Optical system for a thin, low-chin, projection television

A micro-mirror based projection display system in an enclosure with minimum chin and depth measurements is disclosed. A solid-state laser light source generates light of multiple primary colors that is modulated by a digital micro-mirror device. The projection optics of the system include a telecentric rear group of glass lenses with spherical surfaces, followed by a pair of aspheric lenses formed of plastic. A folding mirror is disposed between the aspheric lenses, to reduce the depth of the enclosure, and an aspheric mirror projects the image onto a TIR Fresnel projection screen. The aspheric lenses are magnifying, to reduce the magnification required of the aspheric mirror, and the aspheric lenses and mirror are clipped to reduce enclosure volume.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

Not applicable.

BACKGROUND OF THE INVENTION

This invention is in the field of projection display systems, and is more specifically directed to the arrangement of optical elements in such a display system.

As is evident from a visit to a modern electronics store, the number of flat-panel (i.e., non-CRT) televisions has vastly increased in recent years, while the purchase price for such sets continues to fall. This tremendous competition is due in large part to the competing technologies for the display of high-definition television content. As known in the art, three major current display technologies for flat-panel televisions include liquid-crystal display (LCD), plasma display, and digital micromirror (DMD) based displays. The micromirror-based displays, and some LCD displays, are projection displays, in that a light source illuminates a spatial light modulator formed by the micromirror or LCD panel, with the modulated light then optically projected to a display screen. Plasma displays, on the other hand, are not projection displays; rather, each pixel at the display screen includes red, green, and blue phosphors that are individually excitable by way of argon, neon, and xenon gases, producing the image. Some LCD televisions involve “direct-view” displays, rather than projection displays, such that the liquid crystal elements are at the display screen and are directly energized to produce the image.

In modern micromirror-based projection displays, such as DLP® projection displays now popular in the marketplace using technology developed by Texas Instruments Incorporated, a digital micromirror device spatially modulates light from a light source according to the content to be displayed. An optical “engine”, which includes lens and mirror elements, projects the modulated light onto the display screen. As known in the industry, micromirror-based projection displays are advantageous from the standpoint of brightness, clarity, and color reproduction, as compared with other flat-panel televisions and displays. In addition, the use of micromirror spatial light modulators enable higher-speed modulation of light than many LCD systems, and micromirror-based systems have been observed to be extremely reliable over time.

However, conventional micromirror-based projection systems typically require larger “form factor” enclosures, than do LCD and plasma flat-panel systems of similar screen size and resolution. Two important measures of the enclosure for flat-panel display systems are referred to in the art as the “chin” dimension and the “depth” of the case.FIG. 1aillustrates the conventional definition of the “chin” of a flat-panel television, whileFIG. 1billustrates the “depth” of the system.

As shown in the front elevation view ofFIG. 1a, display screen2is housed within enclosure4. The portion of enclosure4that extends below screen2constitutes the “chin” of the display system.FIG. 1aillustrates dimension CHIN as the distance from the bottom edge of screen2to the bottom of enclosure4.FIG. 1billustrates, in connection with a side view of enclosure4, the dimension DEPTH as the measurement between the front of enclosure4and the back of a rear-ward extending portion of enclosure4. In micromirror-based projection display systems, the system components of the light source, digital micromirror, and the system projection optics, reside within the “chin” and the rearward extending portions of enclosure4.

Consumers are attracted to televisions and display systems that are thin, from front to back, and for which the enclosure only minimally extends beyond the dimensions of the display screen itself. Indeed, it has been observed that the consumer buying decision is often based on the size of the enclosure for a given screen size. As mentioned above, the enclosures of modern plasma and direct-view LCD display systems can typically involve minimal chin and depth, because they are not rear projection systems and as such do not require enclosure of the light source, modulator, and projection optics required by projection systems, especially conventional micromirror-based systems. As such, these conventional micromirror-based projection systems are at a competitive disadvantage in the marketplace in this regard. And therefore, it is desirable for micromirror-based projection systems to also minimize the chin and depth of their enclosures, to attain and preserve market share.

In addition to the physical volume required for enclosures of projection display systems such as those based on micromirrors, other constraints also have resulted in substantial chin and depth dimensions. One such constraint is due to the TIR (Total Internal Reflection) Fresnel display screens that are now commonly used in projection display systems. As known in the art, TIR Fresnel display screens are capable of receiving light at a relatively steep angle from the normal, and of directing that light into the direction normal to the display screen, analogous to Fresnel lenses as used in traffic lights and lighthouses. This construction permits the source of the projected light to reside off-axis with the display screen, which greatly reduces the depth of projection display systems.FIG. 1cshows the rear projection of an image from source8(which may be a plane mirror, for example) to display screen2, which is constructed as a conventional TIR Fresnel display screen. The angle of incidence of light from source8to the bottom of screen2is at a minimum angle φmfrom the normal, while the angle of light from source8to the top of screen2is at a maximum angle of incidence φx. It has been observed that, for conventional TIR Fresnel display screens, the minimum angle of incidence φmshould be above 50° from the normal, to avoid flare and reduced contrast in portions of the displayed image. However, in order to achieve such a large minimum angle of incidence, it is therefore often necessary to construct an enclosure having substantial “chin”, as evident fromFIG. 1c. In addition, if a plane mirror is used as source8, to reflect the projected image to display screen2, as shown in the conventional system ofFIG. 1c, the minimum angle constraint commonly requires the height of this mirror to on the order of one-half the vertical dimension of display screen2, especially as the depth of enclosure4is minimized.

