WO2006072866A1

WO2006072866A1 - Emissive display device

Info

Publication number: WO2006072866A1
Application number: PCT/IB2005/054422
Authority: WO
Inventors: Alan G. Knapp
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-01-05
Filing date: 2005-12-29
Publication date: 2006-07-13
Also published as: TW200629199A; GB0500149D0

Abstract

A light emitting display device comprising: a sensing means for sensing a brightness of ambient light; and a processing means for mapping input pixel brightness levels to output pixel brightness levels for use in driving an output of the display device, wherein the processing means varies the mapping in response to the sensed ambient light brightness, such that a spacing between adjacent lower output pixel brightness levels is increased and a spacing between adjacent higher output pixel brightness levels is decreased in response to an increase in the sensed ambient light brightness.

Description

DESCRIPTION

EMISSIVE DISPLAY DEVICE

This invention relates to an emissive display device, and a method of driving an emissive display device.

Emissive display devices are well known. Examples include cathode ray tubes, backlit liquid crystal display devices and polymer light emitting diode display devices. The common feature of all these display devices is that light is emitted from a display surface of the device to form a visible image.

A known problem associated with emissive display devices is degradation in the subjective appearance of visible images when the display surface of the device is exposed to high levels of ambient illumination. Specifically, ambient light is scattered, or reflected, by the display surface of the device and combines with the light that is emitted to form the image. The reflected ambient light can be greater than the emitted light coming from the darker parts of the displayed image, and this can cause the darker brightness levels of the displayed image to be indistinguishable by human eyesight.

For example, under exposure to strong sunlight, the lowermost (darkest) 20% of brightness levels of a display device may be indistinguishable by human eyesight.

A known way of reducing the above described problem is to increase the peak brightness of the display device. However, this approach leads to increased power dissipation and hence an increased operating temperature of the device. This approach may also lead to other undesirable effects such as reduced life of the device. Increased power dissipation is particularly undesirable in mobile applications as it results in reduced operating times between battery replacements or re-charging.

According to a first aspect of the invention, there is provided a light emitting display device comprising: a sensing means for sensing a brightness of ambient light; and a processing means for mapping input pixel brightness levels to output pixel brightness levels for use in driving an output of the display device, wherein the processing means varies the mapping in response to the sensed ambient light brightness, such that a spacing between adjacent lower output pixel brightness levels is increased and a spacing between adjacent higher output pixel brightness levels is decreased in response to an increase in the sensed ambient light brightness.

The invention provides a display device in which a spacing (i.e. the pitch or ratio between adjacent pixel brightness levels) is differentially varied across the range of pixel brightness levels when the display device is exposed to strong ambient light such as sunlight.

Lower (darker) pixel brightness levels are more difficult to visually distinguish when the display device is exposed to the strong ambient light and the spacing between adjacent ones of these lower pixel brightness levels is therefore increased, thereby enabling visual distinction between the adjacent pixel brightness levels.

Upper (brighter) pixel brightness levels are more visually distinguishable when the display device is exposed to the strong ambient light, i.e. the degradation is less than that of the lower pixel brightness levels. The spacing between adjacent ones of these upper pixel brightness levels is therefore reduced to compensate for the above described variation of the lower pixel brightness levels, but ensuring that adjacent upper pixel brightness levels remain visually distinguishable.

The invention thus provides an emissive display device in which pixel brightness levels are varied to compensate for the effect of reflected ambient light, without increasing a peak pixel brightness level of the device.

The display may be a directly modulated light sources such as an array of active matrix polymer light emitting diodes. Alternatively, the display device may be a indirectly modulated light source. For example, the display device may be an active matrix liquid crystal display device.

The display device preferably comprises a display driving means for driving the display pixels to 2ⁿ different pixel brightness levels, where n is a positive integer greater than 1. For example, the display driving means may be arranged to drive the display pixels to 64 different pixel brightness levels. The display driving means may be part of the processing means or may alternatively be a distinct element of the display device.

The sensing means preferably comprises at least one light sensor located about the periphery of the display device output. By providing several light sensors about the periphery of the display device output, an average measure of the brightness of ambient light may be obtained, thereby leading to greater accuracy. The provision of several light sensors may also be used to minimize the undesirable effect of concentrated environmental light beams, such as from reflections, on the measured brightness of ambient light.

The light sensors may be photodiodes or phototransistors. The sensing means preferably comprises a low pass filter for removing high frequency fluctuations in the sensed brightness of ambient light. In this way, short-term fluctuations in the brightness of ambient light are not reflected in the sensed value used to vary the different brightness levels.

