Patent Publication Number: US-2020301016-A1

Title: Portable panoramic laser mapping and/or projection system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a division of U.S. patent application Ser. No. 14/747,832, filed Jun. 23, 2015 and entitled “Portable Panoramic Laser Mapping and/or Projection System,” the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     With new developments in areas such as self-driving cars, computer animation, 3D printing, and construction, there is an ever increasing demand for the ability to accurately map an environment (e.g., a person&#39;s surroundings) more quickly and/or at lower cost. Lasers are often used to map an environment. However, conventional laser mapping systems typically measure the distance to each point in the environment thousands of times in order to average out noise. Such repetitive measuring may substantially increase an amount of time that is consumed to generate a map and/or a cost associated with generating the map. Moreover, conventional laser mapping systems usually consume a substantial amount of power (e.g., tens of watts) to measure the distances to the points, which are used to generate a map. The relatively high power consumption of such conventional systems may result in a relatively high cost. The relatively high cost and/or time consumption associated with conventional laser mapping systems may render those systems unsuitable for some applications. 
     SUMMARY 
     Various approaches are described herein for, among other things, forming a depth map and/or projecting an image onto object(s) based on the depth map. A depth map is a three-dimensional representation of an environment. Forming a depth map may involve scanning a beam of laser light from a central reference location over a grid of points within an environment. For example, at each point within a grid of points, locating information such as distance and velocity is measured. During each measurement, the point being measured is referred to as the current point. Determining distance and/or velocity from the locating information at the current point may utilize a progressive resolution refinement (PRR) technique. In accordance with this example, the locating information may be coordinated with the scan to form the depth map. 
     An example portable panoramic laser mapping system is described. The portable panoramic laser mapping system includes a depth measurement subsystem, a microelectromechanical systems-based (MEMS-based) scanning subsystem, and a controller. The depth measurement subsystem is configured to measure a distance between a reference location and a current point. The depth measurement subsystem includes a laser source, splitting optics, a light detecting structure, and a signal processing circuit. The laser source is configured to generate coherent light. The coherent light is capable of being modulated. The splitting optics are configured to create a reference beam of light and a detection beam of light from the coherent light. The light detecting structure is configured to convert the reference beam and a reflected detection beam into electrical signals. The reflected detection beam results from reflection of the detection beam from the current point. The signal processing circuit is optionally configured to determine locating information based on the electrical signals in accordance with a progressive resolution refinement technique. The locating information indicates the distance between the reference location and the current point. The MEMS-based scanning subsystem includes mirror(s) and a light redirecting element that has a microelectromechanical structure. The microelectromechanical structure is configured to perform a scan of the current point within a field of view using the mirror(s). The controller is configured to coordinate the locating information with the scan of the current point over the field of view to form a depth map. 
     An example method of adapting a pixel size and/or a measurement resolution on a pixel-by-pixel basis is described. In accordance with this method, a laser is used to generate an emission of coherent light. The emission is split into a reference beam of light and a detection beam of light. A scan is performed. The scan comprises a series of distance measurements using the detection beam as the detection beam is scanned over a line or over an area. A range of frequencies and/or a period of time over which the emission is modulated during the scan is altered for a subset of the distance measurements in the scan. 
     In an aspect of this method, a plurality of operations may be performed for each distance measurement in the scan. For instance, the plurality of operations may include modulating the emission over the range of frequencies and over the period of time. The plurality of operations may include orienting the detection beam toward a point on an object. The plurality of operations may include reflecting the detection beam off of the point on the object to provide a reflected detection beam. The plurality of operations may include combining the reference beam and reflected detection beam on a detector to produce an electrical signal. The electrical signal has a beat frequency. The plurality of operations may include signal processing the electrical signal to determine the beat frequency. The beat frequency is a measurement of a distance to the point on the object. 
     An example method of performing progressive resolution refinement is described. In accordance with this method, a first measurement with a relatively low resolution is performed using an electrically modulated laser source. The first measurement is processed electrically to determine low-resolution locating information. A second measurement with a relatively high resolution is performed. The second measurement is processed electrically using the low-resolution locating information to enable the processing of the second measurement to determine high-resolution locating information. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Moreover, it is noted that the invention is not limited to the specific embodiments described in the Detailed Description and/or other sections of this document. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate embodiments of the present invention and, together with the description, further serve to explain the principles involved and to enable a person skilled in the relevant art(s) to make and use the disclosed technologies. 
         FIG. 1  is a block diagram of an example portable panoramic laser mapping and/or projection system in accordance with an embodiment described herein. 
         FIG. 2  depicts an example implementation of a depth measurement subsystem shown in  FIG. 1  in accordance with an embodiment described herein. 
         FIG. 2 a    depicts an example implementation of a voltage-controlled oscillator (VCO) architecture in accordance with an embodiment described herein. 
         FIG. 2 b    depicts an example implementation of a phase-locked loop (PLL) architecture in accordance with an embodiment described herein. 
         FIG. 2 c    depicts an example implementation of an amplitude modulated continuous wave (AMCW) architecture in accordance with an embodiment described herein. 
         FIG. 3  depicts an example implementation of a microelectromechanical systems-based (MEMS-based) scanning subsystem shown in  FIG. 1  in accordance with an embodiment described herein. 
         FIG. 4  depicts an example implementation of a microelectromechanical structure shown in  FIG. 3  in accordance with an embodiment described herein. 
         FIG. 5  depicts a flowchart of an example method for adapting a pixel size and/or a measurement resolution on a pixel-by-pixel basis in accordance with an embodiment described herein. 
         FIG. 6  depicts a flowchart of an example method for performing a scan in accordance with an embodiment described herein. 
         FIGS. 7-9  depict flowcharts of example methods for performing progressive resolution refinement in accordance with embodiments described herein. 
         FIG. 10  depicts a flowchart of an example method for using a depth mapping apparatus in accordance with an embodiment described herein. 
         FIG. 11  is a block diagram of a computing system that may be used to implement various embodiments. 
     
    
    
     The features and advantages of the disclosed technologies will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION 
     I. Introduction 
     The following detailed description refers to the accompanying drawings that illustrate exemplary embodiments of the present invention. However, the scope of the present invention is not limited to these embodiments, but is instead defined by the appended claims. Thus, embodiments beyond those shown in the accompanying drawings, such as modified versions of the illustrated embodiments, may nevertheless be encompassed by the present invention. 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” or the like, indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Furthermore, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the relevant art(s) to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     II. Example Embodiments 
     Example embodiments described herein are capable of forming a depth map and/or projecting an image onto object(s) based on the depth map. A depth map is a graphical three-dimensional representation of an environment. Forming a depth map may involve scanning a beam of laser light from a central reference location over a grid of points within an environment. For example, at each point within a grid of points, locating information such as distance and velocity is measured. During each measurement, the point being measured is referred to as the current point. Determining distance and or velocity from the locating information at the current point may utilize a progressive resolution refinement (PRR) technique. In accordance with this example, the locating information may be coordinated with the scan to form the depth map. 
     Example techniques described herein have a variety of benefits as compared to conventional laser mapping techniques and conventional laser projection techniques. For instance, the example mapping techniques may be capable of accurately mapping an environment more quickly and/or at lower cost than conventional laser mapping techniques. As an illustration, the example mapping techniques may take one or two measurements per point in in the environment; whereas, conventional techniques may take thousands of measurements per point. The example mapping techniques may reduce an amount of time that is consumed to generate a map and/or a cost associated with generating the map, as compared to conventional laser mapping techniques. The example mapping techniques may require less (e.g., substantially less) laser power output than conventional laser mapping techniques. For example, the example mapping techniques may require less than 1 Watt of laser output power; whereas, the conventional laser mapping techniques may require tens of watts. In an aspect of this example, the example mapping techniques may output less than 1 watt (W) while maintaining a detectable signal at a target distance of at least 200 m, 300 m, 400 m, or 500 m. The example projection techniques may be capable of modifying an image that is to be projected on object(s) in the field of view within a range of 0.1-10 m to compensate for variations in surface(s) of the object(s). 