Other design and manufacturing constraints also affect the design of conventional display “engines” for micromirror-based projection displays. These other constraints involve the nature of the light source (i.e., the “etendue” of the light), the extent of lens groups and numbers of lenses required to obtain a high resolution and minimum distortion image at the display.

By way of further background, a current trend in the construction of projection display systems is the use of non-telecentric lenses in the projection optics, between the spatial light modulator and the display screen. As known in the art, “non-telecentric” refers to lens arrangements that receive light from an image or source (i.e., the modulator) that is larger than the lenses; as such, the chief rays of light from various locations of the image are not parallel to, or not collimated with, one another. The use of non-telecentric lenses is popular in projection systems because the diameter of the lenses can be much smaller than the image or light source. Not only is the physical size of the lenses reduced, but the f-number of the lenses required for efficient light transfer is also kept relatively high, further reducing the cost of the lenses. As known in the art, large lenses of low f-numbers are relatively expensive to produce, especially for applications in which high image quality and resolution is important, as in high-definition television. It has also been observed that higher image contrast is generally attained by non-telecentric projection lenses. In addition, display systems using micro-mirror based spatial light modulators in combination with non-telecentric lenses can omit the “total internal reflection” (TIR) prism for separating “on” and “off” pixel light that is otherwise generally necessary with telecentric projection lens systems. Non-telecentric projection lenses are thus popular in modern projection display systems.

By way of further background, however, non-telecentric projection lenses are known to present certain limitations in projection display systems. Defocus caused by thermal or alignment effects at the SLM plane is made evident as dramatic magnification changes in the displayed image (“overfill” or “underfill”) in systems using non-telecentric projection lenses, even if the f-number of the projection lens group is relatively high (slow).

Another trend in the design of projection display systems is the use of wide-angle, high-magnification, aspheric mirrors as the element reflecting the projected image onto the display screen (e.g., as source8in the arrangement ofFIG. 1c). It has been reported that the use of a high magnification aspheric mirror is believed to suppress color aberration.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a micromirror-based projection display system having minimal chin and minimal depth.

It is a further object of this invention to provide such a display system in which distortion is minimized and resolution is maximized.

It is a further object of this invention to provide such a display system that is competitive, from the standpoint of form factor, with modern LCD and plasma displays.

It is a further object of this invention to facilitate correction of aberration in the image to be displayed, in a manner consistent with a small volume form factor.

It is a further object of this invention to provide such a display system that provides more stability in its displayed image quality relative to defocus at the SLM plane, for example as may be caused by variations in temperature or humidity, or by variations in alignment during manufacture or use.

Other objects and advantages of this invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.

The present invention may be implemented into a projection display system using a laser light source, for example one or more arrays of solid-state lasers emitting primary color light incident on a digital spatial light modulator. The spatial light modulator may be one or more digital micro-mirror devices (DMDs), or a spatial light modulator of another type, such as a liquid crystal on silicon (LCOS) SLM, a high temperature polysilicon (HTPS) SLM, a transmissive LCD SLM, and a diffractive 1-D SLM. The modulated light is projected to the display screen using telecentric projection lenses in a first group, followed by a medium-to-wide angle aspheric projection lens formed of plastic with >1.0 magnification. A plastic aspheric mirror reflects the projected image to the display screen.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be described in connection with its preferred embodiment, namely as implemented into a micromirror-based projection television display system, as it is contemplated that this invention will be especially beneficial in such a system application. It is also contemplated, however, that this invention may be beneficial in other applications, and variations on the described television application. Accordingly, it is to be understood that the following description is provided by way of example only, and is not intended to limit the true scope of this invention as claimed.

FIG. 2schematically illustrates the functional elements of projection display system15according to the preferred embodiments of this invention. The physical arrangement and construction of these elements will be described in further detail below; the illustration ofFIG. 2is presented in a functional manner, to provide functional context for that detailed description.

As shown inFIG. 2, projection display system15includes projection screen12, upon which the displayed image is projected from behind (i.e., from the opposite side of screen12from viewer V). In this preferred embodiment of the invention, screen12is preferably a total internal reflection (TIR) Fresnel screen, to permit the image to be projected from an offset position from the center point of screen12. In this case, the displayed image is projected by aspheric mirror10from below and behind screen12.

According to this embodiment of the invention, light source16directs light of multiple primary colors at spatial light modulator (SLM)18in the conventional manner. Light source16is preferably a laser light source that directs light of at least three primary colors (e.g., red, green, blue) at SLM18in a time-multiplexed manner. As known in the art, other sequential primary color light sources can be constructed as a bulb-and-reflector type of white light source that illuminates a rotating color wheel having multiple colored filters; however it is contemplated, according to this invention, that a laser light source will be preferred, as will be apparent from the following description.

