Abstract:
A scanning beam assembly comprising: a beam generator to generate a beam of radiation, an oscillating reflector configured to deflect the beam at varying angles of excursion to yield a scanned beam that scans a field of view, an optical detector that detects light reflected from the field of view, the detector including at least one of an adjustable gain and adjustable sensitivity, and a controller programmable to control the gain and/or sensitivity of the detector.

Description:
FIELD OF INVENTION 
   This invention relates to improvements in scanning beam assemblies of the type that employ an oscillating reflector to control the scanning beam and a detector to detect the beam on a point by point basis as it is reflected from the field of view (FOV). The invention further related to scanning beam imaging systems incorporating such scanning assemblies and to corresponding improvements in devices, and still more particularly medical devices, including such an assembly. 
   BACKGROUND OF THE INVENTION 
   U.S. Published application 2005/0020926A1 discloses a scanning beam imager which is reproduced in  FIG. 1  herein. This imager can be used in applications in which cameras have been used in the past. In particular it can be used in medical devices such as video endoscopes, laparoscopes, etc. 
     FIG. 1  shows a block diagram of one example of a scanned beam imager  102 . An illuminator  104  creates a first beam of light  106 . A scanner  108  deflects the first beam of light across a field-of-view (FOV) to produce a second scanned beam of light  110 , shown in two positions  110   a  and  110   b . The scanned beam of light  110  sequentially illuminates spots  112  in the FOV, shown as positions  112   a  and  112   b , corresponding to beam positions  110   a  and  110   b , respectively. While the beam  110  illuminates the spots  112 , the illuminating light beam  110  is reflected, absorbed, scattered, refracted, or otherwise affected by the object or material in the FOV to produce scattered light energy. A portion of the scattered light energy  114 , shown emanating from spot positions  112   a  and  112   b  as scattered energy rays  114   a  and  114   b , respectively, travels to one or more detectors  116  that receive the light and produce electrical signals corresponding to the amount of light energy received. Image information is provided as an array of data, where each location in the array corresponds to a position in the scan pattern. The electrical signals drive an image processor  118  that builds up a digital image and transmits it for further processing, decoding, archiving, printing, display, or other treatment or use via interface  120 . 
   Illuminator  104  may include multiple emitters such as, for instance, light emitting diodes (LEDs), lasers, thermal sources, arc sources, fluorescent sources, gas discharge sources, or other types of illuminators. In some embodiments, illuminator  104  comprises a red laser diode having a wavelength of approximately 635 to 670 nanometers (nm). In another embodiment, illuminator  104  comprises three lasers: a red diode laser, a green diode-pumped solid state (DPSS) laser, and a blue DPSS laser at approximately 635 nm, 532 nm, and 473 nm, respectively. Light source  104  may include, in the case of multiple emitters, beam combining optics to combine some or all of the emitters into a single beam. Light source  104  may also include beam-shaping optics such as one or more collimating lenses and/or apertures. Additionally, while the wavelengths described in the previous embodiments have been in the optically visible range, other wavelengths may be within the scope of the invention. Light beam  106 , while illustrated as a single beam, may comprise a plurality of beams converging on a single scanner  108  or onto separate scanners  108 . 
   In a resonant scanning beam imager (SBI), the scanning reflector or reflectors oscillate such that their angular deflection in time is approximately a sinusoid. One example of these scanners employs a MEMS scanner capable of deflection at a frequency near its natural mechanical resonant frequencies. This frequency is determined by the suspension stiffness, and the moment of inertia of the MEMS device incorporating the reflector and other factors such as temperature. This mechanical resonant frequency is referred to as the “fundamental frequency.” Motion can be sustained with little energy and the devices can be made robust when they are operated at or near the fundamental frequency. In one example, a MEMS scanner oscillates about two orthogonal scan axes. In another example, one axis is operated near resonance while the other is operated substantially off resonance. Such a case would include, for example, the non-resonant axis being driven to achieve a triangular, or a sawtooth angular deflection profile as is commonly utilized in cathode ray tube (CRT)-based video display devices. In such cases, there are additional demands on the driving circuit, as it must apply force throughout the scan excursion to enforce the desired angular deflection profile, as compared to the resonant scan where a small amount of force applied for a small part of the cycle may suffice to maintain its sinusoidal angular deflection profile. 
