Patent Application
20160091708
The invention relates to the field of reflective displays. Such reflective displays may be used in tablets, e-readers, mobiles, watches and other wearable or portable displays.
Reflective displays are already well known in the prior art, with potential advantages over more conventional emissive displays including better readability in high ambient light conditions such as sunlight, and lower power operation.
U.S. Pat. No. 8,643,935B2 (Miles, Feb. 4, 2014) discloses a reflective display which has an array of devices. Each device comprises of a substrate; a transparent conducting layer disposed over the substrate; a semiconductor layer disposed over the substrate; and a movable metallic reflector disposed over the substrate. The metallic reflector is movable between a first position and a second position upon application of a voltage between the transparent conducting layer and the metallic reflector. This device can be used in an interferometric reflective display.
U.S. Pat. No. 7,643,203B2 (Gousev et al., Jan. 5, 2010) discloses a reflective interferometric display using a moveable reflective layer, and an absorbing layer.
U.S. Pat. No. 8,115,987B2 (Bita et al., Feb. 14, 2012) discloses an optically resonant cavity comprising a first reflective layer, a partially reflective second layer and a dielectric stack supporting and separating these layers. This optically resonant cavity uses optical interference. This optically resonant cavity is partially filled with a fluid, which can be absorbing, with the fluid coverage controlled by an external voltage.
US20130010341A1 (Hagwood et al., Jan. 10, 2013) discloses using dye molecules in solution in a display using moving shutters. The dye molecules absorb some or all of the visible light to increase the contrast of the display.
US20130278991A1 (Daniel et al., Oct. 24, 2013) discloses a display with a transparent actuator element on a layer of colored fluid. Upon actuation of the transparent actuator, the thickness of the fluid is modulated, modulating the optical density. The actuation mechanism may be electrostatic.
For widespread adoption of reflective displays, they are likely to require the following properties: high reflectivity, high frame rates, good colour depth, high reliability and ease of manufacture. Commercially available reflective displays do not simultaneously fulfil all of these properties
A device and method in accordance with the present invention can simultaneously provide high reflectivity, high frame rates, good colour depth, high reliability and ease of manufacture.
In accordance with the present invention, a reflective display panel comprises a plurality of display pixels wherein each display pixel includes a reflective membrane formed on a first substrate. The display panel further comprises a second substrate arranged in opposition to the first substrate so that the reflective membrane is positioned between the first and second substrates. The first and second substrates form a cell that is filled with an absorbing fluid. At least one of the first and second substrates is substantially transparent. The reflective membrane in each display pixel of the plurality of display pixels may be individually controlled to move to at least two different positions. These positions differ in the membrane's average separation from a transparent substrate. Actuation of the reflective membrane to move to at least one position may be electro-statically controlled.
In one embodiment, a reflective pixel for a display device includes: a first substrate; a membrane formed relative to the first substrate; a fluid arranged between the first substrate and the membrane; and an actuator mechanism operative to move the membrane between at least a first and second position that differ in average separation distance relative to the first substrate to produce a low reflective state and a high reflective state of the pixel.
In one embodiment, the membrane is a reflective membrane and the fluid is an optically absorbing fluid.
In one embodiment, the membrane is an optically absorbing membrane and the fluid is a reflective fluid.
In one embodiment, the actuator mechanism includes: a membrane electrode electrically connected to the membrane; a control electrode electrically isolated from the membrane electrode; and a controller operatively coupled to at least one of the membrane electrode or the control electrode and operative to electro-statically actuate the membrane through application of a potential difference between the membrane electrode and the control electrode.
In one embodiment, the controller is configured to produce an intermediate reflective state of the pixel by varying a time duration in which the membrane is in the low reflective state and the high reflective state.
In one embodiment, the controller is configured to generate an average intermediate reflective state by placing the pixel in one reflective state and at least one adjacent pixel in a different reflective state.
In one embodiment, the membrane includes a reflective layer formed on at least a portion of the membrane.
In one embodiment, the membrane is formed from a reflective material.
In one embodiment, the first substrate is substantially transparent to visible light.
In one embodiment, the reflective pixel display includes a second substrate arranged relative to the first substrate, wherein the reflective membrane is arranged between the first substrate and the second substrate, and at least one of the first and second substrate is substantially transparent to visible light.
In one embodiment, the fluid arranged between the membrane and the first substrate is also arranged between the second substrate and the membrane.
In one embodiment, the reflective pixel display includes a membrane support attached to the membrane and one of the first substrate or the second substrate.
