Abstract:
A method for optically communicating, from a user to a laser tracker, a command to control tracker operation includes providing a rule of correspondence between commands and temporal patterns, and selecting by the user a first command. Also, projecting a first light from the tracker to the retroreflector, reflecting a second light from the retroreflector, the second light being a portion of the first light, obtaining first sensed data by sensing a third light which is a portion of the second light, creating by the user, between first and second times, a first temporal pattern which includes a decrease in the third optical power followed by an increase in the third optical power, the first temporal pattern corresponding to the first command, determining the first command based on processing the first sensed data per the rule of correspondence and executing the first command with the tracker.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 13/090,889, filed on Apr. 20, 2011, which claims the benefit of U.S. Provisional Application No. 61/326,294, filed on Apr. 21, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a point by sending a laser beam to the point, where it is intercepted by a retroreflector target. The instrument finds the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter (ADM) or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. An example of such a device is a laser tracker. Exemplary laser tracker systems are described by U.S. Pat. No. 4,790,651 to Brown et al., incorporated by reference herein, and U.S. Pat. No. 4,714,339 to Lau et al. 
     A coordinate-measuring device closely related to the laser tracker is the total station. The total station, which is most often used in surveying applications, may be used to measure the coordinates of diffusely scattering or retroreflective targets. Hereinafter, the term laser tracker is used in a broad sense to include total stations. 
     Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The apex of the cube corner, which is the common point of intersection of the three mirrors, is located at the center of the sphere. It is common practice to place the spherical surface of the SMR in contact with an object under test and then move the SMR over the surface being measured. Because of this placement of the cube corner within the sphere, the perpendicular distance from the apex of the cube corner to the surface of the object under test remains constant despite rotation of the SMR. Consequently, the 3D coordinates of a surface can be found by having a tracker follow the 3D coordinates of an SMR moved over the surface. It is possible to place a glass window on the top of the SMR to prevent dust or dirt from contaminating the glass surfaces. An example of such a glass surface is shown in U.S. Pat. No. 7,388,654 to Raab et al., incorporated by reference herein. 
     A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. The position of the light that hits the position detector is used by a tracker control system to adjust the rotation angles of the mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) the SMR. 
     Angular encoders attached to the mechanical azimuth and zenith axes of the tracker may measure the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR. 
     As mentioned previously, two types of distance meters may be found in laser trackers: interferometers and absolute distance meters (ADMs). In the laser tracker, an interferometer (if present) may determine the distance from a starting point to a finishing point by counting the number of increments of known length (usually the half-wavelength of the laser light) that pass as a retroreflector target is moved between the two points. If the beam is broken during the measurement, the number of counts cannot be accurately known, causing the distance information to be lost. By comparison, the ADM in a laser tracker determines the absolute distance to a retroreflector target without regard to beam breaks, which also allows switching between targets. Because of this, the ADM is said to be capable of “point-and-shoot” measurement. Initially, absolute distance meters were only able to measure stationary targets and for this reason were always used together with an interferometer. However, some modern absolute distance meters can make rapid measurements, thereby eliminating the need for an interferometer. Such an ADM is described in U.S. Pat. No. 7,352,446 to Bridges et al., incorporated by reference herein. 
     In its tracking mode, the laser tracker will automatically follow movements of the SMR when the SMR is in the capture range of the tracker. If the laser beam is broken, tracking will stop. The beam may be broken by any of several means: (1) an obstruction between the instrument and SMR; (2) rapid movements of the SMR that are too fast for the instrument to follow; or (3) the direction of the SMR being turned beyond the acceptance angle of the SMR. By default, following the beam break, the beam remains fixed at the point of the beam break or at the last commanded position. It may be necessary for an operator to visually search for the tracking beam and place the SMR in the beam in order to lock the instrument onto the SMR and continue tracking. 
     Some laser trackers include one or more cameras. A camera axis may be coaxial with the measurement beam or offset from the measurement beam by a fixed distance or angle. A camera may be used to provide a wide field of view to locate retroreflectors. A modulated light source placed near the camera optical axis may illuminate retroreflectors, thereby making them easier to identify. In this case, the retroreflectors flash in phase with the illumination, whereas background objects do not. One application for such a camera is to detect multiple retroreflectors in the field of view and measure each in an automated sequence. Exemplary systems are described in U.S. Pat. No. 6,166,809 to Pettersen et al., and U.S. Pat. No. 7,800,758 to Bridges et al., incorporated by reference herein. 
     Some laser trackers have the ability to measure with six degrees of freedom (DOF), which may include three coordinates, such as x, y, and z, and three rotations, such as pitch, roll, and yaw. Several systems based on laser trackers are available or have been proposed for measuring six degrees of freedom. Exemplary systems are described in U.S. Published Patent Application No. 2010/0128259 to Bridges, incorporated by reference herein; U.S. Pat. No. 7,800,758 to Bridges et al., U.S. Pat. No. 5,973,788 to Pettersen et al.; and U.S. Pat. No. 7,230,689 to Lau. 
     User Control of Laser Tracker Functionality 
     Two common modes of operation of the laser tracker are tracking mode and profiling mode. In tracking mode, the laser beam from the tracker follows the retroreflector as the operator moves it around. In profiling mode, the laser beam from the tracker goes in the direction given by the operator, either through computer commands or manual action. 
     Besides these modes of operation that control the basic tracking and pointing behavior of the tracker, there are also special option modes that enable the tracker to respond in a manner selected by the operator ahead of time. The desired option mode is typically selected in software that controls the laser tracker. Such software may reside in an external computer attached to the tracker (possibly through a network cable) or within the tracker itself. In the latter case, the software may be accessed through console functionality built into the tracker. 
     An example of an option mode is the Auto Reset mode in which the laser beam is driven to a preset reference point whenever the laser beam is broken. One popular reference point for the Auto Reset option mode is the tracker Home Position, which is the position of a magnetic nest mounted on the tracker body. The alternative to Auto Reset is the No Reset option mode. In this case, the laser beam continues pointing in the original direction whenever the laser beam is broken. A description of the tracker home position is given in U.S. Pat. No. 7,327,446 to Cramer et al., incorporated by reference herein. 
     Another example of a special option mode is PowerLock, a feature offered by Leica Geosystems on their Leica Absolute Tracker™. In the PowerLock option mode, the location of the retroreflector is found by a tracker camera whenever the tracker laser beam is broken. The camera immediately sends the angular coordinates of the retroreflector to the tracker control system, thereby causing the tracker to point the laser beam back at the retroreflector. Methods involving automatic acquisition of a retroreflector are given in international application WO 2007/079601 to Dold et al. and U.S. Pat. No. 7,055,253 to Kaneko. 
     Some option modes are slightly more complex in their operation. An example is the Stability Criterion mode, which may be invoked whenever an SMR is stationary for a given period of time. The operator may track an SMR to a magnetic nest and set it down. If a stability criterion is active, the software will begin to look at the stability of the three-dimensional coordinate readings of the tracker. For instance, the user may decide to judge the SMR to be stable if the peak-to-peak deviation in the distance reading of the SMR is less than two micrometers over a one second interval. After the stability criterion is satisfied, the tracker measures the 3D coordinates and the software records the data. 
     More complex modes of operation are possible through computer programs. For example, software is available to measure part surfaces and fit these to geometrical shapes. Software will instruct the operator to move the SMR over the surface and then, when finished collecting data points, to raise the SMR off the surface of the object to end the measurement. Moving the SMR off the surface not only indicates that the measurement is completed; it also indicates the position of the SMR in relation to the object surface. This position information is needed by the application software to properly account for the offset caused by the SMR radius. 
     A second example of complex computer control is a tracker survey. In the survey, the tracker is driven sequentially to each of several target locations according to a prearranged schedule. The operator may teach these positions prior to the survey by carrying the SMR to each of the desired positions. 
     A third example of complex software control is tracker directed measurement. The software directs the operator to move the SMR to a desired location. It does this using a graphic display to show the direction and distance to the desired location. When the operator is at the desired position, the color on the computer monitor might, for example, turn from red to green. 
     The element common to all tracker actions described above is that the operator is limited in his ability to control the behavior of the tracker. On the one hand, option modes selected in the software may enable the operator to preset certain behaviors of the tracker. However, once the option modes have been selected by the user, the behavior of the tracker is established and cannot be changed unless the operator returns to the computer console. On the other hand, the computer program may direct the operator to carry out complicated operations that the software analyzes in a sophisticated way. In either case, the operator is limited in his ability to control the tracker and the data collected by the tracker. 