SLM18spatially modulates the incident light from light source16, in response to control signals from graphics driver14. In this preferred embodiment of the invention, SLM18is of the digital micromirror (DMD) type, in which a large number of individually controllable micromirrors each correspond to one pixel of the resulting image, and are each controlled in a time-sequential fashion to selectably reflect light in the desired light path according to be displayed. DMD devices suitable for use as SLM18are well-known in the art, for example those DMD devices in the DLP® product family available from Texas Instruments Incorporated. While one SLM18is illustrated inFIG. 2, and modulates light of multiple primary colors to produce a full-color displayed image, it is contemplated that this invention is also applicable to systems that implement multiple SLM devices18(e.g., three SLMs18, one each for red, green, and blue light), as will be evident to those skilled in the art having reference to this specification.

Alternatively, SLM18may be realized according to other technologies. Such alternative technology SLM devices include liquid crystal on silicon (LCOS) SLMs, high temperature polysilicon (HTPS) SLMs, transmissive LCD SLMs, and diffractive 1-D SLM devices. While the following description will generally refer to SLM18as using reflective technology, such as a DLP digital micromirror device as described above, it is contemplated that those skilled in the art having reference to this specification will be readily able to adapt this invention for use with transmissive SLM devices such as those incorporating the technologies described above.

Typically, SLM18will be controlled by graphics driver14in a pulse-width-modulated manner, to precisely control the brightness of light reflected from SLM18along the desired path to be displayed on screen12, for each primary color for each pixel. Incident light from light source16that is not to be part of the displayed image (i.e., that light that is directed away for dark or darker pixels) is preferably recycled for efficiency. In this manner, SLM18spatially modulates the light that is eventually projected onto screen2, with the modulation being controlled according to the information in the image to be displayed.

The light reflected from SLM18received by projection optics20. Projection optics20, as will be described in further detail below, preferably includes multiple telecentric lenses arranged in multiple groups. The purpose of projection optics20is to provide a focused pattern of light of the desired size and resolution upon aspheric mirror10. That focused pattern will, as mentioned above, reflect from aspheric mirror10onto the backside of screen12. Projection optics20also compensate and correct for aberrations in the light pattern, and those aberrations resulting from the shape of aspheric mirror10. The detailed construction of projection optics20according to the preferred embodiments of the invention will be described in further detail below.

The functional arrangement of elements shown inFIG. 2results in a displayed image on screen12of high resolution, high contrast, and excellent brightness. In addition, according to this preferred embodiment of the invention, the physical elements of the light source16, SLM18, projection optics20, and aspheric mirror10are arranged and constructed to reduce the “chin” and depth dimensions of the display system enclosure.

FIGS. 3aand3billustrate the construction of light source16, and its arrangement with SLM18according to the preferred embodiment of the invention. As will be apparent from the following description, light source16is a laser-based light source, as the attributes of laser illumination are especially beneficial in display system15according to this preferred embodiment of the invention, for reasons discussed below. While it is contemplated that other light source types and arrangements may be used in connection with this invention, it is contemplated that a laser-based light source, such as light source16ofFIGS. 3aand3b, will be especially beneficial.

The arrangement of light source16as shown inFIG. 3ais believed to be conventional in the art. However, its construction and arrangement is described herein, to provide context and understanding for the preferred embodiment of the invention described below.

Laser array22provides the light energy involved in the projection of images in display system15, according to this embodiment of the invention. In this example, laser array22includes one or more lines of solid-state lasers, for each of three or more colors. Typically, the three “primary” colors of red, blue, and green are used in projection display systems; as such, laser array22includes one or more lines of lasers in an array for each of these colors. As suggested byFIG. 3a, these arrays22R,22G,22B (for red, green, blue, respectively) are spatially separated from one another, such that the collimated monochrome light from each array22R,22G,22B travels in a plane, parallel but not coplanar with the light from the other arrays22R,22G,22B. The length of each of arrays22R,22G,22B (i.e., the number of solid-state laser emitters in each) corresponds to the corresponding dimension of SLM18, so that each array22R,22G,22B can illuminate a portion of SLM18across its width (i.e., corresponding to the width of the projected image). The planes of collimated monochromatic light from arrays22R,22G,22B are directed by corresponding mirrors23R,23G,23B, respectively, to rotating mirror24.

Rotating mirror24, in this embodiment of the invention, is a rotating mirror having multiple reflective surfaces. In this example, rotating mirror24has a hexagonal cross-section, and is of sufficient length (in the direction normal to the page ofFIG. 3a) to direct the entire width of the output from each of arrays22R,22G,22B. Mirror25re-directs the reflected collimated light of each color from rotating mirror24to TIR prism28. TIR prism28is a conventional “total internal reflection” prism element, which reflects or transmits incident light depending on whether the angle of incidence exceeds or is less than a critical angle for the material, as known in the art. As conventional in the art, TIR prism28is formed of two prism elements adjacent to each other, with a small air gap between them. TIR prism28reflects the collimated multiple color light to different regions of SLM18. In this manner, it is intended that the light of each of arrays22R,22G,22B will illuminate one or mores in separate one-third regions of SLM18. SLM18is synchronously controlled, by graphics driver14as discussed above, to spatially modulate the collimated light of the appropriate primary color, according to the information in the image to be displayed. The modulated light is then passed by TIR prism28to projection optics20.

