Projection Liquid Crystal Displays

Fundamental Principles

A projection liquid crystal display is a device that projects an image written on a liquid crystal cell onto a screen after magnification. In this case, the liquid crystal cell does not function as a display component but rather as a light control device, similar to a valve used to control the flow of liquids or gases, hence it is generally referred to as a light valve. This technology is crucial for creating a high-quality large lcd screen projection with exceptional clarity and color accuracy.

Liquid crystal light valves can use either transmissive or reflective liquid crystal cells. Furthermore, full-color images written in full-color display cells equipped with sub-pixels for the three primary colors can also be projected. When magnified through projection, the pixels expand, and the size of the sub-pixels also increases. In the juxtaposed additive color mixing method, each sub-pixel of the three primary colors must be sufficiently fine to exceed the spatial resolution of the human eye and achieve color mixing. It is easy to imagine that projection displays using this method often have limited color mixing effects, especially when the magnification is increased. This limitation makes the technology challenging for applications requiring a large lcd screen with precise color reproduction.

Therefore, modern projection displays generally use three liquid crystal cells, each containing a monochrome image corresponding to one of the three primary colors. In this configuration, the images of the three primary color pixels are superimposed on the screen to form a single full-color pixel. Projection displays that use three liquid crystal cells corresponding to the three primary colors as light valves are called three-panel displays. Unlike direct-view (non-projection) full-color displays, where the three primary colors are smoothly mixed from the viewer's perspective, three-panel projection displays mix the three primary colors on the screen. This approach is known as simultaneous additive color mixing, as opposed to the juxtaposed additive color mixing used in direct-view displays, and is particularly effective for creating a vibrant large lcd screen projection.

Three-Panel LCD Projector Configuration

Figure 1: Basic structure of a three-panel LCD projector

Figure 6-37 illustrates the basic configuration of a three-panel liquid crystal projector. White light from the light source first passes through dichroic mirrors to separate into red (R), green (G), and blue (B) primary colors. Dichroic mirrors have the function of transmitting only specific wavelengths of light while reflecting others, a critical component for achieving accurate color representation in a large lcd screen projection.

The image to be displayed is decomposed into red, green, and blue primary color images, which are written into three liquid crystal light valves respectively. Each liquid crystal light valve receives monochromatic light, and the emitted light from each valve is combined into a single output beam by a color combining prism. The combined output light is magnified by a projection lens and projected onto a screen, resulting in the large lcd screen viewers see.

There are also devices where both color separation and color synthesis are accomplished using two dichroic mirrors. It is worth noting that in three-panel projection displays, liquid crystal light valves do not require color filters, unlike current direct-view full-color liquid crystal displays where color filters are essential. Color filters typically absorb light of unwanted wavelengths, resulting in low light utilization efficiency. In contrast, dichroic mirrors separate white light into the three primary colors, which significantly improves light utilization efficiency – a key advantage for maintaining brightness in a large lcd screen projection.

Light Sources for Projection Displays

Various light sources are used in projection systems, including halogen lamps, xenon lamps, metal halide lamps, and ultra-high pressure mercury lamps. These sources differ in color temperature and luminous efficiency (lm/W). High-brightness projectors with 200W or more typically use metal halide lamps (with efficiency of 60-80 lm/W), which provide the necessary luminosity for a bright large lcd screen even in well-lit environments. Smaller projectors often use halogen lamps (20-35 lm/W).

Ultra-high pressure mercury lamps, which are close to point light sources (60 lm/W), can also be used in 100W projectors. Projectors requiring particularly high output use xenon lamps (25-40 lm/W). If the light source is close to a point source, the light incident on the liquid crystal cell is parallel, which can improve light collection efficiency and contribute to higher display brightness – a crucial factor for ensuring a vivid large lcd screen even in challenging lighting conditions.