The processing means is preferably arranged so that an uppermost brightness level is not varied, i.e. the mapping of the uppermost pixel brightness level is fixed regardless of the sensed values of ambient light. In this way, the image quality in high levels of ambient light may be improved without any appreciable increase in power consumption of the display device. The processing means is also preferably arranged so that a lowermost brightness level is not varied.

The processing means may be arranged to map the input pixel brightness levels to the output pixel brightness levels by applying a set of correction factors to the input pixel brightness levels. The correction factors preferably vary the pixel brightness levels by different amounts, i.e. some brightness levels are increased by more than others.

In some embodiments, the processing means comprises a memory device for storing at least one correction function, the set of correction factors then being derivable from the transfer function. For example, the transfer function may be a polynomial function. Parameters of the function may be determined by the sensed brightness of ambient light. In other embodiments, the processing means comprises a memory device for storing at least one look up table, the set of correction factors being derivable from the look up tables.

The processing means is preferably arranged to select the set of correction factors from a plurality of different sets of correction factors in dependence on the sensed brightness of ambient light. The magnitude of the selected set of corrective factors preferably increases with the sensed brightness of the ambient light. Thus, the corrective effect of the correction factors increases as the brightness of the ambient light increases.

In a preferred embodiment, the processing means is arranged to select between sets of correction factors with hysteresis. Thus, threshold values of ambient light brightness at which sets of correction factors are selected are higher when the ambient light brightness is increasing and lower higher when the ambient light brightness is decreasing. In this way, frequent switching between sets of correction factors may be avoided.

Where the display driving means is part of the processing means, the input pixel brightness levels may be for supply to the display driving means. In this case, the processing means may be is arranged to map the input pixel brightness levels to the output pixel brightness levels by varying a number of reference voltages which are used by the display driving means for deriving driving voltages. The driving voltages then correspond to the different brightness levels.

The invention also provides a portable electronics device comprising the display device described above.

According to a second aspect of the invention, there is provided a method of driving a light emitting display device, the method comprising: sensing a brightness of ambient light; mapping input pixel brightness levels to output pixel brightness levels for use in driving an output of the display device; and driving an output of the display device using the output pixel brightness levels, wherein the mapping is varied in response to the sensed ambient light brightness, such that a spacing between adjacent lower output pixel brightness levels is increased and a spacing between adjacent higher output pixel brightness levels is decreased in response to an increase in the sensed ambient light brightness.

The second aspect of the invention is essentially a method corresponding to the device of the first aspect of the invention.

Embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which:

Figure 1 is a graph showing the effect of different levels of ambient illumination on the ratio of adjacent pixel brightness levels of a known display device;

Figure 2 is a schematic representation of a first display device according to the invention;

Figure 3 is a graph illustrating operation of the display device shown in Figure 2;

Figures 4A and 4B are graphs showing the pixel brightness levels of a known display device and the display device shown in Figure 2;

Figure 5 is a graph showing ratios of adjacent pixel brightness levels of a known display device and the display device shown in Figure 2;

Figure 6 is a schematic representation of a second display device according to the invention;

Figure 7 is a graph illustrating operation of the display device shown in Figure 6; and

Figure 8 shows a portable electronics device according to the invention.

The effect of different levels of ambient illumination on the contrast and ratios of adjacent brightness levels of a known emissive display device will first be described with reference to Figure 1.

A known emissive display device has a luminance L(N), where N is a driving signal applied to display pixels of the device. If the display device is digitally driven display with six bit resolution, N ranges from 0 to 63. The overall contrast of the display device will therefore be L(63)/L(0) when the display device is not exposed to any ambient light, i.e. in the dark. Under exposure to ambient light, the display device reflects a fraction F of the ambient light A into the eyes of the viewer, which reflected light is added to light emitted from the display device. In this case, the perceived contrast ratio will be degraded from L(63)/L(0) to (L(63) + FA)/(L(0) + FA).

This degradation in image quality can be reduced by designing the display so that the fraction F is as small as possible and the peak luminance L(63) is as large as possible. However, there are practical limits on the reduction in the fraction F, and increasing the peak luminance L(63) introduces other detrimental effects such as increased power consumption and reduced life of the display device.

In addition to the above described reduction in the overall contrast ratio the ambient illumination has a further detrimental effect which specifically affects the lower, i.e. darker, brightness levels (grey levels) of the display device. In the dark (no ambient illumination), adjacent ones of the brightness levels have a brightness ratio of L(N+1)/L(N). However, under ambient illumination, this brightness ratio is reduced to a lower value, (L(N+1 ) + FA)/(L(N) + FA).

