Patent Application
20090257030
The present invention is related generally to projection of optical images, and, more particularly, to optical-image projectors subject to space limitations.
A trend in personal portable devices (such as cell phones and personal digital assistants) is to add new features while keeping the devices small. Many of the new features, such as photograph sharing and video downloading, depend upon a high resolution, easy-to-read display screen. However, manufacturers cannot simply keep increasing the size of their display screens because that would eventually run counter to the desire to keep the devices small and portable.
Recently, “microprojectors,” a new category of display device, have been designed to address this conflict between greater display area and smaller device size. An image, either still or moving, is projected from the device onto a convenient surface (e.g., a projection screen or an office wall). The maximum size of the image is then effectively constrained by the amount of available wall space rather than by the size of the device itself. Using a microprojector-equipped device, several people can simultaneously view a photograph, for example, or review a full page of text, neither of which can be readily done with even the largest displays on current personal portable devices.
Promising as they are, microprojectors raise new headaches when engineers attempt to fit them into personal portable devices. While the overall size of the projected image may be effectively unlimited, expanding the image size is of little use if the resolution of the projected image is severely constrained. What customers want is a projected image that is both larger overall and has much greater resolution than a device's display screen. But, generally, the overall size of a microprojector grows with the amount of resolution it provides. This is especially true when a microprojector uses a microdisplay imager as its image source. The trend toward very thin personal portable devices renders it a challenge to fit in a microprojector that provides usefully high resolution.
Power use is another challenge. By its nature, a microprojector uses a significant amount of power to light a large display area. Reducing the physical size of the microprojector exacerbates the power problem because the optics in microprojectors become less power-efficient as they become smaller. Designers of battery-based personal portable devices are already concerned about their power budgets and look askance at any new feature that threatens to reduce the utility of the device by reducing how long the device can operate between charges.
The above considerations, and others, are addressed by the present invention, which can be understood by referring to the specification, drawings, and claims. According to aspects of the present invention, a microprojector uses a “reduced-height” imager to sequentially display a series of partial images within one frame time. The partial images visually combine on a projection surface (e.g., a screen or a wall) into one high-resolution projected image. As a result, the microprojector projects an image with a resolution equal to the sum of the resolutions of the individual partial images while avoiding the use of very small imager optics with their lowered efficiency.
For example, one embodiment projects exactly two partial images during each frame. During a first state of operation, a “half-height” imager displays the odd-numbered lines of the projected image. During a second state of operation, the imager displays the even-numbered lines of the projected image. By quickly cycling through these two states (e.g., performing each state 24 or 32 times per second), no image flickering between phases is visible, and the combined image appears as a seamless whole.
By increasing the number of operational states, the projected image can be divided into more partial images, and the imager can be made even thinner.
Line-processing optics are used in some embodiments to reduce the vertical “thickness” of the horizontal lines of the projected partial image. Then, the lines projected during one state of operation can visually fit between the lines projected during the other states of operation.
Some embodiments employ a switchable beam shifter to position the partial images with respect to one another so that, when projected, the partial images interlace to form a seamless image without overlap. In an embodiment where two partial images are projected, for example, the beam shifter in one state raises the odd-numbered lines just enough (in conjunction with the narrowing produced by the line-processing optics, if any) so that the even-numbered lines fit between them.
Because the height of the imager is smaller than the height of a monolithic imager that could project an image with the same resolution, the imager can fit into a very thin device. The combined image has a resolution equal to the sum of the resolutions of the partial images. That is, the combined image has a horizontal resolution equal to that of the imager and a vertical resolution equal to the vertical resolution of the imager multiplied by the number of partial images projected during one frame.
While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which:
a is a simplified schematic view of an exemplary time-sequential microprojector with a reduced-height imager, the microprojector being in a first phase of operation;
b shows the image produced by the microprojector of
c is a schematic of the same microprojector as in
d shows the image produced by the microprojector of
Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable environment. The following description is based on embodiments of the invention and should not be taken as limiting the invention with regard to alternative embodiments that are not explicitly described herein.
In
The resolution of a digital image is defined as the product of its horizontal resolution and its vertical resolution. Resolution is measured in number of pixels. In
In a projector, an “imager” is a device that modulates light in order to imprint image information into a projected light beam. Generally, the resolution of a projected image is equal to the resolution of the imager that creates the image. Traditionally, including within the personal portable device 102 an imager that provides acceptable resolution for the projected image 104 makes the personal portable device 102 both thick and bulky. The present invention addresses this issue by allowing a small and thin personal portable device 102 to project a large, high resolution image 104.
a gives an example of how a microprojector made according to aspects of the present invention can achieve a high resolution in the projected image 104.