     Need for Remote Tracker Commands 
     A laser tracker operator performs two fundamental functions. He positions an SMR during a measurement, and he sends commands through the control computer to the tracker. However, it is not easy for one operator to perform both of these measurement functions because the computer is usually far away from the measurement location. Various methods have been tried to get around this limitation, but none is completely satisfactory. 
     One method sometimes used is for a single operator to set the retroreflector in place and walk back to the instrument control keyboard to execute a measurement instruction. However, this is an inefficient use of operator and instrument time. In cases where the operator must hold the retroreflector for the measurement, single operator control is only possible when the operator is very close to the keyboard. 
     A second method is to add a second operator. One operator stands by the computer and a second operator moves the SMR. This is obviously an expensive method and verbal communication over large distances can be a problem. 
     A third method is to equip a laser tracker with a remote control. However, remote controls have several limitations. Many facilities do not allow the use of remote controls for safety or security reasons. Even if remote controls are allowed, interference among wireless channels may be a problem. Some remote control signals do not reach the full range of the laser tracker. In some situations, such as working from a ladder, the second hand may not be free to operate the remote control. Before a remote control can be used, it is usually necessary to set up the computer and remote control to work together, and then only a small subset of tracker commands can ordinarily be accessed at any given time. An example of a system based on this idea is given in U.S. Pat. No. 7,233,316 to Smith et al. 
     A fourth method is to interface a cell phone to a laser tracker. Commands are entered remotely by calling the instrument from the cell phone and entering numbers from the cell phone keypad or by means of voice recognition. This method also has many shortcomings. Some facilities do not allow cell phones to be used, and cell phones may not be available in rural areas. Service requires a monthly service provider fee. A cell phone interface requires additional hardware interfacing to the computer or laser tracker. Cell phone technology changes fast and may require upgrades. As in the case of remote controls, the computer and remote control must be set up to work together, and only a small subset of tracker commands can ordinarily be accessed at a given time. 
     A fifth method is to equip a laser tracker with internet or wireless network capabilities and use a wireless portable computer or personal digital assistant (PDA) to communicate commands to the laser tracker. However, this method has limitations similar to a cell phone. This method is often used with total stations. Examples of systems that use this method include U.S. Published Patent Application No. 2009/017618 to Kumagai et al., U.S. Pat. No. 6,034,722 to Viney et al., U.S. Pat. No. 7,423,742 to Gatsios et al., U.S. Pat. No. 7,307,710 to Gatsios et al., U.S. Pat. No. 7,552,539 to Piekutowski, and U.S. Pat. No. 6,133,998 to Monz et al. This method has also been used to control appliances by a method described in U.S. Pat. No. 7,541,965 to Ouchi et al. 
     A sixth method is to use a pointer to indicate a particular location in which a measurement is to be made. An example of this method is given in U.S. Pat. No. 7,022,971 to Ura et al. It might be possible to adapt this method to give commands to a laser tracker, but it is not usually very easy to find a suitable surface upon which to project the pointer beam pattern. 
     A seventh method is to devise a complex target structure containing at least a retroreflector, transmitter, and receiver. Such systems may be used with total stations to transmit precise target information to the operator and also to transmit global positioning system (GPS) information to the total station. An example of such a system is given in U.S. Published Patent Application No. 2008/0229592 to Hinderling et al. In this case, no method is provided to enable the operator to send commands to the measurement device (total station). 
     An eighth method is to devise a complex target structure containing at least a retroreflector, transmitter and receiver, where the transmitter has the ability to send modulated light signals to a total station. A keypad can be used to send commands to the total station by means of the modulated light. These commands are decoded by the total station. Examples of such systems are given in U.S. Pat. No. 6,023,326 to Katayama et al., U.S. Pat. No. 6,462,810 to Muraoka et al., U.S. Pat. No. 6,295,174 to Ishinabe et al., and U.S. Pat. No. 6,587,244 to Ishinabe et al. This method is particularly appropriate for surveying applications in which the complex target and keypad are mounted on a large staff. Such a method is not suitable for use with a laser tracker, where it is advantageous to use a small target not tethered to a large control pad. Also it is desirable to have the ability to send commands even when the tracker is not locked onto a retroreflector target. 
     A ninth method is to include both a wireless transmitter and a modulated light source on the target to send information to a total station. The wireless transmitter primarily sends information on the angular pose of the target so that the total station can turn in the proper direction to send its laser beam to the target retroreflector. The modulated light source is placed near the retroreflector so that it will be picked up by the detector in the total station. In this way, the operator can be assured that the total station is pointed in the right direction, thereby avoiding false reflections that do not come from the target retroreflector. An exemplary system based on this approach is given in U.S. Pat. No. 5,313,409 to Wiklund et al. This method does not offer the ability to send general purpose commands to a laser tracker. 
     A tenth method is to include a combination of wireless transmitter, compass assembly in both target and total station, and guide light transmitter. The compass assembly in the target and total station are used to enable alignment of the azimuth angle of the total station to the target. The guide light transmitter is a horizontal fan of light that the target can pan in the vertical direction until a signal is received on the detector within the total station. Once the guide light has been centered on the detector, the total station adjusts its orientation slightly to maximize the retroreflected signal. The wireless transmitter communicates information entered by the operator on a keypad located at the target. An exemplary system based on this method is given in U.S. Pat. No. 7,304,729 to Wasutomi et al. This method does not offer the ability to send general purpose commands to a laser tracker. 
     An eleventh method is to modify the retroreflector to enable temporal modulation to be imposed on the retroreflected light, thereby transmitting data. The inventive retroreflector comprises a cube corner having a truncated apex, an optical switch attached to the front face of the cube corner, and electronics to transmit or receive data. An exemplary system of this type is given in U.S. Pat. No. 5,121,242 to Kennedy. This type of retroreflector is complex and expensive. It degrades the quality of the retroreflected light because of the switch (which might be a ferro-electric light crystal material) and because of the truncated apex. Also, the light returned to a laser tracker is already modulated for use in measuring the ADM beam, and switching the light on and off would be a problem, not only for the ADM, but also for the tracker interferometer and position detector. 
     A twelfth method is to use a measuring device that contains a bidirectional transmitter for communicating with a target and an active retroreflector to assist in identifying the retroreflector. The bidirectional transmitter may be wireless or optical and is part of a complex target staff that includes the retroreflector, transmitter, and control unit. An exemplary system of this type is described in U.S. Pat. No. 5,828,057 to Hertzman et al. Such a method is not suitable for use with a laser tracker, where it is advantageous to use a small target not tethered to a large control pad. Also the method of identifying the retroreflector target of interest is complicated and expensive. 
     There is a need for a simple method for an operator to communicate commands to a laser tracker from a distance. It is desirable that the method be: (1) useable without a second operator; (2) useable over the entire range of the laser tracker; (3) useable without additional hardware interfacing; (4) functional in all locations; (5) free of service provider fees; (6) free of security restrictions; (7) easy to use without additional setup or programming; (8) capable of initiating a wide range of simple and complex tracker commands; (9) useable to call a tracker to a particular target among a plurality of targets; and (10) useable with a minimum of additional equipment for the operator to carry. 
     SUMMARY 
     According to an embodiment of the present invention, a method for optically communicating, from a user to a laser tracker, a command to control operation of the laser tracker with steps includes providing a rule of correspondence between each of a plurality of commands and each of a plurality of temporal patterns; and selecting by the user, a first command from among the plurality of commands; projecting a first light from the laser tracker to the retroreflector and reflecting a second light from the retroreflector, the second light being a portion of the first light. The method further includes obtaining first sensed data by sensing a third light, the third light being a portion of the second light; creating by the user, between a first time and a second time, a first temporal pattern, the first temporal pattern including at least a decrease in the third optical power followed by an increase in the third optical power, the first temporal pattern corresponding to the first command. The method also includes determining the first command based at least in part on processing the first sensed data according to the rule of correspondence; and executing the first command with the laser tracker. 
     According to another embodiment of the present invention, a method for optically communicating, from a user to a laser tracker, a command to direct a beam of light from the laser tracker to a retroreflector and lock onto the retroreflector includes projecting a first light from a light source disposed on the laser tracker to the retroreflector; reflecting a second light from the retroreflector, the second light being a portion of the first light. The method further includes obtaining first sensed data by sensing a third light, the third light being a portion of the second light, wherein the first sensed data is obtained by imaging the third light onto a photosensitive array disposed on the laser tracker and converting the third light on the photosensitive array into digital form; generating by the user, between a first time and a second time, a predefined temporal pattern, the predefined temporal pattern including at least a decrease in the third optical power followed by an increase in the third optical power, the predefined temporal pattern corresponding to the command. The method further includes determining by the laser tracker that the first sensed data corresponds to the predefined temporal pattern; pointing the beam of light from the laser tracker to the retroreflector and locking onto the retroreflector with the beam of light from the laser tracker. 
