not required and, therefore, each of the temperature correcting circuits for each of the light emitting devices is made unnecessary. Each of the light emitting devices and each of the photo-detecting devices are mounted in the same package and in the vicinity of each other in a one-to-one relationship and, therefore, even if there are variations in luminance characteristic among the light emitting devices and the photo-detecting devices, luminance of light to be emitted from each of the light emitting devices can be made uniform by appropriately setting a targeted value of luminance. Each of the light emitting devices is mounted so as to correspond to each of the photo-detecting devices in a one-to-one relationship and, therefore, whichever color light out of R, G, and B is emitted by each of the light emitting devices, changes in color balance can be compensated for without using a color filter. As a result, when the flat lighting source is used as a backlight for a transmissive-type liquid crystal panel, it is made possible to provide illuminating light with uniform luminance to an entire display region of the liquid crystal panel.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages, and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram showing electrical configurations of main components of a flat lighting source according to a first embodiment of the present invention;
FIG. 2 is a circuit diagram showing one example of electrical configurations of LED driving/correcting circuits and driving/detecting circuits of FIG. 1;
FIG. 3 is a diagram showing one example of layout of the LEDs, photodiodes, set of LED driving/correcting circuits and driving/detecting circuits of FIG. 1;
FIG. 4 is a diagram showing another example of layout of the LEDs, photodiodes, LED driving/correcting circuits formed integrally with the driving/detecting circuits of FIG. 1;
FIG. 5 is a cross-sectional view showing layout of main components of the flat lighting source of the second embodiment of the present invention;
FIG. 6 is a cross-sectional view showing layout of main components of the flat lighting source of the third embodiment of the present invention;
FIG. 7 is a cross-sectional view showing layout of main components of the flat lighting source of the fourth embodiment of the present invention;
FIG. 8 is a cross-sectional view showing layout of main components of the flat lighting source of the fifth embodiment of the present invention;
FIG. 9 is a cross-sectional view showing layout of main components of the flat lighting source of the sixth embodiment of the present invention;
FIG. 10 is a cross-sectional view showing layout of main components of the flat lighting source of the seventh embodiment of the present invention;
FIG. 11 is a circuit diagram showing electrical configurations of luminance correcting circuits to be used in the flat lighting source of the eighth embodiment of the present invention;
FIG. 12 is a diagram showing electrical configurations of main components of an illuminating device disclosed in Patent Reference 1.
FIG. 13 is a diagram explaining operations of the illuminating device of FIG. 12;
FIG. 14 is a diagram showing one example of configurations of a conventional LED backlight.
FIG. 15 is a diagram showing configurations of LEDs used as a conventional backlight disclosed in Patent Reference 2;
FIG. 16 is a diagram showing another configurations of LEDs used as a conventional backlight disclosed in the Patent Reference 2;
FIG. 17 is a cross-sectional view of a backlight in which the LEDs shown in FIG. 15 or FIG. 16 are mounted; and
FIG. 18 is a block diagram showing configurations of a correcting circuit to correct luminance characteristic of the LEDs shown in FIG. 15 or FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Best modes of carrying out the present invention will be described in further detail using various embodiments with reference to the accompanying drawings. A flat lighting source is provided in which light having given luminance emitted from each LED used as a light emitting device is converted, in a one-to-one relationship, into a luminance detecting signal which is fed back to each luminance correcting circuit and luminance correcting circuits to be used in the flat lighting source are provided.

