Abstract:
A method and apparatus for controlling light intensity from two or more light sources. A timing scheme is used to modulate the light sources. Light from the light sources is combined to form a beam and a photo-sensor senses the beam. In a time interval when only one of the light sources is activated, the signal from the photo-sensor is monitored and used in a feedback control circuit to control the active light source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to co-pending application CML02735T, U.S. patent application Ser. No. ______, entitled Method and Apparatus for Intensity Control of Multiple Light Sources, which is filed even date herewith. 
     
    
     BACKGROUND  
       [0002]     An ultra-compact projection display, called a micro-projector, can provide an image larger than the hosting projecting device dimension. Such displays have the potential to overcome the display size limitation on mobile devices and can be used to display documents and images, for example. The displays can also help to overcome the input interface size limitation on mobile devices by displaying virtual input devices, such as keyboards.  
         [0003]     One technology used for micro-projection is laser scanning, in which red, green and blue laser beams are scanned across a surface to form a full color image.  
         [0004]     It is known that an individual semi-conductor or solid-state laser requires dynamic intensity stability control, which is usually performed by monitoring the laser output with a photo-diode and using a feedback control loop to control the laser intensity.  
         [0005]     When micro-projection is used in a mobile device, the environmental conditions, such as temperature, will be subject to large changes. These changes increase the need for intensity control.  
         [0006]     In a full color micro-projector, the laser sources are very close together and some techniques used for feedback control of a single laser may be ineffective due to cross-talk between the lasers. For example, one prior approach is to monitor the main beam reflection in the system, either from the housing or from the lens, to provide a feedback signal. Another approach is to measure leakage from a back mirror of the laser cavity. However, this is not a direct measure of laser power and reduces laser efficiency. Neither of these approaches will work when multiple lasers are used, because they do not provide feedback for the individual lasers, or they result in increased overall size of the system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as the preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawing(s), wherein:  
         [0008]      FIG. 1  is single laser with intensity control of the prior art.  
         [0009]      FIG. 2  is diagram of an illumination system with intensity control consistent with certain embodiments.  
         [0010]      FIG. 3  is a diagram of a beam combiner consistent with certain embodiments.  
         [0011]      FIG. 4  is an exemplary plot of light intensity in a collimated beam.  
         [0012]      FIG. 5  shows photo-sensors located relative to a collimated beam consistent with certain embodiments.  
         [0013]      FIGS. 6, 7 , and  8  are diagrams of illumination systems consistent with certain embodiments. 
     