     The example portable panoramic laser mapping systems described herein may be characterized by a relatively low manufacturing cost. For instance, the described portable panoramic laser mapping systems may be made using commoditized lasers. The portable panoramic laser mapping systems may combine a depth measurement subsystem, a microelectromechanical systems-based (MEMS-based) scanning subsystem, and a controller into a single portable package. In an example implementation, a laser projection subsystem also may be included in the single portable package. The depth measurement subsystem, the MEMS-based scanning subsystem, and/or the laser projection subsystem may share circuitry, thereby further reducing the manufacturing cost of the described portable panoramic laser mapping systems. 
     The example portable panoramic laser mapping systems may be usable in more applications than conventional laser mapping systems and conventional laser projection systems. For instance, combining laser mapping functionality (e.g., mapping surroundings) and laser projection functionality (e.g., projecting images overlaid on the mapped surroundings) within a portable panoramic laser mapping system enables the portable panoramic laser mapping system to be used in applications beyond those in which a laser mapping system or a laser projection system alone may be used. For example, the device may be used in a construction application in which images of proposed remodeling designs, hidden facilities, or drill patterns or illumination to install equipment, such as an HVAC aperture, in a structure are overlaid on objects (e.g., walls) inside the structure. In another example, the device may be used to take a relatively detailed 3D scan of a small object (such as a coffee mug) and print meta-information on the object or in a vicinity of the object to indicate how to order another one or more of the object, and further optionally capture feedback from a user via gesture or other detected motion within the device&#39;s field of view. 
     In order to achieve a lowest-cost design point without sacrificing performance, the example techniques described herein may provide improvements in multiple areas, as compared to conventional techniques. For instance, through the use of a progressive resolution refinement technique, the example techniques may reduce complexity and/or cost of signal processing, as compared to conventional techniques. Through the use of flexural-based MEMS systems, the example techniques may achieve a substantially lower scanning cost than conventional techniques. The example techniques may utilize higher-performance MEMS materials to achieve relatively wide scan angles. 
       FIG. 1  is a block diagram of an example portable panoramic laser mapping and/or projection system  100  (hereinafter “system  100 ”) in accordance with an embodiment described herein. Generally speaking, system  100  operates to perform laser mapping and/or projection. For example, system  100  may perform laser mapping to form a depth map, which is a three-dimensional representation of an environment. In another example, system  100  may perform laser projection to project an image onto object(s) based on such a depth map. It will be recognized that projection (a.k.a. image projection or laser projection) as described herein is a generalization of projecting any visible two-dimensional information, including but not limited to a single still image, multiple still images, video data, and/or partially or fully synthesized visible information (e.g., augmented reality). Detail regarding techniques for performing laser mapping and/or projection is provided in the following discussion. 
     As shown in  FIG. 1 , system  100  includes a depth measurement subsystem  102 , a microelectromechanical systems-based (MEMS-based) scanning subsystem  104 , a laser projection subsystem  106 , a controller  108 , and a reference fiber optic loop  110 . Depth measurement subsystem  102  is configured to measure a distance between a reference location  132  and a current point  134  in a field of view  130 . For instance, depth measurement subsystem  102  provides a detection beam  126 , which is split from coherent light, to MEMS-based scanning subsystem  104  so that MEMS-based scanning subsystem  104  may provide the detection beam  126  from the reference location  132  to the current point  134 . Depth measurement subsystem  102  receives a reflected detection beam  128 , which results from reflection of the detection beam  126  from the current point  134  at a surface of an object  112 . Depth measurement subsystem  102  compares a representation of the reflected detection beam  128  and a representation of a reference beam, which is split from the coherent light, to determine locating information. The locating information includes a measurement of the distance between the reference location  132  and the current point  134 . 
     Depth measurement subsystem  102  is capable of modulating the coherent light from which the detection beam  126  and the reference beam are split. For example, depth measurement subsystem  102  may modulate the coherent light based on a modulation signal  118  that is received from controller  108 . In accordance with this example, the modulation signal  118  may indicate a type of modulation (e.g., amplitude modulation or frequency modulation) to be applied to the coherent light and/or a manner in which such modulation is to be applied (e.g., the amplitudes and/or frequencies to be used). Depth measurement subsystem  102  may provide a measurement signal  122  to controller  108 . For instance, the measurement signal  122  may include information regarding the distance between the reference location  132  and the current point  134 . 
     Depth mapping using coherent light can take any of a variety of forms in amplitude and/or frequency modulation techniques. Given the superior noise rejection capability and reduced issues with multiple reflections compared to amplitude modulation techniques, the discussion herein is focused more on frequency modulation techniques. However, it will be recognized that the embodiments described herein may utilize any suitable amplitude and/or frequency modulation techniques. Some example techniques for achieving Frequency Modulated Continuous Wave (FMCW) depth mapping are described in U.S. Pat. No. 4,611,912 to Falk et al. and U.S. Pat. No. 4,830,486 to Goodwin, both of which are incorporated herein by reference in their entireties. 
     MEMS-based scanning subsystem  104  is configured to scan the current point  134  over the field of view  130 . During the scan, MEMS-based scanning subsystem  104  provides the detection beam  126  from the reference location  132  to the current point  134 , causing the reflected detection beam  128  to be reflected toward depth measurement subsystem  102 . The detection beam  126  travels a distance D before coming into contact with object  112  at the current point  134 . 
     Laser projection subsystem  106  is configured to project an image onto object(s), such as object  112 , by raster scanning a beam of modulated laser light typically sourced from the combination of one to three visible laser outputs. U.S. Pat. No. 8,416,482 to Desai et al., the entirety of which is incorporated herein by reference, presents such a projection system. The combined output is referred to as visible light  124 . Laser projection subsystem  106  may project the image in response to receiving a modification signal  114  from controller  108 . For example, the modification signal  114  may include a modified version of the image. In another example, the modification signal  114  may include attribute(s) and/or instructions for the laser projection subsystem  106  to modify the image prior to projection of the image onto the object(s). For instance, the image may be modified to compensate for variations in distances between the reference location  132  and the surface(s) of the object(s). 
     Controller  108  is configured to coordinate the locating information with the scan of the current point over the field of view  130  to form a depth map  138 . Controller  108  is shown in  FIG. 1  to include a store  136  for storing the depth map  138  for illustrative purposes and is not intended to be limiting. It will be recognized that controller  108  need not necessarily include a store  136 . 
     Controller  108  may be further configured to control depth measurement subsystem  102 , MEMS-based scanning subsystem  104 , and/or laser projection subsystem  106 . For instance, controller  108  may control any one or more of the aforementioned subsystems  102 ,  104 , and  106  based on measurement  122 . In one example implementation, controller  114  generates the modification signal  114  in response to receipt of measurement  122  from depth measurement subsystem  102 . For example, controller  108  may generate the modification signal  114  to accommodate the distance between the reference location  132  and the current point  134 , as reflected by measurement  122 . In another example implementation, controller  108  controls MEMS-based scanning subsystem  104  using control signal  116 . For instance, controller  108  may use the control signal  116  to control a rate at which MEMS-based scanning subsystem scans the current point  134  over the field of view  130 . In another example implementation, controller  108  controls depth measurement subsystem  102  using progressive resolution refinement control signal  140 . For instance, controller  108  may use the progressive resolution refinement control signal  140  to control the electronic signal processing of the locating information associated with the current point  134 . 
     Controller  108  may be configured to calibrate depth measurement subsystem  102 . For example, controller  108  may be configured to calibrate depth measurement subsystem  102  using a measurement of the distance from the reference location  132  to a reference object (e.g., object  112 ) in the field of view  130 . In accordance with this example, the distance from the reference location  132  to the reference object in the field of view  130  is a known distance. For instance, the distance from the reference location  132  to the reference object may be known prior to the measurement of the distance from the reference location  213  to the reference object being taken. 
