Optical: systems and elements – Lens – With variable magnification
Utility Patent
1999-08-26
2001-01-02
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S684000, C359S689000
Utility Patent
active
06169636
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to projection lenses and, in particular, to zoom projection lenses which can be used, inter alia, to form an image of an object composed of pixels, e.g., an LCD, a reflective LCD, a DMD, or the like.
DEFINITIONS
As used in this specification, the following terms shall have the following meanings:
(1) Telecentric
Telecentric lenses are lenses which have at least one pupil at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis (a) in object space, if the entrance pupil is at infinity, or (b) in image space, if the exit pupil is at infinity. Since light can propagate through a lens in either direction, the pupil at infinity can serve as either an entrance or an exit pupil depending upon the lens' orientation with respect to the object and the image. Accordingly, the term “telecentric pupil” will be used herein to describe the lens' pupil at infinity, whether that pupil is functioning as an entrance or an exit pupil.
In practical applications, the telecentric pupil need not actually be at infinity since a lens having an entrance or exit pupil at a sufficiently large distance from the lens' optical surfaces will in essence operate as a telecentric system. The principal rays for such a lens will be substantially parallel to the optical axis and thus the lens will in general be functionally equivalent to a lens for which the theoretical (Gaussian) location of the pupil is at infinity.
Accordingly, as used herein, the terms “telecentric” and “telecentric lens” are intended to include lenses which have at least one pupil at a long distance from the lens' elements, and the term “telecentric pupil” is used to describe such a pupil at a long distance from the lens' elements. For the projection lenses of the invention, the telecentric pupil distance will in general be at least about 10 times the lens' focal length.
(2) Pseudo-Aperture Stop
The term “pseudo-aperture stop” is used herein in the same manner as it is used in commonly-assigned U.S. Pat. No. 5,313,330 to Ellis Betensky, the contents of which are incorporated herein by reference.
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 basic structure of such a system is shown in
FIG. 2
, where
10
is a light source (e.g., a tungsten-halogen lamp),
12
is illumination optics which forms an image of the light source (hereinafter referred to as the “output” of the illumination system),
14
is the object which is to be projected (e.g., an LCD matrix 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 viewing screen
16
. The system can also include a field lens unit, e.g., a Fresnel lens, in the vicinity of the pixelized panel to appropriately locate the exit pupil of the illumination system.
For front projection systems, the viewer will be on the left side of screen
16
in
FIG. 2
, while for rear projection systems, the viewer will be on the right side of the screen. For rear projection systems which are to be housed in a single cabinet, a mirror is often used to fold the optical path and thus reduce the system's overall size.
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, for example, a single panel having red, green, and blue pixels. In some cases, e.g., large image rear projection systems, multiple panels and multiple projection lenses are used, with each panel/projection lens combination producing a portion of the overall image. In either case, projection lenses used with such systems generally need to have a relatively long back focal length to accommodate the prisms, beam splitters, color wheels, 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 and rear projection systems which are used as computer monitors. 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. Lightweight compact rear-projection systems can also be achieved through the use of these devices.
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 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 microdi
Epps Georgia
Klee Maurice M.
Thompson Tim
U.S. Precision Lens Incorporated
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