First Embodiment

FIG. 1 is a block diagram showing electrical configurations of main components of a flat lighting source according to the first embodiment of the present invention. The flat lighting source of the embodiment includes is used as a backlight of a transmissive-type liquid crystal panel mounted in a liquid crystal display panel and, as shown in FIG. 1, LEDs 71, photodiodes 72 as a photo-detecting device (luminance detecting device), LED driving/correcting circuits 73, and driving/detecting circuits 74. In FIG. 1, only one LED 71 is shown, however, the flat lighting source is made up of a plurality of LEDs serving as a backlight and arranged in a manner to form a planar surface. Each of the photodiodes 72 is mounted, in a one-to-one relationship, for every LED 71 and receives light emitted from each of the LEDs 71 and generates a luminance detecting voltage “a” having a level corresponding to the luminance of the received light. Each of the driving/detecting circuits 74 is mounted, in a one-to-one relationship, for every photodiode 72 and supplies power to each of the photodiodes 72 and sends out the luminance detecting voltage “a” generated by each of the photodiodes 72 as a luminance detecting voltage V2 to each of the LED driving/correcting circuits 73.
Each of the LED driving/correcting circuits 73 is mounted, in a one-to-one relationship, for every LED 71 and supplies driving power “c” to each of the LEDs 71 and detects, based on the luminance detecting voltage V2 fed from each of the driving/detecting circuits 74, a deviation from each targeted value (value corresponding to a luminance setting voltage V1) of luminance of light emitted from each of the LED and corrects the driving power “c” so as to compensate for the deviation from each of the targeted luminance value. In this embodiment in particular, each of the LED driving/correcting circuits 73 increases a current to be fed, for example, as luminance of light emitted from each of the LED 71 decreases. Each of the photodiodes 72, LED driving/correcting circuits 73, and driving/detecting circuits 74 make up each luminance correcting circuit. Each of the luminance correcting circuits sets luminance of light to be emitted from each of the LEDs 71 at a targeted value and detects an amount of deviation from the targeted value of the luminance of light emitted from each of the LEDs 71 and then makes the luminance of light emitted from each of the LEDs 71 coincide with the targeted value, based on the amount of deviation from the targeted value.
FIG. 2 is a circuit diagram showing one example of electrical configurations of each of the LED driving/correcting circuits 73 and driving/detecting circuits 74 of FIG. 1. Each of the LED driving/correcting circuits 73, as shown in FIG. 2, includes resistors 81, 82, 83, and 84, an operating amplifier 85, resistors 86, 87, and 88, an operating amplifier 89, a resistor 90, and an operating amplifier 91, and a resistor 92. Each of the driving/detecting circuits 74 includes a constant current circuit 93 and an operating amplifier 94.
FIG. 3 is a diagram showing one example of layout of each of the LEDs 71, photodiodes 72, set of LED driving/correcting circuits 73 and driving/detecting circuits 74 of FIG. 1. As shown in FIG. 3, On the PCB 75 is arranged one each of the LEDs 71, photodiodes 72, set of LED driving/correcting circuits 73 and driving/detecting circuits 74. In this state, each of the LEDs 71 has various shapes, that is, each of the LEDs 71 may be sealed hermetically or may have a shape of a bare chip. In the case of the bare chip, part or all of a portion of the PCB 75 in which each of the LEDs 71 is mounted is preferably sealed hermetically with a resin or a like. In the embodiment in particular, each of the LEDs 71 is mounted, in a one-to-one relationship, in the vicinity of each of the photodiodes 72 and each of the LED driving/correcting circuits 73 is integrally formed with each of the driving/detecting circuits 74.
FIG. 4 is a diagram showing another example of layout of the LEDs 71, photodiodes 72, and LED driving/correcting circuits 73 formed integrally with the driving/detecting circuits 74 of FIG. 1. As shown in FIG. 4, on the PCB 75 are arranged 20 sets of the LEDs 71, photodiodes 72, and LED driving/correcting circuits 73 formed integrally with driving/detecting circuits 74. In this case, if any one of the photodiodes 72 in any one of the arrangement sets receives light from any one the LEDs 71 contained in an adjacent arrangement set, the photodiode 72 received light is affected by light emitted from any one the LEDs other than the LED 71 that originally has to provide light and, therefore, it is preferable that a structural measure to shield the photodiodes 72 from light emitted from other LEDs 71 is taken.
According to the luminance correcting method to be used for the flat lighting source of the present invention, luminance of light emitted from each of the LEDs 71 is set at a specified targeted value and an amount of deviation from the targeted value occurring when each of the LEDs 71 is turned ON is detected and, based on the amount of the deviation, the luminance of light emitted from each of the LEDs 71 is corrected so as to coincide with the targeted value already set. At this time point, a current or voltage is supplied by each of the LED driving/correcting circuits 73 shown in FIG. 4 to each of the LEDs 71. In this case, either of constant current driving or constant voltage driving may be applied as a driving method. Part of light components with given luminance emitted from each of the LEDs 71 is received by each of the photodiode 72 and is converted into electrical signals and a resistance value of each of the photodiodes 72 decreases in proportion to the luminance of received light accordingly. That is, if luminance of light emitted from each of the LEDs 71 increases, a resistance value of each of the photodiodes 72 decreases while the luminance of light emitted from each of the LEDs 71 decreases, the resistance value of each of the photodiodes 71 increases. The resistance value is detected by each of the driving/detecting circuits 74 and the resistance value is fed back to each of the LED driving/correcting circuits 73. Each of the LED driving/correcting circuits 73 changes a driving current or driving voltage so that luminance of light emitted from each of the LEDs 71 becomes a level corresponding to a luminance setting voltage V1.
As shown in FIG. 2, a luminance setting voltage V1 is input to determine a driving current of each of the LEDs 71 and luminance of light emitted from each of the LEDs 71 is set according to the luminance setting voltage V1. Then, a current I0 is fed from each of the LED driving/correcting circuit 73 to each of the LEDs 71. The current I0 is shown by the following equation (1):
I0=V3/RSC (1)
where “RSC” denotes a resistance value of the resistor 90. The voltage V3 that determines the current I0 is a voltage output from an adding and subtracting circuit made up of the operational amplifiers 85 and is given by the following equation (2):
V3=(−R2/R1)×V1+(R4/R3)×V2 (2)
where “R1” denotes a resistance value of the resistor 81, “R2” a resistance value of the resistor 82, “R3” a resistance value of the resistor 83, and “R4” a resistance value of the resistor 84.
The luminance detecting voltage V2 is the driving voltage of each of the photodiodes 72 that has already been fed through each of the operational amplifiers 94 serving as a voltage follower to each of the LED driving/correcting circuit 73. Each of the photodiodes 72, since it is driven by each constant current circuit 93, has a resistance value corresponding to luminance of light emitted from each of the LEDs 71. That is, when luminance of light emitted from each of the LEDs 71 decreases, a resistance value of each of the photodiodes 72 increases and a driving voltage of each of the photodiodes 72 increases, thus causing an increase in the luminance detecting voltage V2. When the luminance voltage V2 increases, the voltage V3 is increased to become a voltage being R4/R3 times larger than the luminance detecting voltage V2 according to the equation (2). An increase in the voltage V3 causes an increase in a driving current of each of the LEDs 71 and further an increase in the luminance of light emitted from each of the LEDs 71. Therefore, by selecting the resistance values R1, R2, R3, and R4 and a current Id of each of the constant current circuits 93, a voltage is fed back from each of the photodiode 72 and, as a result, a change in luminance of light emitted from each of the LEDs 71 is compensated for.
Thus, in the flat lighting source of the first embodiment, since light having given luminance emitted from each of the LEDs 71 is converted by each of the photodiodes 72, in a one-to-one relationship, into a luminance detecting voltage V2 and the luminance detecting voltage V2 is fed back to each of the driving/detecting circuit 74, variations and changes in luminance of light emitted from each of the LEDs 71 are automatically compensated for. Moreover, since light having the given luminance emitted from each of the LEDs 71 is fed back, a process of extracting only an amount of temperature change for feeding-back is not required, there is no need for mounting a temperature correcting circuit for each of the LEDs 71. Also, since each of the LEDs 71 is mounted, in a one-to-one relationship, in the vicinity of each of the photodiodes 72, luminance of light emitted from each of the LEDs 71 is individually corrected and, therefore, even if there are variations in luminance characteristic among the LEDs 71 and among the photodiodes 72, luminance of light emitted from each of the LEDs 71 is made uniform by appropriately setting the luminance setting voltage V1. Furthermore, each of the LEDs 71 corresponds to each of the photodiodes 72 and, therefore, whichever color out of R (Red), G (Green), and B (Blue) is emitted from each of the LEDs 71, changes in color balance are compensated for without using a color filter.

