Patent Application
20080106493
The present invention relates generally to laser projectors.
During the past decade handheld electronic devices such as mobile telephones, portable video player, personal digital assistants (PDA) and portable game consoles, have come into widespread use. Moreover, continued progress in electronic integration, has enabled the development of ever more powerful devices, to wit the handheld devices of today have processing power comparable to personal computers of a decade ago. Thus, it is possible for handheld electronic devices to run many useful applications that are run on personal computer, such as web browsers, image viewers and video players, for example. One limiting factor, in regards to handheld devices is their small screen size. The small screen size somewhat discourages prolonged use of text and graphics intensive applications. To address the small screen size, it has been proposed to incorporate small laser based image projectors within handheld devices. In order to be useful, especially in relatively brightly lit environments such as offices, a certain minimum screen brightness must be achieved. A particular chosen screen brightness and projected image size dictates a certain optical power from the laser. The inherent electrical-to-optical conversion efficiency of the laser in-turn dictates a certain electrical input power for the laser. In a handheld device this electrical input power must be supplied by a battery, fuel cell or other small power source, which also must provide power for other systems (e.g., the cellular radio) of the handheld device. Thus, there is a need for more efficient laser projector systems. More efficient laser projectors allow larger, brighter projected images, and/or extended battery life.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to laser displays. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.
It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of laser displays described herein. The non-processor circuits may include, but are not limited to, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method to perform image signal processing for laser displays. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.
One or more video clocks 204, e.g., a pixel clock, a row clock and frame clock are coupled to the screen buffer 202 and to a beam scanner 206 (discussed further below). The video clocks 204 clock the pixel brightness values out of the screen buffer 202, into red channel electronics 208, green channel electronics 210, and blue channel electronics 212. Alternatively, one color is used for a monochrome display, two colors are used for a two color display, or more than three colors are used to achieve a display with an increased color gamut. The color channel electronics 208, 210, 212 are discussed in more detail below with reference to
The red, green and blue color channel electronics 208, 210, 212 are coupled respectively to a red laser diode 214, a green laser diode 216 and a blue laser diode 218. Briefly, the color channel electronics 208, 210, 212 serve to generate drive signals to drive the laser diodes 214, 216, 218 based on the pixel brightness values received from the screen buffer 202. Rather than using the light emitted by one or more of the laser diodes 214, 216, 218 directly, the light can be frequency multiplied (e.g., doubled) or used to pump another laser (e.g., a solid state laser).
Laser beams emitted by the red, green and blue laser diodes 214, 216, 218 are coupled through a red channel lens, 220, a green channel lens 222 and a blue channel lens 224 to a first mirror 226, a second, dichroic mirror 228, and a third, dichroic mirror 230. The red, green and blue channel lenses 220, 222, 224 serve to collimate or establish designed angles of divergence of the laser beams. In some cases, e.g., if the beams produced by the three laser diodes 214, 216, 218 are similar in diameter and divergence one or more of the channel lenses may be eliminated. The mirrors 226, 228, 230 serve to combine the laser beams emitted by the three laser diodes 214, 216, 218 into a single beam.
The combined single beam passes through an optional final lens 232 before impinging a beam scanner 206. The optional final lens 232 may be used if the red, green and blue channel lenses 220, 222, 224 are not used or may used in combination with the red, green and blue channel lenses. The beam scanner 206, can for example take the form of one or more piezoelectric mirror devices, MicroElectroMechanical System (MEMS) mirror devices, or rotating mirrors, for example. The beam scanner 206 scans the combined beam over a viewing screen or other surface 234. The beam scanner 206 suitably scans the combined beam in a raster pattern, but may alternatively use a vector pattern. The beam scanner 206 is kept in sync with pixel brightness values coming out of the screen buffer by supplying one or more signals from the video clocks 204 to the beam scanner 206.
The optics module 122 includes the laser diodes 214, 216, 218 channel lenses 220, 222, 224, mirrors 226, 228, 230, final lens 232, and beam scanner 206. The video clocks 204, screen buffer 202 and channel electronics 208, 210, 212 are embodied in the integrated circuits 118 and discretes 120 mounted on the circuit board 104.
Once skilled in the art will appreciate that many variations on the optical layout shown in
A second plot 306 gives power dissipation versus current. As shown even at the laser threshold B0 which corresponds to approximately zero emitted light intensity, the laser diode is dissipating substantial power. In a handheld electronic device this substantial power dissipation is undesirable as it leads to faster depletion of the energy source 110.
A third plot 308 represents rise time as a function of current. Above the laser threshold B0, plot 306 represents the small signal rise time, whereas below the laser threshold B0 the third plot 308 represents rise time from the current indicated on the abscissa for the third plot 306 to a lasing state at the lasing threshold B0. As shown rise time increases as the starting current decreases, and increases markedly below the lasing threshold B0. For some laser diodes, and for some starting currents the rise time to lasing at the laser threshold may exceed a pixel duration (inverse pixel rate). (Plot 306 is qualitative, not based on measured data.)
