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Single Panel DMD projector with improved gamma and high brightness

IP.com Disclosure Number: IPCOM000014657D
Original Publication Date: 2000-Mar-01
Included in the Prior Art Database: 2003-Jun-20

Publishing Venue

IBM

Abstract

Single Panel DMD projector having improved color depth and high brightness

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Single Panel DMD projector with improved gamma and high brightness

Single Panel DMD projector having improved color depth and high brightness

Disclosed is a technique to improve color depth of
a Single Panel Digital Micromirror Device (DMD)
projector without sacrificing brightness.

In a single Panel DMD projection system, a color
wheel is used to create a full color projected
image ( Fig-2). The color wheel is a red, green
and blue, filter system which spins at 60 Hz to
give 180 color fields a second (Fig-3). As DMD is a
bi-stable device, gray scale is achieved through a
digital technique called pulse width modulation
(PWM). With PWM, grayscales are generated by
varying the the time mirror is ON (Fig-4). Color
depth is limitted by the micromirror switching
time. For computor image such as business graphics,
gamma correction is not required and 8-bit color
depth is achieved by PWM (Fig-5a). But video
signals typically have gamma correction applied to
them to compensate for the non-linear
characteristics of the CRT receiver. Degamma will
be achieved by controlling ON time duty in the
conventional single DMD projector. But it will
reduce the color depth (Fig-5b). Adding a Rotating
Neutral Density (ND) wheel (Fig-6,Fig-7,Fig-8) will
introduce natural degamma characteristics without
reducing color depth (Fig-9). But a half of light
will be lost at ND filter and the projection image
becomes dark.

Disclosed technique is to improve color depth with
ND wheel, preventing brightness loss, by utilizing
Two-zoned DMD.

The DMD is monolithically fabricated by CMOS-like
process over a CMOS memory. Each light switch has
an aluminum mirror which can rotate around the
hinge locating on the diagonal. Each mirror 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
difference developed by the mirror and the
underlying cell. With the memory cell in ON state,
the mirror rotates +10 degrees. In this state, the
mirror will reflect the incident light to the
projection lens. This light will forms a "ON" pixel
on the screen. With the memory cell in OFF state,
the mirror rotates -10 degrees. In this state, the
mirror will reflect incident light -40 degrees from
the optical axis. This light will be absorbed.

In the conventional DMD, all mirrors have the same
hinge direction. In the new technique, mirrors are
divided into two zones. In the left zone, hinges
direction is +45 degrees, and in the right zone,
hinges direction is -45 degrees (Fig-10).

The white light from lamp [1] is collimated into
parallel beam by parab...