LCOS technical knowledge collection

LCOS technical knowledge collection

Principle of LCOS Optomechanics

LCOS (Liquid Crystal on Silicon) is a new type of reflective micro LCD projection technology. It uses a CMOS integrated circuit chip coated with liquid crystal silicon as the substrate of the reflective LCD. It is polished with advanced technology and coated with aluminum as a mirror. A CMOS substrate is formed, and then the CMOS substrate is bonded to a glass substrate containing transparent electrodes, and then injected into a liquid crystal package. LCOS puts the control circuit behind the display device, which can improve the light transmittance, so as to achieve greater light output and higher resolution.

The following figure is a cross-sectional analysis diagram of the LCOS liquid crystal packaging level:

LCOS can also be regarded as a type of LCD. The traditional LCD is made on a glass substrate, and LCOS is made on a silicon wafer. The former usually uses the transmissive projection method, the light utilization efficiency is only about 3%, and the resolution is not easy to improve; LCOS uses the reflective projection, the light utilization efficiency can reach more than 80%, and its biggest advantage is that it can be widely used at present 3. Cheap CMOS production technology to produce without additional investment, and with the rapid miniaturization of semiconductor manufacturing process, gradually improve the resolution. On the other hand, high-temperature polysilicon LCD requires separate investment in equipment, and it belongs to a special process, so the cost is not easy to reduce. The structure of the LCOS panel is somewhat similar to that of TFT LCD. Spacer is distributed between the upper and lower two layers of substrates for isolation, and then filled with liquid crystal to form a light valve between the substrates. The circuit switch is used to drive the rotation of liquid crystal molecules to determine the picture Light and dark. The upper substrate of the LCOS panel is ITO conductive glass, and the lower substrate is a CMOS substrate coated with liquid crystal silicon. The biggest feature of the LCOS panel is that the material of the lower substrate is single crystal silicon, so it has good electron mobility and single crystal silicon can be formed Compared with the existing LCD and DLP projection panels, LCOS is a new type of projection technology that can easily achieve high resolution.

The principle of luminescence of LCOS

LCoS TV sets need to go through several steps to produce pictures. This process involves a high-intensity light bulb, a series of reflectors and micro-devices arranged in a cube, a prism and a projection lens. Here is what happened in the whole process:

1. The bulb produces a white light.
2. The light beam passes through a condenser lens (responsible for focusing and aligning the light). It also passes through a filter (only visible light is allowed to pass through), which protects other components from damage.
3. This white light is decomposed into red, green and blue light in one of two ways:
1. The beam passes through a polarizing beam splitter (PBS), which splits the light into three beams, which pass through filters that add red, green, and blue, respectively.
2. The light beam passes through a series of dichroic mirrors. These dichroic mirrors can reflect light of certain wavelengths while allowing the rest of the light to pass through. For example, a dichroic mirror can separate red light from white light, leaving blue and green light, while another dichroic mirror can separate green light and leave only blue light.
4. The color beam just generated is in contact with one of the three LCoS microdevices at the same time-they correspond to red light, green light and blue light respectively. In the next section, we will introduce these devices.
5. The light reflected by these micro devices passes through a prism, which can combine the light.
6. The prisms then project the light rays (they now produce a full-color image) into a projection lens, which magnifies the image and displays it on the screen.

LCoS micro device

The LCoS microdevice puts the liquid crystal layer between a transparent thin-film transistor (TFT) and a silicon semiconductor, instead of putting liquid crystal between two polarized panels like an LCD. This semiconductor has a distorted surface that reflects light. The light emitted by the bulb is projected onto the micro-device through a polarizing filter, and the liquid crystal acts like a door or valve to control the amount of light reaching the reflecting surface. The higher the voltage received by the crystal of a particular pixel, the more light the crystal allows to pass through. Several layers of different materials are required to complete the entire process.

The components and functions of LCoS micro-devices are described below from bottom to top:

Printed circuit board (PCB): transmits commands and current from the TV to the micro device silicon (chip or sensor): uses data from the TV pixel driver to control the liquid crystal, usually using a transistor reflective coating for each pixel: Reflect the light to produce the picture liquid crystal: control the amount of light reaching or leaving the reflective coating. Alignment layer: enable the liquid crystal to be properly aligned, so as to accurately calibrate the light. Transparent electrode: together with silicon and liquid crystal to form a complete circuit cover Protect and seal the entire system

The exact materials and construction vary by manufacturer. Some micro devices use nematic liquid crystal, while others use ferroelectric liquid crystal. Some use organic alignment layers, and they will decompose with extended use time and high intensity exposure. Some use photosensitive materials and light to control the pulse to the liquid crystal.

The orientation of the crystal relative to the reflective surface will change under the action of current. Most are almost orthogonal to the reflective surface when the current is off, and obliquely cross it when the current is on.

In general, there are only very small gaps between pixels in LCoS devices. The pixel pitch-the horizontal distance between two pixels of the same color-is 8 to 20 microns (10-6). This can reduce or eliminate the "screen door" effect that appears on some DLP TV sets, thereby helping to keep the image smooth and uniform.

LCOS optical engine architecture

LCOS optical engine architecture can be roughly divided into two types, three-piece optical engine and single-chip optical engine.

