Microdisplay FAQ:
Is a Microdisplay a "Minidisplay"?
What are the key advantages?
What is the basic structure and operation of LCoS microdisplays?
How does an LCoS microdisplay modulate light?
Are there optics available for virtual?
What color depth can be delivered by the displays?
What contrast is possible?
How low a power consumption is possible?
What is required to drive the microdisplay?
What is the difference between analog and digital backplane offerings?
Why is Three-Five's microdisplay an exciting approach?
Is a Microdisplay a "Minidisplay"?
Formally, a minidisplay is classified as any display less than 1.3" diagonal, which includes microdisplays. However minidisplays usually refer to small direct view panels, such as on the back of newer camcorders. Microdisplays refer to displays that require a magnification system.
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What are the key advantages?
The key advantages of microdisplays are that the device can be low cost, low weight and small in size, yet rich in information content.
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What is the basic structure and operation of LCoS microdisplays?
LCoS microdisplays consist of a silicon backplane, a cover glass and an intervening liquid crystal layer. All LCoS microdisplays are arrays of individual pixels. To incident light, each pixel is a liquid crystal cell above a reflecting mirror. By changing the LC state, the incident light can be made to change its polarization. Thus the LCoS microdisplay works on the light by altering its polarization.
The cover glass has a transparent conductor (typically Indium Tin Oxide ITO) on the inside of the glass. The silicon backplane is an array of pixels, typically 10 to 20 microns in pitch. Each pixel has a mirrored surface that occupies most of the pixel area (in TFS case, this is over 90% of the area). The mirrored surface is also a conductor. Thus each pixel has a stationary mirror and above it, a liquid crystal volume that can experience a voltage difference between top and bottom electrodes.
The CMOS circuitry under each pixel performs functions of responding to address and storage of pixel information. The information can be stored as an analog voltage on a capacitor; it is similar to a DRAM except the voltage stored is analog, not digital. The circuitry may otherwise store a digital state. The liquid crystal behavior to applied voltage depends upon the actuation scheme and the type of liquid crystal. The schemes all share in common the changing of the LC's ability to polarize light. Liquid crystal can be thought of as a fluid suspension of long molecules whose physical orientation can be made to change with applied voltage. When the molecules are vertical with respect to the array plane, incident light polarization is not affected and the LC simply allows the light to pass through unaffected. When molecules are in plane, the cell can be designed such that the molecules stack up in a helix. In this state, incident light polarization is altered.
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How does an LCoS microdisplay modulate light?
LCoS light modulated is based on the principle of cross polarizers. Light passes through two polarizers if their orientation is parallel, and light is blocked if one is 90 degrees rotated with respect to the other. At intermediate rotations, partial light passes through. A microdisplay acts as an array of independently adjustable polarizers, adjusting the angle of rotation of incident light. When the microdisplay is unactivated, each cell rotates the light 90 degrees. When the microdisplay is fully activated, the "twist" of the liquid crystal is cancelled, and light passes through unaffected. Partial polorization rotation occurs at intermediated levels of activation. In this fashion, the consort of independently adjusted pixels generates a gray-scale image.
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Are there optics available for virtual?
Optics are available for virtual applications. The optics are among the best demonstrated in industry - 34 degree FOV in a 17 mm deep small-cube package.
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What color depth can be delivered by the displays?
The analog displays deliver 10-bit resolution analog information of which 8 bits is used, for 24-bit color information delivery to the display. The digital backplane stores 3 bits of information - on/off for red, green, and blue, and so it delivers 2e3 = 8 possible colors in storage mode. In continuous addressing mode the same backplane can display 8e3=512 colors. Spacial or temporal dithering techniques can increase the color depth further at the expense of resolution.
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What contrast is possible?
The microdisplays in projections systems have demonstrated over 400:1 contrast. The microdisplays themselves are capable of up to 1000:1 contrast, depending on the f/number of the viewing system. TFT flat panel displays typically deliver 100:1 contrast by comparison, but in typical ambient light conditions, such high contrast is not viewable.
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How low a power consumption is possible?
Because all the display information can be written to the digital display and support electronics subsequently shut down, a digital display showing a static image consumes less than 100mW power, including illumination (three color LED). When the display is constantly updated, regardless of being analog or digital, power dissipation is higher, dominated by the digital support ASIC.
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What is required to drive the microdisplay?
For 3-valve projection development, TFS can supply a PCB that performs gamma correction and buffers the pixel's information going to the display. A custom digital and analog ASIC chip set will be available for driving the analog microdisplays. One of each type ASIC is needed per display. Available now is a PCB containing FPGA and analog discretes that will later be replaced by the ASICs. The virtual display does not require this signal processing overhead and needs only a single custom support chip.
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What is the difference between analog and digital backplane offerings?
Three-Five Systems is currently developing two microdisplay designs, designated as "analog" and "digital." These names merely refer to how information is sent to the microdisplay - analog or digital input. The analog displays are available in - SXGA (1280 x 1024). The SXGA analog display supports 3-valve operation only. The digital display is SVGA. This design results from our alliance/part-ownership with InViso (formerly Siliscape). A key difference between the two types of displays is addressing. The analog displays are addressed raster mode, while the digital display is addressed like memory, random access. Random access allows updating only cells needing update, saving on processing and associated power consumption.
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Why is Three-Five's microdisplay an exciting approach?
Reflective displays are capable of using near 100% of the pixel area as a light-modulating surface, unlike transmissive displays where some of the useful area blocks the light. LCoS type displays use industry standard processes, allowing it to capitalize on the existing infrastructure and also general industry improvements in manufacturing and technology. Furthermore, Three-Five's approach over other LCoS devices has these advantages: 1) TFS can use it's own installed manufacturing and manufacturing talents for low cost and high volume, and rapid development; 2) TFS specializes in the LC and assembly and allows for more than one kind of silicon "backplane" to be used; and 3) TFS has the high volume subassembly experience to provide a total visual system solution, for more of a "turnkey" product for rapid introduction into the market and lower cost.
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