    
     DETAILED DESCRIPTION  
       [0014]     While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one or more specific embodiments, with the understanding that the present disclosure is to be considered as exemplary of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawings.  
         [0015]     The present invention relates to intensity control in an illumination system employing multiple light sources. The invention is described with reference to an embodiment in a full color micro-projection system using laser light sources. However, other embodiments of the invention will be apparent to those of ordinary skill in the art.  
         [0016]      FIG. 1  is single laser with intensity control of the prior art. The laser  100  includes a support structure  102 , which is coupled to body  104  and supports semi-conductor laser element  106 . The laser element emits light  108  in the forward direction. A photo-diode  110  collects light  112  that is emitted through a back mirror of laser element  106  or reflected by the metal case. This reduces efficiency because of leakage through the back mirror, and results in increased threshold current and decreased slope efficiency. Further, this approach will substantially increase the volume of the overall system with multiple lasers.  
         [0017]      FIG. 2  is diagram of a multiple light source illumination system with intensity control consistent with certain embodiments of the present invention. Referring to  FIG. 2 , the system  200  includes at least three light sources ( 202 ,  204  and  206 ). These may be lasers, for example, each producing light at a primary color wavelength. For example lasers that generate light at 440 nm, 532 nm and 635 nm wavelengths may be used. Other combinations of lasers may be used. Some applications may require only two light sources, while other applications require three or more light sources. In some applications, the light sources are chosen so that a combination of the light from the sources can produce white light. Referring again to  FIG. 2 , the light sources ( 202 ,  204 ,  206 ) emit light uni-directionally in three separate beams. Light from the three separate beams is combined in beam combiner  208  to produce a single collimated beam  210 . The collimated beam  210  is directed towards a 2-axis scanner  212  that scans the beam onto a screen  220 . The light sources and scanner may be located inside projector such as a micro-projector. The scanned beam passes through a beam region  214  close to the scanner  212 . The beam region  214  is inside the projector, while the screen  220  is outside of the projector. Various compact scanners are known to those of ordinary skill in the art.  
         [0018]     A photo-sensor  216  (or a combination of photo-sensors) may be positioned before the scanner  212 . Alternatively, a photo-sensor  218  (or a combination of photo-sensors) may be placed after the scanner  212 . The photo-sensor  218  is located at the edge of the beam region  214 .  
         [0019]     An example beam combiner is shown in  FIG. 3 . The beams from light sources  202 ,  204  and  206  are passed through optical elements  302 ,  304  and  306  and are combined into a collimated and co-linear beam  210  using dichroic mirrors  308 ,  310  and  312 . Each dichroic mirror reflects one primary color wavelength and transmits other wavelengths. Thus, for example, the dichroic mirror  310  reflects light from light source  204  but transmits light from light source  202 . As used herein, the terms collimated and co-linear are meant to include instances of substantially collimated and co-linear beams.  
         [0020]     Photo-sensor before scanner. The beam  210  has varying intensity in its cross section. In one embodiment, the beam  210  has a substantially Gaussian intensity profile.  FIG. 4  is a plot  402  of the intensity of the collimated beam as a function of the normalized distance from the center of the beam. The intensity is plotted relative to the maximum intensity of the beam (which occurs at the center). The radius is normalized relative to the radius at which the intensity falls to 1/e 2  of the maximum intensity. Only the central portion beam (for example, between lines  404  where the intensity is greater than 13.5% of the maximum intensity) is used for projection, so the photo-sensor  216  may be placed outside of the central region of beam, in the fringe of the beam, without shielding useful light. The photo-sensor  216  may be a single sensor that is sensitive to light from any of the light sources, or it may be a set of red, green and blue wavelength sensitive sensors.  
         [0021]     For a Gaussian beam, 13.5% of the total energy of the beam intensity lies outside of the central beam with normalized radius R=1. Most of this energy is available for capture by one or more photo-sensors  216  in the fringe region.  
         [0022]      FIG. 5  shows a cross section through the collimated beam  210 . The beam has an inner region, inside the circle  404 , in which the light has a high intensity (&gt;13.5% of the peak, in this example), and an intermediate region, or fringe, bounded by the circles  502  and  404  where the light has a reduced, but still significant, intensity. Photo-sensors  216 ,  216 ′ and  216 ″ are placed in the fringe of the beam. Thus, the photo-sensors do not interfere with the high intensity region of the beam. Each photo-sensor is responsive to a different range of wavelengths. These wavelengths correspond to the wavelengths of one of the light sources, so each photo-sensor provides an electrical signal that can be used in a feedback loop to control one of the light sources. Further, a timing scheme may be used in which during a particular interval only one of the light source is activated. During this time interval, a measurement of the light intensity may be made at one of the sensors. In one embodiment, a single sensor  216  is used to provide feedback signals for all of the light sources.  
         [0023]     Photo-sensor after scanner When the photo-sensor  218  is used, it is positioned at the edge of the beam region near the scanner ( 214  in  FIG. 2 ). In one embodiment, the scanner and beam region are inside a micro-projector. The photo-sensor may be placed at a corner of the beam region in an area that is usually blanked out to avoid the ‘pin cushion effect’. The dimension SC of the space in the beam region corresponding to the area available for the photo-sensor at a corner of the beam region is given by
 
 SC=d *tan( a )*[(1+ r   2  tan 2 ( a )) 1/2 −1]  (1)
 
 where d is the distance from the scanner to the beam region, a is the maximum angle or rotation of the scanning mirror from the center line and r is the image aspect ratio. 
 
         [0024]     The width of the beam region is W=2*d*tan(a) so the dimension of the available space is  
               SCN   =         N   2     ⁡     [         (     1   +       r   2     ⁢       tan   2     ⁡     (   a   )           )       1   /   2       -   1     ]       ⁢           ⁢   pixels       ,           (   2   )             
 
 where Nis the width of the image in pixels. 
 