     According to another embodiment of the present invention, a laser measurement system includes a laser tracker having a structure rotatable about a first axis and a second axis; a first light source that launches a first light beam from the structure; a distance meter; a first angular encoder that measures a first angle of rotation about the first axis; a second angular encoder that measures a second angle of rotation about the second axis; a processor; and a camera system. The system further includes a communication device that includes a second light source and an operator-controlled device that controls emission of light from the second light source; a retroreflector target separate from the communication device, wherein the camera system is operable to receive the light emitted from the second light source and to convert the light into a digital image, and wherein the processor is operable to determine a command to control operation of the laser tracker based, at least in part, on a change in the optical power of the second light source between a first time and a second time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  shows a perspective view of an exemplary laser tracker; 
         FIG. 2  shows computing and power supply elements attached to exemplary laser tracker; 
         FIGS. 3A-3E  illustrate ways in which a passive target can be used to convey gestural information through the tracking and measuring systems of the laser tracker; 
         FIGS. 4A-4C  illustrate ways in which a passive target can be used to convey gestural information through the camera system of a laser tracker; 
         FIGS. 5A-5D  illustrate ways in which an active target can be used to convey gestural information through the camera system of a laser tracker; 
         FIG. 6  is a flow chart showing the steps carried out by the operator and laser tracker in issuing and carrying out a gestural command; 
         FIG. 7  is a flow chart showing the optional and required parts of a gestural command; 
         FIGS. 8-10  show a selection of laser tracker commands and corresponding gestures that might be used by the operator to convey these commands to the laser tracker; 
         FIGS. 11A-11F  show alternative types of gestures that might be used; 
         FIG. 12  shows an exemplary command tablet for transmitting commands to a laser tracker by means of gestures; 
         FIG. 13  shows an exemplary method for using gestures to set a tracker reference point; 
         FIG. 14  shows an exemplary method for using gestures to initialize the exemplary command tablet; 
         FIG. 15  shows an exemplary method for using gestures to measure a circle; 
         FIG. 16  shows an exemplary method for using gestures to acquire a retroreflector with a laser beam from a laser tracker; 
         FIG. 17  shows an exemplary electronics and processing system associated with a laser tracker; 
         FIG. 18  shows an exemplary geometry that enables finding of three dimensional coordinates of a target using a camera located off the optical axis of a laser tracker; 
         FIG. 19  shows an exemplary method for communicating a command to a laser tracker by gesturing with a retroreflector in a spatial pattern; 
         FIG. 20  shows an exemplary method for communicating a command to a laser tracker by indicating a position with a retroreflector; 
         FIG. 21  shows an exemplary method for communicating a command to a laser tracker by gesturing with a retroreflector in a temporal pattern; 
         FIG. 22  shows an exemplary method for communicating a command to a laser tracker by measuring a change in the pose of a six DOF target with a six DOF laser tracker; 
         FIG. 23  shows an exemplary method for communicating a command to point the laser beam from the laser tracker to a retroreflector and lock onto the retroreflector, the communication based on a gesture involving a spatial pattern created with the retroreflector; 
         FIG. 24  shows an exemplary method for communicating a command to point the laser beam from the laser tracker to a retroreflector and lock onto the retroreflector, the communication based on a gesture involving a temporal pattern in the optical power received by the laser tracker; and 
         FIG. 25  shows an exemplary method for communicating a command to point the laser beam from the laser tracker to a retroreflector and lock onto the retroreflector, the communication based on a gesture involving a change in the pose of a six DOF probe. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary laser tracker  10  is illustrated in  FIG. 1 . An exemplary gimbaled beam-steering mechanism  12  of laser tracker  10  comprises zenith carriage  14  mounted on azimuth base  16  and rotated about azimuth axis  20 . Payload  15  is mounted on zenith carriage  14  and rotated about zenith axis  18 . Zenith mechanical rotation axis  18  and azimuth mechanical rotation axis  20  intersect orthogonally, internally to tracker  10 , at gimbal point  22 , which is typically the origin for distance measurements. Laser beam  46  virtually passes through gimbal point  22  and is pointed orthogonal to zenith axis  18 . In other words, laser beam  46  is in the plane normal to zenith axis  18 . Laser beam  46  is pointed in the desired direction by motors within the tracker (not shown) that rotate payload  15  about zenith axis  18  and azimuth axis  20 . Zenith and azimuth angular encoders, internal to the tracker (not shown), are attached to zenith mechanical axis  18  and azimuth mechanical axis  20  and indicate, to high accuracy, the angles of rotation. Laser beam  46  travels to external retroreflector  26  such as the spherically mounted retroreflector (SMR) described above. By measuring the radial distance between gimbal point  22  and retroreflector  26  and the rotation angles about the zenith and azimuth axes  18 ,  20 , the position of retroreflector  26  is found within the spherical coordinate system of the tracker. 
     Laser beam  46  may comprise one or more laser wavelengths. For the sake of clarity and simplicity, a steering mechanism of the sort shown in  FIG. 1  is assumed in the following discussion. However, other types of steering mechanisms are possible. For example, it would be possible to reflect a laser beam off a mirror rotated about the azimuth and zenith axes. An example of the use of a mirror in this way is given in U.S. Pat. No. 4,714,339 to Lau et al. The techniques described here are applicable, regardless of the type of steering mechanism. 
     In exemplary laser tracker  10 , cameras  52  and light sources  54  are located on payload  15 . Light sources  54  illuminate one or more retroreflector targets  26 . Light sources  54  may be LEDs electrically driven to repetitively emit pulsed light. Each camera  52  comprises a photosensitive array and a lens placed in front of the photosensitive array. The photosensitive array may be a CMOS or CCD array. The lens may have a relatively wide field of view, say thirty or forty degrees. The purpose of the lens is to form an image on the photosensitive array of objects within the field of view of the lens. Each light source  54  is placed near camera  52  so that light from light source  54  is reflected off each retroreflector target  26  onto camera  52 . In this way, retroreflector images are readily distinguished from the background on the photosensitive array as their image spots are brighter than background objects and are pulsed. There may be two cameras  52  and two light sources  54  placed about the line of laser beam  46 . By using two cameras in this way, the principle of triangulation can be used to find the three-dimensional coordinates of any SMR within the field of view of the camera. In addition, the three-dimensional coordinates of the SMR can be monitored as the SMR is moved from point to point. A use of two cameras for this purpose is described in U.S. Published Patent Application No. 2010/0128259 to Bridges. 
     Other arrangements of one or more cameras and light sources are possible. For example, a light source and camera can be coaxial or nearly coaxial with the laser beams emitted by the tracker. In this case, it may be necessary to use optical filtering or similar methods to avoid saturating the photosensitive array of the camera with the laser beam from the tracker. 
     Another possible arrangement is to use a single camera located on the payload or base of the tracker. A single camera, if located off the optical axis of the laser tracker, provides information about the two angles that define the direction to the retroreflector but not the distance to the retroreflector. In many cases, this information may be sufficient. If the 3D coordinates of the retroreflector are needed when using a single camera, one possibility is to rotate the tracker in the azimuth direction by 180 degrees and then to flip the zenith axis to point back at the retroreflector. In this way, the target can be viewed from two different directions and the 3D position of the retroreflector can be found using triangulation. 
     A more general approach to finding the distance to a retroreflector with a single camera is to rotate the laser tracker about either the azimuth axis or the zenith axis and observe the retroreflector with a camera located on the tracker for each of the two angles of rotation. The retroreflector may be illuminated, for example, by an LED located close to the camera.  FIG. 18  shows how this procedure can be used to find the distance to the retroreflector. The test setup  900  includes a laser tracker  910 , a camera  920  in a first position, a camera  930  in a second position, and a retroreflector in a first position  940  and a second position  950 . The camera is moved from the first position to the second position by rotating the laser tracker  910  about the tracker gimbal point  912  about the azimuth axis, the zenith axis, or both the azimuth axis and the zenith axis. The camera  920  includes a lens system  922  and a photosensitive array  924 . The lens system  922  has a perspective center  926  through which rays of light from the retroreflectors  940 ,  950  pass. The camera  930  is the same as the camera  920  except rotated into a different position. The distance from the surface of the laser tracker  910  to the retroreflector  940  is L 1  and the distance from the surface of the laser tracker to the retroreflector  950  is L 2 . The path from the gimbal point  912  to the perspective center  926  of the lens  922  is drawn along the line  914 . The path from the gimbal point  916  to the perspective center  936  of the lens  932  is drawn along the line  916 . The distances corresponding to the lines  914  and  916  have the same numerical value. As can be seen from  FIG. 18 , the nearer position of the retroreflector  940  places an image spot  942  farther from the center of the photosensitive array than the image spot  952  corresponding to the photosensitive array  950  at the distance farther from the laser tracker. This same pattern holds true for the camera  930  located following the rotation. As a result, the distance between the image points of a nearby retroreflector  940  before and after rotation is larger than the distance between the image points of a far away retroreflector  950  before and after rotation. By rotating the laser tracker and noting the resulting change in position of the image spots on the photosensitive array, the distance to the retroreflector can be found. The method for finding this distance is easily found using trigonometry, as will be obvious to one of ordinary skill in the art. 