     In another example, controller  108  uses reference fiber optic loop  110  to calibrate depth measurement subsystem  102 . In accordance with this example, controller  108  calibrates depth measurement subsystem  102  using a measurement of the distance through reference fiber optic loop  110 . In further accordance with this example, the distance through reference fiber optic loop  110  is a known distance. For instance, the distance through reference fiber optic loop  110  may be known prior to the measurement of the distance through reference fiber optic loop  110  being taken. The distance through reference fiber optic loop  110  may be measured simultaneously with the measurement of the distance from the reference location  132  to each current point  134  in the field of view  130 , though the scope of the example embodiments is not limited in this respect. 
     Controller  108  may calibrate depth measurement subsystem  102  once per N measurements of the current point  134  in the field of view  130 , N times per linear scan of the current point  134  in the field of view  130 , N times per scan of the entire field of view  130 , once per N scans of the entire field of view  130 , etc. N is an integer (e.g., a predetermined integer), such as 1, 2, 3, 4, or 5. 
     In an example embodiment, controller  108  utilizes the depth map  138  to provide a modified image. In accordance with this embodiment, laser projection subsystem  106  is configured to generate the visible light  124  for projecting the modified image onto object(s), such as the object  112 . In an aspect of this embodiment, laser projection subsystem  106  may use a light redirecting element in MEMS-based scanning subsystem  104  that is configured to perform the scan of the current point  134  over the field of view  130  to project the modified image onto the object(s). In another aspect of this embodiment, MEMS-based scanning subsystem  104  may further include a second light redirecting element, which is different from the light redirecting element configured to perform the scan of the current point  134  over the field of view  130 . In accordance this this aspect, laser projection subsystem  106  uses the second light redirecting element in MEMS-based scanning subsystem  104  to project the modified image onto the object(s). 
     Controller  108  may be configured to determine velocity of at least one point in the field of view  130  based on the locating information. For example, controller  108  may be configured to determine a gesture based on velocities of at least two points in the field of view  130 . In an aspect of this example, controller  108  may determine the gesture based on a relative velocity between the at least two points. Examples of a gesture include but are not limited to a hand being waved and a finger being pointed. Controller  108  may be configured to determine that an object is moving relative to the system  100  and/or a rate at and/or a direction in which an object is moving relative to system  100 . 
     It will be recognized that system  100  may not include one or more of depth measurement subsystem  102 , MEMS-based scanning subsystem  104 , laser projection subsystem  106 , controller  108 , and/or reference fiber optic loop  110 . Furthermore, system  100  may include components in addition to or in lieu of depth measurement subsystem  102 , MEMS-based scanning subsystem  104 , laser projection subsystem  106 , controller  108 , and/or reference fiber optic loop  110 . 
       FIG. 2  is a block diagram of an example depth measurement subsystem  200  in accordance with an embodiment described herein. Depth measurement subsystem  200  is an example implementation of a depth measurement subsystem  102  shown in  FIG. 1 . Depth measurement subsystem  200  includes a laser source  202 , splitting optics  204 , a light detecting structure  206 , progressive resolution refinement (PRR) circuitry  220 , and a signal processing circuit  210 . 
     Laser source  202  is configured to generate coherent light  244 . For instance, the coherent light  244  may be an infrared laser with emission wavelengths between 800 nm-2000 nm. In accordance with this example, the infrared laser may have a wavelength of 850 nanometers (nm), 940 nm, 1310 nm, 1550 nm, or any other suitable value. For instance, wavelengths from 1300 to 2000 nm may provide reduced absorption and scattering from dust. The output power of laser source  202  may be less than 100 milliwatts (mW) for mapping regions of 10 m or less. For regions in excess of 10 m, higher powers may be needed to achieve sufficiently high reflected signals for determining locating information. 
     Laser source  202  is capable of modulating the coherent light  244 . For instance, laser source  202  may modulate the coherent light  244  in frequency and/or amplitude. Laser source  202  may modulate the coherent light  244  in response to (e.g., based on) receipt of the modulation signal  118 , though the scope of the example embodiments is not limited in this respect. By modulating a current supply to laser source  202 , the wavelengths of the coherent light  244  can be swept anywhere from thousandths of a nanometer to multiple nanometers. The sweep in wavelength can produce large changes in the optical emission frequency. As an example, tenths of a nanometer corresponds to several gigahertz (GHz) changes in optical emission frequency. 
     In the case of frequency modulation, the modulation signal  118  may be swept over a linear saw-tooth profile with a period in a range of 5 nanoseconds (ns) to 500 milliseconds (ms). Frequency modulation changes in a range of 150 MHz to 150 GHz may be utilized depending on the speed and range resolution that are needed. 
     There are many ways to modulate amplitude and/or frequency of a laser&#39;s emission. For that reason, we refer to the combination of the laser and the modulator as the laser source  202  herein for the purpose of discussion. An example for frequency modulation would be a distributed feedback laser (DFB) diode laser powered by a current source. By linearly ramping the current source output in time, the frequency of the laser&#39;s emission can be linearly modulated. Varying the temperature of a diode laser is yet another way to modulate a diode laser&#39;s emission frequency, though it may not be well suited for the time constants associated with depth mapping. An example for amplitude modulation would be a diode laser followed by an optical chopper. 
     Splitting optics  204  are configured to create a reference beam  246  of light and the detection beam  126  of light from the coherent light  244 . For instance, splitting optics  204  may collimate and optically split the coherent light  244  to create the reference beam  246  and the detection beam  126 . Accordingly, splitting optics  204  may include collimation optics, a splitter to split the coherent light  244 , and one or more polarizing filters for altering the detection beam  126  and/or the reference beam  246  for proper interaction between the reflected detection beam  128  and the reference beam  246  at a surface of light detecting structure  206 . 
     Light detecting structure  206  is configured to convert the reference beam  246  and the reflected detection beam  128  into electrical signals. The reflected detection beam  128  results from reflection of the detection beam  126  from the current point  134 , as shown in  FIG. 1 . The electrical signals may include a beat signal  248 . For instance, when the reference beam  246  and the reflected detection beam  128  combine at light detecting structure  206 , the beat signal  248  is produced. The beat signal  248  is an electrical result of optical mixing of the reference beam  246  and the reflected detection beam  128  at a surface of light detecting structure  206 . 
     In an example embodiment, depth measurement subsystem  200  is configured to perform Frequency Modulated Continuous Wave (FMCW) depth mapping with a linear ramp in frequency over an interval Δf, referred to as a chirp frequency excursion. In accordance with this embodiment, the beat signal  248  has a beat frequency, which represents a measurement of the distance between the reference location  132  and the current point  134 . The beat frequency is directly proportional to the distance D traveled by the detection beam  126 , as shown in  FIG. 1 , in accordance with the following equation: 
     
       
         
           
             
               
                 
                   
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     where D is the distance from the reference location  132  to the current point  134 ; c is the speed of light; and T is the duration of the linear frequency ramp (i.e., “chirp period”). 
     FMCW signals facilitate the determination of both the distance between the reference location  132  and each current point  134  in the field of view  130 , and the speed of a point in the field of view  130  as it moves relative to the reference location  132  due to the Doppler effect. To carry out both distance and speed measurements, a saw-tooth profile with rising and falling linear ramps in frequency may be used. Two beat frequencies may be created whose average and difference can be used to compute both relative speed (e.g., velocity) of an object (e.g., object  112 ) and distance to the object. FMCW allows ranging with resolution proportional to the bandwidth (Δf/ΔT) within the pulse window, allowing range to be determined with a single pulse per point in the field of view  130 . 