Figure 1 shows the effect of ambient illumination on the ratio of adjacent grey levels as a function of grey level for four different ambient illumination levels, namely 0, 300, 1000 and 3000. Figure 1 also shows a visibility threshold which represents the minimum brightness ratio that can be visually distinguished by human eyesight.

It should be noted that the Figure is intended for the purpose of explaining the general effect of the ambient illumination, and the units of ambient illumination and the level of the visibility threshold are arbitrary.

The grey levels in the Figure are arranged logarithmically, i.e. the value of L(N+1 )/L(N) for the display device is the same for all N, each grey level being a constant factor times as bright as the level below it. This is generally accepted as the way the human eyesight perceives equal brightness increments. Note that real display devices may be set up in different ways due to details of the implementation of the electronics. For example, a common approach is to make the luminance of the display device a power law function of N, such as L(N) = KN³. Having a different form of the pixel brightness versus N curve does not alter the principles discussed herein.

The visibility threshold is the level of the brightness ratio below which the human eyesight cannot perceive any difference in the brightness levels. In practice, this ratio depends on the external conditions among other factors, but the exact value of the visibility threshold does not alter the principles discussed herein. The actual value of the visibility threshold is often 1% to 2%. For the purpose of this discussion, it is assumed that 2% is the minimum noticeable brightness increment.

It is clear from Figure 1 that, when there is no ambient illumination, the ratios of brightness for all grey levels is constant and lies well above the visibility threshold. As the ambient illumination level increases, more ratios fall below the visibility threshold. At a level of 300, all adjacent grey levels below N=4 are indistinguishable, at 1000 units all those below N=17 are indistinguishable, and at an illumination of 3000 units all those below N=28 are indistinguishable.

The net effect of the ambient illumination is that a viewer cannot see any detail in the darker parts of an image displayed on the display device. As the ambient illumination level rises, this effect gets worse and affects an increasing range of brightness levels.

The invention provides an emissive display device in which the drive signals corresponding to the different pixel brightness levels of the device are varied so as to improve the perceived image quality under high ambient lighting conditions. In particular, the device according to the invention reduces the above described effects of ambient light without increasing the peak display brightness and hence with little penalty in power consumption.

Figure 2 shows a first emissive display device 1 according to the invention. The emissive display device is a backlit active matrix liquid crystal display device. The device 1 comprises an array of display pixels 3, each display pixel in the array being drivable to a varying brightness.

The display device 1 also comprises driving means 5 for independently driving the display pixels 3 to a plurality of different pixel brightness levels. The driving means comprises conventional row and column driver circuits, the structure and operation of which will be known to those skilled in the art.

The driving means 5 receives digital data signals, converts the digital data signals into analogue driving voltages, and addresses the display pixels 3 with the analogue driving voltages. The levels of the driving voltages correspond to the brightness levels of the display pixels 3. There are 64 brightness levels, corresponding to 6 bit resolution. For the purposes of explanation, the brightness levels are numbered 1 to 63.

The device also comprises sensing means 7 for sensing a brightness of ambient light to which the display pixels 3 are exposed. The sensing means comprises four photodiodes positioned around the periphery of the array 3. Signals representing the sensed brightness of ambient light from the four light sensors are averaged and the averaged signal is passed through a low pass filter to eliminate short-term fluctuations.

The averaged signal is then provided to a processing means 9. The processing means 9 is arranged to dynamically vary at least some of the plurality of different pixel brightness levels of the display device in response to changes in the sensed brightness of ambient light. Specifically, the processing means 9 varies mapping between input analogue driving voltages received from the driving means 5 and output analogue driving voltages for supply to the display pixels 3. The mapping is varied in response to the sensed ambient light brightness, such that a spacing between adjacent lower output pixel brightness levels is increased and a spacing between adjacent higher output pixel brightness levels is decreased in response to an increase in the sensed ambient light brightness.

The processing means 9 comprises a memory device 9a in which a plurality of look up tables are stored. Each look up table contains a set of correction factors which may be applied to respective different input analogue driving voltages from the driving means so as to vary the brightness levels of the display device. The corrected driving voltages are then provided to the display pixels 3 to generate an image having modified brightness levels. The corrective factors of a look up table are only applied to the input analogue driving voltages when the sensed brightness of ambient light is above a threshold value. As the sensed brightness increases, different look up tables are selected in turn by the processing means 9, and correction factors of successive look up tables have an increasingly corrective effect on the input analogue driving voltages, and hence the brightness levels.

