Projection lenses for use with reflective pixelized panels

Details Projection lenses for use with reflective pixelized panels Projection lenses for use with reflective pixelized panels

2001-02-28

2002-11-05

Sugarman, Scott J. (Department: 2873)

Optical: systems and elements

Lens

With field curvature shaping

C359S752000, C359S753000, C359S762000, C359S770000

Reexamination Certificate

active

06476974

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to projection lenses and, in particular, to projection lenses for use in forming a large image of a small reflective object composed of pixels, such as, a reflective liquid crystal display (LCD), a digital mirror device (DMD), or the like.
BACKGROUND OF THE INVENTION
Projection lens systems (also referred to herein as “projection systems”) are used to form an image of an object on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection). The projection lenses of the present invention are specifically tailored for use in very compact front projectors, where the projected image emerges from the projector and is sent onto an external wall or screen.
To achieve a high level of compactness, the illumination for such front projectors is preferably fed in from the side near the object end (short conjugate end) of the projection lens. In the case of DMDs, the pixelized panel is also offset in order to provide the appropriate illumination geometry and to allow the dark-field light to miss the entrance pupil of the lens. This dark-field light corresponds to the off position of the pixels of the DMD.
The basic structure of such a system is shown in
FIG. 5
, where
10
is a light source (e.g., a metal halide or a high pressure mercury vapor lamp),
12
is illumination optics which forms an image of the light source (the “output” of the illumination system),
14
is the object which is to be projected (e.g., a Texas Instruments DMD of on and off pixels), and
13
is a projection lens, composed of multiple lens elements, which forms an enlarged image of object
14
on a viewing screen (not shown).
As shown in
FIG. 5
, the illumination optics can include multiple lens elements
15
,
16
,
17
, a light tunnel
18
(e.g., a light tunnel constructed in accordance with commonly-assigned U.S. Pat. No. 5,625,738), and a mirror
19
for folding the optical axis
20
of the illumination system and thus reduce the overall size of the projector. As also shown in this figure, the optical axis
20
of the illumination system intersects the optical axis
22
of projection lens
13
at an acute angle.
Projection lens systems in which the object is a pixelized panel are used in a variety of applications. Such systems preferably employ a single projection lens which forms an image of either a single panel which is used to produce red, green, and blue images or of three panels, one for red light, a second for green light, and a third for blue light. In either case, projection lenses used with such systems generally need to have a relatively long back focal length to accommodate the auxiliary optical systems, such as color wheels, beam splitters, etc., normally used with pixelized panels.
A particularly important application of projection lens systems employing pixelized panels is in the area of microdisplays, e.g., front projection systems which are used to display data. Recent breakthroughs in manufacturing technology has led to a rise in popularity of microdisplays employing digital light valve devices such as DMDs, reflective LCDs, and the like.
Projection displays based on these devices offer advantages of small size and light weight. As a result, a whole new class of ultra portable lightweight projectors operating in front-projection mode and employing digital light valves has appeared on the market.
To display images having a high information content, these devices must have a large number of pixels. Since the devices themselves are small, the individual pixels are small, a typical pixel size ranging from 14-17 &mgr; for DMD displays to approximately 8 &mgr; or even less for reflective LCDs. This means that the projection lenses used in these systems must have a very high level of correction of aberrations. Of particular importance is the correction of chromatic aberrations and distortion.
A high level of chromatic aberration correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. These problems are typically most severe at the edges of the field.
All of the aberrations of the system need to be addressed, with lateral color, chromatic variation of coma, astigmatism, and distortion typically being most challenging. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction, including correction of secondary lateral color, is thus needed from the projection lens.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center. Moreover, projection lenses are often used with offset panels. In such a case, the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
Low distortion and a high level of color correction are particularly important when an enlarged image of a WINDOWS type computer interface is projected onto a viewing screen. Such interfaces with their parallel lines, bordered command and dialog boxes, and complex coloration, are in essence test patterns for distortion and color. Users readily perceive and object to even minor levels of distortion or color aberration in the images of such interfaces.
The above-mentioned microdisplays and, in particular, microdisplays employing DMDs, typically require that the light beam from the illumination system is fed in from the side near the short conjugate side of the projection lens (see the discussion of
FIG. 5
above). This leads to a requirement that the entrance pupil
24
of the projection lens
13
is located at or near the lens' short conjugate side or, equivalently, that the lens' aperture stop is located near the short conjugate side. Such a location for the aperture stop exacerbates the optical design problem. In particular, the nearly external entrance pupil means that there is almost no internal lens symmetry for facilitating the correction of “odd” optical aberrations such as lateral color and coma. (Note that the nearly external entrance pupil can have the advantage of reducing heat buildup within the lens during operation.)
In addition to the foregoing, there is an ever increasing demand for greater compactness of projection lens systems. In terms of the projection lens, this translates into a requirement that the lens has a wide field of view in the direction of the image (screen). This requirement makes it even more difficult to correct the lateral color of the lens. Similarly, the requirement for a long back focal length also makes it more difficult to correct lateral color.
Achieving a short focal length (e.g., a focal length of around 30 millimeters), a long back focal length (e.g., a back focal length which is at least

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