     Light detecting structure  206  may be configured in many ways. For example, light detecting structure  206  may be mounted adjacent to MEMS-based scanning subsystem  104  and may receive light from the field of view  130 . In another example, light detecting structure  206  may be positioned in the optical path of splitting optics  204  and may receive only light from the field of view  14  that passes back through MEMS-based scanning subsystem  104 . In yet another example, light detecting structure  206  may be integrated onto MEMS-based scanning subsystem  104  as part of a composite mirror system through a layer transfer process. In accordance with this example, the medium of light detecting structure  206 , which may be specially designed, may be bonded to MEMS-based scanning subsystem  104  for both mechanical and electrical connection. 
     Light detecting structure  206  may be made out of any of a variety of types of devices, depending on the application and the wavelengths of the coherent light  244  being used for depth measurement. Example devices that may be used to make light detecting structure  206  include but are not limited to an avalanche photodiode, a Metal-Semiconductor-Metal Schottky photodiode, a photoconductive switch, and an ultra-fast p-i-n photodiode. 
     Equation 1 reveals the proportional relationship between the chirp frequency excursion and the beat frequency. As an example, consider the case of a 10 m distance, a 20 GHz chip range, and a chirp duration of 1 microseconds (μs). This set of conditions would produce a 1.33 GHz beat frequency. Although it is possible to measure such a frequency, electronics used to process signals below 500 MHz typically are much less expensive. To reduce the signal processing requirements, one could reduce the chirp frequency excursion to 2 GHz; however, there is a resulting penalty in range resolution according the following equation: 
     
       
         
           
             
               
                 
                   
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     where δR is the range resolution; Δf is the chirp frequency excursion; and c is the speed of light. It can be seen from Equation 2 that a 10×reduction in chirp frequency excursion results in a 10×increase in the minimum range resolution. If in the example above, the chirp frequency excursion were decreased to 2 GHz, the resulting beat frequency would be decreased to 133 MHz; however, the range resolution would be negatively impacted by a factor of 10. The progressive resolution refinement technique was designed to achieve the higher range resolution depth maps, but at a substantially lower cost and/or complexity of system  100 . 
     A progressive resolution refinement technique is a technique in which a first (e.g., relatively lower-resolution) measurement is determined, and a second (e.g., relatively higher-resolution) measurement is determined utilizing the first measurement. By utilizing the first measurement to perform the second measurement, the cost and/or complexity of system  100  may be substantially lowered. 
     For example, in one embodiment of progressive resolution refinement, a first measurement may be performed where modulation signal  118  modulates laser source  202  to produce a 2 GHz chirp frequency excursion, which as shown in the prior example, results in a beat signal  248  having a 133 MHz beat frequency for an example current point with a 10 m distance from the reference location  132 . Processing of the locating information from the beat signal  248  starts with progressive resolution refinement block  220  shown in  FIG. 2 , and detailed in  FIG. 2   a.  Referring to  FIG. 2   a,  for the first measurement, switch  261  would be opened and switch  262  would be closed in order to bypass high frequency signal mixer  240  and feed directly into signal processing circuit  210 . Signal processing circuit  210  may include element  241 , which includes an analog-to-digital conversion circuit coupled with a digital-signal-processing (i.e., DSP) block that carries out a fast Fourier transform (i.e., FFT), to determine the value of the beat frequency in the beat signal  248 . In another embodiment, signal processing circuit  210  may include a phased-locked loop (i.e., PLL) circuit  242  to determine a measure of the beat frequency in the beat signal  248 , a measure being the voltage required to lock the phased locked-loop&#39;s internal voltage controlled oscillator on the beat frequency. It will be recognized that depth measurement subsystem  200  need not necessarily include element  241  and/or PLL circuit  242 . 
     A second measurement using the progressive resolution refinement technique may be performed where modulation signal  118  modulates laser source  202  to produce a 20 GHz chirp frequency excursion, which as shown in the prior example, results in a beat signal  248  having a 1.33 GHz beat frequency for an example current point with a 10 m distance from the reference location  132 . In this second measurement, switch  261  would be closed and switch  262  would be opened in order to send the beat signal with the higher beat frequency through high frequency signal mixer  240 . Based on the results of the first measurement, controller  108  may send progressive resolution refinement control signal  140  to select a voltage-controlled oscillator (VCO) signal  263  from a VCO clock tree  264 . When VCO signal  263  is mixed with beat signal  248  using high frequency signal mixer  240 , low frequency beat signal  250  results. Low pass filter (i.e. LPF)  290  is used to filter out other signal products of signal mixer  240  not related to the low frequency beat signal  250 . Low frequency beat signal  250  has a beat difference frequency equal to the difference between the frequency of the VCO signal  263  and the beat frequency of the beat signal  248 . Beat difference frequency of low frequency beat signal  250  is a measure of the distance to the current point  134 . Signal processing circuit  210  may be used to construct measurement  122  from the beat difference frequency of low frequency beat signal  250 . For example, signal processing circuit  210  may use the aforementioned PLL or FFT circuits to determine the value of the beat difference frequency of low frequency beat signal  250 . 
     VCO clock tree  264  may include a progression of VCO clock signals, wherein the difference in frequency between any two successive VCO clock signals are within the signal processing capabilities of signal processing circuitry  210 . Based on the results of the first measurement, controller  108  may have sufficient information to predict the expected beat frequency of beat signal  250  during the second measurement and thereby select the appropriate VCO clock signal  263  to produce low frequency beat signal  250  that is within the signal processing capabilities of signal processing circuitry  210 . 
     The same process may be used for all the other pixels within the field of view. For example, the locating information for each pixel may be determined in a first (e.g., relatively lower-resolution) measurement to bin the approximate distance for each pixel and then using the approximate distance from the first measurement, perform a second (e.g., relatively high-resolution) measurement to obtain more accurate and/or precise locating information. 
     In another embodiment of progressive resolution refinement, the phased locked loop architecture  270  shown in  FIG. 2 b    is used. Referring to  FIG. 2   b,  in order to determine locating information from the beat signal  248 , the beat signal  248  is fed into a PLL circuit  271 . Controller  108  may send progressive resolution refinement control signal  140  to PLL circuit  271  in order to set the center frequency of the phased locked loop such that the PLL circuit  271  can lock on the beat frequency of the beat signal  248 . Without high expense or complexity, for a single fixed center frequency, it is unlikely that PLL circuit  271  could be designed to span relatively high and relatively low resolution measurements over ranges of 50 m, 100 m, 200 m, 300 m, 400 m, or 500 m. In such examples, beat frequencies may need to be measureable from 10 MHz to 50 GHz. High frequency PLL circuits exist, but the range of frequencies over which the PLL can rapidly lock is typically limited. In an example embodiment of progressive resolution refinement, the phased locked loop architecture  270  does not need to span the entire range. 
     For example, a first measurement may be performed with a relatively low resolution where the modulation signal  118  modulates laser source  202  to produce a 2 GHz chirp frequency excursion, which as shown before, results in a beat signal  248  having a 133 MHz beat frequency for an example current point with a 10 m distance from the reference location  132 . Processing of the locating information from the beat signal  248  starts with the progressive resolution refinement block  220  detailed in  FIG. 2   b.  For the first measurement, controller  108  may provide a progressive resolution refinement control signal  140  to PLL circuit  271  to set a low center frequency thereby enabling PLL circuit  271  to lock onto relatively low beat frequency of the beat signal  248 . Signal processing circuit  210  may include an analog-to-digital conversion circuit to measure the control voltage for the voltage controlled oscillator within PLL circuit  271 , the control voltage being a measure of the beat frequency of the beat signal  248 . Furthermore, the beat frequency provides a measure of the distance to the current point  134 . 