   As illustrated in  FIG. 2 , in one embodiment the scanner employs a concave objective lens or dome  212  having a partially reflective surface  214 . The area of this surface will be appropriate for the medical application and the device design. In one embodiment it may be about 8 mm or less in diameter and in another embodiment it may be about 100-300 micron in diameter. This reflective surface may be integral to the dome  212 , located centrally on the lens surface, as shown in  FIG. 2  or it may be suspended or mounted on the incident side of the dome. The dome  212  has optical power and shapes the scanned beam  110  as it passes through the dome. In one embodiment, in order to view the areas directly behind the surface  214 , the surface  214  is a material that is only partially reflective. The beam  208  emitted from the optical fiber  204  is directed to the reflector  214  via the shaping optic  210 . The major portion (note  FIG. 3 ) of the beam radiation is reflected by the reflector to the oscillating reflector  108  and from reflector  108  into the FOV as scanned beam  110 . A smaller portion of the beam passes through the surface  214 . This “leakage” radiation passing through the surface  14  is reflected from the field of view (FOV). This light is diffuse and when the reflector is not directing light/radiation through the surface  214 , the light is not correlated with the point in the FOV that the scanner is interrogating. In this case it constitutes a source of noise that negatively impacts the SNR (signal to noise ratio). As shown in  FIG. 1 , a portion of the radiation reflected/scattered from the FOV, travels to one or more detectors  116  that receive the light and produce electrical signals corresponding to the amount of light energy received. 
     FIG. 3  is a graph of normalized detected beam intensity versus beam angle for an integral central reflective surface or “dot” reflector of the type illustrated in  FIG. 2  having reflection coefficients of 0.75 and 0.95 respectively.  FIG. 3  shows that less light is transmitted to and then returned to the detectors from the scene in the portion of the FOV covered by the projection of the reflective dot. For a reflective dot having a reflection coefficient of 0.75, 80% of the beam is reflected whereas for a reflective dot having a reflection coefficient of 0.95, less than 20% of the beam is reflected. Regardless of the reflection coefficient, as the angle at which the scanned beam is defected from the scanning reflector increases, e.g., at beam angles greater than about 25°, the beam intensity decreases, thereby also reducing the SNR of these wider angle beams. 
   SUMMARY 
   In accordance with this disclosure, scanning beam imagers are provided with a detector having adjustable sensitivity and/or gain, and a controller that adjusts the gain and/or the sensitivity of the detector to compensate for the effects described above. In more particular embodiments, the detector may include a controllable power source and/or a variable gain amplifier. 
   One embodiment provides for a timed increase in the gain of the detector during those periods of time in which the scanned beam is at its wider angles. Another embodiment provides for a timed increase in the sensitivity of the detector during those periods of time in which the scanned beam intersects the partially reflective surface. Another embodiment provides both functions. 
   A further embodiment of the invention is an endoscope or laproscope that employs an imager that is constructed and operated as described herein. 
   Another embodiment is a method for operating a scanning beam imager as described herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a scanning beam imager known in the art from Published application 2005/0020926A1. 
       FIG. 2  is a schematic diagram of a optical system employing a concave lens with a central reflector dot. 
       FIG. 3  is a graph of normalized detected beam intensity versus beam angle for one example of a dome incorporating a partially reflective area  214  having reflection coefficients (R) of 0.75 and 0.95 (taking into account typical detector design and efficiency). 
       FIG. 4  is one non-limiting example of a timing diagram for adjusting gain and/or detector sensitivity as described herein. 
       FIG. 5  is a control diagram showing processes for controlling detector gain and sensitivity in accordance with one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Before explaining the several embodiments of the present invention in detail, it should be noted that each embodiment is not limited in its application or use to the details of construction and arrangement of parts and steps illustrated in the accompanying drawings and description. The illustrative embodiments of the invention may be implemented or incorporated in other embodiments, variations and modifications, and may be practiced or carried out in various ways. Furthermore, unless otherwise indicated, the terms and expressions employed herein have been chosen for the purpose of describing the illustrative embodiments of the present invention for the convenience of the reader and are not for the purpose of limiting the invention. 
   It is further understood that any one or more of the following-described embodiments, examples, etc. can be combined with any one or more of the other following-described embodiments, examples, etc. 
   In accordance with certain embodiments of the invention, scanner  108  is a MEMS scanner. MEMS scanners can be designed and fabricated using any of the techniques known in the art as summarized in the following references: U.S. Pat. Nos. 6,140,979, 6,245,590, 6,285,489, 6,331,909, 6,362,912, 6,384,406, 6,433,907, 6,512,622, 6,515,278, 6,515,781, and/or 6,525,310, all hereby incorporated by reference. In one embodiment, the scanner may be a magnetically resonant scanner as described in U.S. Pat. No. 6,151,167 of Melville, or a micromachined scanner as described in U.S. Pat. No. 6,245,590 to Wine et al. 
   In accordance with one embodiment of the invention, a scanning beam assembly of the type described in U.S. Published application 2005/0020926A1 is used. 
   The assembly is constructed with a detector having adjustable gain or sensitivity or both. In one embodiment, the detector may include a detector element that is coupled with a means for adjusting the signal from the detector element such as a variable gain amplifier. In another embodiment, the detector may include a detector element that is coupled to a controllable power source. In still another embodiment, the detector may include a detector element that is coupled both to a controllable power source and a variable gain or voltage controlled amplifier. Representative examples of detector elements useful in certain embodiments of the invention are photomultiplier tubes (PMT&#39;s), charge coupled devices (CCD&#39;s), photodiodes, etc. 
