Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2003-03-28
2004-07-20
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S651000, C359S649000, C353S122000
Reexamination Certificate
active
06765731
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to projection lenses and, in particular folded, telecentric projection lenses for use in forming an image of an object composed of pixels, such as, a DMD, a reflective LCD, a transmissive LCD, or the like.
DEFINITIONS
As used in this specification and in the claims, 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.
In practical applications, a 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 a 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 20 times the lens' focal length.
(2) Effective Back Focal Length
The effective back focal length (BFL) of a projection lens/pixelized panel combination is the distance between the front surface of the pixelized panel and the vertex of the back surface of the rearward-most lens element of the projection lens which has optical power when (1) the image of the pixelized panel is located at infinity and (2) the projection lens is located in air, i.e., the space between the rearward-most lens element of the projection lens and the pixelized panel is filled with air as opposed to the glasses making up the prisms, beam splitters, etc. normally used between a projection lens and a pixelized panel.
BACKGROUND OF THE INVENTION
A. Projection Systems
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. 4
, 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 (i.e., for the lenses of the present invention, a 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
.
For front projection systems, the viewer will be on the left side of screen
16
in
FIG. 4
, 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, one or more mirrors are often used between the projection lens and the screen to fold the optical path and thus reduce the system's overall size.
Projection systems in which the object is a pixelized panel (also known in the art as a “digital light valve” or a “microdisplay”) are used in a variety of applications. Such systems preferably employ a single projection lens which forms an image of a single panel used to produce (either sequentially or simultaneously) the red, green, and blue components of the final image or, in some cases, an image of three panels, one for red light, a second for green light, and a third for blue light. For certain applications, 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. Irrespective of the details of the application, the projection lens generally needs to have a long effective back focal length to accommodate the prisms, beam splitters, and other components normally used with pixelized panels.
A particularly important application of projection systems employing pixelized panels is in the area of rear projection systems which can used as large screen projection televisions (PTVs) and/or computer monitors. Improvements in the technology used to manufacture microdisplays has led to a rise in the popularity of projection systems employing such displays. To compete effectively with the established cathode ray tube (CRT) technology, projection systems based on microdisplays need to be smaller in size and lower in weight than CRT systems having the same screen size.
B. Optical Performance
To display images having a high information content (e.g., to display data), a microdisplay 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. 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 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 particular, in the case of DMDs, an offset is typically needed 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.
When a panel is offset 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
3M Innovative Properties Company
Black Bruce E.
Epps Georgia
Thompson Tim
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