     A second measurement using the progressive resolution refinement technique may be performed where modulation signal  118  modulates laser source  202  to produce a 20 GHz chirp frequency excursion, which as shown in the prior example, results in a beat signal  248  having a 1.33 GHz beat frequency for an example current point with a 10 m distance from the reference location  132 . Based on the results of the first measurement, controller  108  may have sufficient information to predict the expected beat frequency of the low frequency beat signal  250  during the second measurement and thereby select the appropriate PLL center frequency for PLL circuit  271  to enable it to lock on to the beat frequency of the beat signal  248 . Controller  108  may set the appropriate PLL center frequency for PLL circuit  271  using the progressive resolution refinement control signal  140 . Signal processing circuit  210  may include an analog-to-digital conversion circuit to measure the control voltage for the voltage controlled oscillator within PLL circuit  271 , the control voltage being a measure of the beat frequency of the beat signal  248 . Signal processing circuit  210  may be used to construct measurement  122  from the measure of the beat frequency of the beat signal  248 . 
     It will be recognized that depth measurement subsystem  200  need not necessarily include VCO clock tree  264 , signal mixer  240 , and/or phased locked loop architecture  270  to enable the progressive resolution refinement technique. Other circuit topologies to accomplish the measurement task are possible and known to those skilled in the art of circuit design. Furthermore, the first measurement need not be a measurement of the current point  134 . Instead, the first measurement may be an estimate of the distance from the reference location  132  to the current point  134  based on measurements of one or more other points within the field of view (e.g., another point that is adjacent to the current point  134 ). 
     In an example progressive resolution refinement technique embodiment, the first measurement of the distance between the reference location  132  and the current point  134  is used to narrow a frequency range over which the beat frequency is to be searched in the second measurement by more than a factor of two. For example, in one embodiment, if a relatively high resolution second measurement produces a beat frequency of a beat signal  248  of 2.5 GHz, VCO architecture  259  shown in  FIG. 2 a    may produce a low frequency beat signal  250  with a difference beat frequency of 500 MHz, 500 MHz being more than a factor of two reduction from the original 2.5 GHz beat frequency. In another embodiment, for example, if a relatively high resolution second measurement produces a beat frequency of a beat signal  248  of 2.5 GHz, PLL circuit  271  shown in  FIG. 2 b    may be controlled by progressive resolution refinement signal  140  to have a center frequency near 2.5 GHz. Furthermore, PLL circuit  271  may have a frequency lock range of 500 MHz, 500 MHz being a range of frequencies more than a factor of two reduction from the original 2.5 GHz beat frequency. 
     In an aspect of this embodiment, laser source  202  shown in  FIG. 2  is configured to perform a periodic chirp in which the coherent light  244  is modulated (e.g., linearly) in frequency over a chirp period of time and over a chirp frequency excursion. In accordance with this aspect, controller  108  may be configured to adjust the chirp frequency excursion to adapt resolution of the locating information. Controller  108  may be configured to adapt the resolution for a single point within a scan, a subset of the points within a scan, or all of the points within a scan. 
     In one example of this implementation, the current point  134  has a pixel size, which is a distance over which the current point  134  scans during the chirp period. In the MEMS-based scanning subsystem  104  described above with reference to  FIG. 1 , one or more of the axes may be driven at mechanical resonance. Accordingly, if a constant chirp period were used, the effective pixel size may vary throughout the scan due to the sinusoidal motion of the microelectromechanical structure therein. One technique for rectifying this issue is to adapt the chirp period to the angular velocity of the microelectromechanical structure to maintain a constant pixel size or a specified pixel size. According to Equation 1, as a secondary result, the beat frequency of the beat signal  240  will be modified as the chirp period is modified. In accordance with this technique, the progressive range resolution circuitry in VCO architecture  259  shown in  FIG. 2 a    could accommodate this chirp period modification by stretching or compressing the base VCO within the VCO clock tree  264 . In doing so, each VCO signal  263  selected would accommodate a constant range of distances despite varying chirp periods. 
     Other frequency modulation schemes may be used, in addition to or in lieu of the FMCW scheme described above. For instance, Amplitude Modulated Continuous Wave (AMCW) is another scheme in which laser modulation can be utilized to determine both the relative speed of an object and distance to the object. 
     In AMCW, one or more simultaneous carrier signals in the form of fundamental sinusoidal modulation or pulse trains are emitted by laser source  202 . The modulation frequencies are chosen based on an unambiguous range to the object. Multiple frequencies of varying amplitude ratios may be emitted either simultaneously or in a predefined sequence to facilitate enhanced range resolution and determination of relative reflectivity of surfaces of the object. 
     In an embodiment of the AMCW technique, in accordance with the progressive resolution refinement technique, for a first measurement of each point in the entire field of view  130 , each point may first be scanned using a modulation frequency chosen as the maximum frequency for the maximum unambiguous range for which the system is configured to operate. Locating information from this first measurement will necessarily be of lower resolution; however, using the locating information from the first measurement, a second measurement may employ different modulation frequencies either emitted simultaneously or in a predefined sequence to obtain a relatively higher resolution measurement. 
     In both the first measurement and the second measurement, the distance to a given point in the field of view  130  may be given by the instantaneous phase angle of a demodulated representation of the reflected detection beam. Demodulation and determination of the instantaneous phase angle may be accomplished through digital signal processing or through analog homodyne mixing in an I/Q detector. 
     For example,  FIG. 2 c    shows an AMCW architecture  259  in accordance with the progressive resolution refinement technique. As shown in  FIG. 2   c,  multiple changes to system  100  are made to accommodate AMCW compared to FMCW, including but not limited to reference beam  248  becomes reference signal  248 A, which is an electrical representation of the amplitude modulation signal; only reflected detection beam  128  impinges upon light detecting structure  206 ; and beat signal  248  becomes amplitude signal  248 B. 
     Internal to the AMCW architecture  259  is an I/Q detector. Reference signal  248 A provides the reference for multipliers  282  and  283 . After multipliers  282  and  283 , low pass filter (i.e. LPF)  292  and LPF  293  pass the low frequency phase information. On one leg of the I/Q detector, phase delay  281  is used to provide the quadrature signal reference to multiplier  282 . Analog-to-digital converter (i.e., ADC)  284  and ADC  285  digitize the analog signals and provide Q-Data  286  and I-Data  287 , respectively. From Q-Data  286  and I-Data  287 , digital signal processor (DSP)  288  is able to compute the relative phase between the reference signal  248 A and the reflected detection beam  128 , the relative phase being a measure of the distance to the current point  134 . DSP  288  utilizes the first measurement and the second measurement in order to determine the relatively high-resolution locating information contained within measurement  122 . 
     AMCW architecture  259  is shown to include an I/Q detector for illustrative purposes and is not intended to be limiting. It will be recognized that there are a variety of techniques for determining the phase of an AMCW signal. 
       FIG. 3  is a block diagram of an example microelectromechanical systems-based (MEMS-based) scanning subsystem  300  in accordance with an embodiment described herein. MEMS-based scanning subsystem  300  is an example implementation of a MEMS-based scanning subsystem  104  shown in  FIG. 1 . MEMS-based scanning subsystem  300  includes a light redirecting element  302  and mirror(s)  304 . Light redirecting element  302  has a microelectromechanical structure  306  that is configured to perform a scan  352  of the current point  134  over the field of view  130  using the mirror(s)  304 . For instance, microelectromechanical structure  306  may perform the scan  352  in response to receipt of the control signal  116 . Mirror(s) are configured to reflect the detection beam  126  from the reference location  132  to the current point  134  during the scan  352 . 
     The visible light  124  may define an image that is to be projected onto object(s). Microelectromechanical structure  306  may be configured to project the visible light  124  onto the object(s) using the mirror(s)  304 . For instance, microelectromechanical structure  306  may project the visible light  124  onto the object(s) in response to receipt of the control signal  116 . 
     In the presence of moving objects, a Doppler shift may induce a frequency shift in the beat signal that is ambiguous with range determination in the case of a linearly increasing chirp. Rising and falling chirps may be used to cause respective beat frequencies, which may be used to compute both velocity and depth of points in the field of view  130 . The average of the beat frequencies represents the distance D between the reference location  132  and the current pixel  134 , and the difference between the beat frequencies represents the relative velocity between the points. 