In one embodiment, the membrane support is laterally separated from the membrane and further comprising a linking element connected between the membrane and the membrane support.
In one embodiment, the reflective pixel display includes a plurality of electronic control elements integrated into a surface of one of the first substrate or the second substrate.
In one embodiment, at least a first portion of the membrane or linking element lies in a first plane, and a second portion of the membrane or linking element lies in a second plane different from the first plane.
In one embodiment, the membrane comprises at least one hole that enables the fluid to pass therethrough.
In one embodiment, the membrane is attached to one of the first substrate or the second substrate, and the control electrode is attached to the same first substrate or second substrate.
In one embodiment, the membrane is attached to one of the first substrate or the second substrate, and the control electrode is attached to the other of the first substrate or the second substrate.
In one embodiment, the reflective pixel display includes: a return electrode arranged on one of the first substrate or the second substrate, the return electrode electrically isolated from the membrane; and a drive electrode arranged on the other of the first substrate or the second substrate, the drive electrode electrically isolated from the membrane, where when in the first position the membrane is separated from the drive electrode by a first average separation distance, and when in the second position the membrane is separated from the drive electrode by a second average separation distance, the first average separation distance being different from the second average separation distance.
In one embodiment, the membrane is configured to diffusely reflect light.
In one embodiment, the reflective pixel display includes an optical diffuser.
In one embodiment, the reflective pixel display includes a color filter.
In one embodiment, a display device includes a plurality of reflective pixels as described herein.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
In accordance with the present invention, a reflective display panel comprises a plurality of display pixels wherein each display pixel includes a membrane formed relative to a first substrate via an actuator mechanism. A fluid is arranged between the first substrate and the membrane. The actuator mechanism can move the membrane between at least two positions that differ in average separation distance relative to the first substrate to produce a low reflective state and a high reflective state of the pixel. The membrane in each display pixel of the plurality of display pixels may be individually controlled.
The display panel may further comprise a second substrate arranged in opposition to the first substrate so that the membrane, via an actuator mechanism, is positioned between the first and second substrates. Fluid may fill the cell formed between the first and second substrates. At least one of the first and second substrates may be substantially transparent. The membrane in each display pixel of the plurality of display pixels may be individually controlled to move to at least two different positions. These positions differ in the membrane's average separation from a transparent substrate. Actuation of the reflective membrane to move to at least one position may be electro-statically controlled.
The membrane may be a reflective membrane, and the fluid an absorbing fluid. The membrane may be an absorbing membrane and the fluid may be a reflecting fluid.
For the purpose of this invention, a membrane is defined as a solid layer which is attached to a substrate via support structures, but is not attached to the substrate elsewhere. Over 20% of the membrane's side nearest the substrate therefore has a void between it and the substrate. This void can be filled with fluid. The membrane may have a thickness in the range 0.01-100 microns. The membrane may have a size in the range 1-1000 microns.
For the purpose of this invention, a reflective membrane is a membrane in which at least part of one or more of the membrane surfaces is substantially reflective to visible light, forming a reflective surface. The reflection from the reflective surface can be due to the membrane material itself, at least one additional optional layer formed on at least part of one of the surfaces of the membrane, or a combination of the above. The at least one additional layer may be formed over the whole or part of the membrane. The reflective surface has a reflectivity >10% averaged over visible wavelengths. The reflective surface may have a reflectivity >50% averaged over visible wavelengths. The reflected light may be specular or diffuse in nature.
For the purpose of this invention, an absorbing fluid is defined as a fluid which absorbs >50% of light averaged over visible wavelengths in a distance of 100 microns. An absorbing fluid may comprise a fluid which is intrinsically absorbing, or a fluid which contains a suspension of absorbing particles, or a fluid which contains a solution of absorbing material or any combination of the above.
For the purpose of this invention, an absorbing membrane is defined as a membrane which absorbs or transmits >50% of light incident on its surface, averaged over its surface. Thus reflection from the absorbing membrane is <50% of light averaged over visible wavelengths at normal incidence. The absorbing membrane may reflect <10% of light averaged over visible wavelengths. The absorption or transmission through the absorbing membrane may occur on part of the membrane only. The absorption or transmission through the absorbing membrane may occur due to the membrane material itself or due to additional layers on at least part of the membrane or to some combination of the two.
