3 Types of Projector Technology: LCD, DLP and LCoS
There are three types of technology used in projectors. They are Liquid Crystal Display, Digital Light Processing and Liquid Crystal on Silicon.
Liquid Crystal Display – LCD
Liquid Crystal Display is commonly referred to as LCD. There are three separate LCD glass panels, using the standard RGB** color configuration. An LCD projector is a modern analog of the slide projector or overhead projector.
Vivitek’s 1080p LED projector, HC7500A
It typically send light from a metal halide lamp through a prism that separates light into three poly silicone panels, one each for red, green and blue components of the video signal. As the polarized light passes through the panels, individual pixels either opens or closes, to allow or block the light from passing through. The combination of open and closed pixels projects an image with a wide range of colors and shades.
LCD projectors have the ability to project an image on any flat surface, and they can be small and portable. The best surface for projection is either a blank white, grey or black surface, and dedicated projection screens are mostly white.
Early LCD systems were used with existing overhead projectors. The LCD system did not have a light source of its own. The overhead projector was built on a large “plate” that sat on top of the projector in place of transparencies during the era when computer was not yet a universal display medium. The LCD technology was later employed in some rear projection television consoles.
It is normal, but not compulsory, for LCD projectors to use SCART connectors, especially for European models. SCART is designed to carry composite video and RGB video signals.
** There is now a new technology using RGBA, with the A representing an Alpha mode. It is actually a simple use of the RGB color mode with an alpha channel, normally used as an opacity channel.
Digital Light Processing – DLP
Digital Light Processing, or DLP, is a trademark owned by Texas Instrument. It is currently one of the leading technologies used in digital cinema projection.
DLP technology with a single chip.
In DLP projectors, the image is created by mirrors laid out in a matrix on a semiconductor chip, known as a Digital Micromirror Device. These mirrors are microscopically small, and each represents one or more pixels in the projected image. The number of mirror corresponds to the resolution of the projected image, and the common resolution of 1280 x 720 and 1920 x 1080 (both HDTV, usually referred to as 720p and 1080p) are some common DMD sizes.
There are two methods by which DLP projection system creates a color image. The first uses a single-chip DLP where colors are either produced by placing a color wheel between the lamp and the DLP chip, or by using individual light sources to produce the primary colors. Light sources may include LEDs or lasers. The color wheel is divided into multiple sectors, which includes the primary colors of red, green and blue, and very often includes the secondary colors of cyan, magenta, yellow and white.***
*** The use of secondary colors is a new color performance system called BrillianColor, which processes the primary colors and secondary colors together to create a broader spectrum of possible color combinations for projection.
Meanwhile, a three-chip DLP projector uses a prism to split light from the lamp, and each primary color light is routed to its own DLP chip that can produce 35 trillion colors, while the human eye can detect about 16 million colors. However, three-chip DLP projectors do not display the entire gamut of colors that the human eye can distinguish. Ironically, single-chip DLP projectors have an advantage of allowing any number of primary colors in a sufficiently fast color filter wheel and improve color gamut.
LED-based DLPs eliminate the use of color wheel. It has the advantage of a longer lamp replacement time as opposed to the traditional use of metal halide lamps. The LED illumination helps increase color saturation and improve color gamut. This was the technology used by Vivitek for the first 1080p projector in the market.
LASER-based DLP also eliminates the use of a color wheel. In this technology, three LASERs illuminate the DMD in projection televisions to produce richer and more vibrant color palettes. The first commercially available was a DLP HDTV by Mitsubishi, L65-A90 LASERVUE, in 2008.
Liquid Crystal on Silicon – LcoS
The Liquid Crystal on Silicon, LCoS, is a “micro-projection” technology applied in projection tv. The theory of this technology is similar to DLP, but instead of using micromirrors, it uses liquid crystals. The liquid crystals of LCoS are applied directly to the surface of a silicon chip coated with an aluminized layer and a passivation layer, which is highly reflective.
Technically, it produces higher resolution and higher contrast images than LCD and plasma display technologies, and it is less expensive too. In December 2005, the first projector using 3 LCoS chips by Sony was introduced. The Sony-VPL-VW100, or affectionately called the “Ruby” projector has a contrast ratio of 15,000:1 using a dynamic iris, and each of the LCoS chips support a native resolution of 1080p.
There are two types of LCoS displays. First, there is a three-panel design, where ther is one display chip per color, and the images are combined optically. Second, there is the single-panel design with one display chip showing the red, green and blue components in succession, relying on the viewer’s eye to combine the color stream. A color wheel is present, illuminating the display with only red, green or blue light with each color presented. Single panel projectors are less expensive but it requires higher speed display elements to process all three colors during a single frame time. If a color field frequency is lower than 540Hz, color breakup happens where false colors are briefly perceived when either the image or the viewer’s eye is moving.
* There is also another projector technology, the Cathode Ray Tube, or CRT. Currently, this technology is no longer available for general customer use. Many older units are still in operation, and it is only available for some commercial and industrial settings. Hence, it is not covered here.