         [0025]     In one exemplary embodiment, the beam region is located 10 mm in front of an SVGA scanner that generates an image of 800×600 pixels. The scanning angle is a=30° and the aspect ratio is r=600/800, so the dimension of the space available (given by equation 1) is 0.518 mm. Thus, a photo-sensor of dimension 0.5 mm×0.5 mm may be used. This is approximately 35 pixels at the pin cushion corner (from equation 2).  
         [0026]     In order to allow an individual light source intensity to be measured, a timing scheme is used in which the light sources are modulated. In certain time intervals, only one light source is activated. The intensity of the activated light source can be monitored by the photo-sensor  218  during these intervals. Each light source is activated in turn, allowing the intensity of each of the light sources to be monitored. In this embodiment, the scanned beam is periodically directed onto the photo-sensor  218  at the edge of the beam region. This portion of the beam region is usually blanked out to avoid a ‘pin-cushion’ shaped image. A single sensor may be used, or a set of red, green and blue sensors. In certain time periods, full intensity light from a single light source is directed at the sensor.  
         [0027]     A feedback control circuit is responsive to the photo-sensor output signals. The feedback control circuit may be synchronized with the light source timing scheme, so that intensity of each light source can be adjusted in turn in response to the appropriate photo-sensor signal.  
         [0028]     An SVGA image comprises 800×600=480,000 pixels. At a 75 frames per second, the beam is at each pixel for an average of 28 ns. In one embodiment, the photo-sensor is a photo-diode. For example, a photo-diodes can have a response time in the region of 3 ns, so the photo-sensor can be scanned at the same rate as the rest of the image or treated as a single pixel. Further, controlled independent readings for one or more light sources can be made in a single scan.  
         [0029]     In one embodiment, semi-conductor, diode or frequency-doubled lasers are used as light sources and Si-photodiodes are used as photo-sensors.  
         [0030]     Conventional feedback electronics (not shown in  FIG. 2 ) may be used adjust the light source intensity dependent upon the signal from the corresponding photo-sensor.  
         [0031]      FIG. 6  is a diagram of an illumination system consistent with certain embodiments. The illumination system includes a first light source  202  for generating light at a first wavelength, a second light source  204  for generating light at a second wavelength and a third light source  206  for generating light at a third wavelength. The light sources are controlled by a feedback control circuit  602  that is responsive to photo-sensor  216  or  218 . The photo-sensor produces a monitor signal  604 . During an interval when the light source  202  is activated and the light sources  204  and  206  are not activated, the feedback control circuit  602  generates a control signal  606  to control the intensity of light source  202 . Similarly, during an interval when of the three light source only the light source  204  is activated, the feedback control circuit  602  generates a control signal  608  to control the intensity of light source  204 , and when only the light source  206  is activated, the feedback control circuit  602  generates a control signal  610  to control the intensity of light source  206 .  
         [0032]     A timing circuit  612  controls modulation of the light sources  202 ,  204  and  206  via signal  614 , and controls the feedback control circuit  602  via signal  616 . In operation, during a time interval when only one of the light sources is activated, the photo-sensor signal is monitored and is used to adjust the intensity of the activated light source. If the photo-sensor  218  is used, the timing circuit  612  also synchronizes the scanner  212  to direct the scanner output to the photo-sensor at the appropriate time using signal  618 .  
         [0033]     In one embodiment, the feedback control circuit comprises three independent control circuits, and sensor signals are passed to each circuit in turn.  
         [0034]     Single photo-sensor integrated with beam combiner.  FIG. 7  is diagram of a multiple light source illumination system with intensity control consistent with certain embodiments. Referring to  FIG. 7 , the system  200  includes three light sources  202 ,  204  and  206 . These may be lasers, for example, each producing light at a primary color wavelength. For example lasers that generate light at 440 nm, 532 nm and 635 nm wavelengths may be used. Referring again to  FIG. 7 , the light sources  202 ,  204  and  206  emit light uni-directionally in three separate beams. The beams are combined into a collimated and co-linear beam  210  in beam combiner  208  using dichroic mirrors  308 ,  310 ,  702  and  704 . Dichroic mirrors  308  and  704  reflect substantially all light from source  202 , while dichroic mirrors  310  and  702  reflect substantially all light from the source  204 . Mirrors  702  and  704  transmit substantially all light from source  206 . Mirror  704  transmits substantially all light from source  206 .  