     Another possibility is to switch between measuring and imaging of the target. An example of such a method is described in U.S. Pat. No. 7,800,758 to Bridges et al. Other camera arrangements are possible and can be used with the methods described herein. 
     As shown in  FIG. 2 , auxiliary unit  70  is usually a part of laser tracker  10 . The purpose of auxiliary unit  70  is to supply electrical power to the laser tracker body and in some cases to also supply computing and clocking capability to the system. It is possible to eliminate auxiliary unit  70  altogether by moving the functionality of auxiliary unit  70  into the tracker body. In most cases, auxiliary unit  70  is attached to general purpose computer  80 . Application software loaded onto general purpose computer  80  may provide application capabilities such as reverse engineering. It is also possible to eliminate general purpose computer  80  by building its computing capability directly into laser tracker  10 . In this case, a user interface, possibly providing keyboard and mouse functionality is built into laser tracker  10 . The connection between auxiliary unit  70  and computer  80  may be wireless or through a cable of electrical wires. Computer  80  may be connected to a network, and auxiliary unit  70  may also be connected to a network. Plural instruments, for example, multiple measurement instruments or actuators, may be connected together, either through computer  80  or auxiliary unit  70 . 
     The laser tracker  10  may be rotated on its side, rotated upside down, or placed in an arbitrary orientation. In these situations, the terms azimuth axis and zenith axis have the same direction relative to the laser tracker as the directions shown in  FIG. 1  regardless of the orientation of the laser tracker  10 . 
     In another embodiment, the payload  15  is replaced by a mirror that rotates about the azimuth axis  20  and the zenith axis  18 . A laser beam is directed upward and strikes the mirror, from which it launches toward a retroreflector  26 . 
     Sending Commands to the Laser Tracker from a Distance 
       FIGS. 3A-3E ,  4 A- 4 C, and  5 A- 5 D demonstrate sensing means by which the operator may communicate gestural patterns that are interpreted and executed as commands by exemplary laser tracker  10 .  FIGS. 3A-3E  demonstrate sensing means by which the operator communicates gestural patterns that exemplary laser tracker  10  interprets using its tracking and measuring systems.  FIG. 3A  shows laser tracker  10  emitting laser beam  46  intercepted by retroreflector target  26 . As target  26  is moved side to side, the laser beam from the tracker follows the movement. At the same time, the angular encoders in tracker  10  measure the angular position of the target in the side-to-side and up-down directions. The angular encoder readings form a two dimensional map of angles that can be recorded by the tracker as a function of time and analyzed to look for patterns of movement. 
       FIG. 3B  shows laser beam  46  tracking retroreflector target  26 . In this case, the distance from tracker  10  to target  26  is measured. The ADM or interferometer readings form a one-dimensional map of distances that can be recorded by tracker  10  as a function of time and analyzed to look for patterns of movement. The combined movements of  FIGS. 3A and 3B  can also be evaluated by laser tracker  10  to look for a pattern in three-dimensional space. 
     The variations in angle, distance, or three-dimensional space may all be considered as examples of spatial patterns. Spatial patterns are continually observed during routine laser tracker measurements. Within the possible range of observed patterns, some patterns may have associated laser tracker commands. There is one type of spatial pattern in use today that may be considered a command. This pattern is a movement away from the surface of an object following a measurement. For example, if an operator measures a number of points on an object with an SMR to obtain the outer diameter of the object and then moves the SMR away from the surface of the object, it is clear that an outer diameter was being measured. If an operator moves the SMR away from the surface after measuring an inner diameter, it is clear that the inner diameter was being measured. Similarly, if an operator moves an SMR upward after measuring a plate, it is understood that the upper surface of the plate was being measured. It is important to know which side of an object is measured because it is necessary to remove the offset of the SMR, which is the distance from the center to the outer surface of the SMR. If this action of moving the SMR away from an object is automatically interpreted by software associated with the laser tracker measurement, then the movement of the SMR may be considered to be a command that indicates “subtract the SMR offset away from the direction of movement.” Therefore, after including this first command in addition to other commands based on the spatial patterns, as described herein, there is a plurality of commands. In other words, there is a correspondence between a plurality of tracker commands and a plurality of spatial patterns. 
     With all of the discussions in the present application, it should be understood that the concept of a command for a laser tracker is to be taken within the context of the particular measurement. For example, in the above situation in which a movement of the retroreflector was said to indicate whether the retroreflector target was measuring an inner or outer diameter, this statement would only be accurate in the context of a tracker measuring an object having a circular profile. 
       FIG. 3C  shows laser beam  46  tracking retroreflector target  26 . In this case, retroreflector target  26  is held fixed, and tracker  10  measures the three-dimensional coordinates. Certain locations within the measurement volume may be assigned special meanings, as for example when a command tablet, described later, is located at a particular three-dimensional position. 
       FIG. 3D  shows laser beam  46  being blocked from reaching retroreflector target  26 . By alternately blocking and unblocking laser beam  46 , the pattern of optical power returned to tracker  10  is seen by the tracker measurement systems, including the position detector and the distance meters. The variation in this returned pattern forms a pattern as a function of time that can be recorded by the tracker and analyzed to look for patterns. 
     A pattern in the optical power returned to the laser tracker is often seen during routine measurements. For example, it is common to block a laser beam from reaching a retroreflector and then to recapture the laser beam with the retroreflector at a later time, possibly after moving the retroreflector to a new distance from the tracker. This action of breaking the laser beam and then recapturing the laser beam may be considered to be a simple type of user command that indicates that the retroreflector is to be recaptured after it is moved to a new position. Therefore, after including this first simple command in addition to other commands based on the temporal variation in optical power, as described herein, there is a plurality of commands. In other words, there is a correspondence between a plurality of tracker commands and a plurality of patterns based on variations in optical power received by a sensor disposed on the laser tracker. 
     A change in optical power is often seen during routine measurements when the laser beam is blocked from returning to the laser tracker. Such an action may be interpreted as a command that indicates “stop tracking” or “stop measuring.” Similarly, a retroreflector may be moved to intercept a laser beam. Such simple actions might be interpreted as commands that indicates “start tracking ” These simple commands are not of interest in the present patent application. For this reason, commands discussed herein involve changes in optical power that include at least a decrease in optical power followed by an increase in optical power. 
       FIG. 3E  shows laser beam  46  tracking retroreflector  26  with a six degree-of-freedom (DOF) probe  110 . Many types of six-DOF probes are possible, and the six-DOF probe  110  shown in  FIG. 3E  is merely representative, and not limiting in its design. Tracker  10  is able to find the angle of angular tilt of the probe. For example, the tracker may find and record the roll, pitch, and yaw angles of probe  110  as a function of time. The collection of angles can be analyzed to look for patterns. 
       FIGS. 4A-4C  demonstrate sensing means by which the operator may communicate gestural patterns that exemplary laser tracker  10  interprets using its camera systems.  FIG. 4A  shows cameras  52  observing the movement of retroreflector target  26 . Cameras  52  record the angular position of target  26  as a function of time. These angles are analyzed later to look for patterns. It is only necessary to have one camera to follow the angular movement of retroreflector target  26 , but the second camera enables calculation of the distance to the target. Optional light sources  54  illuminate target  26 , thereby making it easier to identify in the midst of background images. In addition, light sources  54  may be pulsed to further simplify target identification. 
       FIG. 4B  shows cameras  52  observing the movement of retroreflector target  26 . Cameras  52  record the angular positions of target  26  and, using triangulation, calculate the distance to target  26  as a function of time. These distances are analyzed later to look for patterns. Optional light sources  54  illuminate target  26 . 
       FIG. 4C  shows cameras  52  observing the position of retroreflector target  26 , which is held fixed. Tracker  10  measures the three-dimensional coordinates of target  26 . Certain locations within the measurement volume may be assigned special meanings, as for example when a command tablet, described later, is located at a particular three-dimensional position. 