It is contemplated that those skilled in the optics art will be readily able to design rotating mirror24, and TIR prism28, and the various mirrors using conventional design techniques.

In operation, the rotation of rotating mirror24temporally “scrolls” colored light across SLM18. For the position shown inFIG. 3a, red light from array22R illuminates one or more rows of micromirrors in the upper third of SLM18, green light from array22G illuminates one or more rows of micromirrors in the center third of SLM18, and blue light from array22B illuminates one or more rows of micromirrors in the lower third of SLM18. This situation is illustrated inFIG. 3bby DMD state18(1). Upon rotating mirror24rotating in the direction illustrated, the particular rows of micromirrors illuminated by the collimated light also scroll across SLM18. The order in which light is directed to SLM18changes as a vertex of rotating mirror24passes below the point at which red light from array22R impinges, after which the red light is reflected at a shallower angle, and is directed via mirror25and TIR prism28to one or more rows of micromirrors in the lower third of SLM18; meanwhile, the steeper reflection angle of the surface of rotating mirror24at this time will direct green light to one or more rows of micromirrors in the upper third of SLM18, and blue light to one or more rows of micromirrors in the center third of SLM18, as shown inFIG. 3bby DMD state18(2). The scrolling continues in this manner, with blue light eventually being directed to one or more rows of micromirrors in the upper third of SLM18, during which time red light is directed to one or more rows of micromirrors in the center third of SLM18, and green light is directed to one or more rows of micromirrors in the lower third of SLM18. The process then repeats.

As known in the art and as mentioned above, during such time as light of a particular color is directed to micromirrors of SLM18, graphics driver14synchronously controls the operation of those micromirrors to selectively modulate the incident light into or out of the light path to be projected onto screen2, depending upon the content of the image to be displayed. In this manner, the operation of SLM18is controlled to include information in the light reflected through TIR prism28to projection optics20.

Projection optics20, according to this preferred embodiment of the invention, includes a rear group of glass lenses of spherical curvature, and a front group of aspheric plastic lenses, with one or more mirrors disposed within the lenses of these groups, as will be described below. As will be evident from the following description, the construction and arrangement of the lenses within projection optics20are a significant factor in permitting the enclosure of display system15to have minimal chin and depth dimensions.

Referring now toFIG. 4a, the construction and arrangement of rear group20aof projection optics20will be described in detail, following the path of light in the direction from SLM18to screen2. A first element of rear group20ais window18W of SLM18; as typical in the art, window18W is merely a transparent protective glass window, and does not affect the path of transmitted light. According to this preferred embodiment of the invention, the light from both “on” pixels and “off” pixels are transmitted by window18W to the next element of rear group20a, which is total internal reflection (TIR) prism32.

As known in the art, TIR prism32serves to direct light into different paths, depending upon the angle of incidence of that light as compared to a critical angle. TIR prisms, such as TIR prism32, are conventionally used at the output of a DMD device, to pass desired light beams from those pixels that are to be displayed, and to reflect away the light beams from those pixels that are to be “dark”. TIR prism32may be constituted as including TIR prism28of light source16, with an additional surface to reflect and transmit the “off” and “on” pixel light modulated by SLM18, or alternatively may be a separate TIR prism from prism28. In either case, the modulated light reflected from SLM18is directed at a high incident angle such that it passes through the TIR surface of the TIR prism and through the air gap therein. If the angle of incidence of a given light beam at the TIR surface of TIR prism32is less than the critical angle, it is reflected away as shown inFIG. 4a(“OFF” PIXEL LIGHT). Modulated light at an angle of incidence greater than the critical angle is transmitted by TIR prism32along the projection path, toward the lens elements33through39of rear group20a.

Lenses33through39are each spherical glass lenses, arranged on the optical axis of the light from SLM18through TIR prism32. None of lenses34through39are tilted, and the aperture of none of these lenses is clipped. Details of the construction of lenses33through39, according to this example of the preferred embodiment of the invention, are provided in Table 1:

As evident fromFIG. 4a, and as those skilled in the art will realize from the example of the lens construction of Table 1, lenses33through39of rear group20aconstitute a “telecentric” multi-element lens group, in that the chief rays for all points across the object defined by window18W are parallel to the optical axis through lenses34through39. As known in the art, the optical properties of telecentricity are beneficial in many ways. For purposes of this invention, one important benefit of telecentricity is that distortion due to position across the object plane (window18W) is eliminated. In addition, position of the object plane (window18W) relative to the lenses does not affect the image size in a telecentric lens system, reducing the sensitivity of the relative position of these elements to one another and thus facilitating manufacture of the system. As such, the design of a telecentric lens system such as lenses33through39in this embodiment of the invention, in combination with a collimated light source such as laser light source16, reduces the sensitivity of projection optics20to defocus and to misalignment of SLM18.

However, as discussed above, the current trend in projection display systems is to use non-telecentric projection lens systems. Because the non-parallel chief rays from the object plane converge in a non-telecentric lens system, the diameter of the projection lenses can be greatly reduced, relative to the dimensions of the object plane. This enables “slower” (i.e., larger f-number) lenses to be used in the projection lens system, which greatly reduces aberrations from the lenses themselves, reduces the complexity of the projection lens system, and greatly reduces the cost of the lenses themselves.

