Understanding How LCoS Works

For many, the cathode ray tube (CRT) television is what they grew up with. While bulky and heavy, CRT TVs provided an excellent picture as long as the signal was clear. The CRT remains the iconic image when people think of a TV.

However, if you've been shopping for a TV lately, you’ve likely noticed many more options. CRTs are still effective for screens up to 40 inches. But for larger screens, flat-panel TVs, widescreen models, or HDTV compatibility, you'll need to choose from different types of sets, including liquid crystal display (LCD), digital light processing (DLP), and liquid crystal over silicon (LCoS).

While LCoS isn't an entirely new technology, its availability has only recently increased. In this article, we will delve into the technology behind LCoS, how it ensures a clear picture, and how manufacturers have tackled issues related to black levels and contrast.

Review of LCD and DLP

LCoS is commonly used in front- and rear-projection TVs. The setup is similar to that of a DLP system, which uses a digital micromirror device (DMD) to create an image by assembling small square tiles like a mosaic. The DMD contains millions of tiny mirrors that reflect light from a lamp, with each mirror creating one pixel of the image.

The mirrors rapidly switch between their "on" and "off" positions. When on, they point toward a projection lens, with the longer the mirror stays in the on position, the brighter the pixel it forms. Mirrors that create black pixels stay off. In most DLP TVs, a color wheel spins between the lamp and the DMD, adding red, green, and blue light to the image. The viewer’s eyes combine these colors to form the complete picture.

LCoS operates on a very similar principle. Like DMDs, LCoS devices are extremely small—most measuring less than one inch square. Both technologies are reflective, meaning they bounce light from a source to a lens or prism that collects the light and projects the image. However, instead of using tiny mirrors that flip on and off, LCoS relies on liquid crystals to control the amount of reflected light.

A liquid crystal is a substance in a mesomorphic state—it's neither a liquid nor a solid. The molecules usually maintain a solid-like structure but can move like a liquid. For instance, nematic liquid crystals align in loose parallel lines. The majority of LCDs use twisted nematic (TN) crystals, which straighten when an electrical charge is applied.

When placed between two polarized panels, the twisted crystals manipulate the light's path. By altering the light's direction, the crystals can either allow or block it from passing through the second panel. This ability to change the light's path is essential to the function of both LCD and LCoS technologies.

In their twisted state, liquid crystals control the light’s direction so it can pass through the second polarized panel.

Ferroelectric liquid crystals (FLCs), sometimes found in LCoS devices, align at a fixed angle to the normal in orderly rows. These crystals also develop electrical polarity when exposed to an electric charge. Ferroelectric chiral smectic C crystals can quickly change their orientation. To learn more about smectic and nematic liquid crystals, visit Kent State University’s Liquid Crystal Institute.

The liquid crystal layer in an LCoS microdevice regulates the light intensity for each pixel, similar to the mirrors in a DMD. However, creating the picture requires more than just the microdevice—it also involves lenses, mirrors, and prisms.

Projection and Color

Creating an image in an LCoS television involves multiple steps. These include a high-intensity lamp, a set of mirrors and microdevices arranged in a cube, a prism, and a projection lens. Here’s the process from start to finish:

This process is typically used in most rear-projection LCoS televisions. Some projectors employ a linear setup instead of a cube, with the white light hitting surfaces that color it red, green, and blue before it reaches the microdevices. A few systems use just one microdevice, combined with other methods for adding color, such as color wheels like those in DLP systems or transmissive dyes on the microdevices. Some systems also use additional polarizers or filters to enhance picture quality and contrast.

Without the projection lens, the image created in this process would be too small to see properly. This is why LCoS technology is categorized as microdisplays—displays that are too small to be visible without magnification of some kind.

The LCoS Microdevice

Unlike an LCD, which places liquid crystals between two polarized panels, an LCoS microdevice uses a liquid crystal layer situated between a transparent thin-film transistor (TFT) and a silicon semiconductor. The silicon semiconductor has a reflective, pixelated surface. Light from the lamp passes through a polarizing filter and illuminates the device, while the liquid crystals act as gates or valves to regulate how much light reaches the reflective surface. The more voltage applied to a specific pixel’s crystal, the more light it allows to pass through. This process involves several layers of distinct materials working together.

Here’s a breakdown of the components in an LCoS microdevice, starting from the bottom to the top, and their functions:

The materials and designs used vary from one manufacturer to another. Some manufacturers use nematic liquid crystals, while others opt for ferroelectric crystals. Some incorporate organic alignment layers, which can degrade over time due to use and exposure to the high-intensity light from the lamp. Others employ photosensitive materials and light to manage the impulses to the liquid crystal.

LCoS devices typically have a very narrow gap between pixels. The pixel pitch — the horizontal distance separating one pixel from the next of the same color — ranges from 8 to 20 microns (10). This minimal gap reduces or even eliminates the 'screen door' effect that can sometimes be seen in DLP televisions, contributing to a smoother and more uniform image.

While LCoS systems generally produce a high-quality picture, they come with both advantages and drawbacks. Let's explore these next.

Pros and Cons

The physical attributes of the LCoS microdevice, such as its lack of a color wheel and high fill factor, generally result in a high-quality image with minimal artifacts. LCoS pixels also tend to be smoother than those in other display systems, which some viewers believe leads to more natural-looking images. LCoS technology does not suffer from the rainbow and screen door effects often associated with DLP televisions. Unlike LCD systems, LCoS devices are also less susceptible to screen burn-in.

However, many LCoS systems struggle to achieve a strong black level, meaning they have difficulty producing the color black effectively. Televisions with poor black levels tend to have less contrast and detail compared to those with better black levels. LCoS projectors and televisions often use three microdevices instead of one, making them heavier and bulkier. Additionally, these systems typically require regular lamp replacements, which can cost several hundred dollars.

Moreover, LCoS systems are not as widespread as other types of display technologies. The main reason for this is that manufacturing LCoS microdevices is a challenging process, with each device requiring three microdevices per system. Several companies, including Intel, have attempted to develop LCoS systems but eventually abandoned their efforts due to persistently low manufacturing yields.

Check out the following links for more detailed information on liquid crystals, television technology, and related subjects.