       FIGS. 5A-5D  demonstrate sensing means by which the operator may communicate gestural patterns that exemplary laser tracker  10  interprets by using its camera systems in combination with an active light source.  FIG. 5A  shows cameras  52  observing active retroreflector target  120 . Active retroreflector target comprises retroreflector target  126  onto which are mounted light source  122  and control button  124  that turns light source  122  on and off. The operator presses control button  124  on and off in a prescribed pattern to illuminate light source  122  in a pattern that is seen by cameras  52  and analyzed by tracker  10 . 
     An alternative mode of operation for  FIG. 5A  is for the operator to hold down control button  124  only while gesturing a command, which might be given, for example, using side-to-side and up-down movements. By holding down control button  124  only during this time, parsing and analysis is simplified for tracker  10 . There are several ways that the tracker can obtain the pattern of movement, whether control button  124  is held down or not: (1) cameras  52  can follow the movement of light source  122 ; (2) cameras  52  can follow the movement of retroreflector  126 , which is optionally illuminated by light sources  54 ; or (3) tracking and measurement systems of laser tracker  10  can follow the movement of retroreflector  126 . In addition, it is possible for the tracker to follow retroreflector  126  in order to collect measurement data while the operator is at the same time pressing control button  124  up and down to produce a temporal pattern in the emitted LED light to issue a command to the tracker. 
       FIG. 5B  shows cameras  52  observing light source  132  on six DOF probe  130 . Six-DOF probe  130  comprises retroreflector  136 , light source  132 , and control button  134 . The operator presses control button  134  on and off in a prescribed manner to illuminate light source  132  in a pattern seen by cameras  54  and analyzed by tracker  10 . 
     An alternative mode of operation for  FIG. 5B  is for the operator to hold down control button  134  only while gesturing a command, which might be given, for example, using side-to-side and up-down movements or rotations. By holding down control button  134  only during this time, parsing and analysis is simplified for tracker  10 . In this case, there are several ways that the tracker can obtain the pattern of movement: (1) cameras  52  can follow the movement of light source  132 ; (2) cameras  52  can follow the movement of retroreflector  136 , which is optionally illuminated by light sources  54 ; or (3) tracking and measurement systems of laser tracker  10  can follow the movement or rotation of six-DOF target  130 . 
       FIGS. 5A ,  5 B can also be used to indicate a particular position. For example, a point on the spherical surface of the active retroreflector target  120  or a point on the spherical surface of the six-DOF probe  130  can be held against an object to provide a location that can be determined by the cameras  52 . Certain locations within the measurement volume may be assigned special meanings, as for example when a command tablet, described in reference to  FIG. 12 , is located at a particular three-dimensional position. 
       FIG. 5C  shows cameras  52  observing light source  142  on wand  140 . Wand  140  comprises light source  142  and control button  144 . The operator presses control button  144  on and off in a prescribed manner to illuminate light source  142  in a temporal pattern seen by cameras  54  and analyzed by tracker  10 . 
       FIG. 5D  shows cameras  52  observing light source  142  on wand  140 . The operator presses control button  144  on wand  140  to continuously illuminate light source  142 . As the operator moves wand  140  in any direction, cameras  52  record the motion of wand  140 , the pattern of which is analyzed by tracker  10 . It is possible to use a single camera  52  if only the pattern of the transverse (side-to-side, up-down) movement and not the radial movement is important. 
     As explained above, tracker  10  has the ability to detect spatial positions, spatial patterns, and temporal patterns created by the operator through the use of retroreflector target  26 , six-DOF target  110  or  130 , active retroreflector target  120 , or wand  140 . These spatial or temporal patterns are collectively referred to as gestures. The particular devices and modes of sensing depicted in  FIGS. 3A-3E ,  4 A- 4 C,  5 A- 5 D are specific examples and should not be understood to limit the scope of the invention. 
       FIG. 6  shows flow chart  200 , which lists steps carried out by the operator and laser tracker  10  in issuing and carrying out gestural commands. In step  210 , laser tracker  10  scans continuously for commands. In other words, the tracker uses one or more of the modes of sensing shown in  FIGS. 3A-3E ,  4 A- 4 C,  5 A- 5 D to record positions, spatial patterns, and temporal patterns. In step  220 , the operator signals a command. This means that the operator creates a gesture by taking a suitable action on an object such as retroreflector target  26 , six-DOF target  110  or  130 , active retroreflector target  120 , or wand  140 . An appropriate action might involve movement to a particular absolute coordinate or movement to create a particular spatial or temporal pattern. 
     In step  230 , tracker  10  intercepts and parses the command just signaled by the operator. It intercepts the command by sensing and recording spatial and temporal information from the moving objects. It parses the command by using computing power, possibly within the tracker, to break the stream of data into appropriate subunits and identify the patterns formed by the subunits according to an algorithm. Types of algorithms that might be used are discussed hereinafter. 
     In step  240 , the tracker acknowledges that a command has been received. The acknowledgement might be in the form of a flashing light located on the tracker, for example. The acknowledgement might take several forms depending on whether the command was clearly received, garbled or incomplete, or impossible to carry out for some reason. The signal for each of these different conditions could be given in a variety of different ways. For example, different colors of lights, or different patterns or durations of flashes might be possible. Audible tones could also be used as feedback. 
     In step  250 , tracker  10  checks whether the command is garbled. In other words, is the meaning of the received command unclear? If the command is garbled, the flow returns to step  210 , where tracker  10  continues to scan for commands. Otherwise the flow continues to step  260 , where tracker  10  checks whether the command is incomplete. In other words, is more information needed to fully define the command? If the command is incomplete, the flow returns to step  210 , where tracker  10  continues to scan for commands. Otherwise the flow continues to step  270 . 
     In step  270 , tracker  10  executes whatever actions are required by the command. In some cases, the actions require multiple steps both on the part of the tracker and the operator. Examples of such cases are discussed below. In step  280 , tracker  10  signals that the measurement is complete. The flow then returns to step  210 , where the tracker continues to scan for commands. 
       FIG. 7  shows that step  220 , in which the operator signals a command, comprises three steps: step  222 -prologue, step  224 -directive, and step  226 -epilogue. The prologue and epilogue steps are optional. The directive part of the command is that part of the command that conveys the instructions to be followed. The prologue part of the command indicates to the tracker that the command is starting and the directive will soon be given. The epilogue part of the command indicates to the tracker that the command is over. 
       FIGS. 8-10  show two exemplary sets of gestures (“Example 1 gesture” and “Example 2” gesture) that correspond to an exemplary set of commands. The leftmost columns of  FIGS. 8-10  show the exemplary set of commands. Some of these commands are taken from FARO CAM2 software. Other commands are taken from other software such as SMX Insight software or the Utilities software shipped with the FARO laser tracker. Besides these examples, commands may be taken from other software or simply created for a particular need. In each of  FIGS. 8-10 , the second column shows a software shortcut in the CAM2 software, if available. An operator may press this software shortcut on the keyboard to execute the corresponding command. The third and fourth columns of  FIGS. 8-10  show some spatial patterns that might be used to represent a certain command. The two dimensional spatial patterns might be sensed using methods shown in  FIGS. 3A ,  4 A, or  5 D, for example. 
     For each of the gestures in the third and fourth columns in  FIGS. 8-10 , the starting position is indicated with a small circle and the ending position is indicated with an arrow. The gestures in the third column of  FIGS. 8-10  are simple shapes-circles, triangles, or squares. The 28 shapes shown in this column are distinguished from one another by their orientations and starting positions. In contrast, the shapes in the fourth column of  FIGS. 8-10  are suggestive of the command to be carried out. The main advantage of the shapes in the third columns is that these are easier for the computer to recognize and interpret as commands. This aspect is discussed in more detail below. The main advantage of the shapes in the fourth columns is that these may be easier for the operator to remember. 
       FIGS. 11A-11F  show some alternative spatial patterns that might be used in gestures.  FIG. 11A  shows single strokes;  FIG. 11B  shows alphanumeric characters;  FIG. 11C  shows simple shapes;  FIG. 11D  shows a simple path with the path retraced or repeated once;  FIG. 11E  shows a compound path formed of two or more simpler patterns; and  FIG. 11F  shows patterns formed of two or more letters. 
       FIG. 12  shows an exemplary command tablet  300 . The operator carries command tablet  300  to a convenient location near the position where the measurement is being made. Command tablet  300  may be made of stiff material having the size of a sheet of notebook paper or larger. The operator places command tablet  300  on a suitable surface and may use a variety of means to hold the target in place. Such means may include tape, magnets, hot glue, tacks, or Velcro. The operator establishes the location of command tablet  300  with the frame of reference of laser tracker  10  by touching fiducial positions  310 ,  312 , and  314  with retroreflector  26 . It would be possible to use multiple command tablets in a given environment. An exemplary procedure for finding the command tablet location is discussed below. 
