Microdisplay describes a group made up of HDTV display formats types. The HD Guru explains how they work and describes the differences between the three competing microdisplay technologies.

What is it?

A Microdisplay is a device or chip smaller than a 1 inch diagonal found inside all lamp- driven projection HDTVs. The device(s) creates all of the picture elements. Light is either transmitted through (LCD), or reflected off the device (DLP, LCoS) and then sent through projection lenses onto a special optical screen for viewing. When you look at microdisplay rear projection TV you are looking at a magnified view of the image produced by the device(s).

The Three Types of Microdisplays and How They Work

The three types of microdisplays are LCD, DLP and LCoS. Each creates images differently.

LCD is a transmissive technology, meaning that light travels through the chip to form an image, and then bounces off a mirror and onto a screen. Each picture element uses microscopic crystals suspended within a solution. A drive transistor around each pixel sends varying voltage levels to its adjacent sub-pixel. A voltage increase or decrease causes the tiny crystals to rotate. Polarizing the projection lamp’s light, it is then split it into the three primary colors (Red, Blue and Green), with each color beam then passing through its own LCD chip. As the light passes, each pixel acts like a tiny shutter, allowing the light to either pass or be absorbed. The three colored light beams then recombine to form a full color image, which is then projected onto the screen. LCD rear projectors today are only available in the 720p format (720 progressively scanned lines) with a resolution of 1280 x 720. LCD RPTV is a mature and proven technology that’s been available for five years.

DLP is a reflective technology utilizing between 900,000+ and 1,000,000+ microscopic mirrors housed within a single tiny chip. Each independently functioning hinged mirror can pivot up and down, and thus reflect different amounts of light. Current DLP chips contain either 1280 x 720 (720p) or 960 x1080 mirrored pixels. An optical shifting technique allows the 960×1080 chip to project 1920 x 1080 pixels on the screen. DLP’s mirrors are spaced closer together than LCD’s pixels, producing greater “active area” within the chip. The amount of “active area” within the chip, relative to its overall size, is called its “fill factor.” DLP’s “fill factor” is higher than LCD’s.

All DLP RPTVs use a color wheel placed in the light path after it has been reflected off the DLP chip, to create a full color image from a single chip. Color wheels today include 3 to 6 color elements. All have red, blue and green primary segments, with the more sophisticated ones adding cyan, magenta and yellow. The spinning color wheel, powered by a tiny, high speed electric motor creates a full color image on the screen, using a process know as sequential color.

Early versions of DLP suffered from what’s called the “rainbow effect,” which caused some viewers to occasionally see the coherent color image break up into quick flashes of red, blue and green stripes. Higher speed wheels and six color segments have virtually eliminated this problem for the vast majority of viewers. DLP “light engines” introduced in 2006 utilizing red, blue and green light emitting diodes (LEDs) capable of high speed operation, eliminate both the color wheel and expensive bulbs. DLP is also a mature RPTV technology, available for more than five years.

LCoS ( Liquid Crystal on Silicon), combines aspects of LCD and DLP. Like LCD, the LCoS chip utilizes microscopic crystals and polarized light as well three chips to create a full color image. However, unlike LCD, which passes light through the crystal, the LCoS chip’s crystal layer is mounted on a reflective layer, behind which are the switching transistors, allowing the individual pixels to be placed closer together than either LCD or DLP and giving LCoS better “fill factor” than either DLP or LCD.

Two LCoS variants, DiLA, or Digital Image Amplifier (JVC) and SXRD, or Silicon Crystal Reflective Display (Sony) are currently manufactured. DiLA chips are available in projectors with resolutions of 1280 x 720 (720p) and 1920 x 1080 91080p) full HD. Sony’s SXRD RPTV are available only in full HD 1920 x 1080 (1080p) resolution.

LCoS is a relatively new display technology. JVC’s current three chip DiLA is less than three years old, while Sony’s SXRD is less than two years old. The HD Guru strongly recommends purchasing an extended warranty on any display device with a core technology less than three years old. Otherwise you are assuming the high risk of problems often associated with an unproven technology.

Microdisplay technology allows designers to make slimmer projectors. Current projectors have depths from around 20” to down to 10” with screen sizes ranging from 42” –73”.
DLP rear projectors are available from Mitsubishi, RCA, Samsung, Toshiba, Zenith and Nuvision. LCD rear projectors are available from Sony and Hitachi. LCoS is only available from JVC and Sony.

Coming Soon: The HD Guru Picks the Top Microdisplay Rear Projectors
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