A frustratable total internal reflection (FTIR) image display is potentially a much faster switching reflective display technology that enables web browsing and video applications. FTIR display technology utilizes TIR of a front sheet or film comprising of, for example, convex or hemispherical protrusions or micro-prisms to create a bright state. A dark state is created by frustration of TIR when light absorbing particles are moved adjacent the front sheet into the evanescent wave region. The switching speed of an FTIR-based display can be faster than conventional dual particle electrophoretic display technology. This is due to the modulation of particles of only one charge. The particles need to be moved in and out of the evanescent wave region at the hemisphere surface. This distance is much shorter than the movement distance in conventional electrophoretic displays.
FTIR-based displays may be addressed to move the light absorbing charged particles. The movement of the charged particles from one electrode to another creates images. The charged particles may be moved using different methods such as direct drive addressing of a patterned electrode array, active matrix addressing of a thin film transistor (TFT) array and passive matrix addressing of a grid array of electrodes.
In direct drive displays, a display is divided into a plurality of segments in a patterned array. Each display segment has an individual lead to control the segment. Although the patterned array and drive electronics are less expensive to fabricate, direct drive displays are greatly limited. As the number of segments in the display increases, the number of leads also increases thereby making the display difficult or even impossible to fabricate.
Thin film transistor (TFT) arrays are commonly used in current liquid crystal display (LCD) technologies and contain a plurality of transistors and capacitors. Each capacitor and transistor is connected to a single pixel, which actively maintains the pixel state while other pixels are being addressed. The advantage of the TFT approach is that the capacitor/transistor combination provides a threshold voltage that enables individual pixels to be addressed using row/column drivers. This is needed if the electro-optical system (e.g., the liquid crystal (LC), the electrophoretic suspension, etc.) does not have an intrinsic voltage threshold. TFT systems are faster and have better voltage control. The fundamental advantage of the TFT array is the ability to control each pixel with the threshold voltage. TFT arrays provide drive systems for displays requiring fine structure and detail. However, the TFT arrays are costly to manufacture.
Passive matrix driven displays are composed of an array of electrodes in a grid structure. The grid structure is made of rows and columns with each respective row and column connected to an integrated circuit (IC). The ICs supply charge to the row and column electrodes to address individual pixels at locations where the rows and columns intersect. Passive matrix displays are simple and low cost to manufacture. Passive matrix displays can provide fine structure and image quality but they have major drawbacks. For example, passive matrix driven displays have slow response times and poor voltage control. In addition, the electro-optical systems of such displays require an intrinsic threshold behavior in the LC or electrophoretic suspension portion of the display. Despite the slow response time, passive matrix displays can be used in a variety of applications that require fine image structure without the need for video rate. Such applications include: electronic shelf labels, billboards and other types of display signage that would be cheaper to fabricate than with TFT drive electronics. Poor voltage control, another drawback, can lead to poor image quality.
FIG. 1 schematically illustrates a portion of a conventional passive matrix grid 100 of electrodes containing a first plurality 102 of rows of individual electrodes 104. Opposing the plurality of row electrodes 102 is a second plurality 106 of columns of individual column electrodes 108 in a perpendicular direction to the first plurality of row electrodes 102. The individual pixels are located where the row and column electrodes intersect. In order to address, for example, the middle pixel (the pixel is highlighted by a dotted line box) of the grid array 100, a first voltage is applied at +10V at the middle column electrode while the other electrodes remain at 0V. A second applied voltage bias of −10V is applied at the middle row electrode while the other row electrodes remain at 0V to form an electromagnetic field therebetween. The voltage difference leads to an overall voltage bias at the desired middle pixel of +20V. An undesired voltage bias of +10V is also applied to the adjacent pixels. Preferably, these pixels would not be addressed at +10V but as mentioned in preceding paragraphs, passive matrix displays exhibit poor voltage control. Regardless of the pixel addressed in a specific row or column, all other pixels in the same row or column of said pixel are addressed by an applied voltage, albeit at a lower voltage than the desired addressed pixel.
In the schematic example in FIG. 1, the desired pixel is addressed at +20V and activated while all of the other pixels in the same row and column are addressed at +10V. Unwanted partial activation of the pixels being addressed at +10V may result. A key method to circumvent this problem is to implement a threshold into the display such that the pixels are not activated when a voltage of +10V is applied. Instead, pixels are activated only when a voltage of >10V, such as when +20V is applied. This method, however, has many drawbacks.