     Command tablet  300  may be divided into a number of squares. In addition to the squares for fiducial positions  310 ,  312 , and  314 , there are squares for commands in  FIGS. 8-10 , and other squares corresponding to target type, nest type, direction, and number. The layout and contents of exemplary command tablet  300  is merely suggestive, and the command tablet may be effectively designed in a wide variety of ways. A custom command tablet may also be designed for a particular job. 
     To gesture a command to laser tracker  10 , the operator touches the retroreflector to the desired square on command tablet  300 . This action by the operator corresponds to step  220  in  FIG. 200 . Sensing of the action may be carried out by methods shown in  FIGS. 3C  or  4 C, for example. If a sequence involving multiple numbers is to be entered—for example, the number 3.50-then the squares 3, point, 5, and 0 would be touched in order. As is discussed below, there are various ways of indicating to the tracker that a square is to be read. One possibility is to wait a preset time-say, for at least two seconds. The tracker will then give a signal, which might be a flashing light, for example, indicating that it has read the contents of the square. When the entire sequence of numbers has been entered, the operator may terminate the sequence in a predetermined way. For example, the agreed upon terminator might be to touch one of the fiducial points. 
     Command tablet  300  may also be used with an articulated arm CMM instead of a laser tracker. An articulated arm CMM comprises a number of jointed segments attached to a stationary base on one end and a probe, scanner, or sensor on the other end. Exemplary articulated arm CMMs are described in U.S. Pat. No. 6,935,036 to Raab et al., which is incorporated by reference herein, and U.S. Pat. No. 6,965,843 to Raab et al., which is incorporated by reference herein. The probe tip is brought into contact with the squares of command tablet  300  in the same way as the retroreflector target is brought into contact with the squares of command tablet  300  when using a laser tracker. An articulated arm CMM typically makes measurement over a much smaller measurement volume than does a laser tracker. For this reason, it is usually easy to find a convenient place to mount command tablet  300  when using an articulated arm CMM. The particular commands included in command tablet  300  would be adapted to commands appropriate for the articulated arm CMM, which are different than commands for the laser tracker. The advantage of using a command tablet with an articulated arm CMM is that it saves the operator the inconvenience and lost time of setting down the probe, moving to the computer, and entering a command before returning to the articulated arm CMM. 
     We now give four examples in  FIGS. 13-16  of how gestures may be used.  FIG. 13  shows gestures being used to set a reference point for exemplary laser tracker  10 . Recall from the earlier discussion that Auto Reset is a possible option mode of a laser tracker. If the laser tracker is set to the Auto Reset option, then whenever the beam path is broken, the laser beam will be directed to the reference position. A popular reference position is the home position of the tracker, which corresponds to the position of a magnetic nest permanently mounted on the body of the laser tracker. Alternatively, a reference point close to the work volume may be chosen to eliminate the need for the operator to walk back to the tracker when the beam is broken. (Usually this capability is most important when the tracker is using an interferometer rather than an ADM to make the measurement.) 
     In  FIG. 13 , the actions shown in flow chart  400  are carried out to set a reference point through the use of gestures. In step  420 , the operator moves the target in the pattern shown for “Set Reference Point” in  FIG. 10 . The target in this case may be retroreflector  26 , for example, as shown in  FIG. 3A . In step  430 , laser tracker  10  intercepts and parses the command and acknowledges that the command has been received. In this case, the form of acknowledgement is two flashes of the red light on the tracker front panel. However, other feedback such as a different color or pattern, or an audible tone may be used. In step  440 , the operator places SMR  26  into the magnetic nest that defines the reference position. Laser tracker  10  continually monitors position data of SMR  26  and notes when it is stationary. If the SMR is stationary for five seconds, tracker  10  recognizes that the operator has intentionally placed the SMR in the nest, and the tracker begins to measure. A red light on the tracker panel, for example, may be illuminated while the measurement is taking place. The red light goes out when the measurement is completed. 
     In  FIG. 14 , the actions shown in flow chart  500  are carried out to establish the position of exemplary command tablet  300  in three-dimensional space. Recall from the earlier discussion that command tablet  300  has three fiducial positions  310 ,  312 , and  314 . By touching a retroreflector target to these three positions, the position of command tablet  300  in three-dimensional space can be found. In step  510 , the operator moves the target in the pattern shown for “Initialize Command Tablet” in  FIG. 9 . The target in this case may be retroreflector  26 , for example, as shown in  FIG. 3A . In step  520 , laser tracker  10  intercepts and parses the command and acknowledges that the command has been received by flashing the red light twice. In step  530 , the operator holds SMR  26  against one of the three fiducial points. Laser tracker  10  continually monitors position data of SMR  26  and notes when the SMR is stationary. In step  540 , if SMR  26  is stationary for five seconds, tracker  10  measures the position of SMR  26 . In step  550 , the operator holds SMR  26  against a second of the three fiducial points. In step  560 , if SMR  26  is stationary for five seconds, tracker  10  measures the position of SMR  26 . In step  570 , the operator holds SMR  26  against the third of the three fiducial points. In step  580 , if SMR  26  is stationary for five seconds, tracker  10  measures the position of SMR  26 . Now tracker  10  knows the three-dimensional positions of each of the three fiducial points, and it can calculate the distance between these three pairs of points from these three points. In step  590 , tracker  10  searches for an error by comparing the known distances between the points to the calculated distances between the points. If the differences are too large, a signal error is indicated in step  590  by a suitable indication, which might be flashing of the red light for five seconds. 
     In  FIG. 15 , the actions shown in flow chart  600  are carried out to measure a circle through the use of gestures. In step  610 , the operator moves the target in the pattern shown for “Measure a Circle” in  FIG. 8 . The target in this case may be retroreflector  26 , for example, as shown in  FIG. 3A . In step  620 , laser tracker  10  intercepts and parses the command and acknowledges that the command has been received by flashing the red light twice. In step  630 , the operator holds retroreflector  26  against the workpiece. For example, if the operator is measuring the inside of a circular hole, he will place the SMR against the part on the inside of the hole. Laser tracker  10  continually monitors position data of retroreflector  26  and notes when the SMR is stationary. In step  640 , after retroreflector  26  is stationary for five seconds, the red light comes on and tracker  10  commences continuous measurement of the position of retroreflector  26 . In step  650 , the operator moves retroreflector  10  along the circle of interest. In step  660 , when enough points have been collected, the operator moves retroreflector  26  away from the surface of the object being measured. The movement of retroreflector  26  indicates that the measurement is complete. It also indicates whether retroreflector target  26  is measuring an inner diameter or outer diameter and enables the application software to remove an offset distance to account for the radius of retroreflector  26 . In step  670 , tracker  10  flashes the red light twice to indicate that the required measurement data has been collected. 
     In  FIG. 16 , the actions shown in flow chart  700  are carried out to acquire a retroreflector after the laser beam from laser tracker  10  has been broken. In step  710 , the operator moves the retroreflector in the pattern shown for “Acquire SMR” in  FIG. 10 . The target in this case may be retroreflector  26 , for example, as shown in  FIG. 4A . At the beginning of this procedure, the SMR has not acquired the SMR and hence the modes shown in  FIGS. 3A-3E  cannot be used. Instead cameras  52  and light sources  54  are used to locate retroreflector  26 . In step  720 , laser tracker  10  intercepts and parses the command and acknowledges that the command has been received by flashing the red light twice. At the same time, it drives the laser beam  46  toward the center of retroreflector  26 . In step  730 , tracker  10  checks whether the laser beam has been captured by retroreflector  26 . In most cases, the laser beam is driven close enough to the center of retroreflector  26  that it lands within the active area of the position detector within the tracker. In this case, the tracker servo system drives the laser beam in a direction that moves the laser beam toward the center of the position detector, which also causes the laser beam to move to the center of retroreflector  26 . Normal tracking occurs thereafter. If the laser beam is not driven close enough to the center of retroreflector  26  to land on the position detector within the tracker, then one possibility is to perform a spiral search, as shown in step  740 . Laser tracker  10  carries out a spiral search by aiming the laser beam in a starting direction and then directing the beam in an ever widening spiral. Whether or not to perform a spiral search can be set as an option with the laser tracker or the application software used with the laser tracker. Another option, which might be appropriate for a rapidly moving target, is to repeat step  720  repeatedly until the laser beam is captured by the retroreflector or until there is a timeout. 
