Display device comprising an adjustable light source

Abstract

The display device (DD) comprises an adjustable light source (BL), a display panel (DP) with display pixels for modulating light originating from the light source (BL) and processing circuitry (P) coupled to the display panel (DP) and the light source (BL). The processing circuitry (P) has an input for receiving an input signal (V1) representing gray levels of pixels of an image to be displayed on the display panel (DP). The processor (P) comprises:

• • selecting circuitry (S) for selecting a dimmed brightness level of the light source (BL) in dependence on the gray levels of the image pixels, and
  • adaptation circuitry (A) for adapting the input signal (V1) in dependence on the dimmed brightness level.

Description

a

The display panel DP may have a gamma characteristic Gd that is different from the pre-correction function Gs. When driven by an adapted gray level x′ the display panel DP generates a light output L of:
L=(Lbdim/Lbmax)·Gd(x′).
In order to match the light output L to the brightness Ls of the image source the adapted gray level x′ should be:
x′=Gdi(Lbmax/Lbdim·Ls)=Gdi(Lbmax/Lbdim·Gsi(x)),
with Gdi and Gsi representing the inverse functions of Gd and Gs, respectively
The term (x−x₁) represents the deviations of the displayed gray value from the gray value x of the input signal, hence is a measure for the amount of clipping. The weighting may be linear by selecting b=1 or non-linear by selecting b>1. All embodiments provided for the weighting function f(x) apply mutatis mutandis for the weighting function g(x).

Another embodiment with soft clipping is illustrated in FIG. 4. Like in FIG. 3, the light output L as function of the gray level x is shown with x_max being the maximum gray level. The first curve C1 shows again the curve for an ideal panel. A fourth curve C4 illustrates the soft clipping for a dimmed brightness level Lbdim. Between gray levels x₃ and x₂ the fourth curve C4 follows substantially the first curve C1, similar to the third curve C3 as shown in FIG. 3. However the difference with the third-curve C3 is that below the gray level x₃ the relation of the light output L versus the gray level x is gradually flattened. When the gray level x approaches zero, the light output approaches the minimum brightness level Lbdim/CR. In the third curve C3 shown in FIG. 3, all gray levels between zero and x_thresd are rendered with the same minimum brightness level Lbdim/CR. In the fourth curve C4 with soft clipping as shown in FIG. 4, the gray levels between zero and x_thresd are rendered with a different brightness level. So, the gray levels below x_thresd remain discernable, thereby improving the perceived image quality. Similarly, the fourth curve

 As result of the fifth step M5 the value of the selected smoothed dimmed brightness level LbdimS(n) for the n^th image is available. This value is translated into a light source drive signal BLD which generates this brightness level when applied to the light source BL. Moreover, in the fifth step M5 the adaptation drive signal AD is generated, which, for example, comprises information about the gray level x₁ corresponding to the selected smoothed dimmed brightness level Lbdim.

Finally in a sixth step M6 the gray level x₁ corresponding to the selected smoothed dimmed brightness level Lbdim is used to determine according to which curve the gray levels of the input signal V1 should be adapted, for example according to the third curve C3 as shown in FIG. 3. Optionally soft clipping may be applied by as explained hereinbefore by applying the fourth curve C4 as shown in FIG. 4.

The smoothing as described in the fifth step M5 may be set to respond faster to an increase of the dimmed brightness level Lbdim(n) during subsequent images. This may be achieved by selecting a different constant q during the increase. The advantage is, that clipping in white areas is reduced for images with white areas following a sequence of dark images. At the same time the relatively slow response during a decrease of the dimmed brightness level Lbdim(n) for subsequent images, ensures that flickering of the displayed image is avoided. A suitable value for the constant q is 0.95 during a sequence of increasing dimmed brightness levels Lbdim(n) and q=0.05 during other sequences.

The smoothing may also take into account lamp parameters, for example at what rate a lamp is able to change its light output, or any limitations required to ensure an adequate lifetime of a lamp. The smoothing may also take into account actual operating conditions or historical operating conditions of a lamp.

