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Definition
Large-screen, high-brightness electronic projection displays serve four broad
areas of application: (1) electronic presentations (e.g., business, education,
advertising), (2) entertainment (e.g., home theater, sports bars, theme parks,
electronic cinema), (3) status and information (e.g., military, utilities, transportation,
public, sports) and (4) simulation (e.g., training, games). The electronic presentation
market is being driven by the pervasiveness of software that has put sophisticated
presentation techniques (including multimedia) into the hands of the average PC
user.
A
survey of high-brightness (>1000 lumens) electronic projection displays for
comparing the already existing three types of projection display technologies
namely, Oil film, CRT-LCD, and AM-LCD was conducted. Developed in the early 1940s
at the Swiss Federal Institute of Technology and later at Gretag AG, oil film
projectors (including the GE Talaria) have been the workhorse for applications
that require projection displays of the highest brightness. But the oil film projector
has a number of limitations including size, weight, power, setup time, stability,
and maintenance. In response to these limitations, LCD-based technologies have
challenged the oil film projector. These LCD-based projectors are of two general
types: (1) CRT-addressed LCD light valves and (2) active-matrix (AM) LCD panels.
LCD-based projectors have not provided the perfect solution for the entire range
of high-brightness applications. CRT-addressed
LCD light valves have setup time and stability limitations. Most active-matrix
LCDs used for high-bright-ness applications are transmissive and, because of this,
heat generated by light absorption cannot be dissipated with a heat sink attached
to the substrate. This limitation is mitigated by the use of large-area LCD panels
with forced-air cooling. However, it may still be difficult to implement effective
cooling at the highest brightness levels.
In response to these and other limitations,
as well as to provide superior image quality under the most demanding environmental
conditions, high-brightness projection display systems have been developed based
on Digital Light Processing technology. DLP is based on a micro electro mechanical
system (MEMS) device known as the Digital Micro mirror Device (DMD). The DMD,
invented in 1987 at Texas Instruments, is a semiconductor-based array of fast,
reflective digital light switches that precisely control a light source using
a binary pulse modulation technique. It can be combined with image processing,
memory, a light source, and optics to form a DLP system capable of projecting
large, bright, seamless, high-contrast color images.
The
Mirror as a Switch
The DMD light switch is a member of a class of
devices known as micro electromechanical systems. Other MEMS devices include pressure
sensors, accelerometers, and micro actuators. The DMD is monolithically fabricated
by CMOS-like processes over a CMOS memory. Each light switch has an aluminum mirror,
16 µm square that can reflect light in one of two directions depending on
the state of the underlying memory cell. Rotation of the mirror is accomplished
through electrostatic attraction produced by voltage differences developed between
the mirror and the underlying memory cell. With the memory cell in the on state,
the mirror rotates to +10 degrees. With the memory cell in the off state, the
mirror rotates to .10 degrees. A close-up of DMD mirrors operating in a scanning
electron microscope (SEM). By combining the DMD with a suitable light source and
projection optics (Figure 6), the mirror reflects incident light either into or
out of the pupil of the projection lens by a simple beam-steering technique. Thus,
the state of the mirror appears bright and the state of the mirror appears dark.
Compared to diffraction-based light switches, the beam-steering action of the
DMD light switch provides a superior tradeoff between contrast ratio and the overall
brightness efficiency of the system.
By
electrically addressing the memory cell below each mirror with the binary bit
plane signal, each mirror on the DMD array is electrostatically tilted to the
on or off positions. The technique that determines how long each mirror tilts
in either direction is called pulse width modulation (PWM). The mirrors are capable
of switching on and off more than 1000 times a second. This rapid speed allows
digital gray scale and color reproduction. At this point, DLP becomes a simple
optical system. After passing through condensing optics and a color filter system,
the light from the projection lamp is directed at the DMD. When the mirrors are
in the on position, they reflect light through the projection lens and onto the
screen to form a digital, square-pixel projected image.
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