1. Three-piece optical engine

The LCOS optical engine is currently dominated by a three-piece type. The three-piece type divides the light source into red, blue, and green light through a beam splitter prism, and then separately projects the light beam into three LCOS panels to project the three-color image. After the optical system converges and combines to form a color image. In addition to the three-piece LCOS optical engine, in addition to the three panels, combined with multiple splitting and combining systems, the volume is larger and the cost is higher. However, due to the high optical efficiency and high image quality The characteristics of it are mainly for the development of high-end professional uses such as large screens.

The following figure is the principle diagram of LCOS optical machine:

As shown in the above figure is the LCOS projection structure system, using UHP (cold light source lamp) as the light source, using parabolic reflectors to filter ultraviolet light and infrared light, and then using cold reflectors to filter infrared light, through the focusing lens and the compound eye lens to obtain uniform parallel The light is then split by a dichroic mirror, and then polarized light is obtained through PBS, synthesized by reflection from the LCOS chip, and projected onto the screen through a zoom lens to form an image.

The process of displaying signals by optomechanics: Connect the display and signals (including video signals, computer signals, etc.) to the LCOS optomechanical motherboard through a cable. The motherboard converts the signals into electronic signals, and then adds the electronic signals to the LCOS chip. On the drive board, the signal is converted into a digital signal by the board, and the digital signal is added to the LCOS chip through the LCOS drive board, and the signal is added to the optical machine beam by the LCOS chip, and the image is formed by the PBS, the light combining system and the imaging lens Projected on the screen, that is, the image we see.

The main structure of the LCOS projector and the LCD projector are similar in the design of the light guide and the beam splitting part, but in the LCOS projector system, more PBS is added in front of the LCOS panel.

The light emitted by the light source is divided into R, G and B three-color light after passing through the Dichroic Mirror (two-color mirror). After passing through the respective PBS, the three-color light will reflect S polarized light into the LCOS panel. The polarized light will be changed to P polarized light, and finally the three polarized lights combined with dichroic prism (Dichroic Prism) will be projected onto the screen to obtain the image.

Simply put, LCOS is directly related to CRT projection technology, high-temperature poly-silicon liquid crystal (Poly-Si LCD) transmission projection technology, DMD (Digital Micromirror Device) data optical processing (DLP; Digital Light Processing) reflective technology . These three technologies have matured, so LCOS can become the new mainstream of projection display technology.

2. Single-chip optical engine

The single-chip Color Wheel optical engine uses the fast-rotating ColorSwitch to form white light into sequential red, blue, and green lights. The three primary colors and the red, blue, and green screens generated by the driver are synchronized to form a color-separated image. Due to the persistence of the human eye's vision, a colorful projection screen is finally produced in the human brain.

The biggest advantage of the monolithic type is that because the number of panels is only one, and the system architecture of splitting and combining light is relatively simple, it is more competitive in cost and the space of the optical engine is relatively small. However, at present, there are some technical difficulties. In terms of Color Wheel, the polarized white light source is only 1/3 of the previous one, and the brightness is significantly reduced. In addition, because the LCOS panel has to be fast in red, blue and green pictures The composite image is switched, so the response speed of the panel is more demanding, which makes the difficulty of production increase.

Features of LCOS projection technology

Compared with LCD and DLP projector technologies, LCOS projection technology has the potential for high resolution, high brightness, and low cost. It is the star of tomorrow in projection technology.

1. High resolution and high definition

The biggest feature of LCOS projection technology is that the lower substrate of the panel uses a silicon wafer CMOS substrate. Because the material of the lower substrate is single crystal silicon, it has good electron mobility, and single crystal silicon can form thinner circuits, so it is easier Produce high-resolution panels that display high-definition picture quality.

2. High brightness

LCOS is a reflective technology, unlike LCD optical engines, which will greatly reduce the light utilization rate due to light penetrating the panel, so the light utilization rate can be increased to 80%, which can be reduced compared to 3% for transmissive LCD Power consumption and can produce higher brightness.

3. Bright and full colors

The development characteristics of the LCOS reflective liquid crystal display range in color is far higher than the three-shot (CRT) projector and other display technologies, and will almost be close to the NTSC color reproduction standard. Therefore, the high color saturation of the overall picture can be presented, so that the colors are colorful and full.

There are three LCOS chips in the optical machine, each of which has about 1.5 million pixels, and three of them are synchronized to form R, G, and B monochrome images. RGB three-color images are combined into a 24-bit (16777216-color) true color image by optical components, and then enlarged to the screen by a high-resolution wide-angle lens. The display of LCOS silicon crystal TV is more accurate and realistic, and the ultra-high brightness makes the picture performance even better.

4. Low cost

The LCOS optical engine has the advantage of low cost because of the simple parts of the product. Coupled with the large investment of domestic manufacturers, compared with the LCD panel supplied by Epson, Sony, and the DLP panel exclusively supplied by TI, LCOS has The trend of rapid cost reduction.

LCOS projector adopts full digital signal processing technology, uses LCOS chip (liquid crystal on silicon) as reflection imaging device, uses digital light processing technology to modulate computer and video signals, and drives LCOS chip reflection imaging device system, which is obtained by combining light system and projection lens Large screen image. This technology has the characteristics of full image display color, high definition and high resolution of the picture, high brightness, high stability and maintenance-free. At present, a single LCOS projector can be widely compatible with multiple resolution signals. Multiple combined splicing can achieve ultra-high resolution display. The overall wall resolution is the superposition of the resolution of each projection display unit.

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