According to the preferred embodiment of this invention, however, the use of laser-based light source16enables the construction of rear group20aof projection optics20as a telecentric lens system, within the physical constraints of the enclosure of projection system15and at reasonable cost. The concept of “etendue” is useful in the optical art, and refers to the geometric or spatial capability of an optical system to transmit or receive light. In the context of an SLM-based system, the source of light incident on the SLM has an etendue that corresponds to the size and directionality of the source; the SLM also has an etendue that corresponds to its size and ability to receive light from various directions. As known in the art, lamp and LED light sources tend to have large source etendue values, relative to the SLM that is being illuminated. This mismatch indicates that only a fraction of the light emitted by the source is useful in the projection system. In other words, the light “cone” emitted by both lamp and LED light sources subtends a wide angle, relative to the aperture defined by the SLM. As such, for a desired brightness level, the lamp or LED light source is required to be of relatively high power.

In contrast, a laser-based light source, such as light source16discussed above, has a relatively low source etendue level, for example on the order of 100 times smaller than that of a conventional lamp or LED light source. As such, the source etendue of laser light source16is preferably well-matched to the etendue of SLM18, in projection system15according to this preferred embodiment of the invention.

From an optical standpoint, the lower source etendue of laser-based light source16corresponds to a narrower angle of the incident light “cone”. In other words, the effective diffraction “aperture” defined by each pixel of SLM18is narrower for light from a lower etendue source, such as laser-based light source16, than it is for a higher etendue source, such as a lamp or LEDs. It has been observed, in connection with this invention, that this smaller or narrower aperture permits “slower” lenses (i.e., lenses with higher f-numbers) for optical correction and focus to be used, for a given resolution. This effect of low source etendue on the f-number of the projection lenses compensates for the lower f-number lenses, and more complex arrangement of such lenses, that are required to realize a telecentric lens system. For example, the f-number of rear group20aof projection optics20, constructed to include lenses33through39as described above, is about f/2.8 or higher (i.e., slower), for the case of a laser-based light source16and a DMD-based SLM18measuring 0.45″ in width. It has been demonstrated and observed, in connection with this invention, that the use of laser-based light source16enables such relatively slow f-number optics as rear group20a, in consideration with other factors such as geometrical lens aberration (i.e., spherical aberration that is dependent on aperture) on one hand, and MTF diffraction effects that “blur” the pixel resolution from slower lenses, on the other hand. As such, the use of laser-based light source16, as described above, enables rear group20aof projection optics20to be telecentric at virtually no cost, while keeping the lens speed and size reasonable, and while permitting the “throw” distances of the projection lenses to also be modest. Indeed, it is believed that the use of a lamp or LED light source with the telecentric projection lens rear group20awould not result in a projected image of optimum quality (i.e., adequate quality for today's high-definition television marketplace), without greatly increasing the throw distances of the lenses beyond the desired size of the enclosure of projection system15. Accordingly, the combination of laser-based light source16with the telecentricity of rear group20aof projection optics20, provides important advantages in the construction of a mirror-based projection display system, especially in the form factor of such a system as will be described below.

Alternatively, light source16may be realized by way of one or more light-emitting diodes (LEDs), for example one or more LEDs for each of the primary colors. Conventional LED-based light sources or “engines” are known that provide one LED for each of the red, green, and blue primary colors, or an array of LEDs for each primary color (e.g., six LEDs for each of red, green, and blue). As mentioned above, the source etendue of an LED-based light source16is greater than a laser-based source, which requires a wider aperture for rear group20aof projection optics20, and perhaps a larger SLM18. For example, it is contemplated that use of an LED-based light source16will require the f-number of rear group20ato be f/2.0, for a DMD-based SLM18measuring 0.65″ in width. But even with this additional constraint on rear group20a, it is contemplated that an LED-based light source16may be implemented in projection system15, according to this invention, while still providing an enclosure with low “chin” and “depth” measurements as will be described below.

Referring back toFIG. 4a, the light for “on” pixels that is transmitted through lenses33through39is then directed at optical actuator40. Optical actuator40is a fully-reflective plane mirror that redirects the path of the light projected from last lens39. According to the preferred embodiment of the invention, optical actuator40is slightly “dithered” between two angles relative to the optical axis of lenses33through39. In this regard, it is contemplated that optical actuator40includes a motor or other mechanism for controllably positioning its reflective surface at a selected one of at least two different angles, relative to the optical axis of rear group20a. It is contemplated that this motor or mechanism will be controlled by circuitry within projection system15, for example by graphics driver14itself, or by other circuitry that is synchronized to graphics driver14.

As known in the art in connection with the SMOOTH PICTURE™ technology developed and available from Texas Instruments Incorporated, odd-numbered image pixels can be assigned to one subframe of an image frame, and even-numbered image pixels can be assigned to a second subframe. The timing control signals applied to SLM18can be similarly divided. In displaying the image, optical actuator40is placed at one angle relative to the optical axis of lenses34through39for one subframe, and is placed at a second angle relative to the optical axis for the next subframe; the angles of optical actuator40are selected so that the difference between these two positions, in projected light path at screen2, is about one-half pixel width. Typically, the pixels of SLM18are diamond-shaped, such that the light beam or ray from a given pixel is shifted in the direction orthogonal to that defined by optical actuator40, also by one-half the pixel width. As such, optical actuator40not only directs the projected light along its path in a different direction from that of lenses34through39, but also implements the SMOOTH PICTURE™ technology so that the resulting resolution of the displayed image is greatly improved.