Second Embodiment

FIG. 5 is a cross-sectional view showing layout of main components of the flat lighting source of the second embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the first embodiment shown in FIG. 4. In the flat lighting source of the second embodiment, as shown in FIG. 5, each of the LED 71, photodiodes 72, LED driving/correcting circuits 73, and driving/detecting circuits 74 is mounted in the same package 76. In the packet 76, terminals or a like (not shown) to be used fro connection to external components are mounted. According to the flat lighting source of the second embodiment, each of the LEDs 71 is arranged, in a one-to-one relationship, in the vicinity of each of the photodiode 72 in the same package 76 and each of the LED driving/correcting circuits 73 and each of the driving/detecting circuits 74 are integrally formed and, therefore, the second embodiment has the same advantage as the first embodiment.

Third Embodiment

FIG. 6 is a cross-sectional view showing layout of main components of the flat lighting source of the third embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the second embodiment shown in FIG. 5. In the lighting source of the third embodiment, as shown in FIG. 6, each of the photodiode 72, each of the LED driving/correcting circuit 73 and each of the driving/detecting circuits 74 are integrally formed to operate as one IC (Integrated Circuit) 77. As in the case of the first embodiment, each of the LEDs 71 is arranged, in a one-to-one relationship, in the vicinity of the photodiode 72 and, therefore, the same advantage as obtained in the first embodiment can be achieved in the third embodiment as well.