FIG. 9 shows a block diagram of an embodiment of the display device according to the invention, which uses feedback. The diagram is the same as the one shown in FIG. 1, except that the input for the selection circuitry S is now the output signal V2. This means a feedback loop is present, wherein the selection circuitry S determines a dimmed brightness level Lbdim(n) for an n^th image of a sequence of images and a corresponding adaptation drive signal AD on the basis of gray levels of the output signal V2 corresponding to the (n−1)^th image.

When dimming the light source BL and simultaneously compensating for the dimming by adapting the input signal V1, the display panel operates at a higher transmission (or reflection) rate. Especially for LCD panels, at this higher transmission rate the viewing angle increases. This means that dimming in combination with adapting the input signal V1 as described hereinbefore has the additional advantage of improving the viewing angle for the dimmed images. Moreover a lamp of the light source requires less power when dimmed, so power is saved when dimming is applied. At the same time the lifetime of the lamp may be extended.

A further advantage is, that when the amount of dimming takes into account the operating limits of the light source, it is possible to select firstly the amount of dimming which the light source is able to follow, and then to determine the corresponding adaptation of the input signal V1. So, any mismatch, caused by the fact that the light source is not able to follow the requested brightness changes of subsequent images, is avoided, thereby avoiding picture quality deterioration.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. For example, the allocation of the features in the various blocks of software or hardware may be changed without departing from the scope of the appended, claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A method of adjusting a light source of a display device, the display device comprising a display panel having display pixels for modulating light originating from the light source; and processing circuitry coupled to the display panel and the light source, the processing circuitry having an input for receiving an input signal representing gray levels of pixels of an image to be displayed on the display panel, the method comprising:

selecting a dimmed brightness level of the light source in dependence on the gray levels of the image pixels, the selecting step comprising:

selecting the dimmed brightness level from a plurality of temporary dimmed brightness levels, said selecting in dependence on: (i) a number of occurrences of a gray level corresponding to a brightness level of display pixels above the temporary dimmed brightness levels, and/or (ii) a number of occurrences of a gray level corresponding to a brightness level of display pixels below a predetermined brightness level being a fixed or adjustable level determined in dependence on the temporary dimmed brightness levels, and

adapting the input signal in dependence on the dimmed brightness level.

2. The method as claimed in claim 1, the selecting step further comprising substantially minimize an error function including one or more weighted numbers of occurrences formed by multiplying each of the one or more numbers of occurrences by a weighting factor.

3. Processing circuitry having:

an input for receiving an input signal representing gray levels of pixels of an image to be displayed on a display panel of a display device, the display device comprising an adjustable light source, the display panel having display pixels for modulating light originating from the light source;

outputs for coupling to the display panel and the light source;

means for selecting a dimmed brightness level of the light source in dependence on the gray levels of the image pixels, the means for selecting being capable of:

selecting the dimmed brightness level from a plurality of temporary dimmed brightness levels, said selecting in dependence on: (i) a number of occurrences of a gray level corresponding to a brightness level of display pixels above the temporary dimmed brightness levels, and (ii) a number of occurrences of a gray level corresponding to a brightness level of display pixels below a predetermined brightness level being a fixed or adjustable level determined in dependence on the temporary dimmed brightness levels,; and

means for adapting the input signal in dependence on the dimmed brightness level.

4. A display device comprising the adjustable light source; the display panel with display pixels for modulating light originating from the light source; and the processing circuitry as claimed in claim 3.

5. A The display device as claimed in claim 4, the predetermined brightness level being formed by the maximum contrast ratio of the display panel and each of the dimmed brightness levels.

6. A The display device as claimed in claim 4, the processing circuitry further comprising means for determining a smoothed dimmed brightness level (Lbdim S(n)) for an image in dependence on the selected dimmed brightness level (Lbdim (n)) of the image and a previous smoothed dimmed brightness level (LbdimS(n−1)) of a previous image, wherein n is a sequence number of successive images.

