Projection TV Guide - How-it-works Projection Television
Updated: December 15, 2012

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A How-it-Works Guide to Video Projection Technology


We discuss available video projection technologies and explain how the different projection systems operate to produce huge high-resolution images on a projection screen - whether that being hung on a wall in the case of a front projection setup, or in a TV box in the case of a rear projection HDTV.


 

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Video Projection Technology Basics

An awareness of the basic operational principles behind the different projection television technologies would surely come to assistance when selecting a rear projection TV or home theater projector.

In principle, video projection systems work by forming a small image on a device inside the projector and then shines that image onto a large screen located elsewhere; the traditional image source used to be the CRT or an LCD panel.

However, development in technology led to the use of other 'non-traditional' image sources such as the use of Digital Micromirror Devices,  also referred to as DMD, in order to serve as the image generating component inside a video projector unit.

Irrespective of the projection technology in use, projection television systems consist of three basic elements or components, the arrangement of which varies with the different projector solutions; these are:

- The actual Projector Unit
- Projection Screen
- Control Electronics

It is important to note here that there are two basic design approaches with respect to the deployment of the 'projector' unit in a video projection system: 'Rear' and 'Front' projection set-ups.

Rear Projection Television Systems:  Here the image is projected onto a mirror within the projection 'cube' itself. The mirror bounces the image on a screen located at the front of the cube (or box), housing the various components of the video projection system. This set-up renders a rear projection television similar in appearance to a large-screen conventional television - the main difference being the deeper profile of a rear projection TV over a flat-panel HDTV.

Front Projection Television Systems: In front projection, the image is projected onto a screen located elsewhere within the room. In the case of front projection, the projected image size is normally limited only by the size of the room itself.

The 'Projector Unit' within a projection television system may be further sub-divided into two main categories:

Transmissive Projectors: These shine light through the image-forming element prior to being projected onto a screen; this can be either a CRT or LCD type.

Reflective Projectors: Here, the light is bounced off the image-forming element, to be projected onto a screen; DMD and LCOS projectors use this principle of operation.

Note: The terms 'Transmissive' and 'Reflective' as used here should not be confused with 'rear' and 'front' projection. Rear and front projection refer to the arrangement of the projector unit within the projection television system itself; on the other hand, the terms 'transmissive' and 'reflective' refer to the optical processes used within the projector unit to build up the projected image.

Transmissive-Type Video Projectors:

Transmissive-type projectors use either CRT tubes or an LCD type display to generate the image.

Projection television - how it works

Transmissive Projection Set-Up

Photo courtesy: Philips Research

CRT Type:

Typically, these employ small 9" tubes similar to those used in conventional TV sets except that these are extremely bright and expensive!

These projector units may use any one of the following set-ups:

Single Color CRT Tube: This is the simplest of all projector implementation.

Single Black and White CRT Tube with a rapidly rotating tri-color filter wheel in front of the tube to generate the color information.

Three CRT tubes: Three tubes are used - one for each color red, green, and blue - with a separate lens for each tube, and aligned in such a manner to produce a single image onto a projection screen.

Though all three configurations are possible, yet it is the three-tube CRT configuration that is most commonly employed due to its capability to produce brighter image projections.

LCD-Type Projectors:

These make use of a small color LCD display panel to generate the image; a bright light source is used to backlit the LCD and an optical lens arrangement is used to project the image formed by the LCD onto a projection screen.

LCD video projectors are among the cheapest and represent most of today's entry-level home theater projectors.

This type of projector is very similar in operation to a conventional slide projector with the LCD acting very much like the photo-slide itself. Some set-ups make use of three separate mono-LCD screens — hence the terms 3-LCD video projector — one for each color in a similar manner to the three-CRT tube arrangement.

Reflective-Type Video Projectors

It is in the area of reflective projector technology that we have experienced the most exciting advancements in projection television systems during these last years. Main technologies include:

Microelectromechanical systems (MEMS) such as the Digital Micromirror Device (DMD) and the Grating light valve (GLV)

Liquid Crystal on Silicon - simply referred to as LCOS

Microelectromechanical Systems

The main characteristic of MEMS-based devices is that these have a movable or deformable reflective surface on top of a semiconductor chip. The chip generates voltages in response to digital information which then deforms the reflective surface rapidly and in a controlled way to produce the image that was encoded by the digital information.

