Methods for driving video electro-optic displays

Abstract

Video displays using relatively low frame rates of about 10 to about 20 frames per second, but having acceptable video quality are described. The displays may use bistable media, and may be driven such that the medium, when driven, changes its optical properties continuously during the driving of each frame. The displays may use an electro-optic medium such that the frame period is from about 50 to about 200 per cent of the switching time of the electro-optic medium at the driving voltage used.

Claims

1. A method of driving a bistable electro-optic display comprising a bistable electro-optic medium wherein a frame period is from about 50 to about 200 percent of the switching time of the bistable electro-optic medium, wherein the bistable electro-optic medium undergoes a series of smooth, largely uninterrupted changes in optical state between successive images.

2. A method of driving a bistable electro-optic display comprising a bistable electro-optic medium wherein a frame period is from about 75 to about 150 percent of the switching time of the bistable electro-optic medium, wherein the bistable electro-optic medium undergoes a series of smooth, largely uninterrupted changes in optical state between successive images.

3. A method of driving a bistable electro-optic display comprising a bistable electro-optic medium wherein a frame period is from about 50 to about 200 percent of the switching time of the bistable electro-optic medium, and wherein the bistable electro-optic medium undergoes a series of smooth, largely uninterrupted changes in optical state between successive images and comprises a rotating bichromal member or electrochromic medium.

4. A method of driving a bistable electro-optic display comprising a bistable electro-optic medium wherein a frame period is from about 50 to about 200 percent of the switching time of the bistable electro-optic medium, and wherein the bistable electro-optic medium undergoes a series of smooth, largely uninterrupted changes in optical state between successive images and comprises an electrophoretic medium, which itself comprises a plurality of electrically charged particles disposed in a fluid and capable of moving through the fluid under the influence of an electric field.

5. A method according to claim 4 wherein the electrically charged particles and the fluid are confined within a plurality of capsules or microcells.

6. A method according to claim 4 wherein the electrically charged particles and the fluid are present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material.

7. A method according to claim 4 wherein the fluid is gaseous.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 of the accompanying drawings is a graph showing schematically how the optical properties of a single pixel of a prior art liquid crystal display vary with time during a series of transitions in a video.

(2) FIG. 2 is a graph similar to FIG. 1 but showing the optical properties of a pixel of an electrophoretic display of the present invention undergoing a similar series of transitions in a video.

DETAILED DESCRIPTION

(3) Conventional video rate displays using non-bistable media, such as the phosphors on cathode ray tubes and conventional liquid crystal displays, require frame rates in excess of about 25 frames per second (fps) to provide acceptable video quality. (Video display at 15 fps is common on internet videos but results in a noticeable lack of video quality.) It has now very surprisingly been found that bistable, and certain other, electro-optic displays can produce good quality images at frame rates substantially below 25 fps, and in the range of about 10 to about 20 fps, preferably about 13 to about 20 fps. Experienced observers have determined that encapsulated electrophoretic displays running at 15 fps can produce video quality which appears substantially equal to that produced by non-bistable displays running at about 30 fps.

(4) Although the reasons for this unexpectedly high video quality at low frame rates are not at present entirely understood (and the invention is not limited by any particular explanation for the phenomenon), it appears that part of the explanation lies in the manner in which the persistent image on a bistable display assists the eye in blending successive images to create the illusion of motion. All video displays rely upon the ability of the eye to blend a series of still images to create the illusion of motion. However, many types of video display actually introduce transient intervening images which hinder the blending process. For example, a motion film display using a mechanical film projector actually places a first static image on the screen, then displays a blank screen for a very short period as the projector advances the film to the next frame, and thereafter displays a second static image.

(5) Other types of video displays (for example, cathode ray tubes and non-bistable liquid crystals) do not introduce an intermediate image but change an image by writing a first image very rapidly on the display during a small proportion of the frame period, and then allowing this first image to undergo a substantial amount of fading during the remaining part of the frame period before a second image is written. This type of behavior is illustrated in a highly schematic manner in FIG. 1 of the accompanying drawings.

(6) FIG. 1 illustrates schematically the variation with time of the gray levels of a single pixel of an 8 gray level liquid crystal display, the gray levels being designated 0 (black) to 7 (white). (In practice, commercial liquid crystal displays normally have a considerably larger number of gray levels.) In a first frame, the liquid crystal is driven from black (gray level 0, corresponding to a non-transmissive liquid crystal material) to white (gray level 7, corresponding to a transmissive liquid crystal material). As shown at 102 in FIG. 1, typically the liquid crystal material undergoes a very rapid transition from gray level 0 to gray level 7, and thereafter there is, over the remaining major portion of the frame period, a gradual relaxation to (say) about gray level 6, as indicated at 104 in FIG. 1.