7. A The display device as claimed in claim 6, the smoothing having a faster response time to an increasing dimmed brightness level of subsequent images than to a decreasing dimmed brightness level of subsequent images.

8. A The display device as claimed in claim 4, wherein the means for selecting a dimmed brightness level are further adapted to select the dimmed brightness level in dependence on a content of a part of the image.

9. A product comprising the display device as claimed in claim 4, and signal processing circuitry for providing the input signal.

10. The display device as claimed in claim 4, the means for selecting being adapted to substantially minimize an error function including one or more weighted numbers of occurrences formed by multiplying each of the one or more numbers of occurrences by a weighting factor.

11. A The display device as claimed in claim 10, the error function being formed by an addition of the one or more weighted numbers of occurrences.

12. A The display device as claimed in claim 11, the error function being substantially:

13. A The display device as claimed in claim 12, the weighting functions (f(x), g(x)) being substantially equal to one.

14. A The display device as claimed in claim 5 12, at least one of the weighting functions (f(x), g(x)) being formed by a sum (f_ij) of deviations (d(k1,k2)) of gray levels between a pixel and its neighboring pixels, with k1, k2 being indices identifying the neighboring pixels.

15. A The display device as claimed in claim 14, the pixel being the pixel having the highest sum (f_ij) of all pixels with this gray level in an image.

16. A The display device as claimed in claim 5 12, at least one of the weighting functions (f(x), g(x)) being formed by a deviation of the gray level from a gray level corresponding to each of the temporary dimmed brightness levels or by a deviation from a gray level corresponding to the predetermined brightness level.

17. A The display device as claimed in claim 10, the input signal comprising color components (R1, G1, M1) of the image, a component error function being determined for each of the color components (R1, G1, B1), the error function being formed by adding the component error functions.

18. Processing circuitry having:

an input for receiving an input signal representing gray levels of pixels of an image to be displayed on a display panel of a display device, the display device comprising an adjustable light source, the display panel having display pixels for modulating light originating from the light source;

outputs for coupling to the display panel and the light source;

means for selecting a dimmed brightness level of the light source in dependence on the gray levels of the image pixels, the means for selecting being capable of:

selecting the dimmed brightness level from a plurality of temporary dimmed brightness levels, said selecting in dependence on: (i) a number of occurrences of a gray level corresponding to a brightness level of display pixels above the temporary dimmed brightness levels, and (ii) a number of occurrences of a gray level corresponding to a brightness level of display pixels below a predetermined brightness level being a fixed or adjustable level determined in dependence on the temporary dimmed brightness levels, and

means for adapting the input signal in dependence on the dimmed brightness level;

wherein the means for selecting being adapted to substantially minimize an error function including one or more weighted numbers of occurrences formed by multiplying each of the one or more numbers of occurrences by a weighting factor; and,

wherein, the error function being substantially:

19. Processing circuitry comprising:

an input to receive an input signal representing gray levels of pixels of an image to be displayed on a display panel of a display device, the display device comprising an adjustable light source, the display panel having display pixels for modulating light originating from the light source;

outputs to couple to the display panel and the light source;

a processor configured to select a dimmed brightness level of the light source in dependence on the gray levels of the image pixels, the processor further configured to:

select the dimmed brightness level from a temporary dimmed brightness level, said selecting in dependence on

at least one of: (i) a number of occurrences of a gray level corresponding to a brightness level of display pixels above the temporary dimmed brightness level, or (ii) a number of occurrences of a gray level corresponding to a brightness level of display pixels below a predetermined brightness level being a fixed or adjustable level determined in dependence on the temporary dimmed brightness level, and

adapt the input signal in dependence on the dimmed brightness level.

20. The processing circuitry as claimed in claim 19, the processor further configured to substantially minimize an error function including one or more weighted numbers of occurrences formed by multiplying each of the one or more numbers of occurrences by a weighting factor.

21. The processing circuitry as claimed in claim 20, the error function being formed by an addition of the one or more weighted numbers of occurrences.