Projected light bounces off the reflective surface and gets collected by the projector lens.

MEMS devices include Digital Micromirror devices and Grating Light valves.

Digital Micromirror Devices (DMD)

Also know as DMD or DLP (short for Digital Light Processing Device), this represents the only true reflective imaging technology.

The diagram below shows a typical DLP-based video projector with the various components and the DMD imaging chip.

Click on image to enlarge

DLP™ technology is based on an optical semiconductor device called Digital Micromirror Device, or DMD chip, which was invented in 1987 by Texas Instruments. To-days DLPs consist of a small chip with up 2 million individually controlled hinge-mounted microscopic mirrors; the number of mirrors being dependent on the pixel resolution of the projected image.

These micromirrors, each measuring just 16 micrometers square in size (that's one-fifth the width of a human hair!) - rest on support hinges; space between individual mirrors is less than 1 micron. Each individual mirror is connected to an array of controlling electrodes.

The support structure is such that each mirror can tilt from some +10 degrees to –10 degrees corresponding to ‘on’ and ‘off’ in a digital signal. This switching on and off can occur several thousands of times per second.

A light source is used to shine on the DLP chip surface; a lens is aligned such as to project the resulting image for 'positive' tilts. For 'negative' tilts, light is reflected onto a light absorber. 

The process to eventually produce the image projection requires that each frame of a movie is digitally processed, separated into its red, blue, and green components, and digitized into one to two million samples (the number of samples depends on chip pixel resolution) for each color. Each mirror in the system is controlled by one of these samples.

By using a color filter wheel between the light and the DMD (click on above image for an enlarged picture), and by varying the amount of time each individual DMD mirror pixel is on, a full-color, digital picture is projected onto the screen.

Current DMD chips can produce up to 1024 shades of gray, which when combined with a 6-panel color wheel (2 x RGB), produce more than 16.7 million colors. Pro systems (such as those used in cinemas) based on the use of 3-DMD chips, can reproduce over 35 trillion colors!

DLP chips are extremely robust and come with a MTBF (mean time between failures) of 100,000hrs - the highest around for any projector technology.

Grating Light Valves (GLV)

Projection technology - GLV chipDiagram of a single grating light valve pixel on a GLV chip
(by Silicon Light Machines)

Earlier on, we have mentioned the Grating Light Valve. This represents one of the latest advancement in reflective projector technology and consists of tiny reflective ribbons mounted over a silicon chip.

Response time for these ribbons is extremely short making it possible to generate a complete picture by simply aligning the GLV pixels into a single line rather than an array. Three lasers are used – one for each color – to scan these pixels and project the resultant image. GLV chips are capable of generating high resolution images at relatively low cost.

Liquid Crystal on Silicon (LCOS)

This is a relatively new LCD-based technology that uses reflective optics instead of the usual transmissive based setup normally associated with LCD type projectors. LCOS are capable of a higher resolution than normal LCD technology and are used both in projection television systems, as well as in near-eye applications.

In contrast to conventional (nematic twisted-type) LCDs, in which the crystals and electrodes are sandwiched between polarized glass plates, LCOS devices have the crystals coated over the surface of a silicon chip. The electronic circuits that drive the formation of the image are etched into the chip, which is coated with a reflective (aluminized) surface. Light is then bounced off the LCOS chip surface. The polarizers are located in the light path both before and after the light bounces off the chip.

Color information is reproduced using either three separate chips - one for each color, or using a color filter wheel in a similar arrangement to DLP type set-ups.

LCOS-based projectors are easier to manufacture than conventional LCD displays. In addition, the LCOS chip can support a higher resolution because several million pixels can be etched onto a single chip. This also renders LCOS devices much smaller than conventional LCD displays.

These characteristics render LCOS devices suitable for both projection television systems as well as for micro-displays used in near-eye applications - like wearable computers.

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