(7) In the second frame, it is desired to change the pixel to gray level 3. Since liquid crystals are only driven in one direction, from dark to light, the change from gray level 6 to gray level 3 is effected by reducing the electric field across the liquid crystal to a suitably low value, and allowing the liquid crystal to relax to the desired gray level, as indicated at 106 in FIG. 1.

(8) In the third frame, it is desired to return the pixel to gray level 7. The resultant 3-7 gray level transition is generally similar to the 0-7 gray level transition, with a very rapid initial increase in gray level, indicated at 108, followed by a gradual relaxation to about gray level 6, as indicated at 110.

(9) Many types of prior art display, for example cathode ray tubes using phosphors, use a similar rewriting process in which the rewriting occupies only a small part of each frame period. The increase in emission from a phosphor struck by an electron beam may occur in less than 1 millisecond, while modern non-bistable liquid crystals may be rewritten in about 2 to 5 milliseconds. Since the pixel remains in the same optical state throughout the greater part of the frame, subject of course to any fading which occurs between rewrites, the effect is similar to that achieved with a mechanical motion picture projector, in which a series of fixed images are displayed successively, with no blending between successive images.

(10) Furthermore, the relaxation or fading illustrated at 104 and 110 causes its own problems. Since a new image is normally written line by line by scanning across the display, each line in turn goes from being part of the darkest portion of the display to being the brightest portion immediately after rewriting. This continual change in brightness of the various lines of the display is perceived by the human eye as a flicker on the display. In many cases, annoying flicker can only be reduced to an acceptable level by using a frame rate higher than that required to give the illusion of motion. For example, television broadcasts (which were originally designed to be watched on cathode ray tubes, although several other technologies are now in use) use a frame rate of 30 fps but also use an interlacing technique whereby only alternate lines on the display are rewritten on each scan, with the second half of the lines being rewritten on the next scan, so that the display shows 60 half-frames per second. Liquid crystal computer monitors typically have to be driven at frame rates of at least 60 fps (non-interlaced) to avoid flicker, although 30 fps is normally sufficient to give the illusion of motion.

(11) FIG. 2 of the accompanying drawings illustrates the changes in optical state of an electrophoretic medium undergoing the same 0-7-3-7 optical transitions as in FIG. 1. (Although FIGS. 1 and 2 both show three frame periods, it is not intended to imply that these frame periods are of the same duration in both cases. Typically, the frame period for writing an electrophoretic display is substantially longer than for rewriting a liquid crystal display.) Note that, as shown at 202 in FIG. 2, during the 0-7 gray level transition in the first frame period, the optical state of the pixel changes linearly during the entire frame period, so that gray level 7 is only reached at the end of the frame period and there is no opportunity for later fading, which in any case would not occur since the display is bistable. (FIG. 2 is somewhat over-simplified. The change in optical state of an electrophoretic medium is not necessarily linear with time. Also, in practice to keep the controller simple and inexpensive, as described in several of the patents and applications referred to in the Reference to Related Applications section above, the controller may only be able to apply a single drive voltage, which may be turned off and on repeatedly during a single transition, so that the change in optical state during a transition may be jerkier than illustrated in FIG. 2.)

(12) In the second frame, a 7-3 gray level transition is effected. Unlike a liquid crystal medium, where a transition from a light state to a darker state is effected simply by relaxation of the liquid crystal medium, a bistable electrophoretic medium needs to be driven in both directions (i.e., in both black-going and white-going transitions), and hence, as illustrated at 204 in FIG. 2, the 7-3 transition is generally similar to the earlier 0-7 transition in that the optical state changes essentially linearly during a major proportion of the frame period. However, FIG. 2 does illustrate the point that, in some cases, the transition may not occupy the whole of the frame period and there may be a short period, as shown at 206, in which the medium is not being driven and simply remains in substantially the same optical state by virtue of its bistability.

(13) Finally, in the third frame period a 3-7 gray level transition is effected. As shown at 208 in FIG. 2, this transition is substantially similar to the 0-7 transition effected in the first frame period, and the optical state of the medium simply increases smoothly with time until gray level 7 is reached at the end of the frame period.