For the purpose of this invention, a reflecting fluid is defined as a fluid which reflects >10% of light averaged over visible wavelengths at normal incidence in a 100 micron thick film. A reflecting fluid may reflect >50% of light averaged over visible wavelengths in a 100 micron thick film. A reflective fluid may include a scattering fluid. A reflecting fluid may comprise a fluid which is intrinsically reflecting or scattering, or a fluid which contains a suspension of reflecting or scattering particles, or a fluid which contains a solution of reflecting or scattering material, or a fluid which contains a mixture or emulsion of at least two different fluids, or any combination of the above. Scattering or reflective particles may include metallic particles or oxides such as titanium dioxide.
For the purpose of this invention, a substantially transparent substrate transmits >10% of light averaged over visible wavelengths through the substrate. Preferably, a substantially transparent substrate transmits >50% of light averaged over visible wavelengths through the substrate.
By predominantly using the phenomenon of optical absorption rather than the phenomenon of optical interference or an optical cavity, the reliability and ease of manufacture can be increased, compared to some conventional reflective type displays, as the required control over device dimensions can be less stringent. The sensitivity of the reflection to viewing angle may also be decreased by using the phenomenon of absorption rather than the phenomenon of optical interference or an optical cavity.
By actuating the solid membranes which then move through the fluid, rather than directly moving the fluid as in an electro-wetting display, there is the potential for higher actuation speeds, increased reliability, and greater flexibility over the choice of fluid. In this invention only a single fluid may be required, which can completely fill the cell formed by the two substrates, including the void between the membrane and the first substrate. This may make the device easier to manufacture, and increases the device's reliability.
The display driver 180 controls the state of the pixels. The display driver may be operationally coupled to but separate from the display panel. Alternatively the display driver may be incorporated into the display panel. Thin film transistors or other electrical control elements may be formed on one or both of the first and second substrates and may constitute some or all of the display driver functions.
The membrane may be controlled to move between two different states, corresponding to different membrane positions relative to the first transparent substrate.
In the relaxed state the first average distance is large enough for the average thickness of the absorbing fluid in the void between the reflective membrane and the control electrode or first transparent substrate to absorb a significant proportion of the incident light. The relaxed state therefore corresponds to a low reflectivity state. In the driven state the second average distance is small enough that the average thickness of absorbing fluid in the void does not absorb a significant proportion of the incident light which is substantially reflected back to the viewer. The driven state therefore corresponds to a high reflectivity state. Actuation of the membrane therefore allows switching between a high reflective state and a low reflective state.
The position of the reflective membrane in the driven state may be such that it is partially or fully in contact with the control electrode. Alternatively, the membrane may not be in contact with control electrode in the driven state. In the latter case there exists an absorbing fluid filled gap between the reflective membrane and the control electrode at all times. Such a non-contact state may be used as an additional intermediate state, and can be actuated by a potential difference between the membrane and the control electrode that is less than the pull in voltage, VP. Such an intermediate state has an average reflection over visible wavelengths that is in between the low and high reflective states. Many intermediate states, with different average reflections, are possible by varying the potential difference between the membrane and control electrode, with the potential difference below the pull-in voltage.
There are a variety of methods to control the array of display pixels to display text, an image, a sequence of moving images, or a combination of the above. The array of display pixels with data and scan line interconnects may be controlled, for example, by passive matrix addressing, in which the scan line interconnects and data line interconnects are directly connected to the display pixel electrodes. An example of the connection scheme for each display pixel 800 is shown in
The operational states of a display pixel with the example connection scheme of
V
M
<|V
SW
|<V
P
|VSE−VSW|<VM
|VSE+VSW|>VP
The operational state of all display pixels in a single row may therefore be set by applying the high level of switch voltage signal VSE to the respective row electrode. The voltage on the data lines then controls the state of the membranes in that row. If the voltage on the data line is +VSW, then the potential difference between the membrane electrode and the control electrode is then VSE+VSW. Since the magnitude of this potential difference is larger than the pull-in voltage the membrane is electrostatically actuated and moves to the driven state. If the voltage on the data line is −VSW, then the potential difference between the membrane electrode and the control electrode is VSE−VSW. Since the magnitude of this potential difference is less than the maintain voltage, the membrane moves by spring action to the relaxed state.
Whilst one row of the array of display pixels is being set, the operational state of the display pixels in all other rows is maintained. In order to achieve this, the low level of the selection voltage signal may be applied to all other rows in the array. Application of the +VSW or −VSW voltage on the column electrode will result in a potential difference between the membrane electrode and the control electrode of magnitude |VSW|. Since this lies between the hold and maintain voltages, the membrane does not move and is maintained in its current position.
