Digital Light Processing

By Team Digit Published Date
01 - May - 2005
| Last Updated
01 - May - 2005
Digital Light Processing
Digital Light Processing (DLP) is a technique used to project images and is based around a specialised optical semiconductor chip-a Digital Micromirror Device (DMD), developed by Texas Instruments (TI). Today, DLP powers general purpose conference room projectors, and over the years, has spawned a new category of small, ultra-portable mobile projectors.

Although initially developed for projection processes, DLP is now used in telecommunications, scientific instrumentation, volumetric displays, holographic data storages, lithography and medical imaging.

Let's take a closer look at what makes a DLP system tick.

The Chip That Rocks The Cradle
The soul of any DLP system is the Digital Micromirror Device (DMD) chip, which was developed by Dr Larry Hornbeck at Texas Instruments (TI) in 1987. Over the years of development, starting from 1977, to the release of the first commercial product in 1996, DLP technology has been perfected to deliver the goods it promises.

A DMD chip consists of an array of two million microscopic hinged mirrors that can be individually tilted using digital signals. The tilting motion allows them to be used as a switch to modulate light in a particular direction, and are often referred to as light switches or Spatial Light Modulators (SLMs).

Each aluminium micromirror is a 16 micrometer square, polished to reflect light in the desired direction. The combination of a micromirror, the hinge assembly and an address pad forms one pixel of the DMD.

The Stuff It's Made Of
The mirror has a central post on which it is pivoted (as shown above: Anatomy Of A Micromirror); the other end of the post is connected to the yoke of the hinge assembly. The hinge assembly consists of a central yoke that is suspended between two support posts via a torsional hinge.

Two address electrodes are placed coinciding with the diagonal ends of the mirror, with a gap of air left between the mirror surface and the address electrode, so as to enable the tilting motion of the mirror.

The hinge assembly, as a whole, is responsible for the tilting motion of the mirror. The address pad has two pads-one feeds the address electrodes with the required signal while the other sends the 'Reset' signal to the yoke in the hinge assembly via the supporting post. The address pad acts as an interface between the mechanical mirrors and the CMOS addressing logic that forms the substrate of a DMD chip.

Since a DMD is an amalgamation of an optical device with a semiconductor controlling base, it is called an Optical Semiconductor. Such devices fall under a category known as Micro-Electronic Mechanical Systems (MEMS). A DMD is a 'MEMS' primarily because it has millions of mechanically moving micromirror's controlled via CMOS electronics.

How Stuff Works
To modulate light in a particular direction, the mirror can be rotated to a maximum of ±12 degrees, depending on the state of the CMOS circuitry lying underneath each mirror. The CMOS applies a voltage to the address electrode of the hinge assembly to create an electrostatic attraction between the electrode and the mirror, which, in turn, results in the tilting of the mirror in the desired direction.

When the CMOS is in the 'ON' state the mirror rotates to 12 degrees, whereas when the CMOS is in 'OFF' state the mirror rotates to -12 degrees. Once the rotation completes, the mirror is electro-mechanically latched in the desired direction, and the state of the CMOS can change without affecting the position of the mirror.

Typically, for projection purposes, a metal-halide lamp is used as a light source. The white light from such a source is projected on the DMD using focusing optics.

The arrangement of the light source, with respect to the DMD chip, is such that all micro mirrors that are in the 'ON' state reflect the incident light to the pupil of the projecting lens, whereas the micromirrors in 'OFF' state reflect the incident light on to a light absorber.

The controller driving the CMOS states (ON or OFF), can change a thousand times per second. In other words, each mirror on the DMD either switches 'ON' or remains 'OFF' a thousand times per second.

When a particular micromirror remains 'ON' more frequently than 'OFF', it will project a grey pixel on the screen whereas the micromirror that remains 'OFF' than 'ON' reflects a darker shade of grey. In this way by precisely controlling the rotation of the mirrors, the DMD is able to produces a grey scale that can go up to 1024 shades of grey.     

