Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-11-22
2004-02-10
Lester, Evelyn (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C345S089000, C345S101000, C345S692000, C345S697000, C345S204000, C345S214000, C345S599000, C348S671000, C355S035000
Reexamination Certificate
active
06690499
ABSTRACT:
This invention relates generally to methods for modulating light and more specifically to methods and arrangements for producing modulated light having linear gray scale in light modulating systems with a plurality of states, wherein the response time for the light modulator to modulate between the states may be longer than the duration of at least one of the time periods used to produce a desired gray scale intensity.
BACKGROUND OF THE INVENTION
It is well known that humans viewing successive images within short time intervals may perceive the images as a single or continuous image. For instance, cinematic motion pictures include a series of individual images; however, the individual images appear as a continuous image when viewed in succession above a certain frame frequency. This frequency has been called the critical flicker frequency and in many systems, the critical flicker frequency is roughly 60 hertz. Thus, in most situations, when the time interval for each image in a series is on the order of {fraction (1/60)}th of a second, the individual images become indiscernible.
Certain display systems exploit this concept to produce images. For example, consider a display system consisting of an array of pixels, each pixel having only two states, ON and OFF. This type of display system is know as a binary display system. In such a system, the pixels switch between the two states, thus modulating light so as to produce images. Binary display systems are used in a variety of applications, including head-mounted, hand-held, desk-top and projection devices. Consider further that this display system is capable of switching the individual pixels between the two states at frequencies much greater than the critical flicker frequency. If a specific pixel is ON for half of the time and OFF for half of the time and the frequency of modulation is less than the critical flicker frequency, the pixel appears to flash. However, if the pixel modulates between ON and OFF at a frequency greater than the critical flicker frequency, then the pixel appears to be ON continuously, but the intensity appears to be half as great as the intensity if the pixel was in the ON state. Likewise, a pixel that is ON for one-fourth of the time and OFF for three-fourths of the time appears to have one-fourth the intensity of the pixel being always in the ON state, assuming the frequency of modulation is greater than the critical flicker frequency.
This intensity variation in light modulating systems such as the one described above is known as gray scale. The greater the number of different intensities the system is able to produce, the greater the level of gray scale the system is said to have. In order to maximize the number of different intensity levels a system produces, the frame—the time period during which a single image is produced—is typically divided into time segments or slots. In one common example, the duration of each slot is determined such that each slot is twice as long as the next shortest slot, and the total duration of all slots combined is equal to the frame duration. Each slot is then assigned to be either ON or OFF. Thus, if the frame is divided into eight slots of unequal duration as explained above, (e.g., having duration ratios of 1:2:4:8:16:32:64:128), the slots may be assigned ON or OFF in 256 ways (2
8
=256) to produce 256 unique intensities. Such a system is called an eight-bit gray scale system since the eight slots may be represented by eight binary bits with, for example, a 1 representing the ON state and a 0 representing the OFF state.
The demand to produce systems with more intensities, or greater levels of gray scale, is increasing as display system applications become more pervasive. However, if the system is incapable of modulating between states instantaneously, the speed with which the system switches between states may limit the level of gray scale the system is able to produce. For instance, if the response time—the time the light modulator takes to changes states—is longer than the shortest slot, then the light may not be displayed for the correct amount of time during that slot to produce the desired intensity.
Display systems are not the only systems that encounter the gray scale limitation caused by the light modulating speed. Any multi-state light modulating system that has a non-zero response time to switch between states may experience this restriction. For example, referring initially to
FIG. 1
, one example of a basic system for modulating light and generally designated by reference numeral
10
is illustrated. Light modulating system
10
includes a light source
12
, a light polarizer
14
and a light modulator
16
. Light source
12
is configured to direct light
18
toward polarizer
14
. Polarizer
14
is configured to pass light of one polarization state, for instance horizontally polarized light (i.e., horizontal with respect to the orientation of the polarizer). Horizontally polarized light H is then directed toward light modulator
16
. For this example, light modulator
16
may be any binary light modulating system that has a non-zero response time to switch between states. In the present example, light modulator
16
has an ON state, wherein horizontally polarized light
20
is allowed to pass through to a viewing area
22
, and an OFF state, wherein no light passes through to viewing area
22
. The state of light modulator
16
is controlled by a drive signal from controller
24
. Thus, light modulating system
10
is configured to produce a temporal pattern of light directed toward viewing area
22
.
Having generally described the configuration and operation of light modulating system
10
, a more detailed method for operating the system will now be described, continuing to refer to FIG.
1
. As previously stated, light modulating system
10
is configured to produce a temporal pattern of modulated light directed toward viewing area
22
. Depending upon the frequency with which the light is modulated, the pattern may appear to a human viewer as a series of flashes. This would occur, for instance, if the frequency of modulator
16
is less that the critical flicker frequency of the human eye. However, if the frequency is greater than the critical flicker frequency, then modulated light
20
would appear continuous and have an intensity corresponding to the fraction of time that modulator
16
is in the ON state. Thus, light modulating system
10
has the ability to vary the intensity of light
20
directed toward viewing area
22
, even though the intensity of light source
12
remains constant.
Light modulating systems such as system
10
and methods for operating them are well known in the art. For example, light modulating system
10
may be a miniature display system of the type disclosed in U.S. Pat. No. 5,596,451, which is incorporated herein by reference. Further, U.S. Pat. No. 5,748,164, which is incorporated herein by reference, discloses several methods for using such a system to produce images having gray scale and/or color. However, as described above, if any slots are deficient—have duration shorter than the response time of the light modulator—the system may not produce the desired intensity when the specific intensity level requires the light to be ON during that slot. Thus, the system may not produce a linear gray scale response. A linear gray scale response occurs when the ratio of any two input signals is equal to the ratio of the output intensities resulting from the two input signals.
For example, consider a four-bit gray scale system, including bits A, B, C, and D, each bit corresponding to a slot. Bit A, the least significant bit (LSB), determines the state (ON or OFF) of the shortest slot and has a time weight of 1; bit D, the most significant bit (MSB), determines the state of the longest slot and has a time weight of 8. The system is capable of providing 16 different intensities (2
4
=16). Assuming a frame time period of {fraction (1/60)}th of a second, or 16.7 milliseconds, the duration of the slots associated
Dallas James M.
Larsen Per H.
Malzbender Rainer M.
Meadows Michael R.
Branch Gene
Crouch Robert G.
Displaytech, Inc.
Lester Evelyn
Marsh & Fischmann & Breyfogle LLP
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