Projector efficiency is determined not only by the efficiency of the light source but also by the product of the color separation efficiency of the dichroic mirrors, the transmission efficiency of the liquid crystal light valve, the color combining efficiency of the color combining prism, and the efficiency of the projection lens. Each component plays a vital role in delivering the final image quality on the large lcd screen.

Liquid Crystal Light Valves & LCOS Technology

The transmission efficiency of a liquid crystal light valve is primarily determined by the aperture ratio of the pixels. In this sense, using a reflective cell structure for the light valve and arranging active devices and data lines behind the reflective electrode are very effective measures. Additionally, compared to direct-view displays, the diagonal size of the liquid crystal cell used in light valves is only 1 inch (25.4mm) or smaller, making them highly efficient at controlling light for the large lcd screen projection.

Therefore, single-crystal silicon (wafer) can be used as the substrate for the liquid crystal cell, and triode arrays and wiring structures can be fabricated using the same process technology as LSI. Compared to amorphous silicon semiconductor films formed on glass substrates, the carrier mobility in single-crystal silicon semiconductor layers is much higher. For n-type semiconductors, electron mobility is approximately 700 cm²/(V·s), and for p-type semiconductors, hole mobility is approximately 300 cm²/(V·s).

Moreover, microfabrication is possible, with deep submicron processing technology for channel widths of 0.35μm being quite mature (in fact, the feature line width of ULSI technology has reached 45nm or even smaller). These advancements ensure higher operating speeds for triodes, which is essential for maintaining image quality when scaled up to a large lcd screen.

Figure 2: MOS transistor structure in LCOS microdisplays

The structure of MOS transistors used to drive liquid crystal pixels in LCOS microdisplays is shown in Figure 6-38. Higher density can be achieved by incorporating peripheral circuits. This technology, or the components produced using this technology, is generally called LCOS (Liquid Crystal on Silicon), meaning liquid crystal integrated on a silicon substrate. Liquid crystal light valves using LCOS technology can achieve an aperture ratio of over 90%, which significantly enhances light efficiency for the large lcd screen projection.

Figure 6-39 shows examples of high-density microdisplay cells (screens) including 0.78-inch WXGA (wide XGA: 1280x768 pixels) and 0.85-inch WUXGA (wide UXGA: 1920x1200 pixels). These high-resolution microdisplays are essential for creating a sharp, detailed large lcd screen projection where individual pixels remain indistinguishable to the human eye.

Front and Rear Projection Systems

Projection liquid crystal displays are categorized into front projection and rear projection types. When the viewer and the projection device are on the same side relative to the screen, it is called front projection. When the viewer and the projection device are on opposite sides, it is called rear projection. Front projection display devices using liquid crystal light valves are called liquid crystal projectors. The optical system used as a component in rear projection systems is called an optical engine. For liquid crystal rear projection displays, a liquid crystal projector essentially functions as the optical engine, ultimately producing the large lcd screen viewers perceive.

Front Projection

In front projection systems, the projector and viewer are positioned on the same side of the projection surface. This configuration is highly flexible for creating a large lcd screen in various environments, from small meeting rooms to large auditoriums.

• Flexible screen size options
• Portable configurations available
• Requires controlled lighting conditions
• Lower space requirements

Rear Projection

Rear projection systems place the projector behind a specially designed screen, with viewers positioned in front. This setup is ideal for dedicated spaces where a permanent large lcd screen display is required.

• Integrated, self-contained units
• Better performance in ambient light
• Fixed installation typically
• Requires more depth space

Figure 6-40 shows the basic configuration of a liquid crystal rear projection display and a schematic diagram of the optical engine. In rear projection systems, the display screen and optical engine are generally integrated into a single unit. Rear projection displays also come in single-panel types using one liquid crystal light valve and three-panel types using three liquid crystal light valves, each offering different advantages for large lcd screen applications.

The choice between front and rear projection depends on various factors including space availability, lighting conditions, portability needs, and budget constraints. Front projection offers more flexibility in terms of large lcd screen size and installation options, while rear projection provides a more integrated solution with better performance in well-lit environments.