The processing means 9 is arranged to select between the look up tables with hysteresis, as shown in Figure 3. Specifically, Figure 3 illustrates the process of selection of look up tables A, B, C and D in increasing and decreasing ambient illumination levels. It can be seen from the Figure that the threshold values of ambient illumination at which different look up tables are selected are greater when the ambient illumination level is rising than when the ambient illumination level is falling. In this way, rapid switching between look up tables is avoided.

As mentioned above, in order to improve the perception of pixel brightness levels, or grey levels, in the darker parts of a displayed image, the invention varies the pixel brightness levels. Essentially, a brightness against input data characteristic of the display is altered so that adjacent lower (darker) data levels have a greater brightness ratio. An example of the characteristic is shown in Figures 4A and 4B, where examples of two pixel brightness versus input data curves are plotted with linear (Figure 4A) and logarithmic (Figure 4B) brightness scales. Figure 4B is included so that the effects at low pixel brightness levels can be seen more clearly.

Curve A in the Figures shows a basic power law brightness (B) versus input data (N) curve of the form B = B₀ + N³ where B₀ the minimum pixel brightness. Curve A is representative of known display devices and the display device of the invention when exposed to low levels of ambient illumination.

Curve B in the Figures shows a modified pixel brightness (B) versus input data (N) curve according to the invention which improves the visibility of the darker grey scales when the display is illuminated with high levels of ambient light. A complex function, or mapping, has been applied to obtain curve B which causes the pixel brightness versus input data curve to have a steeper slope at low data levels but approximates to curve A as the data levels, and hence displayed pixel brightness, increase.

The effect of the modified brightness versus data curve according to the invention is illustrated in Figure 5 where the ratio of brightness L(N+1 )/L(N) of adjacent grey levels is plotted against input data value (N) for displays exposed to a high level of ambient illumination and having the pixel brightness versus data characteristics as shown in Figures 4A and 4B. A visibility threshold corresponding to a 2% brightness difference is also included in Figure 5.

For the basic curve A, all data values below the input data value of 12 fall below the visibility threshold and so would not be properly distinguishable. For the modified curve B, all data values lie well above the visibility threshold and so all grey levels would be distinguishable.

The invention thus provides a display device in which the form of the pixel brightness versus input data curves (L - N curves) are modified according to the ambient illumination level under which the display operates so that, at low ambient light levels one curve (for example, curve A in Figures 4A and 4B) is used and at some higher illumination level the pixel brightness versus input data characteristic is switched to that corresponding to another curve in which the pixel brightness steps at low brightness levels are increased (for example, curve B in Figures 4A and 4B).

Several modified L-N curves are available and the display switches to the most appropriate curve as the ambient illumination level increases, as illustrated in Figure 3.

Figure 6 shows a second display device according to the invention. The second display device is similar to the first display device shown in Figure 1 , and like elements have the same reference numbers.

Instead of storing a plurality of look up tables, the processing means 9 stores a plurality of correction functions in the memory 9a. Each correction function may be applied to a set of reference voltages V_REF before the reference voltages are provided to the display driving means 5. The reference voltages are used by the display driving means 5 to derive the driving voltages which are representative of the brightness levels, as shown in Figure 7.

Referring to Figure 7, nine reference voltages 11 are represented by vertical dashed lines. The reference voltages correspond to the driving voltages for brightness levels numbered 0, 7, 15, 23, 31 , 39, 47, 55 and 63. Intermediate values are linearly interpolated by the driving means so that, for example, if 15<N<23 then V(N) = V(15)+(N-15)*(V(23)-V(15))/8. By applying the correction function to the reference voltages, the shape of the brightness versus data curve can be varied in accordance with the invention.

In Figure 7, two driving voltage versus data value curves are shown for the display device. The dashed curve is representative of a prior art display device and the display device of the invention when exposed to low levels of ambient illumination. The continuous curve is representative of the display device of the invention when exposed to higher levels of ambient illumination. In practice, several curves are provided for by several different correction functions. The selection of a particular correction function is dependent on the sensed brightness of ambient light.

The driving voltages from the driving means are then used to address the display pixels 3, the levels of the driving voltages corresponding to brightness levels of the display pixels 3.

Figure 8 shows a mobile device 13 according to the invention. The mobile device 13 is a mobile telephone comprising a casing 15, data entry means 17, batteries (not shown), electronic circuit (not shown) and a display 19 according to the invention, as described above.