     As discussed previously with reference to  FIG. 7 , the operator signals a command through the use of three steps: an optional prologue, a directive, and an optional epilogue. If tracker  10  is constantly parsing data and can quickly respond when the desired pattern has been produced, then it may be possible to use the directive alone without the prologue or epilogue. Similarly, if the operator touches a position on command tablet  300 , the command should be clear to the tracker without the need for a prologue or epilogue. On the other hand, if the tracker cannot parse quickly enough to respond immediately to the patterns created by the operator, or if there is a chance that the operator might create a command pattern unintentionally, then use of a prologue, epilogue, or both may be needed. 
     An example of a simple prologue or epilogue is simply a pause in the movement of the target, which might be any of the targets shown in  FIGS. 3A-3E ,  4 A- 4 C, and  5 A- 5 D. For example, the operator may pause for one or two seconds before the start of a pattern and one or two seconds at the end of the pattern. By pausing in this way, the starting and ending positions of each gesture, indicated by circles and arrows, respectively, in  FIGS. 8-10  and by circles and squares, respectively, in  FIG. 11  will be more easily understood by the parsing software within the tracker or computer. 
     Another example of a simple prologue or epilogue is rapid blocking and unblocking of the laser beam from the tracker. For example, the operator may splay his fingers so that there is a space between each of the four digits. Then by moving his fingers rapidly across the laser beam, the beam will be broken and unbroken four times in rapid succession. Such a temporal pattern, which might be referred to as the “four finger salute”, is readily recognized by the laser tracker. The modes of sensing based on temporal variations in returned laser power are shown in  FIG. 3D  with a passive target and in  FIGS. 5A-5C  with active targets. 
     Besides the use of a prologue or epilogue in the gestural command, a type of prologue is also sometimes needed at the start of an action by the laser tracker. For example, in the examples of  FIGS. 13-15 , there is a wait of five seconds after a command is given before the tracker measurement is made. The purpose of this wait is to give the operator time to get the retroreflector target into position before beginning the measurement. Of course, the time of five seconds is arbitrary and could be set to any desired value. In addition, it would be possible to use other indicators that the measurement should begin. For example, it would be possible to use a four-finger salute rather than a time delay to indicate readiness for measurement. 
     Active targets such as those shown in  FIGS. 5A-D  are useful in applications such as tool building and device assembly. A tool is a type of apparatus made to assist in the manufacture of other devices. In fields such as automotive and aerospace manufacturing, tools are constructed to exacting specifications. The laser tracker helps both in assembling and in checking such tools. In many cases, it is necessary to align the component elements of a tool with respect to one another. A single retroreflector target, such as retroreflector  26 , can be used to establish a coordinate system to which each element in the tool can be properly aligned. In a complicated tool, however, this can involve a lot of iterative measuring. An alternative is to mount multiple retroreflector targets on the tooling elements and then measure all of these in rapid succession. Such rapid measurement is made possible today by modern tracker technologies such as absolute distance meters and camera systems (such as components  42 ,  44 ). If multiple retroreflectors are mounted directly on tooling, then it may be difficult or inefficient for an operator to use one of these retroreflectors to create gestural commands. It may be more convenient to use a wand such as  140  shown in  FIGS. 5C  or  5 D. The operator can quickly give commands using a wand without disturbing the retroreflectors mounted on the tooling. Such a wand may be mounted on the end of a hammer or similar device to leave the operator&#39;s hands free to perform assembly and adjustment. In some cases, a separate retroreflector or six-DOF probe, like those shown in  FIGS. 5A and 5B , respectively, may be needed during tool building. By adding a light source and control button to the basic SMR or six-DOF probe, the operator can issue commands in a very flexible way. 
     Active targets such as those shown in  FIGS. 5A-D  are also useful in device assembly. A modern trend is flexible assembly using laser trackers rather than automated tooling assembly. An important advantage of the tracker approach is that little advance preparation is required. One thing that makes such assembly practical today is the availability of software that matches CAD software drawings to measurements made by laser trackers. By placing retroreflectors on the parts to be assembled and then sequentially measuring the retroreflectors with a laser tracker, the closeness of assembly can be shown on a computer display using colors such as red to indicate “far away”, yellow to indicate “getting closer”, and green to indicate “close enough”. Using an active target, the operator can give commands to measure selected targets or groups of targets in ways to optimize the assembly process. 
     Multiple retroreflectors are often located in a single measurement volume. Examples for tool building and device assembly with multiple retroreflectors were described above. These examples showed that an active target can be particularly useful. In other cases, the ability of the laser tracker to recognize movements of multiple passive retroreflectors can be useful. For example, suppose that multiple retroreflectors have been placed on a tooling fixture such as a sheet metal stamping press and the operator wants to perform a target survey after each operation of the fixture. The survey will sequentially measure the coordinates of each target to check the repeatability of the tooling fixture. An easy way for the operator to set up the initial survey coordinates is to sequentially lift each retroreflector out of its nest and move it around according to a prescribed gestural pattern. When the tracker recognizes the pattern, it measures the coordinates of the retroreflector in its nest. It is the ability of the tracker cameras to recognize gestural patterns over a wide field of view that enables the operator to conveniently switch among retroreflectors. 
     As mentioned previously, there are several different types of methods or algorithms that can be used to identify gestural patterns and interpret these as commands. Here we suggest a few methods, while recognizing that a wide variety of methods or algorithms could be used and would work equally well. As explained earlier, there are three main types of patterns of interest: (1) single-point absolute position, (2) temporal patterns, and (3) movement patterns. Recognizing single-point absolute position is arguably the easiest of these three categories. In this case, the tracker simply needs to compare measured coordinates to see whether these agree to within a specified tolerance to a coordinate on the surface of command tablet  300 . 
     Temporal patterns are also relatively easy to identify. A particular pattern might consist of a certain number of on-off repetitions, for example, and additional constraints may be placed on the allowable on and off times. In this case, tracker  10  simply needs to record the on and off times and periodically check whether there is a match with a pre-established pattern. It would of course be possible to reduce the power level rather than completely extinguishing the light to send a signal to the tracker. Reduction in the level of retroreflected laser power could be obtained by many means such as using a neutral density filter, polarizer, or iris. 
     Movement patterns may be parsed in one, two, or three dimensions. A change in radial distance is an example of a one-dimensional movement. A change in transverse (up-down, side-to-side) movement is an example of two-dimensional measurement. A change in radial and transverse dimensions is an example of three-dimensional measurement. Of course, the dimensions of interest are those currently monitored by the laser tracker system. One way to help simplify the parsing and recognition task is to require that it occur within certain bounds of time and space. For example, the pattern may be required to be between 200 mm and 800 mm (eight inches and 32 inches) in extent and to be completed in between one and three seconds. In the case of transverse movements, the tracker will note the movements as changes in angles, and these angles in radians must be multiplied by the distance to the target to get the size of the pattern. By restricting the allowable patterns to certain bounds of time and space, many movements can be eliminated from further consideration as gestural commands. Those that remain may be evaluated in many different ways. For example, data may be temporarily stored in a buffer that is evaluated periodically to see whether a potential match exists to any of the recognized gestural patterns. A special case of a gestural movement pattern that is particularly easy to identify is when the command button  124  in  FIG. 5A  is pushed to illuminate light  122  to indicate that a gesture is taking place. The computer then simply needs to record the pattern that has taken place when light  122  was illuminated and then evaluate that pattern to see whether a valid gesture has been generated. A similar approach can be taken when the operator presses command button  134  to illuminate light  132  in  FIG. 5B  or presses command button  144  to illuminate light  142  in  FIG. 5D . 
     Besides these three main patterns, it is also possible to create patterns made using a passive object or a passive object in combination with a retroreflector. For example, the cameras on the tracker might recognize that a particular command is given whenever a passive red square of a certain size is brought within one inch of the SMR. 
     It would also be possible to combine two of the three main patterns. For example, it would be possible to combine both the speed of movement with a particular spatial pattern, thereby combining pattern types two and three. As another example, the operator may signal a particular command with a saw tooth pattern comprising a rapid movement up, followed by a slow return. Similarly acceleration might be used. For example, a flick motion might be used to “toss” a laser beam away in a particular direction around an object. 
     Variations are also possible within types of patterns. For example, within the category of spatial patterns, it would be possible to distinguish between small squares (say, three-inches on a side) and large squares (say, 24 inches on a side). 