Projection optics20of projection system15, according to this embodiment of the invention, also includes front group20bof lenses.FIG. 4billustrates the optical arrangement of front group20b; as will be described below, the physical arrangement of front group20bdiffers from its effective optical path, for purposes of minimization of depth.

As shown inFIG. 4b, front group20bincludes three aspheric elements, namely aspheric meniscus lenses42,44, and aspheric mirror10. According to the preferred embodiment of this invention, as shown inFIG. 4b, each of these aspheric elements is constructed of optical acrylic plastic. This permits each of aspheric elements42,44,10to be physically “clipped” at or near its optical axis of these elements, because the optical path utilizes only a portion of the entire aspheric surface. This greatly facilitates the positioning of these aspheric elements within the enclosure of display system15. In addition, as will also be described in further detail below, sufficient space is provided between aspheric lens42and aspheric lens44for a two-surface folding mirror, which will bend the light path back on itself to save additional form factor volume.

According to this embodiment of the invention, aspheric lenses42,44are constructed to operate as a medium-to-wide angle projection lens system, which reduces the magnification required of aspheric mirror10. The detailed construction of aspheric elements42,44,10will now be discussed in connection with the diagram ofFIG. 4c, in which each of these elements42,44,10are shown in an “unclipped” form, for clarity in the description of their construction.

According to the preferred embodiment of the invention, each of elements42,44,10are formed of optical acrylic plastic, having surfaces that are each defined as a rotationally or axially symmetric polynomial aspheric surface. These surfaces can be described by way of a polynomial expansion of a deviation from a surface that is spherical, or aspheric defined as a conic (i.e., nearly “axiconic”). In this example, the surfaces of elements42,44,10are “even” asphere surfaces, as only even powers of the polynomial expansion are used (odd powers are zeroed) because of the axial symmetry. The lateral distance, or surface “sag”, z from the front apex of the surface to a radial point r from the vertex of the aspheric surface along the optical axis, is commonly defined according to

z=cr21+1-(1+k)⁢c2⁢r2+a1⁢r2+a2⁢r4+a3⁢r6+a4⁢r8+a5⁢r10+a6⁢r12
where k is the “conic coefficient” for the surface, and where c is the curvature (1/radius) of the base sphere (from which the asphere deviates) at the vertex, and the coefficients aiare the aspheric coefficients defining the shape of the asphere.

Specific values used to define the surfaces of elements42,44,10in an example of the preferred embodiment of the invention are specified in Table 2, in which the lens surfaces42S1,42S2,44S3,44S4refer to the surfaces of lenses42,44as shown inFIG. 4b, and in which the surface10S5refers to the substantially axiconic reflective surface of aspheric mirror10:

TABLE 2clear apertureoutputC (reciprocal oflens(~physicalradius of curvatureradius ofsurfaceradius, in mm)(mm)curvature) (mm−1)k42S121.420.8140.04804454−7.43599942S226.324.1020.04148992−0.309590144S351.617.4530.05729674−9.050689544S454.730.0810.03324395−12.30310S591.3−13.516−0.07398622−2.887278lenssurfacea1a2a3a4a5a642S10−3.712239E−052.621195E−07−5.760937E−108.906940E−13−3.383627E−1642S20−9.963297E−053.197044E−07−5.579417E−105.586180E−13−2.430105E−1644S301.732750E−05−2.334335E−081.638530E−11−5.311442E−156.322489E−1944S401.604374E−05−1.786992E−081.062825E−11−3.009051E−153.174648E−1910S509.390947E−087.529221E−12−3.619127E−154.031248E−19−1.584210E−23
It is contemplated that those skilled in the art having reference to this description will be able to readily construct aspheric lenses42,44, and aspheric mirror10as suitable for a particular system application, without undue experimentation. In this example, the optical path length from lens39to aspheric lens42, via optical actuator40, is about 64.78 cm, the optical path length from aspheric lens42to aspheric lens44, via folding mirror48, is about 53 cm, and optical path length from aspheric lens44to aspheric mirror10is about 22 cm. Each of these paths is through air.FIG. 4billustrates the general paths of projected light through lenses42,44, as reflected by aspheric mirror10to screen2.