         [0035]     Photo-sensor  216  (such as a photo-diode or photo-transistor) is positioned behind dichroic mirror  704  and is oriented to detect transmitted green light and reflected red and blue light The intensity of the detected light is in direct proportion to the intensity of the primary light, so the output from the photo-sensor provides a signal for use in a feedback control loop to control the light source intensity.  
         [0036]     More generally, dichroic mirrors are used to direct the light from a light source, either by transmitting or reflecting substantially all of the light at a certain narrow band. When the light is transmitted, the photo-sensor can be placed on the same side of the mirror as the light source so as to collect the small amount not transmitted and not to interfere with the main beam. When the light is reflected by the mirror, the photo-sensor can be placed on the opposite side of the mirror from the light source so as to collect the small amount not reflected and not to interfere with the main beam. In both case, the sensor is place to receive light that is not directed along the main beam. That is, if the main beam emanates from one side of the mirror, the sensor is place on the other side. It will be apparent to those of ordinary skill in the art that this approach can be used for other light directing means, such as prisms.  
         [0037]     Optionally, a prism structure  706  may be used to provide a mechanical support for the photo-sensor  216  and dichroic mirror  704 . That is, dichroic mirror  704  is mounted on one facet of prism structure  706 , while photo-sensor  216  is mounted on another facet of the prism structure  706 .  
         [0038]     In a further embodiment, the dichroic mirror is formed as a coating on the surface of the prism structure  706 .  
         [0039]     The photo-sensor  216  receives light from all of the light sources. As described above, a timing scheme is used to allow independent monitoring of each of the light sources. In certain time intervals, only one light source is activated. Since it is known which light source is activated, the photo-sensor signal may be used in a feedback control circuit to adjust the corresponding light source.  
         [0040]     For applications where full color is not required, the illumination system may use only two light sources. In  FIG. 7 , for example, any one of the light sources could be omitted. However, omitting light source  204  eliminates two dichroic mirrors. For full color applications, three or more light sources may be used.  
         [0041]     The geometric arrangement shown in  FIG. 7  makes efficient use of space, since the photo-sensor is positioned in space behind the dichroic mirror. This space would otherwise be unused. The use of a single sensor in combination with the timing scheme described above reduces the volume further.  
         [0042]     The geometric arrangement results in improved efficiency compared to a system using back facet reflectivity (98% for mirrors versus 90% for back reflectivity). For example, for a red laser diode used with back monitoring, a typical back reflectivity at about 90% is used. If back monitoring is not needed, the back reflectivity can be 95% or higher, this makes the laser more efficient by decreasing the threshold current and increasing the slope efficiency.  
         [0043]     Dual photo-sensors integrated with beam combiner.  FIG. 8  is a diagram of a multiple light source illumination system with intensity control consistent with certain embodiments. Referring to  FIG. 8 , the system  200  includes at least three light sources,  202 ,  204  and  206 . The light sources emit light uni-directionally in three separate beams. The beams are combined into a collimated and co-linear beam  210  in beam combiner  208  using dichroic mirrors  308 ,  310 ,  702  and  704 . Dichroic mirrors  308  and  704  reflect light from source  202 , while dichroic mirrors  310  and  702  reflect light from the source  204 . Mirrors  702  and  704  transmit light from source  206 .  
         [0044]     Photo-sensor  216 ′ is positioned behind dichroic mirror  308  and is oriented to detect the transmitted light. Optionally, a prism structure  802  can be used to provide a mechanical support for the photo-sensor and associated dichroic mirror. The photo-sensor  216 ′ provides an independent measure of light from light source  202 . A second photo-sensor  216  receives light from light sources  204  and  206 . As described above, a timing scheme is used to allow independent monitoring of each of the light sources. In certain time intervals, only one of the two light sources (either  204  or  206 ) is activated. Since it is known which light source is activated at a given time, the photo-sensor signal may be used in a feedback control circuit to adjust the appropriate light source.  
         [0045]     While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.