Fourth Embodiment

FIG. 7 is a cross-sectional view showing layout of main components of the flat lighting source of the fourth embodiment of the present invention. In the flat lighting source of the fourth embodiment, as shown in FIG. 7, each of the LED driving/correcting circuit 73 and each of the driving/detecting circuit 74 are mounted outside the package 76 and on the PCB 75 together with the package 76. The PCB 75 is made of organic materials or inorganic materials. According to the flat lighting source of the fourth embodiment, as in the first embodiment, each of the LEDs 71 is arranged, in a one-to-one relationship, in the vicinity of the photodiode 72 and, therefore, the same advantage as obtained in the first embodiment can be achieved.

Fifth Embodiment

FIG. 8 is a cross-sectional view showing layout of main components of the flat lighting source of the fifth embodiment of the present invention. In the flat lighting source of the fifth embodiment, as shown in FIG. 8, the same package 76 as shown in FIG. 8 is mounted on the PCB 75 and each correcting device 78 is newly mounted on the PCB 75. Each of the correcting devices 78 is made up of, for example, a variable resistor, a thick film printed resistance device to be trimmed by laser, a Zener zapping device consisting of a resistor and a Zener diode, a memory device that can write data corresponding to a targeted value of luminance of each of LEDs, or a like and is configured so as to provide a luminance setting voltage V1 to each of the LED driving/correcting circuit 73. Therefore, even if there are variations in luminance characteristic among the LEDs 71 and among the photodiodes 72, luminance of light emitted from each of the LEDs 71 can be controlled to be uniform by appropriately setting the luminance setting voltage V1 using each of the correcting devices 78.

Sixth Embodiment

FIG. 9 is a cross-sectional view showing layout of main components of the flat lighting source of the sixth embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the fourth embodiment shown in FIG. 7. In the flat lighting source of the sixth embodiment, as shown in FIG. 9, each of the correcting devices 78 shown in FIG. 8 is mounted on the PCB 75 shown in FIG. 7. Therefore, even if there are variations in luminance characteristic among the LEDs 71 and among the photodiodes 72, luminance of light emitted from each of the LEDs 71 can be controlled to be uniform by appropriately setting the luminance setting voltage V1 using each of the correcting devices 78.

Seventh Embodiment

FIG. 10 is a cross-sectional view showing layout of main components of the flat lighting source of the seventh embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the sixth embodiment shown in FIG. 9. In the flat lighting source of the seventh embodiment, as shown in FIG. 10, each of the LED driving/correcting circuits 73, driving/detecting circuits 74, and correcting devices 78 are integrally formed to operate as one IC 79. Therefore, as in the case of the sixth embodiment, even if there are variations in luminance characteristic among the LEDs 71 and among the photodiodes 72, luminance of light emitted from each of the LEDs 71 can be controlled to be uniform by appropriately setting the luminance setting voltage V1 using each of the correcting devices 78.

Eighth Embodiment

FIG. 11 is a circuit diagram showing electrical configurations of a luminance correcting circuit to be used in the flat lighting source of the eighth embodiment of the present invention and the same reference numbers are assigned to components having the same function as those of the first embodiment shown in FIG. 2. Each of the luminance correcting circuits of the eighth embodiment has, as shown in FIG. 11, the LED driving/correcting circuit 73A different from the LED driving/correcting circuit 73 shown in FIG. 2. Each of the LED driving/correcting circuits 73A includes resistors 81, 82, 83, and 84, an operational amplifier 85, an operational amplifier 95, an n-channel type MOSFET (Metal Oxide Semiconductor Field Effect Transistor (nMOS)] 96, and a variable resistor 97.
In this luminance correcting circuit, a current I0 flowing through the LED 71 is shown by the following equation (3).
I0=V4/RICC (3)
where “RICC” denotes a resistance value of the variable resistor 97.
Each of the luminance correcting circuit operates so that a voltage V3 of a non-inverted input terminal (+) of the operational amplifier 95 is equal to a voltage of its inverted input terminal (−) and, as a result, the voltage V3 is almost the same as the voltage V4. The current I0 flowing through each of the LEDs 71 is determined by the voltage V4 and the resistance value RICC of the variable resistor 97. By adjusting the resistance value RICC, a desired current is made to flow through each of the LEDs 71 and each of the LEDs 71 emits light with a desired luminance. Moreover, the operational amplifier 85 makes up an adding/subtracting circuit as in the first embodiment and, therefore, by changing a luminance setting voltage V1, the voltage V3 changes. Thus, in the luminance correcting circuit having configurations different from those in the first embodiment, the same advantage as obtained in the first embodiment can be achieved.
It is apparent that the present invention is not limited to the above embodiments but may be changed and modified without departing from the scope and spirit of the invention. For example, the luminance correcting circuit shown in FIG. 2 or FIG. 11 may have any configuration so long as it has the same functions as the configurations shown in FIG. 2 or FIG. 11 have.
Moreover, the present invention can be applied widely to cases where illuminating light with uniform luminance should be provided to an entire displaying region of a display panel, for example, of a liquid crystal display device in which illuminating light is supplied by a backlight.