In some embodiments, the horizontal resolution of the imager 202 is equal to the horizontal resolution of the projected image 104, but the vertical resolution of the imager 202 is only a fraction (e.g., ½) of the vertical resolution of the projected image 104. To produce the projected image 104, the microprojector system moves sequentially through a cycle of phases of operation (Step 302 of
a through 2d illustrate the operation of an exemplary microprojector system with exactly two phases of operation. During the first phase of operation, as illustrated in
It is important that the lines projected during each phase of operation are not projected on top of the lines projected during other phases of operation. Two functional elements, line-processing optics 206 and a switchable beam shifter 208, are provided in some embodiments to ensure this. The immediately following discussion presents an overview of the functions of these elements, while
During the first phase of operation as illustrated in
The switchable beam shifter 208 also moves sequentially through a cycle of phases. During the first phase as illustrated in
b represents the result of the first phase of operation, in a highly stylized manner. The odd-numbered lines projected during this phase of operation are shown as four horizontal bands making up the partial image 212a. In actual operation, it is expected that these bands will each be only one pixel wide, and that there will be many more than four of them. The resolution of a VGA display, for example, is 640×480 pixels. Then, for a microprojector with exactly two phases of operation, the partial image 212a will consist of 240 horizontal lines of pixels, each 640 pixels wide, with a single pixel-width blank line between each adjacent pair of projected horizontal lines. Embodiments of the present invention are compatible with other image resolutions.
To complete this example of a microprojector with exactly two phases of operation, turn to
d shows, again very stylistically, four even-numbered bands in the partial display 212b.
When the microprojector system moves through its cycle of states very rapidly (e.g., 24 or 32 full cycles are completed in every second), then the human eye cannot distinguish the separate partial images 212a and 212b. Instead, these partial images 212a and 212b combine visually into a seamless, flicker-free, projected image 104.
Because the imager 202 presents multiple partial images during each full cycle of operation, the imager 202 can be shorter in a vertical direction than a monolithic imager of the same overall resolution. This permits the personal portable device 102 to remain small and thin. There is no need to include in the personal portable device 102 room for a single monolithic imager that has the same resolution as the final image 104. Instead, the system of
Other embodiments use more than two phases of operation during a full cycle. This allows the imager 202 to be even thinner, at the possible cost of either decreasing the quality of the projected image 104 or of increasing the cycle rate. With four phases of operation, for example, the imager 202 produces only ¼ of the overall number of horizontal lines per cycle, but there may need to be 48 or more cycles every second in order to produce acceptable image quality.
The imager 202 shown in
For simplicity's sake, the projection lens system 210 is drawn as a single lens in
Note again that “vertical” and “horizontal” are used here merely for convenience' sake and are used with respect to the figure under discussion. In most embodiments, the image 104 is expected to be projected from an end face of the personal portable device 102. The shape of the end face of many personal portable devices 102 approximates a long, thin rectangle. In some embodiments of the present invention, the projected image 104 roughly follows this shape. Thus, to project an image in “landscape” mode (that is, with a greater horizontal than a vertical dimension), the user 100 holds her personal portable device 102 “flat” (with the long edge of the face of the device 102 parallel to the ground). To project an image 104 in the “portrait” mode as shown in
In some embodiments, the imager 202 consists of (1) areas that actually create images separated by (2) “blanks” or areas that do not create any image. In these embodiments, second line-processing optics (not shown) can be placed between the illumination source 200 and the imager 202. These optics serve to concentrate incident light only on the image-producing areas of the imager 202 so that no light is wasted.
The switchable beam shifter 208 of
The projection lens system 210 of
The microprojector system as described so far would, in some embodiments, produce a final image 104 with an incorrect aspect ratio. (The aspect ratio is defined to be the ratio of the horizontal dimension of the image 104 to its vertical dimension.) For example, in the case where the microprojector has exactly two phases of operation during each cycle, the vertical dimension of the final image 104 will only be about half what it should be in relation to the image 104's horizontal dimension. To compensate for this, in some embodiments, the projection lens system 210 is anamorphic. The anamorphic projection lens system 210 expands the set of projected lines more in a vertical direction than in a horizontal direction (Step 310 of
In view of the many possible embodiments to which the principles of the present invention may be applied, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of the invention. For example, the light paths in the figures are only meant to illustrate the functions of the various components and are not meant to be definitive. Other arrangements of the optical components shown in the figures and the addition of other known optical components are possible and may be called for in various environments. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.