(14) Comparing FIG. 2 with FIG. 1 it will be seen that the transitions in FIG. 2 lack the abrupt changes in optical state followed by relatively slow fading characteristic of the first and third transitions shown in FIG. 1; instead, a pixel undergoing changes, as illustrated in FIG. 2 undergoes a series of smooth, largely uninterrupted changes in optical state. Furthermore, as discussed in several of the patents and applications referred to in the Reference to Related Applications section above, bistable displays can be driven by rewriting only the pixels which change between successive images, so that in many cases most of the pixels of an image will not change as the display is rewritten. It is believed that this type of smooth, continuous flow from one image to the succeeding image is more successful in creating to the eye an impression of smooth motion, as compared with the display of unchanging images throughout most if not substantially all of each frame period.

(15) Thus a video display of the present invention using a bistable electro-optic medium does not write any intermediate image on the display; the first image simply persists until the second image is written over it. Furthermore, there is no appreciable fading of a bistable display between successive images, so bistable displays are essentially free from any flicker effects.

(16) Although FIG. 2 has been described above with reference to driving an electrophoretic medium, it will be apparent to those skilled in the technology of electro-optic displays that the advantages resulting from the smooth transitions shown in FIG. 2 are dependent upon the smoothness of the transitions and not upon the nature of the specific electro-optic medium used. Furthermore, the transitions shown in FIG. 2 do not require that the electro-optic medium be bistable in the normal sense of that term. Even if undriven periods such as that indicated at 206 in FIG. 2 are present (and it may often be possible to eliminate such undriven periods by careful control of the waveforms used to drive the display), such undriven periods have a duration of only a fraction of a frame period (say of the order of 25 milliseconds), and provided there is no substantial change in the optical state of the medium during such brief undriven periods, the advantages of the invention are still obtained. Thus, in a second aspect this invention provides a method of driving an electro-optic display at a frame rate of about 10 to about 20 frames per second, wherein the electro-optic medium used in the display, when being driven, changes its electro-optic properties continuously throughout the driving of each frame. For example, since an organic light emitting diode (OLED) responds essentially instantaneously (for practical purposes) to changes in the applied voltage, by careful control of the applied voltage against time curve, an OLED could be caused to mimic the behavior of the electrophoretic display shown in FIG. 2.

(17) It will readily be apparent that, to produce the type of smooth transitions illustrated in FIG. 2, in which the change in optical density continues throughout the frame period, that there should be a controlled relationship between the drive voltage used in the display, the switching speed of the display medium at this drive voltage, and the frame period. It has been found desirable to use a drive voltage such that the frame period is from about 50 to about 200 per cent of the switching time of the electro-optic medium. Preferably, the frame period is from about 75 to about 150 per cent of the switching time. With a frame rate similar to the switching time, at least the pixels which differ between successive images are changing their appearance throughout the frame period, and, as already noted, it is believed that this type of smooth, continuous flow from one image to the succeeding image is more successful in creating to the eye an impression of smooth motion, as compared with the display of unchanging images throughout most if not substantially all of each frame period. If a bistable electro-optic display is driven with a voltage-modulated driver, it may be advantageous to adjust the driving voltage used for each transition such that each transition required at least about one-half of the frame period to be completed.

(18) The video displays of the present invention also have a further advantage when it is desired to record the output from the display using a video camera or similar device. As is well known to those skilled in the art of video photography, when attempting to photograph a cathode ray tube or non-bistable liquid crystal video display, it is necessary to carefully synchronize the frame rate of the camera with that of the display or noticeable video artifacts, often in the form of dark bands which slide up or down the display, will adversely affect the quality of the recording. These dark bands are largely due to the aforementioned fading of the display between successive rewritings. Since the electro-optic displays of the present invention do not suffer significantly from such fading, the output from such a display can be recorded without synchronizing the frame rate of the camera with that of the display and without producing noticeable video artifacts.

(19) The video electro-optic displays of the present invention share most of the advantages of prior art electro-optic displays intended for displaying static images. For example, the video displays of the present invention typically have lower power consumption than prior art video displays, since it is only necessary to rewrite the pixels which change between successive images. (Rewriting of unchanging pixels at long intervals of at least seconds may be needed to cope with slow fading of the displays, but the energy used in rewriting at such long intervals is much less than that required in displays, such as those based on non-bistable liquid crystals, which must be rewritten continuously.) Furthermore, freezing individual frames on a bistable display of the present invention is much simpler than on a prior art display, since on the bistable display one can simply stop rewriting the display leaving the desired frozen image in place.

(20) The displays of the present invention may be used in any application in which prior art video displays have been used. Thus, for example, the present displays may be used in electronic book readers, portable computers, tablet computers, cellular telephones, smart cards, signs, watches, shelf labels and flash drives.

(21) Numerous changes and modifications can be made in the preferred embodiments of the present invention already described without departing from the scope of the invention. Accordingly, the foregoing description is to be construed in an illustrative and not in a limitative sense.