Once you have a grey scale solution in place, producing colour is just a matter of introducing a colour wheel, consisting of red, green and blue filters between the light source and the DMD.

The rotation of the wheel is synchronised with the controller that drives the CMOS of each mirror. Now, for example, to project the colour purple on the screen, a pixel will only project red and blue light.

The colour green is bypassed by ensuring that the micromirror is in the 'OFF' position when the green filter is in position, and turned 'ON' when the Red and Blue filters come into position. Though this actually means that different colours are flashed on the screen a thousand times a second, the naked eye cannot keep up with this and sees a mixture of red and blue-purple.

Now, since each mirror is controlled by a digital signal-issued to the CMOS circuitry by a digital controller-the incident light is said to be digitally modulated, and hence this technique is called Digital Light Processing (DLP). It should be noted that during the whole controlling or modulating process, the incident light never undergoes an optical-to-electrical conversion, as is prevalent in digital cameras and camcorders.  

A Three Chip DLP Projector Configuration

The current generation of DMDs have a mechanical switching time of 15µs (microseconds) and an optical switching of 2µs. Such fast switching times offer the advantage of using a single light modulator i.e., one DMD, to produce 256 shades per primary colour, as compared to other slower modulators which require three separate modulators for each primary colour.

Thus, using a single DMD, smaller and more compact projectors can be made, which offer results on par or even better than projectors based on slower light modulators, as in LCD technologies, for instance.
DLP In Projectors
Depending on the number of DMDs used (one, two, or three), three configurations of DLP projectors are currently available in the market. Remember, the choice of configuration depends upon the intended application, cost, brightness levels, lamp technology, weight and power dissipation.

One-chip configurations are normally used in consumer grade products such as conference room projectors and televisions. The single chip projector is cheaper, offers decent brightness levels and results in smaller, more  portable designs.

Generally, the two chip projectors are used in situations where a longer lamp life is desired since they offer better light efficiency.

Three chip projectors offer the highest optical efficiency, and hence are used in venues where large-screen projection is required, such as public information displays, auditoriums and movie theatres. Shown on the previous page is a functional schematic of a three chip DLP projector.  

Apart from the above-mentioned parameters, 'Resolution' of the projected image is also important. Products using DLP technology capable of SVGA (800 x 600), XGA (1024 x 768) and SXGA (1280 x 1024) are available in the market. Recently, improvements have added high definition capabilities to DMDs, and such products will be available soon.

DLP Cinema
The goal of most multimedia projector manufacturers is to offer a product that can rival the quality of 35 mm film projection. The motion picture industry has long since shifted to digital technology, in terms of the production of movies; however, movie theatres still use last century's technology to show them to us.

In recent years, the motion picture industry has opted for multi-track sound formats such as Dolby or DTS, resulting in a dramatically improved aural experience. The visual experience though, remains outdated.

Moreover, film-based projection has severe limitations-duplication and distribution is expensive, deteriorating film quality with repeated screening, focus flutters, travel ghosting and inconvenient updating of post production content such as advertisements.

An exciting development in DLP technology is all set to change this with the introduction of DLP Cinema. The goal of DLP Cinema is to make the entire motion picture industry a digital industry, right from the production and distribution, to the final presentation of the film.

In 1997, a prototype using a 1280 x 1024 (SXGA) DMD called the DMD1210 was made, for evaluating the rapid upgrading of existing projectors to produce high quality, high brightness images. It involved the technical and creative community of the motion picture industry. The DLP Cinema technology demonstration projector exceeded the expectations of most people.

Alhough, the DLP Cinema has apparent advantages over film based projection, as was proved by the demonstration, there are some hurdles that the technology has to see through. A considerable amount of work is needed in terms of data compression, storage, and media delivery, before the 'Cinema of Tomorrow' can materialise.

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