The operation of a display pixel is further described with reference to the waveform diagram of
In accordance with the second embodiment of the invention, a display pixel comprises a reflective fluid and an absorbing membrane instead of an absorbing fluid and a reflective membrane.
The absorbing membrane can be actuated in the same way as the reflective membrane described in the first embodiment. For example, by applying a potential difference between the control electrode and the membrane that is greater than the pull-in voltage, the actuated state of
In accordance with a third embodiment of the invention, electronic control elements are integrated onto the surface of the first substrate below the membrane. Example electrical control elements may include transistors, capacitors, diodes and resistors. These additional electrical elements may be fabricated on the surface of the first substrate using, for example, standard thin-film processing techniques as are well-known in the display industry. The array of display pixels of the present embodiment may therefore be controlled using an active matrix addressing scheme.
In an active matrix addressing scheme, an example of the voltages applied on the control and membrane electrodes of a display pixel is shown in
To control the voltages on each membrane electrode in an array of display pixels in an active matrix addressing scheme, further electrical control elements are required. An example pixel circuit including electrical control elements is shown in
The operation of this example pixel circuit is described with reference to the waveform diagram of
Since the electrostatic force is only dependent on the potential differences between electrodes, the electrical connections to the control and membrane electrodes may be swapped, and the operation of the membrane will occur in the same manner.
An active matrix addressing scheme can have several advantages over a passive matrix addressing scheme. The use of a storage capacitor means that the time to write data to one row can be shorter than the time to move the membrane. This means that update of the entire array can be quicker, allowing higher frame rates, and the potential for higher colour depth. An active matrix addressing scheme can use lower power than a passive matrix scheme. For example, consider a membrane which does not need to change state on an update of the array of display pixels. In an active addressing scheme, no voltage change is applied to the membrane or control electrode, so no power is dissipated there. For a passive matrix scheme, the voltage may change depending upon the data written to other display pixels in the same column, dissipating power in the capacitance formed between the membrane and the control electrode. In addition active matrix addressing can use lower voltages on the scan lines used to control the data written to the display pixel than the voltages required to move the membrane. This can also reduce the power requirements.
In accordance with a fourth embodiment of the invention, the membrane supports may be laterally separated from the membrane. A plan view of an example of such a membrane is shown in
In accordance with a fifth embodiment of the invention out of plane elements may be added to the membrane or membrane linking elements. These out of plane elements may include ribs. An example of out of plane elements is shown in a plan view in
A membrane structure in accordance with a sixth embodiment of the invention is shown in
In accordance with a seventh embodiment of the invention, the display pixel has the structure shown in
The reflective membrane may be controlled to move between two different states corresponding to a difference in the location of the membrane relative to the second transparent substrate.
Switching from the driven state to the returned state requires that the potential difference between the drive electrode and the membrane, WDM, to be less than a first maintain voltage, VM1, and that potential difference between the return electrode and the membrane electrode, VRM, is greater than the second pull-in voltage, VP2. Switching from the returned state to the driven state requires that the potential difference between the return electrode and the membrane, VRM, is less than a second maintain voltage, VM2, and that potential difference between the return electrode and the membrane, WDM, is greater than the first pull-in voltage, VP1. The electro-mechanical characteristic of the membrane with a potential difference between the membrane electrode and either the drive or the return electrode is illustrated in
In the driven state the first average distance between the drive electrode and the reflective membrane is small enough that the average thickness of absorbing fluid in the void between the reflective membrane and the drive electrode or second transparent substrate does not absorb a significant proportion of the incident light which is substantially reflected back to the viewer. The driven state therefore corresponds to a high reflectivity state. In the returned state the second average distance between the reflective membrane and the drive electrode is large enough for the average thickness of the absorbing fluid in the void to absorb a significant proportion of the incident light. The returned state therefore corresponds to a low reflectivity state. Actuation of the membrane therefore allows switching between a high reflective state and a low reflective state.
The position of the reflective membrane in the returned or driven states may be in contact with the return electrode or drive electrode respectively. Alternatively, the membrane may not be in contact with the return or drive electrodes. In the latter case there exists an absorbing fluid filled gap between the reflective membrane and the drive and return electrodes at all times. Such a non-contact state may be used as an additional intermediate state, and can be actuated by a potential difference between the membrane and the drive or return electrode that is less than the corresponding pull in voltage. The reflective surface may only partially contact the opposite surface, for example the drive electrode. The reflective membrane may contact only one of the drive electrode and return electrode.