       FIG. 4  is a block diagram of an example microelectromechanical structure  400  in accordance with an embodiment described herein. Microelectromechanical structure  400  is an example implementation of a microelectromechanical structure  306  shown in  FIG. 3  in accordance with an embodiment. Microelectromechanical structure  400  includes a frame  402 , a first inner flexure  404   a,  a second inner flexure  404   b,  a first outer flexure  406   a,  a second outer flexure  406   b,  a first frame sensing electrode  408   a,  a second frame sensing electrode  408   b,  a first mirror sensing electrode  410   a,  a second mirror sensing electrode  410   b,  and mirror(s)  412 . 
     Frame  402  provides structural support for inner flexures  404   a - 404   b  and mirror(s)  412 . 
     Inner flexures  404   a - 404   b  and outer flexures  406   a - 406   b  are configured to mount mirror(s)  412 . For instance, inner flexures  404   a - 404   b  are shown to directly mount mirror(s)  412  via direct contact with mirror(s)  412 , and outer flexures  406   a - 406   b  are shown to indirectly mount mirror(s)  412  via indirect contact with mirror(s)  412 , though the scope of the example embodiments is not limited in this respect. Inner flexures  404   a - 404   b  mechanically couple frame  402  to mirror(s)  412 . Inner flexures  404   a - 404   b  enable mirror(s)  412  to rotate about axis  414 , as depicted by arrow  420 . Outer flexures  406   a - 406   b  mechanically couple frame  402  to a substrate  422 . Outer flexures  406   a - 406   b  enable frame  402  to rotate about axis  416 , as depicted by arrow  418 . 
     Mirror(s)  412  is configured to reflect light (e.g., coherent light) that is incident on mirror(s)  412 . The direction in which the light is reflected is based on an extent to which mirror(s)  412  is rotated about axis  414  and an extent to which flame  402  is rotated about axis  416 . 
     Frame sensing electrodes  408   a - 408   b  are configured to sense motion of frame  402 . For instance, frame sensing electrodes  408   a - 408   b  may be configured to sense an extent to which frame  402  rotates clockwise or counterclockwise about the axis  416 . 
     Mirror sensing electrodes  410   a - 410   b  are configured to sense motion of mirror(s)  412 . For instance, mirror sensing electrodes  410   a - 410   b  may be configured to sense an extent to which mirror(s)  412  are rotated clockwise or counterclockwise about the axis  414 . 
     In an example embodiment, inner flexures  404   a - 404   b  and outer flexures  406   a - 406   b  are used in lieu of spinning elements to mount mirror(s)  412 . Accordingly, microelectromechanical structure  400  may not include spinning elements. 
     In another example embodiment, inner flexures  404   a - 404   b  and/or outer flexures  406   a - 406   b  are formed from one or more materials having a fracture toughness of at least 15 MPa(m̂½) and a Young&#39;s modulus of at least 10 Gpa. MPa represents megapascals; GPa represents gigapascals; and m represents meters. 
     In yet another example embodiment, inner flexures  404   a - 404   b  and/or outer flexures  406   a - 406   b  are formed from one or more materials capable of undergoing a strain of two percent without failure. 
     In still another example embodiment, mirror(s)  412  are formed on a substrate material that is different from a material from which inner flexures  404   a - 404   b  and/or outer flexures  406   a - 406   b  are formed. 
     In yet another example embodiment, microelectromechanical structure  400  is configured to pivot at least one mirror (e.g., at least one of mirror(s)  412 ) about one or more axes (e.g., axis  414  and/or axis  416 ) over an optical field of view greater than a threshold angle. For instance, the threshold angle may be 60 degrees, 70 degrees, 80 degrees, or 90 degrees. 
     In still another example embodiment, microelectromechanical structure  400  is configured to pivot at least one mirror (e.g., at least one of mirror(s)  412 ) about one or more axes (e.g., axis  414  and/or axis  416 ) at a frequency greater than a threshold frequency. For instance, the threshold frequency may be 400 Hz, 500 Hz, 600 Hz, 800 Hz, or 1 kHz. 
     Microelectromechanical structure  400  may be configured using a 2-axis mirror system, as shown in  FIG. 4 , or using a pair of single axis mirrors. Selection of the appropriate configuration is based on the application requirements. A 2-axis system may be lower in cost given the reduced assembly requirements; however, a pair of single-axis mirrors may be desirable for a higher performance system given the mirror size and drive frequency requirements. 
     Microelectromechanical structure  400  may be made from silicon (as are most MEMS structures). However, depending on the application requirements which may include large mirrors, higher scan frequencies, and relatively large fields-of-view, higher performance materials such as alloyed titanium may be used in addition to or in lieu of silicon. In one example, if a titanium alloy is used to make inner flexures  404   a - 404   b  and/or outer flexures  406   a - 406   b,  the same sheet of material may be used for forming mirror(s)  412 . Such material may be polished and coated with a second material to increase reflectivity. In another example, if the application&#39;s mirror flatness requirements are relatively high, mirror  412  may be formed on a different substrate than inner flexures  404   a - 404   b  and outer flexures  406   a - 406   b  and later bonded to inner flexures  404   a - 404   b.  One example would be a metal coated piece of silicon bonded to the titanium alloy using a eutectic bond. 
     A field-of-view of 60 degrees implies a peak-to-peak mechanical deflection of 30 degrees. Given the reflection off of a mirror substrate, the optical field of view is twice the mechanical deflection. Peak deflections of greater than 15 degrees are formidable to achieve using MEMS, especially as the frequency requirements exceed 1 kHz. 
     Actuation of microelectromechanical structure  400  could take many forms including electrostatic, Lorentz, or piezoelectric based forcing. For a 2-axis microelectromechanical structure fabricated from silicon, electrostatic actuation may be used given the ease of fabrication, though the scope of the example embodiments is not limited in this respect. U.S. Pat. No. 6,753,638 to Adams et al., the entirety of which is incorporated herein by reference, presents such a 2-axis electrostatically actuated mirror system. 
     The same actuators that were used to drive microelectromechanical structure  400  may be used for sensing the motion of microelectromechanical structure  400 . For instance, frame sensing electrodes  408   a  and  408   b  and/or mirror sensing electrodes  410   a  and  410   b  may be used. Frame sensing electrodes  408   a  and  408   b  may be mounted to the floor below frame  402 . Assuming frame  402  is made of a conductive material, a carrier signal of approximately 100 kHz and 1 volt peak may be applied to frame  402 . Frame sensing electrodes  408   a  and  408   b  may be connected to a differential input trans-impedance amplifier and demodulation circuitry, which are known in the MEMS industry, for sensing the motion of frame  402 . 
     Other example techniques for sensing the motion include optical, Lorentz, piezoelectric, and capacitance-based techniques. For instance, capacitance-based techniques may be relatively simple and have a relatively lower temperature dependence than some other techniques. 
       FIG. 5  depicts a flowchart  500  of an example method for adapting a pixel size and/or a measurement resolution on a pixel-by-pixel basis in accordance with an embodiment described herein. For illustrative purposes, flowchart  500  is described with respect to system  100  shown in  FIG. 1  and depth measurement subsystem  200  shown in  FIG. 2 . Further structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the discussion regarding flowchart  500 . 
     As shown in  FIG. 5 , the method of flowchart  500  begins at step  502 . In step  502 , a laser is used to generate an emission of coherent light. In an example implementation, laser source  202  uses a laser to generate an emission of coherent light  244 . 
     At step  504 , the emission is split into a reference beam of light and a detection beam of light. In an example implementation, splitting optics  204  split the emission into a reference beam  246  of light and a detection beam  126  of light. 
     At step  506 , a scan is performed. The scan comprises a series of distance measurements using the detection beam as the detection beam is scanned over a line or over an area. In an example implementation, MEMS-based scanning subsystem  104  performs the scan. In accordance with this implementation, the scan comprises a series of distance measurements using the detection beam  126  as the detection beam  126  is scanned over a line or over an area. 