According to this preferred embodiment of the invention, it is contemplated that the magnification power of aspheric lenses42and44, in combination is greater than 1.0. For the example of aspheric lenses42,44described above, the magnification power of these two lenses is shown by curve50ofFIG. 8a, as varying between about 2.25 and 1.25 over normalized field of view (FOV) values ranging between about 0.1 and 1.0, with the minimum magnification at about 0.5 of FOV, normalized.FIG. 8cillustrates the definition of normalized field of view, ranging from the radial distance at the optical axis of the lens or mirror element LME (having a normalized FOV value of 0.0) to the point on the surface of the lens or mirror corresponding to the pixel at the diagonal corner of the image displayed on screen2(normalized FOV of 1.0). This >1.0 magnification power of aspheric lenses42,44reduces the magnification power required of aspheric mirror10according to this preferred embodiment of the invention, relative to conventional systems. Curve52ofFIG. 8billustrates the magnification of aspheric mirror10over normalized FOV between about 0.1 and 1.0, as varying from about 15.0 to about 3.5; it is contemplated that this magnification by aspheric mirror10is substantially less than that of conventional projection systems using a substantially larger aspheric mirror than aspheric mirror10in this embodiment of the invention, such conventional systems generally including a single aspheric lens element that does not substantially magnify the image. The arrangement of plastic aspheric lenses42,44according to this embodiment of the invention results in this smaller size for aspheric mirror10, because the complexity of the system of aspheric lenses42,44defines an optimum distance between aspheric mirror10and rear group20a; this optimum distance does not necessarily impact the depth of the enclosure, but enables the smaller size for front aspheric mirror10.

In addition, it has been observed, in connection with this invention, that conventional projection systems involving an aspheric projection mirror will typically have only the final aspheric lens element formed of an optical plastic, with all other lens elements formed in glass. In contrast, the arrangement of this invention enables both aspheric lenses42,44to be formed of optical acrylic plastic, reducing the cost of the system and also permitting “clipping” of these lenses in the manner illustrated in the Figures and described herein.

By way of further description, curve54ofFIG. 8billustrates the total magnification of the “front group” of aspheric lenses42,44in combination with aspheric mirror10. While the magnification at low normalized FOV points are greatly magnified by these aspheric elements42,44,10, it has been observed, in connection with this invention, that projection optics20, including aspheric mirror10, has outstanding distortion performance.FIG. 8dillustrates the distortion exhibited by an example of projection system15according to this preferred embodiment of the invention, as measured over normalized FOV from 0.10 to 1.00 (bottom to top along the y-axis). As evident from this plot, the overall distortion of the system remains within 1.00% over the entire image range, which is excellent performance for this scale of optical system.

As discussed above relative toFIG. 4b, aspheric lenses42,44, and aspheric mirror10are physically clipped to save volume within the form factor of the enclosure of projection system15. This clipping results in the surface cross-section of aspheric lenses42,44, and aspheric mirror10residing substantially within a single half-plane, as shown inFIG. 4b. In addition, also as discussed above relative toFIG. 4b, the distance between aspheric lenses42,44is selected to be sufficient to insert a two-panel folding mirror between these elements, to direct the light path in an efficient manner within the enclosure of projection system15. Referring now toFIG. 5a, the arrangement of rear group20aand front group20bof projection optics20, according to the preferred embodiment of the invention, will now be described.

FIG. 5ais a perspective view of projection optics20, including folding mirror48, which has two surfaces48a,48b, and including screen2(a portion of which is shown in phantom). As illustrated inFIG. 5a. rear group20ais oriented so that its optical axis is generally parallel to the plane of screen2, with the path of its projected light diverted substantially perpendicularly by optical actuator40. The precise angular relationship of this optical axis to screen2is not important, as the light path is controlled by optical actuator40. However, the arrangement of rear group20ato be generally parallel to the plane of screen2permits the depth of an enclosure for display system15to be minimized. Aspheric lens42is positioned so that its optical axis is generally in the perpendicular plane relative to screen2, and receives the projected light as reflected by optical actuator40.

Folding mirror48is constructed as two planar reflective panels that are at a selected angle (generally perpendicular) relative to one another, and that are disposed in the light path between aspheric lens42and aspheric lens44. Aspheric lens42is oriented in substantially the reverse direction, relative to aspheric lens42from the optical arrangement ofFIG. 4b. As shown inFIG. 5a, the presence of folding mirror48enables aspheric lens44to reside substantially above aspheric lens42, within the physical arrangement of projection system15. Aspheric lens44is aimed at aspheric mirror10, which in turn is positioned to direct projected light to screen2.

FIG. 6illustrates the light path from optical actuator40to screen2. In this arrangement, the light projected from rear group20aand reflected by optical actuator40is then transmitted by aspheric lens42toward bottom panel48b, and reflected from bottom panel48bto top panel48a, from which the light is reflected to aspheric lens44. Aspheric lens42magnifies the image of the projected light, as evident from the diverging light rays illustrated inFIG. 6. The light reflected from top folding mirror panel48ais then further magnified by aspheric lens44, and projected onto the surface of aspheric mirror10, which in turn reflects the projected light toward screen2. As discussed above relative toFIGS. 8aand8b, it is contemplated that the magnification applied by aspheric lens44(and, to a lesser extent, by aspheric lens42) reduces the curvature and magnification of aspheric mirror10, improving the overall resolution and fidelity of the projected image.

FIG. 5billustrates the physical arrangement of projection optics20within projection system15in a top-down view, further illustrating the physical relationship of aspheric lenses42,44to one another, and to the other elements.FIG. 5cis a perspective view of these elements from the opposite direction from that shown inFIG. 5a, and further illustrates the reflecting surface of optical actuator40. AndFIG. 5dis an elevation view from the same side as shown inFIG. 5a, but in a direction that is substantially parallel to the plane of screen2.