An example voltage driving scheme for an individual display pixel, such as is shown in
In operation of this example pixel circuit, the display pixel state is written in a row by row manner, in a similar manner to the third embodiment. This is achieved by application of a write-enabling voltage Vw to each scan line interconnect in turn. Upon application of Vw to a scan line interconnect, Vw is applied to the gates of all the transistors in that row of display pixels. This enables current to flow between the source and drain terminals of the transistors in that row. Therefore voltage signals applied to the data line interconnects are written to the membrane electrode and stored in the storage capacitor in that row of display pixels. By either applying Vo or 0 V to the data line interconnects, this voltage is then applied to the membrane electrode, resulting in the actuation of the membrane to two possible states, as shown in the drive scheme for an individual pixel in
In accordance with a ninth embodiment of the invention, the display pixel has the structure shown in
The reflective membrane may be actuated to two different states, as shown in
An advantage of this configuration may be that it is easier to fabricate the membrane closer to the drive electrode than the return electrode. This may make it easier to make a high reflective state with a high reflectivity, as it may be easier to achieve to achieve a large area of contact between the reflective membrane and the drive electrode in the driven state.
In accordance with a tenth embodiment of the invention, the reflected light from a display pixel may substantially reflect light in a diffuse manner. The reflective part of the membrane may substantially reflect light in a diffuse manner by modification of the surface of the membrane or modification of the surface of the optional additional layer which makes part of the membrane reflective. For example the surface may have a random or partially random surface texture. Alternatively, the reflective membrane may substantially reflect light in a specular manner, and an additional optical element at least partially scatters the light in a diffuse manner, acting as an optical diffuser. The optical diffuser may be added in a number of different positions in the device.
In accordance with a eleventh embodiment of the invention the scattering of light may be produced by texturing a surface.
In accordance with a twelfth embodiment of the invention the display pixels may generate intermediate light intensities between a high and low reflective state. The intermediate light intensities may be generated by using membrane positions in between the positions for high and low reflective states, directly generating intermediate light intensities.
In accordance with an thirteenth embodiment intermediate light intensities may be generated by using a time average of different time durations in different states. These time durations are shorter than those which can be perceived by an average human viewer. As a result, the average reflected light intensity is perceived by the viewer as an intermediate light intensity. For example, by adjusting the amount of time a display pixel is in a high reflective state compared to a low reflective state, the time averaged pixel state observed by a viewer may consist of an intermediate reflective state. This may be used to produce a range of reflective intensities, for example to produce a greyscale reflective image. This method can be described as time-sequential greyscale.
In accordance with a fourteenth embodiment of the invention the display pixels may use different states in nearby display pixels to generate apparent average intermediate reflective states to a viewer over a number of display pixels. For example spatial dithering may be used. Spatial dithering may further be combined with time-sequential greyscale techniques to increase the image appearance.
In accordance with a fifteenth embodiment of the invention, a colour filter may be applied between the reflective membrane and the viewer. The colour filter may be applied onto the substrate nearest the viewer. The colour filter may be applied to the membrane. Different colour filters may be used for different display pixels. For example, groups of three display pixels with either a red, blue or green colour filter may be formed. This group of pixels allows colours to be displayed to a viewer. Colour filters may further be combined with intermediate reflective states to produce a large range of colour observable by a viewer. The intermediate reflective states may be created by using at least one of: intermediate states in one pixel, a time average of at least two different states in one pixel and a spatial average over several display pixels.
In accordance with a sixteenth embodiment of the invention the display pixels may be used in a lower power latch mode to maintain their state. For example, in the third and eighth embodiments, once the reflective membrane has been moved to a state either by an electrostatic force or a mechanical restoring force, it is latched to this state. Very little power is required to maintain this state, as any voltage loss across the membrane is driven only by leakage currents, for example across the capacitor, transistor or membrane. Therefore this state should only require an occasional rewriting of the display data, at a frequency much lower than the normal refresh rate of the display. Such a low power state may be useful when displaying static images or text. The reflective display may switch between this low power mode and a higher power mode. A higher power mode may allow higher frame rates for moving images and may allow better colour depth. Different regions of the display may independently switch between low and high power modes.
Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.
The invention finds application in displays for industrial and consumer devices. The invention is especially suited for use in tablets, e-readers, mobiles, watches and other wearable or portable displays.