The display device 19 of the mobile device 13 exhibits improved performance under high levels of ambient illumination, without a significant increase in power consumption.

Specific examples of the invention have been described above. Various modifications may be made to the examples without departing from the scope of the invention as defined by the claims. These modifications will be apparent to those skilled in the art. The above described examples use a plurality of look up tables or correction functions to vary a spacing between pixel brightness levels. However, a single correction function could be used. For example, a polynomial could be used in which the parameters of the function are determined by the sensed ambient illumination level.

The above described examples vary a spacing between pixel brightness levels by varying analogue display driving voltages or analogue reference voltages supplied to a display driving means. However, the invention could be implemented to vary digital display data that is representative of pixel brightness levels. Those skilled in the art will appreciate that this implementation would require a significant increase in the resolution of the digital data, so that sufficient digital levels are available to provide a smooth variation in spacing between levels.

The invention is particularly suitable for battery operated devices, such as portable devices, because it enables an improvement in image quality without requiring increased maximum driving voltages. The display devices in such devices may be matrix array display devices, active or passive.

The display device according to the invention is particularly useful for portable electronics devices which are used outdoors and which are operated from batteries, for example mobile telephones. However, the invention is also applicable to a wide range of other devices which are operated at normal or high levels of ambient light.

Claims

1. A light emitting display device (1 ) comprising: a sensing means (7) for sensing a brightness of ambient light; and a processing means (9) for mapping input pixel brightness levels to output pixel brightness levels for use in driving an output (3) of the display device, wherein the processing means (9) varies the mapping in response to the sensed ambient light brightness, such that a spacing between adjacent lower output pixel brightness levels is increased and a spacing between adjacent higher output pixel brightness levels is decreased in response to an increase in the sensed ambient light brightness.

2. A device according to claim 1 , wherein the sensing means (7) comprises at least one light sensor located about the periphery of the output of the display device.

3. A device according to claim 1 or 2, wherein the sensing means (7) comprises a low pass filter for removing high frequency fluctuations in the sensed ambient light brightness.

4. A device according to any preceding claim, wherein the processing means (9) is arranged so that the mapping for an uppermost input pixel brightness level is not varied.

5. A device according to any preceding claim, wherein the processing means (9) is arranged to map the input pixel brightness levels to the output pixel brightness levels by applying a set of correction factors to the input pixel brightness levels.

6. A device according to claim 5, wherein the processing means (9) comprises a memory device (9a) for storing at least one correction function, the set of correction factors being derivable from the correction function.

7. A device according to claim 5, wherein the processing means (9) comprises a memory device (9a) for storing at least one look up table, the set of correction factors being derivable from the look up tables.

8. A device according to any of claims 5 to 7, wherein the processing means (9) is arranged to select the set of correction factors from a plurality of different sets of correction factors in dependence on the sensed brightness of ambient light.

9. A device according to claim 8, wherein the processing means (9) is arranged to select between sets of correction factors with hysteresis.

10. A device according to any preceding claim, wherein the input pixel brightness levels are driving voltages from a display driving means (5).

11. A device according to any of claims 1 to 4, wherein the processing means (9) comprises a display driving means (5), the input pixel brightness levels being for supply to the display driving means (5), and wherein the processing means (9) is arranged to map the input pixel brightness levels to the output pixel brightness levels by varying reference voltages (V_REF), the reference voltages being for use by the display driving means (5) in deriving driving voltages.

12. A portable electronics device (13) comprising a device (19) according to any of the preceding claims.

13. A method of driving a light emitting display device (1 ), the method comprising: sensing a brightness of ambient light; mapping input pixel brightness levels to output pixel brightness levels for use in driving an output (3) of the display device; and driving an output (3) of the display device using the output pixel brightness levels, wherein the mapping is varied in response to the sensed ambient light brightness, such that a spacing between adjacent lower output pixel brightness levels is increased and a spacing between adjacent higher output pixel brightness levels is decreased in response to an increase in the sensed ambient light brightness.

14. A method according to claim 13, wherein the mapping for an uppermost input pixel brightness level is not varied.

15. A method according to claim 13 or 14, wherein the input pixel brightness levels are mapped to the output pixel brightness levels by applying a set of correction factors to the input pixel brightness levels.

16. A method according to claim 15, wherein the set of correction factors is selected from a plurality of different sets of correction factors in dependence on the sensed brightness of ambient light.

17. A method according to claim 13, wherein the input pixel brightness levels are mapped to the output pixel brightness levels by varying reference voltages (V_REF), the reference voltages being for use in deriving driving voltages.