   In accordance with one embodiment of the invention, the detector gain and/or sensitivity is adjusted to compensate for those areas in the FOV in which the SNR is low. More particularly,  FIG. 4  is a chart showing detector sample timing wherein the times/positions at which the scanned beam intersects the partial reflector (A) and the wider beam angles (E) are respectively designated on Cartesian coordinates. In accordance with one embodiment, the assembly includes a controller that increases the sensitivity of the detector to compensate for the SNR when the beam intersects areas A. In certain embodiments, the scanning beam assembly may not include the partial reflector  214  and hence areas A will not be present in the field of view. For example, those skilled in the art will recognize that optical paths can be designed in which the beam optics are not located within the field of view thereby obviating the need for the partial reflector  214 . In these embodiments, detector adjustments would not be necessary to compensate for the noise levels associated with the partial reflector  214 . 
   In accordance with one embodiment, the assembly includes a controller that increases the sensitivity of the detector when the beam intersects areas A. In accordance with another embodiment, the assembly includes a controller that increases the gain of the detector when the beam intersects areas E. In one embodiment of the invention, areas E correspond to beam excursions of greater than about 25 degrees and in a more particular embodiment, areas E correspond to excursions greater than about 40 degrees and in a still more particular embodiment, areas E correspond to beam excursions greater than about 50 degrees. More particularly, with regard to areas E, when the scanner in includes a partially reflective surface  214 , the beam angle at which it may be advantageous to increase the gain will be a function of the reflection coefficient of surface  214  as is apparent from  FIG. 3 . In one embodiment using a reflective surface  214  having a reflective coefficient of 75%, increasing the gain may not be required until the beam excursions is greater than 40° and in another embodiment using a reflective surface having a reflective coefficient of 95%, increasing the gain may not be required until beam excursion is greater than 50°. In still another embodiment, the controller adjusts the gain and/or sensitivity of the detector when the beam interrogates points in areas A and areas E. 
     FIG. 5  is a control diagram illustrating one embodiment in which the controller  118  is operatively connected to the detector  116  to modify the detector gain through control ports labeled Sensitivity  501  and Gain  502 . The detector  116  includes a detector element  500  that generates a signal in response to radiation. In the illustrated embodiment, the sensitivity port  502  is operably connected to a controllable power source such as a Voltage Controlled Voltage Source (VCVS  504 ). In one embodiment the sensitivity control port  502  employs analog signaling. In another embodiment, the sensitivity control port  502  employs digital signaling. The gain port  501  is operably connected to a voltage controlled amplifier (VCA  506 ). In one embodiment, the gain control port  501  employs analog signaling. In another embodiment, the gain control port  501  employs digital signaling. Based on the current scan path, controller  118  apportions detector gain settings to the sensitivity and gain control ports. The controller  118  must be able to update settings during each detector sample period or during a small number of temporally contiguous sample periods. In one embodiment, a standard lookup table suitably sized to match the number of detector samples taken per fame is used to drive the sensitivity and gain ports  501  and  502 . The lookup table could be pre-calculated for each point in the FOV. To account for manufacturing tolerances, the look-up tables may be customized for each interchangeable scanning mirror device. The output from the VCA  506  in the illustrated embodiment is processed through an analog-to-digital converter  508  to provide the digital output  510 . In one embodiment, the output  510  may be processed by an image processor to produce an image of the field of view. In another embodiment, the output  510  is not necessarily processed to form an image but may be fed to a controller to control directly a therapeutic treatment such as a laser. See, for example, U.S. application Ser. No. 11/615,140 (Attorney&#39;s docket END5904). 
   As one non-limiting illustration of the control function where the partially reflective area has R=0.75, 80% of the illumination incident on the scene is captured by the detector system for a central beam (angle=0). To compensate, the effective gain of the detector system can be adjusted to 1.25 (e.g., 1/0.8) in the areas designated A in  FIG. 4 . On the other hand, the edge of the FOV, for example the areas E in  FIG. 4 , both reflectivities (R=0.75 and R=0.90) yield intensities of about 70%. For this case the gain can be set to about 1.4 (e.g., 1/0.70). 
   There are, according to  FIG. 5 , two controls for gain in the system. One control referred to as Sensitivity, the other referred to as Gain. In a particular detector, an APD or Avalanche Photo Diode, sensitivity can be controlled by the applied bias voltage (controlled by the VCVS in  FIG. 5 ). This type of gain control is relatively slow. In one embodiment, this control can best be used to adjust the gain or “brightness level” of the overall image, not the areas A and E under discussion. Another method to control the gain is to provide a Voltage Controlled Amplifier (sometimes referred to as a Variable Gain Amplifier) just prior to sending the detector output to the A/D converter (ADC in  FIG. 5 ). These circuits have extremely rapid response and can be used to change the gain many times during a single oscillation of the scanning mirror. 
   It will be understood that the foregoing description is provided by way of example, and that other modifications and variations there may be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.