Digital Light Processing
Published on Feb 20, 2020
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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