Claims

1. A flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, comprising:

a plurality of luminance correcting circuits each to set luminance of light to be emitted from each of said plurality of light emitting devices at a targeted value for every light emitting device and to detect an amount of deviation in luminance of light from said targeted value occurring when each of said plurality of light emitting devices is turned ON, and to make said luminance of light coincide with said targeted value, based on the detected amount of deviation.

2. The flat lighting source according to claim 1, wherein said luminance correcting circuit comprises:

a plurality of photo-detecting devices so mounted as to correspond to each of said plurality of light emitting devices in a one-to-one relationship which receives light emitted from each of said plurality of light emitting device and generates a luminance detecting signal having a level corresponding to luminance of light emitted from each of said plurality of light emitting devices; and

a plurality of driving/correcting circuits each being mounted so as to correspond to each of said plurality of light emitting devices in a one-to-one relationship, each of which supplies driving power to each of said plurality of light emitting devices and detects deviation in luminance of light emitted from each of said plurality of light emitting devices from a targeted value, based on a level of said luminance detecting signal, and corrects said driving power so that said deviation is compensated for.

3. The flat lighting source according to claim 2, wherein each of said plurality of said photo-detecting devices is arranged, in a one-to-one relationship, in a vicinity of each of said plurality of said light emitting devices.

4. The flat lighting source according to claim 3, wherein said plurality of light emitting devices and said plurality of photo-detecting devices are mounted together in a same package.

5. A luminance correcting circuit to be used in a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, wherein each of said luminance correcting circuits sets luminance of light to be emitted from each of said plurality of light emitting devices at a targeted value for every light emitting device and detects an amount of deviation in luminance of light from said targeted value occurring when each of said plurality of light emitting devices is turned ON, and makes said luminance of light coincide with said targeted value based on the detected amount of deviation.

6. The luminance correcting circuit according to claim 5, further comprising:

a plurality of photo-detecting devices each being mounted so as to correspond to each of said plurality of light emitting devices in a one-to-one relationship and to receive light emitted from each of said plurality of light emitting devices; and

a plurality of driving/correcting circuits each being mounted so as to correspond to each of said plurality of light emitting devices in a one-to-one relationship and to supply driving power to each of said plurality of light emitting devices and to detect deviation in luminance of light emitted from each of said plurality of light emitting devices from a targeted value, based on a level of said luminance detecting signal, and to correct said driving power so that said deviation is compensated for.

7. A luminance correcting method to be applied to a flat lighting source having a plurality of light emitting devices so arranged as to form a planar surface and letting illuminating light come in a display region of a transmissive-type display panel from its rear, comprising;

setting luminance of light to be emitted from each of said plurality of light emitting devices at a targeted value for every light emitting device;

detecting an amount of deviation in luminance of light from said targeted value occurring when each of said plurality of light emitting devices is turned ON; and

making said luminance of light coincide with said targeted value based on the detected amount of deviation.

8. A flat lighting source comprising:

a plurality of light emitting devices;

a plurality of luminance detecting devices, each of which is provided, in a one-to-one relationship with each of said plurality of said light emitting devices, to receive light emitted from the corresponding light emitting device; and

a plurality of luminance correcting circuits, each of which is provided, in a one-to-one relationship with each of said plurality of said light emitting devices, to correct luminance of light emitted from the corresponding light emitting devices, based on a difference between a predetermined desirable luminance value and a detected luminance value obtained from the corresponding luminance detecting device, so as to maintain uniformity of the luminance of each of said light emitting devices.

9. The flat lighting source according to claim 8, wherein said light emitting devices each comprise a light emitting diode.

10. A liquid crystal display provided with a flat lighting source for backlighting,

the flat lighting source comprising:

a plurality of light emitting devices;

11. The liquid crystal display according to claim 10, wherein said light emitting devices each comprise a light emitting diode.