     At step  508 , a range of frequencies and/or a period of time over which the emission is modulated during the scan are altered for a subset of the distance measurements in the scan. In an example implementation, laser source  202  alters the range of frequencies and/or the period of time over which the emission is modulated during the scan for the subset of the distance measurements in the scan. 
     In accordance with the embodiment of  FIG. 5 , the pixel size is a distance over which the detection beam is scanned during a measurement. 
     In some example embodiments, one or more steps  502 ,  504 ,  506 , and/or  508  of flowchart  500  may not be performed. Moreover, steps in addition to or in lieu of steps  502 ,  504 ,  506 , and/or  508  may be performed. For instance, in an example embodiment, the method of flowchart  500  further includes altering the period of time to obtain a specified pixel size at each measurement in a scan. In an aspect of this embodiment, the specified pixel size is a constant pixel size over an entirety of the scan. In an implementation of this aspect, altering the period of time to obtain the specified pixel size at each measurement comprises altering a primary clock of a VCO clock tree within a VCO architecture to enable the signal processing architecture to track a change in the pixel size at each measurement. 
     In another example embodiment, the method of flowchart  500  further includes altering the range of frequencies to obtain a specified measurement resolution at each measurement in a scan. 
       FIG. 6  depicts a flowchart  600  of an example method for performing a scan in accordance with an embodiment described herein. Flowchart  600  is described as an example implementation of step  506  shown in  FIG. 5  for purposes of illustration. For instance, the steps of flowchart  600  may be performed for each distance measurement in the scan. For illustrative purposes, flowchart  600  is described with respect to system  100  shown in  FIG. 1  and depth measurement subsystem  200  shown in  FIG. 2 . Further structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the discussion regarding flowchart  600 . 
     As shown in  FIG. 6 , the method of flowchart  600  begins at step  602 . In step  602 , the emission is modulated over the range of frequencies and over the period of time. In an example implementation, laser source  202  modulates the emission over the range of frequencies and over the period of time. 
     At step  604 , the detection beam is oriented toward a point on an object. In an example implementation, MEMS-based scanning subsystem  104  orients the detection beam  126  toward a current point  134  on an object  112 . 
     At step  606 , the detection beam is reflected off of the point on the object to provide a reflected detection beam. In an example implementation, MEMS-based scanning subsystem  104  reflects the detection beam  126  off of the current point  134  on the object  112  to provide a reflected detection beam  128 . 
     At step  608 , the reference beam and reflected detection beam are combined on a detector to produce an electrical signal. The electrical signal has a beat frequency. In an example implementation, light detecting structure  206  combines the reference beam  246  and the reflected detection beam  128  thereon to produce beat signal  248 . In accordance with this implementation, the beat signal  248  has the beat frequency. 
     At step  610 , the electrical signal is signal processed to determine the beat frequency. The beat frequency is a measurement of a distance to the point on the object. In an example implementation, signal processing circuit  210  signal processes the beat signal  248  to determine the beat frequency. In accordance with this implementation, the beat frequency is a measurement of a distance to the current point  134  on the object  112 . 
     In some example embodiments, one or more steps  602 ,  604 ,  606 ,  608 , and/or  610  of flowchart  600  may not be performed. Moreover, steps in addition to or in lieu of steps  602 ,  604 ,  606 ,  608 , and/or  610  may be performed. 
       FIGS. 7-9  depict flowcharts  700  and  800  of example methods for performing progressive resolution refinement in accordance with embodiments described herein.  FIG. 10  depicts a flowchart  1000  of an example method for using a depth mapping apparatus in accordance with an embodiment described herein. For illustrative purposes, flowcharts  700 ,  800 ,  900 , and  1000  are described with respect to system  100  shown in  FIG. 1  and depth measurement subsystem  200  shown in  FIG. 2 . Further structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the discussion regarding flowcharts  700 ,  800 ,  900 , and  1000 . 
     As shown in  FIG. 7 , the method of flowchart  700  begins at step  702 . In step  702 , a first measurement with a relatively low resolution is performed using modulated coherent light. In an example implementation, depth measurement subsystem  102  performs the first measurement with the relatively low resolution using the modulated coherent light. 
     At step  704 , the first measurement is processed electrically to determine low-resolution locating information. The low-resolution locating information includes a relatively low resolution estimate of a distance between a reference location and a current point. In an example implementation, signal processing circuit  210  processes the first measurement electrically to determine the low-resolution locating information. In accordance with this embodiment, the low-resolution locating information includes a relatively low resolution estimate of the distance between the reference location  132  and the current point  134 . 
     At step  706 , a second measurement with a relatively high resolution is performed. In an example implementation, depth measurement subsystem  102  performs the second measurement with the relatively high resolution. 
     At step  708 , the second measurement is processed electrically using the low-resolution locating information to enable the processing of the second measurement to determine high-resolution locating information. The high-resolution locating information includes a relatively high resolution estimate of the distance between the reference location and the current point. In an example implementation, signal processing circuit  210  processes the second measurement electrically using the low-resolution locating information to enable the processing of the second measurement to determine the high-resolution locating information. In accordance with this implementation, the high-resolution locating information includes a relatively high resolution estimate of the distance between the reference location  132  and the current point  134 . 
     In some example embodiments, one or more steps  702 ,  704 ,  706 , and/or  708  of flowchart  700  may not be performed. Moreover, steps in addition to or in lieu of steps  702 ,  704 ,  706 , and/or  708  may be performed. For instance, in an example embodiment, the method of flowchart  700  further includes one or more of the steps shown in flowchart  800  of  FIG. 8 . As shown in  FIG. 8 , the method of flowchart  800  begins at step  802 . In step  802 , coherent light is modulated in frequency to provide the modulated coherent light. In an example implementation, laser source  202  modulates the coherent light  244  in frequency to provide the modulated coherent light. 
     At step  804 , the modulated coherent light is split into a reference beam and a detection beam. In an example implementation, splitting optics  204  split the modulated coherent light into the reference beam  246  and the detection beam  126 . 
     At step  806 , a frequency range over which a beat frequency of a beat signal is to be searched is reduced by more than a factor of two to enable the processing of the second measurement to determine the high-resolution locating information. The beat signal is an electrical result of optical mixing of the reference beam and a reflected detection beam at a surface of a light detecting structure. The reflected detection beam results from reflection of the detection beam from the current point. In an example implementation, depth measurement subsystem  102  reduces the frequency range over which the beat frequency of the beat signal is to be searched by more than a factor of two to enable the processing of the second measurement to determine the high-resolution locating information. 
     In another example embodiment, the method of flowchart  700  further includes one or more of the steps shown in flowchart  900  of  FIG. 9 . As shown in  FIG. 9 , the method of flowchart  900  begins at step  902 . In step  902 , coherent light is modulated in amplitude over time to provide the modulated coherent light. In an example implementation, laser source  202  modulates the coherent light  244  in amplitude over time to provide the modulated coherent light. 
     At step  904 , a reference signal and a detection beam are formed. The detection beam is formed from the modulated coherent light. In an example implementation, depth measurement subsystem  102  forms the reference signal  248 A and the detection beam  126 . In accordance with this implementation, the detection beam  126  is formed from the modulated coherent light. 
     At step  906 , a phase difference between the reference signal and a reflected detection beam is measured. The reflected detection beam results from reflection of the detection beam from the current point. In an example implementation, depth measurement subsystem  102  measures a phase difference between the reference signal  248 A and the reflected detection beam  128 . 
     At step  908 , the high-resolution locating information is determined based on the phase difference and the low-resolution locating information. In an example implementation, depth measurement subsystem  102  determines the high-resolution locating information based on the phase difference and the low-resolution locating information. 
