Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
2000-02-18
2001-05-01
Dowling, William (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S094000, C345S046000, C345S083000, C348S742000
Reexamination Certificate
active
06224216
ABSTRACT:
TECHNICAL FIELD
This invention relates to image projection displays and more particularly to an LED light source employed in an optical pathway of such displays.
BACKGROUND OF THE INVENTION
Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
In a common operating mode, multimedia projection systems receive analog video signals from a personal computer (“PC”). The video signals may represent still, partial-, or full-motion display images of a type rendered by the PC. The analog video signals are typically converted in the projection system into digital video signals, and the signals are electronically conditioned and processed to control an image-forming device, such as a liquid crystal display (“LCD”) or a digital micromirror device (“DMD”).
A popular type of multimedia projection system employs a broad spectrum light source and optical path components upstream and downstream of the image-forming device to project the image onto a display screen. An example of a DMD-base multimedia projector is the model LP420 manufactured by In Focus Systems, Inc., of Wilsonville, Ore., the assignee of this application.
Significant effort has been invested into developing projectors producing bright, high-quality, color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projectors in a well-lighted room. The projectors typically use high intensity arc lamps as their light source and then filter out all of the light except for blue, green, and red light and employ three separate light paths or some forms of sequential color modulator to coordinate color image data.
Because LCD displays have significant light attenuation and triple path color light paths are heavy and bulky, portable multimedia projectors typically employ DMD displays in a single light path configuration. Producing a projected color image with this configuration typically requires projecting a frame sequential image through a sequential color modulator, such as a color wheel.
FIG. 1
shows a typical prior art frame sequential color (FSC) display system
10
in which a sensor
12
senses a timing mark
14
to detect a predetermined color index position of a motor
16
that rotates a color wheel
18
having respective red, green, and blue filter segments R, G, and B. A light source
20
projects a light beam
22
through color wheel
18
and a relay lens
24
onto a display device
26
, such as an LCD-based light valve or a DMD. A display controller (not shown) drives display device
26
with sequential red, green, and blue image data that are timed to coincide with the propagation of light beam
22
through the respective filter segments R, G, and B of color wheel
18
. A DC motor rotates color wheel
18
at about 6,650 rpm to about 7,500 rpm. Clearly, successful operation of a FSC display system depends on properly synchronizing the red, green, and blue image data to the angular position of color wheel
18
.
Sensor
12
typically employs any of optoelectrical or electromechanical shaft position or motor armature position detectors and usually requires some means for aligning timing mark
14
to the start of one of the filter segments. This alignment is typically a costly and error prone mechanical adjustment that accounts for angular differences between motor
16
and the mechanical mounting of filter segments R, G, and B. Of course, electrical or mechanical delays associated with sensor
12
further contribute to alignment errors.
The accumulated angular errors open the possibility of synchronization errors between the red, green, and blue image data to the angular position of color wheel
18
, a possibility that prior workers avoided by building a timing duty cycle into the display controller electronics. The timing duty cycle provides for driving display device
26
with the red, green, and blue image data for only a portion of the time when light beam
22
is propagating through each of respective filter segments R, G, and B to prevent presenting display device
26
with an improper color. Unfortunately, the timing duty cycle reduces the total amount of illumination available for displaying each color and, therefore, reduces the brightness of the resultant displayed color image. In addition, color wheels
18
and their associated motors are heavy and noisy.
An alternative technique that substantially eliminates any mechanical, optical, and electrical rotational timing errors that are intrinsic to color wheel systems is, therefore, desired.
SUMMARY OF THE INVENTION
An object of this invention is, therefore, to provide an apparatus and a method for employing an LED light source in a projection display system.
The present invention employs light sources such as LEDs or diode lasers that emit desired wavelengths such as the primary colors rather than starting with a broadband light source and filtering out the undesired wavelengths. The invention eliminates the need for color wheels, color wheel motors, or beam splitters.
In addition, LEDs offer a longer life, fewer thermal issues than HID lamps, better primary colors, redundancy for failed pixels, electronic (faster) sequencing of the colors for better color depth, and dynamic adjustment of the color temperature by directly controlling the amount of blue, green, and red light generated.
One design constraint involves positioning the individual LEDs sufficiently close together to efficiently illuminate the small imaging devices currently used, particularly without creating thermal problems for the LEDs, certain imaging devices, or other sensitive system components. One solution includes mounting bare LED dies together on a thermally conductive substrate, such as alumina, for better heat conduction and employing optical fibers to transmit the light efficiently to the imaging device. Each LED, LED die, or LED array can be coupled to separate optical fibers, which could have ends shaped to a lens surface similar to a ‘fly's eye’ array, and the fibers can then be bundled together into a small group, such that surface area of the array of LEDs can be much larger than that of the imaging device for thermal management, yet appear to be effectively smaller. The bundle of optical fibers can then be coupled to an integrating light pipe to effectively blend the light and uniformly illuminate the imaging device. LED-lighted display projectors of this type would provide a lighter weight, simpler, brighter, and less expensive multimedia projection system.
In a single path embodiment of the present invention, the light emitted from blue, green, and red generally monochromatic LEDs or LED arrays is propagated through optical fibers and then mixed within an optical integrator. A display controller receives image data from an image data source such as a personal computer and converts the data to color frame sequential data delivered to a common display device. The controller synchronizes the data with ON/OFF signals conveyed to the LED power supply. Because the synchronization is entirely electronic, the frame sequential information can be cycled much faster and more accurately than can be accomplished with a color wheel system.
In a triple path embodiment of the present invention, the light emitted from blue, green, and red generally monochromatic LEDs or LED arrays is propagated along separate respective optical paths through optical fibers and then preferably made substantially spatially uniform through an optical integrator. A display controller receives image data from an image data source such as a personal computer and converts the data to color data delivered to respective separate display devices whose output is coupled into a combiner. The display controller synchronizes the dat
Parker Fred
Peterson Mark
Dowling William
InFocus Corporation
Rives LLP Stoel
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