22. The processing circuitry as claimed in claim 21, the error function being substantially:

23. The processing circuitry claimed in claim 22, the weighting functions (f(x), g(x)) being substantially equal to one.

24. The processing circuitry as claimed in claim 22, at least one of the weighting functions (f(x), g(x)) being formed by a sum (f_ij) of deviations (d(k1,k2)) of gray levels between a pixel and its neighboring pixels, with k1, k2 being indices identifying the neighboring pixels.

25. The processing circuitry as claimed in claim 24, the pixel being the pixel having the highest sum (f_ij) of all pixels with this gray level in an image.

26. The processing circuitry as claimed in claim 22, at least one of the weighting functions (f(x), g(x)) being formed by a deviation of the gray level from a gray level corresponding to only one of the temporary dimmed brightness level or by a deviation from a gray level corresponding to the predetermined brightness level.

27. A display device comprising an adjustable light source; a display panel with display pixels for modulating light originating from the light source; and the processing circuitry as claimed in claim 19.

28. The display device as claimed in claim 27, the predetermined brightness level being formed by the maximum contrast ratio of the display panel and the dimmed brightness level.

29. The display device as claimed in claim 27, the processor being configured to determine a smoothed dimmed brightness level (Lbdim S(n)) for an image in dependence on the selected dimmed brightness level (Lbdim (n)) of the image and a previous smoothed dimmed brightness level (LbdimS(n−1)) of a previous image, wherein n is a sequence number of successive images.

30. The display device as claimed in claim 29, the smoothing having a faster response time to an increasing dimmed brightness level of subsequent images than to a decreasing dimmed brightness level of subsequent images.

31. The display device as claimed in claim 27, the processor being further configured to select the dimmed brightness level in dependence on a content of a part of the image.

32. The display device as claimed in claim 27, the processor being configured to substantially minimize an error function including one or more weighted numbers of occurrences formed by multiplying each of the one or more numbers of occurrences by a weighting factor.

33. A method of adjusting a light source of a display device, the display device comprising a display panel having display pixels for modulating light originating from the light source; and processing circuitry coupled to the display panel and the light source, the processing circuitry having an input for receiving an input signal representing gray levels of pixels of an image to be displayed on the display panel, the method comprising acts of:

selecting a dimmed brightness level of the light source in dependence on the gray levels of the image pixels, the selecting act comprising:

selecting the dimmed brightness level from a temporary dimmed brightness level, said selecting in dependence on

at least one of: (i) a number of occurrences of a gray level corresponding to brightness level of display pixels above the temporary dimmed brightness level, or (ii) a number of occurrences of a gray level corresponding to a brightness level of display pixels below a predetermined brightness level being a fixed or adjustable level determined in dependence on the temporary dimmed brightness level; and

adapting the input signal in dependence on the dimmed brightness level.

34. The method as claimed in claim 33, the selecting act further comprising substantially minimizing an error function including one or more weighted numbers of occurrences formed by multiplying each of the one or more numbers of occurrences by a weighting factor.

35. The method as claimed in claim 34, comprising an act of forming the error function by an addition of the one or more weighted numbers of occurrences.

36. The method as claimed in claim 35, the error function being substantially:

37. The method as claimed in claim 36, the weighting functions (f(x), g(x)) being substantantially equal to one.

38. The method as claimed in claim 36, comprising an act of forming at least one of the weighting functions (f(x), g(x)) by a sum (f_ij) of deviations (d(k1,k2)) of gray levels between a pixel and its neighboring pixels, with k1, k2 being indices identifying the neighboring pixels.

39. The method as claimed in claim 38, the pixel being the pixel having the highest sum (f_ij) of all pixels with this gray level in an image.

40. The method as claimed in claim 36, comprising an act of forming at least one of the weighting functions (f(x), g(x)) by a deviation of the gray level from a gray level corresponding to only one of the temporary dimmed brightness level or by a deviation from a gray level corresponding to the predetermined brightness level.