As evident fromFIGS. 5athrough5dandFIG. 6and the above description, the arrangement of projection optics20and aspheric mirror10enables the enclosure of these elements within a volume that is competitive with LCD and plasma display systems, especially in connection with the important dimensions of the “chin” and depth of the system enclosure.FIG. 7illustrates the relative position of projection optics20and aspheric mirror10, in connection with a rear view of display system15. In actual implementation, enclosure50(shown in shadow inFIG. 7) surround the system components, including screen2and projection optics20, aspheric mirror10, light source16(not shown) and the other elements of projection system15.

In this example, screen2is a 44-inch widescreen (16:9 aspect ratio) projection screen, upon which projection optics and aspheric mirror10are capable of projecting a full resolution image. It is contemplated that enclosure50can provide sufficient volume for the elements of projection system15in a manner that is quite efficient. For this example, and given the example of the construction described above, it is contemplated that enclosure50for this 44-inch system15can contain these elements within a “chin” dimension (from the bottom of screen2to the base of enclosure50, as shown) of 6 inches or less (and an “optical” chin dimension, corresponding to the vertical offset from the optical axis of aspheric mirror10to the bottom of screen2, of 4 inches or less), and a depth (from the front of screen2to the rear of the enclosure) of about 6 inches or less. These dimensions, for a 44-inch projection display system15, are similar to current-day LCD and plasma display systems available in the marketplace.

The minimal chin and depth dimensions are attained by projection system15, while meeting other important constraints in the design of the system. One important constraint that is met by projection system15according to this preferred embodiment of the invention is the minimum angle (from the normal) of incident light reflected from aspheric mirror10to screen2. As known in the art and as described above, this minimum angle of incidence is required of TIR Fresnel screens such as screen2, in order to eliminate flare and non-uniform contrast in the projected image. According to this preferred embodiment of the invention, even with the minimal chin and depth measurements specified above, the minimum angle of incidence φm, at the worst case location of the bottom of screen2, is greater than 50°.

Projection system15constructed according to this preferred embodiment of the invention is capable of being housed in enclosure50of this small form factor, while projecting an image of excellent resolution. It has been observed, according to this invention, that projection optics20in projection system15provide excellent (>50%) response even at high spatial frequencies (>0.65 cycles per mm), over the screen2. In addition, because of the telecentricity of rear group20aof projection optics20, excellent response is maintained over relatively wide focus shifts. Distortion and lateral color shift are also minimized according to this design. As such, projection system15according to this embodiment of the invention is fully capable of accurately and precisely projecting modern “high definition” images.

In addition, it has been discovered and observed that this invention provides the additional important benefit of greatly improved stability over variations in temperature and humidity in the system environment. It has been observed that conventional projection systems, for example DMD-based projection systems using only an aspheric mirror, suffer from loss of resolution over variations in temperature and humidity, due to the effects of such environmental variations on the aspheric mirror.FIGS. 9athrough9cillustrate the modulation transfer function (MTF), expressed as the modulus of the DTF ranging from 0.0 to 1.0, over spatial frequency in cycles per millimeter, for a conventional single aspheric mirror system at temperatures of −5° C., +20° C., and +45° C., respectively. As shown inFIG. 9b, the resolution performance is quite good for this conventional system; however, the higher and lower temperature performance is dramatically poorer, with poor resolution (as poor as 0.13 cycles per mm) exhibited for these temperatures

On the other hand, projection system15according to this preferred embodiment of the invention provides relatively good stability over temperature, as exhibited byFIGS. 9dthrough9fat temperatures of −5° C., +20° C., and +45° C., respectively. As evident from these plots, the degradation in resolution over temperature is much reduced for projection system15, relative to the conventional system for which performance is shown inFIGS. 9athrough9c. It is believed, according to this invention, that this improved stability is because any thermal dilation of plastic aspheric mirror10will be compensated by substantially equivalent thermal expansion of plastic aspheric plastic lenses42,44, which presents the opposite effect of the dilation of aspheric mirror10. Conventional systems using only a single plastic aspheric mirror do not have this compensating effect, resulting in the poor thermal performance illustrated inFIGS. 9athrough9c.

It is also contemplated that, because of this improved thermal stability, the plastic aspheric lenses42,44and plastic aspheric mirror10of projection system15according to this invention can be constructed to be stable over temperature in a system using a plastic baseplate, thus reducing manufacturing cost and also reducing mechanical stress due to thermal mismatch between the plastic lenses and the baseplate.

Furthermore, the nature of DMD-based projection systems such as display system15according to this preferred embodiment of the invention lends itself well to scaling to larger screen sizes. As such, it is contemplated that the ratio of chin and depth of enclosure50, to the size of screen2, will be the same or better as the size of screen2is scaled upward. These larger screen display DMD-based display systems are contemplated to be less expensive than corresponding LCD and plasma systems, given the scalability of the DMD projection engine relative to those other technologies.

While the present invention has been described according to its preferred embodiments, it is of course contemplated that modifications of, and alternatives to, these embodiments, such modifications and alternatives obtaining the advantages and benefits of this invention, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein.