The present invention relates to optical displays, such as Dynamic Foil Displays, and in particular to the driving of such displays using subfields.
An optical display is a display in which each pixel independently modulates light from a light source, such as a backlight, a front light, an illumination light, or a light guide, to generate an image.
A Dynamic Foil Display (DFD) typically comprises a display panel having a light guide plate acting as an active plate, a passive plate and a movable foil sandwiched between these plates as well as selection means. The movable foil is arranged with a transparent electrode, to which a foil voltage can be applied. Pixels are typically arranged in a matrix configuration, each pixel being located at the intersection of a horizontal scan electrode arranged on the passive plate and a vertical address electrode arranged on the active plate. Depending on the voltage setup between the scan, address and foil electrodes, electrostatic forces can be created locally forcing the foil either to the active or to the passive plate, resulting in the pixel being either activated or inactivated, respectively. Thus, each pixel is either in an active, light decoupling state or in an inactive, light blocking state, there is no state in-between. In case a pixel is activated, the movable foil is locally brought into contact with the light guide plate and light is consequently decoupled out of the light guide plate into the foil where it scatters out of the display, resulting in a bright pixel. The pixel remains in this active state until it is deactivated, i.e. the contact is interrupted, and vice versa.
A pixel is said to be in an ON-state when the movable foil is locally brought into contact with the light guide, and to be in an OFF-state when the foil is locally in contact with the passive plate. Typically, a display is addressed one row at a time. When designing the addressing scheme for such a display, the time it takes to address, or scan, one row is commonly called a time slot. Thus, the addressing of every pixel in one row needs one time slot. During that time slot, some of the pixels are typically activated into their ON-state while the other pixels are left in their OFF-state. Of course, the duration of the time slot is independent of the number of activated pixels. Similarly, one time slot is needed for erasing a row, i.e. deactivating the pixels. However, the time slot for erasing does not need to have the same duration as the time needed for addressing.
Since the pixels are restricted to be either in an ON-state or in an OFF-state, gray scales are not readily provided for. Thus, in order to create gray scales at pixels, the frame time for each image is divided into a set of subfields. Each subfield comprises an addressing interval, an active interval and an erase interval, each interval having a predetermined duration. In case a pixel is to be active during a particular subfield, it is activated during the activation interval, it decouples light during the active interval and it is deactivated during the deactivation interval. In case the pixel is not to be activated, it is left deactivated during the activation interval and, consequently, does not decouple light during the active interval. Depending on the duration of the active interval, a subfield potentially (in case it is activated) contributes a certain amount of brightness to the image. Typically, the active intervals of a set of subfields have different durations, thus potentially contributing different amounts of light. However, the addressing and erasing intervals are the same for every subfield and thus independent from the duration of the active intervals.
Such a configuration facilitates a large set of gray scale levels to be displayed during a frame time, simply by choosing to activate different combinations of subfields during the frame time. In other words, by displaying all subfields consecutively during one frame time, some of which are active and some of which are inactive, the total fraction of time that light is decoupled during each frame time at a pixel is controlled and gray scales are created. The dullest (and non-zero) gray scale is achieved activating only the subfield having the shortest active interval, and the brightest gray scale is achieved activating all subfields.
A DFD of a general type is known from WO99/28890.
However, the use of subfields in order to provide gray scales is experienced to have certain limitations and drawbacks. For example, it is difficult to provide bright enough images for use in sunshine conditions. Furthermore, when used in dark ambient conditions the number of available gray scales is too limited to provide for the desired image quality. In addition, severe artifacts might affect the image quality. Consequently, there is a need for improved optical display devices in which the above problems are alleviated.
For the purpose of the present invention, it is recognized that a compromise has to be made between the brightness and the quality of the image when operating an optical display. It is furthermore recognized that the critical trade-off factor is the total number of subfields into which each frame time is divided. Basically, a high brightness can be obtained when a low number of subfields are employed, i.e. when the total addressing time (the sum of the activating and deactivating intervals for all subfields in a frame) is low and a large fraction of the frame time can be used for light generation in the active intervals. With a low number of subfields, the image quality is poor, however, due to the occurrence of severe motion artifacts (dynamic false contouring, blur, double images) and/or a limited number of gray levels. When employing a large number of subfields, the number of gray levels can be increased and coding rules can be introduced which results in good image quality. For this purpose, various coding rules are available and well known in the art. However, a larger fraction of the frame time is used for addressing the display and only a limited fraction of the frame time is available for light generation. Thus, the maximum available brightness will be low.
The problems related to subfields are substantially alleviated by the optical display device according to claim 1 and by the method according to claim 7. The appended subclaims provide preferred embodiments of the invention.
Thus, according to a first aspect of the invention an optical electronic information display device is provided which is operative to display images during frame times. The display device is arranged to operate in either one of at least two modes of operation, wherein
For the purpose of the invention, the importance of controlling the number of subfields, thus finding the right balance between for example brightness and other image quality factors, is recognized. Thus, the invention according to claim 1 provides for dynamic adaptation of the number of subfields used for each time frame. Consequently, instead of having to make a final compromise at the time of manufacturing of the device, the number of subfields can be dynamically adapted during the operation of the device. Switching between the different modes of operation can be made manually or automatically.
According to one preferred embodiment the display device comprises an ambient light sensor device and the means for switching is responsive to an output of the sensor device to make the display operate in the first mode of operation when the sensor is exposed to bright ambient conditions and to make the display operate in the second mode of operation when the sensor is exposed to dull ambient conditions.
This embodiment thus facilitates automatic switching between different modes of operation, depending on the ambient light condition. Consequently, the invention provides automatic adaptation of the number of subfields to the amount of ambient light. In a bright environment, it is most important to achieve the maximum brightness, and a limited loss in gray level rendering at the lower gray values is acceptable. In a dull environment, however, the lowest gray values should also be well distinguishable, but the maximum brightness can be reduced. The human visual system has adapted to the dull environment and a reduced maximum brightness is then even preferred.
According to another embodiment, the means for switching is controllable by means of user input. This is advantageous in that it provides manual switching between the different modes of operation. The user can thus choose the mode of operation individually. It is also possible to combine a light sensing function with a user control function, for example by having user input means which, when used, overrides the sensor signal.
According to another embodiment, a larger number of gray scales are provided in the second mode of operation than in the first mode of operation. This is advantageous in that the second mode of operation thus provides for higher image quality.
According to another embodiment, a first set of coding rules are employed in the first mode of operation and a second set of coding rules are employed in the second mode of operation, the first and the second sets of coding rules being different from each other. Thereby, it is possible to dynamically adapt the coding rules to, for example, the ambient light conditions. This is advantageous in that it provides improved image quality. It is, for example, possible to reduce motion artifacts when employing the second mode of operation as compared to the first mode of operation.
According to another embodiment, the first mode of operation provides brighter images than does the second mode of operation. This is advantageous in that the trade-off between image brightness and image quality can be made. For example, the first mode of operation can be used in order to provide bright images in bright ambient conditions, and the second mode of operation can be used in order to provide a larger number of gray scales and/or reduced motion artifacts. Instead of having to make a final selection at the time of manufacturing the device, the number of subfields can be dynamically adapted during the operation of the device.
According to a second aspect of the invention, a method of operating an optical display device that is operative to display images during frame times is provided, comprising the steps of:
Thus, the inventive method provides dynamic adaptation of the mode of operation and thus the number of subfields into which each frame time is divided. The method is advantageous in that the mode of operation can be adapted in response to for example varying driving conditions or user input. The different numbers of subfields facilitate the use of for example different coding rules, different number of gray scales and different brightness for a displayed image.
According to one embodiment, the method further comprises the step of:
Preferably, a mode of operation using a smaller number of subfields is selected for high levels of ambient light and a mode of operation using a higher number of subfields is selected for low levels of ambient light. The ambient light level can be determined by means of an ambient light sensor, which provides a control signal to driver circuitry of the display. This is advantageous in that the adaptation to different ambient light levels can be performed automatically.
According to one embodiment, the step of selecting a mode of operation depends on a trade-off between required brightness for the image and a number a gray scales for the image. If a large number of gray scales has priority, a larger number of subfields is selected, whereas a smaller number of subfields is selected if a high brightness has priority.
Thus, the basic idea underlying the present invention is the insight that the number of subfields can be adapted dynamically in order to adapt the optical display to different driving conditions. The dynamic adaptation is preferably based on a tradeoff between factors such as preferred image brightness, preferred number of gray scales and preferred coding rules. In particular, a reduced number of subfields facilitates brighter images, whereas an increased number of subfields facilitates an increased number of gray scales and/or image quality enhancing coding rules.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The driving of a Dynamic Foil Display relies on bistability or hysteresis. The pixels are addressed by means of a voltage applied to a row electrode and a column electrode related to the pixel. The voltage applied to the electrodes can be classified in three different regions, an ON-region, in which the pixel is urged to its ON-state, an OFF-region, in which the pixel is urged to its OFF-state, and an intermediate, bistable region, in which the pixel remains in its current state.
There are two different principles on which the driving of optical displays can be based. Either a so-called Address-Display-Separated or Flash Light principle is employed, or a so-called Address-While-Display principle is employed. In case an Address-While-Display mode of operation is employed, the light guide is always carrying light and the pixels are thus always emitting light when they are in their ON-state. Such addressing schemes are illustrated in
In case an Address-Display-Separated or Flash Light mode of operation is employed, the light guide is turned off during the addressing intervals and turned on only during active ON intervals. Such an addressing scheme is illustrated in
An exemplifying addressing scheme for a Flash Light mode of operation is now described with reference to
Gray scales are made up by the appropriate combination of binary weighted subfields (BWS), or using subfields with other weights for improved (moving) image quality. The subfields are organized in an addressing scheme, which describes in what order the rows are being addressed with ON and OFF addressing actions. At each addressing slot, one row is being addressed while all other rows are kept at the “unselect” level (in which they stay in their current state). In an Address-While-Display scheme, the light in the light guide is always kept on, and the effect of ON- and OFF-addressing on the pixel's light output is immediate.
As a first example, the invention can be implemented on a Flash-Light-operated DFD with 500 lines. Thus, consider such a display that needs 1.5 ms to address one subfield (0.25 ms for erasing, plus 500 rows times 2.5 μs of addressing time per row). When 10 subfields are used in a 20 ms video field period, 15 ms is needed for addressing and only 5 ms are left for light generation. At high ambient light, the lowest-weighted subfield corresponds to a luminance contribution that is not (or hardly) visible: it is thus possible to drop this subfield and use only 9 subfields. Then 13.5 ms are needed for addressing and 6.5 ms are available for light generation: this allows an increase of brightness of 30%. In a dark environment however, 11 or even 12 subfields are preferred, resulting in a reduction of brightness of 30% (3.5 ms light generation) or 60% (2 ms light generation) respectively, but then the lowest gray level and the gray level resolution (step size) can be reduced by a factor of approximately 2 or 4, respectively. This example is shown schematically in
As a second example, illustrated in
As previously discussed, an ambient light sensor can be used to facilitate automatic, dynamic adaptation of the number of subfields.
The driving of a DFD panel 1007 is schematically illustrated with a block diagram in
The present invention can be applied to various types of subfield-driven displays that are based on binary modulation such as, notably:
1. Micro-mechanical optical systems, such as:—Digital Mirror Devices (MD, DLP);
2. Subfield-driven reflective or transmissive LCDs.
Furthermore, the invention can be implemented on sub-line driven displays (e.g. using Pulse-Width Modulation schemes) in which case each row is addressed using subfields, but only one row at a time is activated.
In conclusion, the present invention relates to the driving of optical displays, such as Dynamic Foil Displays, and provides dynamic adaptation of the number of subfields into which frame times of such displays are divided. The number of subfields can thus be adapted depending on for example the ambient light conditions and image quality requirement. In particular, a smaller number of subfields (605) facilitates brighter images, whereas a larger number of subfields (606) facilitates an increased number of gray scales and/or motion artifact reduction.
Number | Date | Country | Kind |
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02080571.9 | Dec 2002 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB03/05715 | 12/4/2003 | WO | 6/24/2005 |