According to certain embodiments of the invention, a laser diode in a laser projection display system is selectively operated at the current level B0′ which is substantially below the lasing threshold, in response pixel brightness information indicating pixel brightness below a predetermined threshold. According to certain embodiments a laser diode in a laser projection system is operated at the current level B0′ in response to pixel brightness information indicating a pixel brightness of zero. In systems using quantized pixel brightness values, a pixel brightness below the lowest non-zero pixel brightness value is zero. According to certain embodiments B0′ is substantially lower than the lasing threshold B0. According to certain embodiments B0′ is less than two-thirds of the lasing threshold current B0. According to certain embodiments B0′ is less than one-half of the lasing threshold current B0. Setting a low value of B0′ serves to conserve energy of the energy source 110. According to certain embodiments B0′ is greater than zero. According to certain embodiments B0′ is substantially greater than zero and according to certain embodiments B0′ is greater than ⅓ of B0. In certain embodiments, setting a relatively high value of B0′ serves to reduce the total rise time from B0′ to a lasing state, thereby facilitating high pixel rates, which facilitates high resolution and high frame rates, which generally leads to higher display quality, without having to have recourse to the embodiments shown in
An output of the FIFO buffer 506 is coupled to an input 508 of a D/A 510. An output 512 of the D/A 510 is coupled to an input 514 of a laser driver amplifier 516. An output 518 of the laser driver amplifier 516 is coupled to the laser diode (e.g., 214, 216, or 218).
One or more memory locations of the FIFO are coupled to an input 520 of a decision logic block 522. An output 524 of the decision logic block 522 is coupled to a control input 526 of a controllable bias 528. An output 530 of the controllable bias 528 is coupled to the laser diode.
The decision logic block 522 decides whether to raise the laser bias current from an energy conserving level (e.g., B′) to a level that is at least the lasing threshold (e.g., B0) based on one or more of the discrete quantized digital pixel brightness values stored in the FIFO buffer 506. According to one embodiment the decision logic block 522 decides whether to raise the laser current by comparing the pixel brightness value in the first memory location 504 to a threshold (e.g., testing if the pixel brightness value in the first memory location is nonzero.) According to another embodiment the decision logic block 522 decides whether to raise the bias current based on pixel brightness values in multiple (e.g., the first N) memory locations in the FIFO buffer 506. For example, the decision logic block 522 can compare the sum of the pixel brightness values in multiple memory locations in the FIFO buffer 506 to a threshold, and if the sum exceeds the threshold raise the bias current. Alternatively, decision logic block 522 can perform spatial frequency analysis (e.g., using an Finite Impulse Response (FIR), or Fast Fourier Transform (FFT)) on pixel brightness values in multiple memory locations of the FIFO buffer. For example if a low-pass filtered amplitude is below a certain predetermined threshold, the bias current can be maintained at an energy conserving level. Note that the eye is relatively insensitive to variations in intensity of high spatial frequency components. Note that certain decision rules may be better suited for images and certain better suited for text. In cases that the pixel duration (inverse of the pixel clock rate) is smaller than the time required to transition the laser diode from the energy saving bias to a lasing state above the lasing threshold, then length of the FIFO buffer 506 can be set to provide enough time (pixel durations), between the time that a pixel brightness value reaches a last memory location used by the decision logic block 522 and the time that the pixel brightness value is output from the FIFO buffer 506, for the laser to be transitioned to the lasing state.
The decision logic block 522 can be implemented using state logic or a programmed processor for example. An example of the former is multiply-and-accumulate (MAC) unit which can be conveniently used to perform FIR or simple summing operations on pixel brightness values in a sequence of memory locations of the FIFO 506.
Block 604 is a decision block, the outcome of which depends whether the pixel brightness read in block 602 is higher than a predetermined threshold. According to certain embodiments, the predetermined threshold is zero. If the outcome of decision block 604 is negative, then the flowchart branches to decision block 608 which on depends if there are more pixel to be displayed, e.g., if more pixels are entering the FIFO buffer 506. If it is determined in block 608 that there are no more pixels to be displayed then the flowchart terminates. If, on the other hand, there are more pixels to be displayed then block 610 advances to the next pixel (e.g., another pixel brightness value is clocked into the FIFO buffer 506) and the flowchart returns to block 602. If it is determined in block 604 that the pixel brightness value is above the predetermined threshold, then the flowchart branches to block 606 in which the laser diode bias is increased to a predetermined higher level, at or above the laser threshold at least until the pixel corresponding to the pixel brightness value read in block 602 is displayed. Alternatively, the predetermined level may be so slightly below the laser threshold that adding a current increment coded by the lowest non-zero brightness level increases the laser current above the laser threshold. In the aforementioned embodiment in which the FIFO buffer 506 is made just long enough so that a pixel brightness value will be pass through the FIFO buffer in an interval that is at least about equal to the rise time, in block 606 the laser diode bias will be kept high at least until the pixel brightness value read in block 602 is clocked through the FIFO buffer, and converted by the D/A 510 to an analog voltage which is amplified by the laser driver amplifier 516 and drives the laser diode (e.g., 214, 216, 218). Thus, at the time the pixel brightness value is actually used to determine the laser diode current, the laser diode will be properly biased to access light emitting states on the linear portion of the light intensity plot 302 above the knee 304 (lasing threshold).
A decision logic block 704 of the laser operating electronics 700 that is shown in
The OR gate 706 is coupled to a trigger input 708 of a mono-stable multivibrator (“one shot”) 710. The one-shot 710 is a timer that outputs a pulse (active low or active high) of a predetermined duration starting whenever it is triggered. A pulse output 712 of the one-shot 710 is coupled to the control input 526 of the controllable bias circuit 528. In the laser operating electronics 700, the duration (or “width”) of the pulse output by the one-shot 710 is equal to the time required for a pixel magnitude value to propagate from the first memory location 504 through the FIFO buffer 702, and D/A 510 and be used to control driving of the laser diode, plus a pixel duration or more. When the OR gate 706 finds that the pixel magnitude in the first memory location 504 of the FIFO buffer 702 is non-zero it triggers the one-shot 710 which outputs a pulse to the controllable bias 528 which changes the bias on the laser diode from the energy saving bias B0′ to a bias B0 at or above the lasing threshold.
In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