     As shown in  FIG. 10 , the method of flowchart  1000  begins at step  1002 . In step  1002 , a depth map is formed. In an example implementation, controller  108  forms the depth map  138 . For instance, controller  108  may form the depth map  138  based on a scan of a beam of laser light from the reference location  132  over points (e.g., a grid of points) in the field of view  130 . 
     At step  1004 , locations of objects in the depth map are interpreted. In an example implementation, controller  108  interprets the locations of the objects in the depth map  138 . For instance, measured distances from the reference location  132  to respective points in the field of view  130  may indicate the locations of the objects in the depth map  138 . Accordingly, controller  108  may use the measured distances to interpret the locations of the objects in the depth map  138 . 
     At step  1006 , surfaces of the objects in the depth map are identified. For example, the surfaces of the objects may be interpreted based at least in part on the locations of the objects. In accordance with this example, interpolation between points that correspond to the locations of the objects may be performed to identify the surfaces of the objects in the depth map. In an example implementation, controller  108  identifies the surfaces of the objects in the depth map  138 . 
     At step  1008 , an image is modified in response to the locations of the objects and the surfaces of the objects to provide a modified image. In an example implementation, controller  108  and/or laser projection subsystem  106  modifies the image in response to the locations of the objects and the surfaces of the objects to provide the modified image. 
     At step  1010 , the modified image is projected onto one or more of the surfaces. In an example implementation, laser projection subsystem  106  projects the modified image onto the one or more surfaces. For instance, laser projection subsystem  106  may use a light redirecting element in MEMS-based scanning subsystem  104  to project the modified image onto the one or more surfaces. 
     In some example embodiments, one or more steps  1002 ,  1004 ,  1006 ,  1008 , and/or  1010  of flowchart  1000  may not be performed. Moreover, steps in addition to or in lieu of steps  1002 ,  1004 ,  1006 ,  1008 , and/or  1010  may be performed. 
     III. Example Computing System Implementation 
     Example embodiments, systems, components, subcomponents, devices, methods, flowcharts, steps, and/or the like described herein, including but not limited to flowchart  500 , flowchart  600 , flowchart  700 , flowchart  800 , flowchart  900 , and flowchart  1000  may be implemented in hardware (e.g., hardware logic/electrical circuitry), or any combination of hardware with software (computer program code configured to be executed in one or more processors or processing devices) and/or firmware. The embodiments described herein, including systems, methods/processes, and/or apparatuses, may be implemented using well known computing devices, such as computer  1100  shown in  FIG. 11 . For example, each of the steps of flowchart  500 , each of the steps of flowchart  600 , each of the steps of flowchart  700 , each of the steps of flowchart  800 , each of the steps of flowchart  900 , and each of the steps of flowchart  1000  may be implemented using one or more computers  1100 . 
     Computer  1100  can be any commercially available and well known communication device, processing device, and/or computer capable of performing the functions described herein, such as devices/computers available from International Business Machines®, Apple®, HP®, Dell®, Cray®, Samsung®, Nokia®, etc. Computer  1100  may be any type of computer, including a server, a desktop computer, a laptop computer, a tablet computer, a wearable computer such as a smart watch or a head-mounted computer, a personal digital assistant, a cellular telephone, etc. 
     Computer  1100  includes one or more processors (also called central processing units, or CPUs), such as a processor  1106 . Processor  1106  is connected to a communication infrastructure  1102 , such as a communication bus. In some embodiments, processor  1106  can simultaneously operate multiple computing threads. Computer  1100  also includes a primary or main memory  1108 , such as random access memory (RAM). Main memory  1108  has stored therein control logic  1124  (computer software), and data. 
     Computer  1100  also includes one or more secondary storage devices  1110 . Secondary storage devices  1110  include, for example, a hard disk drive  1112  and/or a removable storage device or drive  1114 , as well as other types of storage devices, such as memory cards and memory sticks. For instance, computer  1100  may include an industry standard interface, such a universal serial bus (USB) interface for interfacing with devices such as a memory stick. Removable storage drive  1114  represents a floppy disk drive, a magnetic tape drive, a compact disk drive, an optical storage device, tape backup, etc. 
     Removable storage drive  1114  interacts with a removable storage unit  1116 . Removable storage unit  1116  includes a computer useable or readable storage medium  1118  having stored therein computer software  1126  (control logic) and/or data. Removable storage unit  1116  represents a floppy disk, magnetic tape, compact disk (CD), digital versatile disc (DVD), Blu-ray disc, optical storage disk, memory stick, memory card, or any other computer data storage device. Removable storage drive  1114  reads from and/or writes to removable storage unit  1116  in a well-known manner. 
     Computer  1100  also includes input/output/display devices  1104 , such as touchscreens, LED and LCD displays, keyboards, pointing devices, etc. 
     Computer  1100  further includes a communication or network interface  1120 . Communication interface  1120  enables computer  1100  to communicate with remote devices. For example, communication interface  1120  allows computer  1100  to communicate over communication networks or mediums  1122  (representing a form of a computer useable or readable medium), such as local area networks (LANs), wide area networks (WANs), the Internet, etc. Network interface  1120  may interface with remote sites or networks via wired or wireless connections. Examples of communication interface  1120  include but are not limited to a modem (e.g., for 3G and/or 4G communication(s)), a network interface card (e.g., an Ethernet card for Wi-Fi and/or other protocols), a communication port, a Personal Computer Memory Card International Association (PCMCIA) card, a wired or wireless USB port, etc. Control logic  1128  may be transmitted to and from computer  1100  via the communication medium  1122 . 
     Any apparatus or manufacture comprising a computer useable or readable medium having control logic (software) stored therein is referred to herein as a computer program product or program storage device. Examples of a computer program product include but are not limited to main memory  1108 , secondary storage devices  1110  (e.g., hard disk drive  1112 ), and removable storage unit  1116 . Such computer program products, having control logic stored therein that, when executed by one or more data processing devices, cause such data processing devices to operate as described herein, represent embodiments. For example, such computer program products, when executed by processor  1106 , may cause processor  1106  to perform any of the steps of flowchart  500  of  FIG. 5 , flowchart  600  of  FIG. 6 , flowchart  700  of  FIG. 7 , flowchart  800  of  FIG. 8 , flowchart  900  of  FIG. 9 , and/or flowchart  1000  of  FIG. 10 . 
     Devices in which embodiments may be implemented may include storage, such as storage drives, memory devices, and further types of computer-readable media. Examples of such computer-readable storage media include a hard disk, a removable magnetic disk, a removable optical disk, flash memory cards, digital video disks, random access memories (RAMs), read only memories (ROM), and the like. As used herein, the terms “computer program medium” and “computer-readable medium” are used to generally refer to media (e.g., non-transitory media) such as the hard disk associated with a hard disk drive, a removable magnetic disk, a removable optical disk (e.g., CD ROMs, DVD ROMs, etc.), zip disks, tapes, magnetic storage devices, optical storage devices, MEMS-based storage devices, nanotechnology-based storage devices, as well as other media such as flash memory cards, digital video discs, RAM devices, ROM devices, and the like. Such computer-readable storage media may store program modules that include computer program logic to implement, for example, embodiments, systems, components, subcomponents, devices, methods, flowcharts, steps, and/or the like described herein (as noted above), and/or further embodiments described herein. Embodiments are directed to computer program products comprising such logic (e.g., in the form of program code, instructions, or software) stored on any computer useable medium. Such program code, when executed in one or more processors, causes a device to operate as described herein. 
     Note that such computer-readable storage media are distinguished from and non-overlapping with communication media (do not include communication media). Communication media embodies computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wireless media such as acoustic, RF, infrared and other wireless media, as well as wired media. Embodiments are also directed to such communication media. 
     The disclosed technologies can be put into practice using software, firmware, and/or hardware implementations other than those described herein. Any software, firmware, and hardware implementations suitable for performing the functions described herein can be used. 
     IV. Conclusion 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the scope of the embodiments. Thus, the scope of the embodiments should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.