     The methods of algorithms discussed above are implemented by means of processing system  800  shown in  FIG. 17 . Processing system  800  comprises tracker processing unit  810  and optionally computer  80 . Processing unit  810  includes at least one processor, which may be a microprocessor, digital signal processor (DSP), field programmable gate array (FPGA), or similar device. Processing capability is provided to process information and issue commands to internal tracker processors. Such processors may include position detector processor  812 , azimuth encoder processor  814 , zenith encoder processor  816 , indicator lights processor  818 , ADM processor  820 , interferometer (IFM) processor  822 , and camera processor  824 . It may include gestures preprocessor  826  to assist in evaluating or parsing of gestures patterns. Auxiliary unit processor  870  optionally provides timing and microprocessor support for other processors within tracker processor unit  810 . It may communicate with other processors by means of device bus  830 , which may transfer information throughout the tracker by means of data packets, as is well known in the art. Computing capability may be distributed throughout tracker processing unit  810 , with DSPs and FPGAs performing intermediate calculations on data collected by tracker sensors. The results of these intermediate calculations are returned to auxiliary unit processor  870 . As explained previously, auxiliary unit  70  may be attached to the main body of laser tracker  10  through a long cable, or it may be pulled within the main body of the laser tracker so that the tracker attaches directly (and optionally) to computer  80 . Auxiliary unit  870  may be connected to computer  80  by connection  840 , which may be an Ethernet cable or wireless connection, for example. Auxiliary unit  870  and computer  80  may be connected to the network through connections  842 ,  844 , which may be Ethernet cables or wireless connections, for example. 
     Preprocessing of sensor data may be evaluated for gestures content by any of processors  812 - 824 , but there may also be a processor  826  specifically designated to carry out gestures preprocessing. Gestures preprocessor  826  may be a microprocessor, DSP, FPGA, or similar device. It may contain a buffer that stores data to be evaluated for gestures content. Preprocessed data may be sent to auxiliary unit for final evaluation, or final evaluation of gestures content may be carried out by gestures preprocessor  826 . Alternatively, raw or preprocessed data may be sent to computer  80  for analysis. 
     Although the use of gestures described above has mostly concentrated on their use with a single laser tracker, it is also beneficial to use gestures with collections of laser trackers or with laser trackers combined with other instruments. One possibility is to designate one laser tracker as the master that then sends commands to other instruments. For example, a set of four laser trackers might be used in a multilateration measurement in which three-dimensional coordinates are calculated using only the distances measured by each tracker. Commands could be given to a single tracker, which would relay commands to the other trackers. Another possibility is to allow multiple instruments to respond to gestures. For example, suppose that a laser tracker were used to relocate an articulated arm CMM. An example of such a system is given in U.S. Pat. No. 7,804,602 to Raab, which is incorporated by reference herein. In this case, the laser tracker might be designated as the master in the relocation procedure. The operator would give gestural commands to the tracker, which would in turn send appropriate commands to the articulated arm CMM. After the relocation procedure was completed, the operator could use a command tablet to give gestural commands to the articulated arm CMM, as described above. 
       FIG. 19  shows steps  1900  that are carried out in giving a gesture to communicate a command to the laser tracker according to the discussions that referenced  FIGS. 3A-B ,  4 A-B, and  5 A. Step  1910  is to provide a rule of correspondence between commands and spatial patterns. Step  1920  is for the user to select a command from among the possible commands. Step  1930  is for the user to move the retroreflector in a spatial pattern corresponding to the desired command. The spatial pattern might be in transverse or radial directions. Step  1940  is to project a light from the laser tracker to the retroreflector. This light may be a beam of light emitted along the optical axis of the laser tracker or it may be light emitted by an LED near a camera disposed on the laser tracker. Step  1950  is to reflect light from the retroreflector back to the laser tracker. Step  1960  is to sense the reflected light. The sensing may be done by a photosensitive array within a camera disposed on the tracker; by a position detector in the tracker, or by a distance meter within the tracker. Step  1970  is to determine the command based on the rule of correspondence. Step  1980  is to execute the command. 
       FIG. 20  shows steps  2000  that are carried out in giving a gesture to communicate a command to the laser tracker according to the discussions that referenced  FIGS. 3C ,  4 C, and  5 A. Step  2010  is to provide a rule of correspondence between commands and three-dimensional positions. Step  2020  is for the user to select a command from among the possible commands. Step  2030  is for the user to move the retroreflector to a position corresponding to the desired command, possibly by bringing the retroreflector target in contact with a command tablet. Step  2040  is to project a light from the laser tracker to the retroreflector. This light may be a beam of light emitted along the optical axis of the laser tracker or it may be light emitted by an LED near a camera disposed on the laser tracker. Step  2050  is to reflect light from the retroreflector back to the laser tracker. Step  2060  is to sense the reflected light. The sensing may be done by a photosensitive array within a camera disposed on the tracker; by a position detector in the tracker, or by a distance meter within the tracker. Step  2070  is to determine the command based on the rule of correspondence. Step  2080  is to execute the command. 
       FIG. 21  shows steps  2100  that are carried out in giving a gesture to communicate a command to the laser tracker according to the discussions that referenced  FIGS. 3D and 5A . Step  2110  is to provide a rule of correspondence between commands and temporal patterns. Step  2120  is for the user to select a command from among the possible commands. Step  2130  is to project a light from the laser tracker to the retroreflector. This light may be a beam of light emitted along the optical axis of the laser tracker or it may be light emitted by an LED near a camera disposed on the laser tracker. Step  2140  is to reflect light from the retroreflector back to the laser tracker. Step  2150  is to sense the reflected light. The sensing may be done by a photosensitive array within a camera disposed on the tracker; by a position detector in the tracker, or by a distance meter within the tracker. Step  2160  is for the user to create a temporal pattern in the optical power received by the sensors on the laser tracker. Such a temporal pattern is easily done by blocking and unblocking a beam of light as discussed hereinbelow. Step  2170  is to determine the command based on the rule of correspondence. Step  2180  is to execute the command. 
       FIG. 22  shows steps  2200  that are carried out in giving a gesture to communicate a command to a six DOF laser tracker according to the discussions that referenced  FIGS. 3E and 5B . Step  2210  is to provide a rule of correspondence between commands and pose of a six DOF target. Step  2220  is for the user to select a command from among the possible commands. Step  2230  is to use the six DOF laser tracker to measure at least one coordinate of a six DOF target in a first pose. A pose includes three translational coordinates (e.g., x, y, z) and three orientational coordinates (e.g., roll, pitch, yaw). Step  2240  is for the user to change at least one of the six dimensions of the pose of the six DOF target. Step  2250  is to measure the at least one coordinate of a second pose, which is the pose that results after the user has completed step  2240 . Step  2260  is to determine the command based on the rule of correspondence. Step  2270  is to execute the command. 
       FIG. 23  shows steps  2300  that are carried out in giving a gesture to communicate a command to the laser tracker to point the laser beam from the laser tracker to the target and lock onto the target. Step  2310  is to project light onto the retroreflector. This light may be light emitted by an LED near a camera disposed on the laser tracker. Step  2320  is for the user to move the retroreflector in a predefined spatial pattern. Step  2330  is to reflect light from the retroreflector to the laser tracker. Step  2340  is to sense the reflected light. The sensing may be done, for example, by a photosensitive array within a camera disposed on the tracker. Step  2350  is to determine the command based on the rule of correspondence. Step  2360  is to point the beam of light from the tracker to the retroreflector. Step  2370  is to lock onto the retroreflector with the laser beam from the tracker. 
       FIG. 24  shows steps  2400  that are carried out in giving a gesture to communicate a command to the laser tracker to point the laser beam from the laser tracker to the target and lock onto the target. Step  2410  is to project light onto the retroreflector. This light may be light emitted by an LED near a camera disposed on the laser tracker. Step  2420  is to reflect light from the retroreflector to the laser tracker. Step  2430  is to sense the reflected light. The sensing may be done, for example, by a photosensitive array within a camera disposed on the tracker. Step  2440  is to generate a predefined temporal pattern, as discussed hereinabove. Step  2450  is to determine the command based on the rule of correspondence. Step  2460  is to point the beam of light from the tracker to the retroreflector. Step  2470  is to lock onto the retroreflector with the laser beam from the tracker. 
       FIG. 25  shows steps  2500  that are carried out in giving a gesture to communicate a command to the laser tracker to point the laser beam from the laser tracker to the target and lock onto the target. Step  2510  is to project light onto the retroreflector. This light may be light emitted by an LED near a camera disposed on the laser tracker. Step  2520  is to measure at least one coordinate of a first pose of a six DOF target. As discussed hereinabove, the pose includes three translational and three orientational degrees of freedom. Step  2530  is to change at least one coordinate of a first pose. Step  2540  is to measure the at least one coordinate of a second pose, which is the pose that results after the at least one coordinate of the six DOF probe has been changed. Step  2550  is to determine the rule of correspondence has been satisfied. Step  2560  is to point the beam of light from the tracker to the retroreflector. Step  2570  is to lock onto the retroreflector with the laser beam from the tracker. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.