Mike Akridge, Tim J. Butler, and Graham H. Moss
During the past decade, advancements in image modulators have made electronic projection systems widely available to a variety of large-screen industries. Consequently, projector manufacturers, such as Digital Projection Inc. (DPI; Kennesaw, GA), are diversifying their units and adding proprietary technology, such as the digital micromirror device (DMD; Texas Instruments; Dallas, TX), to proven projection platforms.
A DMD is an array of microscopic mirrors built over a CMOS SRAM cell, fabricated with semiconductor-manufacturing techniques. The DMD chip is approximately 2 x 1.5 cm, or about the size of a postage stamp. Each mirror, 16 µm square, represents one pixel within the displayed image. Commercial DMD resolution has recently been increased to 1024 x 768 mirrors, or 786,432 mirror-pixels per chip.
Projection systems
DMD-based projection systems manufactured by DPI contain a prism, light source, lamp, and general power supplies and signal-processing electronics. The systems use the DMD chips, prism, and formatting electronics of a three-chip DMD light engine, which creates projected images by reflecting light off of each mirror array. Mirrors on the array are independently modulated from the on (or reflect) position to the off (or light-dump position) by applying an address voltage. The amount of time a mirror remains in the on and off positions determines the brightness of the projected pixel.
Light enters the prism assembly and is divided and filtered into each of the three color primaries. The three-chip DMD light engine uses three static dichroic filters to separate the primary colors without any time-division multiplexing. Each DMD chip within the projection system creates a monochrome image for each color primary: red, green and blue. These independent images are recombined within the prism assembly, creating a single integrated, color-accurate image when projected. Signal information is presented to the DMD in a 10-bit digital format to achieve full color depth. Each DMD creates up to 1024 shades of gray; a three-chip projection system is capable of generating more than 1 billion dependable and predictable colors or gray shades.
The densely arranged DMD mirrors provide an on-screen fill factor of greater than 90%. Additionally, the mirrors have a high intrinsic reflectivity greater than 90%. This allows very high system efficiency and provides an on-screen benefit of increasing perceived resolution and very even illumination across the screen. This is a significant improvement compared to traditional display technologies such as liquid-crystal displays (LCDs).
For example, LCD projection panels are transmissive devices that have pathways for control traces between each pixel. This creates a panel that typically uses as much as 50% of its surface area for the creation of images and transmission of light. When magnified on the projection screen, this type of display fills less than half of the screen surface area with the actual program material. Additionally, more than half of the luminance from the light source is restricted or blocked by the control trace pathways, LCD panel, and required polarizers. This 50% fill factor is not efficient or effective for audience view closer than 1.5 times the screen width, and the reduced luminance of the projected image tends to restrict the optimum depth of view to no more than five times the screen width under normal lighting conditions.
For DMD- and LCD-based projection systems with a native resolution of 1024 x 768 pixels displaying the same source material on the same 57-ft-wide screen, the DMD pixel will be 16 mm square with 1 mm of separation between mirrors. The LCD pixel will be approximately 8.5 mm in diameter with an 8.5-mm space between the nearest pixel's edge.
Using the DMD
To achieve a high level of illumination or brightness using a very small image modulator such as the DMD, a number of critical optical-path points must be considered. The first stage of the optical path is the light source, which, for a high-brightness DMD projector, must provide a very-high-lumen output with high efficiency in a small volume. In addition, the diameter and angular distribution of the light output must be constrained to allow the maximum power transfer to the DMD. These requirements can be conflicting, and to achieve best performance and efficiency, DPI uses an optical-integrator rod to convert the circular illumination area from the condenser to the rectangular area of the DMD. This is a solid rod of optical material with a rectangular cross section that matches the format of the DMD. The light from the lamp is focused onto one end and propagates along the rod by a series of total internal reflections, finally exiting at the opposite end.
The action of the rod is to superimpose rotated images of the focused spot, hence removing intensity nonuniformities. The light is spread evenly over the exit face of the rod, thereby producing uniform illumination in the correct format ratio. A relay lens system is then used to image the exit face of the rod onto the DMD.
The second stage is the prism system, which, in addition to the input and output control of the beam, also provides a mechanical surface to mount the DMD accurately for fixed registration. Further, the prism has also been designed to carry unused dump light out of the system without increasing the potential for light scatter. The specific positions of the mirrors reflect the light to a black surface within the prism. The current prism design converts the light to thermal energy. A heat sink positioned on the prism carries the now converted light energy out of the system.
Matching etendue
An interesting benefit of DMD-based projection systems is that as the size of the DMD surface increases, total on-screen luminance increases without increasing illumination power. More specifically, if the area of the DMD is increased, the surface area, or etendue, of the DMD is increased, and the maximum possible efficiency can be increased. Etendue is a geometrical parameter of the optical beam that is conserved in a well-corrected optical system. It is basically the product of the cross-sectional area of the beam and the solid angle of the beam, both measured at the same point in the optical system.
If the etendues of the lens, prism, and condenser system are less than or equal to the DMD etendue, they can be removed from consideration because they do not reduce the throughput of the system. If that is true and the lamp-reflector etendue is equal to the DMD etendue, maximum efficiency is achieved. It is therefore reasonable to expect luminance values achieved through using the 1024 x 768 DMD light engine to increase when the same lamp energy is applied to a 1280 x 1024 DMD light engine.
The choice of a xenon lamp is a reasonably straightforward design choice for use in a DMD-based projector, as xenon lamps produce light with broad and accurate spectral qualities. The color temperature of a xenon lamp also tends to be stable over the life of the lamp, shifting on average by only 200 K. This accurate stable light source, when used with a digitally controlled and accurate DMD-based projection system, creates highly saturated, accurate color that shifts little over its life.
However, it is critical to manage the significant infrared and ultraviolet component of the xenon light to prevent damage to the optical system and DMDs. This is done as light leaves the lamp housing. A custom-designed cold reflector is positioned in the light path at 45° to the lamp projection angle. This cold reflector allows the IR energy from the lamp to pass through to a heat-sink collector. Infrared light is converted to thermal energy and is exhausted from the projector. The light, minus the IR portion, is reflected down the long axis of the projector. Immediately in front of the primary condenser lens and after the cold mirror, is a custom designed UV filter to provide additional filtering of damaging light wavelengths.
MIKE AKRIDGE is director of technical operations, and TIM J. BUTLER is the marketing communications manager at Digital Projection Inc., 55 Chastain Rd., Kennesaw, GA 30144; e-mail: [email protected]. GRAHAM H. MOSS is an optical systems engineer at Digital Projections Ltd., Manchester, England.