Abstract:
A laser based coordinate measuring device measures a position of a remote target. The laser based coordinate measuring device includes a stationary portion, a rotatable portion, and at least a first optical fiber. The stationary portion has at least a first laser radiation source and at least a first optical detector, and the rotatable portion is rotatable with respect to the stationary portion. The first optical fiber system, which optically interconnects the first laser radiation source and the first optical detector with an emission end of the first optical fiber system, has the emission end disposed on the rotatable portion. The emission end emits laser radiation to the remote target and receives laser radiation reflected from the remote target with the emission direction of the laser radiation being controlled according to the rotation of the rotatable portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation Application of U.S. Ser. No. 09/621,645 filed on Jul. 24, 2000, which claims the benefit of U.S. Patent Application Nos. 60/171,474 filed Dec. 22, 1999, 60/145,686 filed Jul. 26, 1999 and 60/145,315 filed Jul. 23, 1999, which are hereby incorporated by reference. The present application also hereby incorporates by reference U.S. patent application Ser. No. 09/285,654 filed Apr. 5, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coordinate measuring device and, more particularly, to a laser based coordinate measuring device. 
     2. Discussion of the Related Art 
     There is a class of instrument that measures the coordinates of a point by sending a laser beam to a retroreflector target that is in contact with the point. The instrument determines coordinates by measuring the distance and the two angles to the retroreflector target. There is another class of instrument that is capable of measuring the three orientation angles (pitch, yaw, and roll) of a retroreflector target. If such an instrument can also measure the three coordinates of a point in space, it is said to measure six degrees of freedom. However, such six degree-of-freedom systems, whether or not they are employing laser techniques, are generally inaccurate, slow, limited in radial or angular range, and/or expensive. Exemplary systems for determining position (three to six degrees of freedom) are described by U.S. Pat. No. 4,790,651 to Brown et al.; U.S. Pat. No. 4,714,339 to Lau et al.; U.S. Pat. No. 5,5059,789 to Salcudean; U.S. Pat. No. 5,367,373 to Busch-Vishniac et al.; U.S. Pat. No. 5,973,788 to Pettersen et al.; and U.S. Pat. No. 5,267,014 to Prenninger, et al. (the disclosures of which are hereby incorporated by reference). 
     The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. To provide a beam-steering mechanism for this tracking function, laser trackers conventionally include a stationary base onto which a rotating stage or platform is mounted. Until now, most laser trackers have used optical elements, such as mirrors or prisms, to steer the laser beam from its source in the base to optics in the rotating stage and through or off those optics toward the retroreflector. These optical elements and their mounts are costly. Also, they are subject to tilting and bending as a result of thermal and/or mechanical stresses that are usually present in tracker work environments. The consequence of these stresses is reduced accuracy and stability. Examples of beam-steering laser trackers are described by Lucy, et al., Applied Optics, pp. 517-524, 1966; Bernard and Fencil, Applied Optics, pp. 497-505, 1966; Sullivan, SPIE, Vol. 227, pp. 148-161, 1980; U.S. Pat. No. 4,020,340 to Cooke; U.S. Pat. No. 4,025,193 to Pond; U.S. Pat. No. 4,386,848 to Clendenin et al.; U.S. Pat. No. 4,436,417 to Hutchin; U.S. Pat. No. 4,457,625 to Greenleaf et al.; U.S. Pat. No. 4,714,339 to Lau et al.; U.S. Pat. No. 4,721,385 to Jelalian et al.; Gennan Patent DE 3205362 A1 to Pfeifer et al. (which are hereby incorporated by reference). An example of a beam-steering mechanism that uses prismatic optical elements is described by U.S. Pat. No. 4,790,65 1 Brown et al. (which is hereby incorporated by reference). 
     A device that is closely related to a laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface. The laser tracker and laser scanner are both coordinate-measuring devices. It is common practice today to use the term laser tracker to also refer to laser scanner devices having distance- and angle-measuring capability. This broad definition of laser tracker, which includes laser scanners, is used throughout this application. 
     An alternative to steering the laser beam with a mirror or prism is to launch the laser beam from an optical fiber mounted on a rigid platform. Although such devices have been built, none has taken full advantage of the simplicity, stability, and flexibility possible with such an approach. For example, such systems usually require separate optical fibers for transmitting and receiving the laser light. An exemplary system that tracks a laser beam launched from an optical fiber is described in Nakamura, et al., Review of Scientific Instruments, pp. 1006-1011, 1994; Takatsuji et al., easurement Science &amp; Technology, pp. 38-41, 1998; Takatsuji, et al., Measurement Science &amp; Technology, pp. 1357-1359, 1998; and Takatsuji, et al., Dimensional Metrology in the 21 st  Century, International Dimensional Metrology Workshop sponsored by Oak Ridge Metrology Center, May 10-13, 1999 (which are hereby incorporated by reference). Non-tracking systems that launch laser beams from optical fibers are numerous in the prior art and include U.S. Pat. No. 4,459,022 to Morey; U.S. Pat. No. 5,095,472 to Uchino, et al.; U.S. Pat. No. 5,198,874 to Bell et al.; U.S. Pat. No. 5,200,838 to Nudelman; U.S. Pat. No. 5,402,230 to Tian, et al.; U.S. Pat. No. 5,508,804 to Furstenau; and U.S. Pat. No. 5,557,406 to Taylor (which are hereby incorporated by reference). 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a laser-based coordinate measuring device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a laser-based coordinate measuring device with improved laser beam steering, six degree of freedom measurements, and capability to locate multiple retroreflectors distributed throughout large volumes. 
     Another object of the present invention is to provide a reliable laser-based coordinate measuring device that is easily manufactured at a low cost without complex beam-steering optics. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a laser based coordinate measuring device for measuring a position of a remote target comprising a stationary portion having at least a first laser radiation source and at least a first optical detector; a rotatable portion that is rotatable with respect to the stationary portion; and at least a first optical fiber system for optically interconnecting the first laser radiation source and the first optical detector with an emission end of the first optical fiber system, the emission end disposed on the rotatable portion for emitting laser radiation to the remote target and for receiving laser radiation reflected from the remote target, wherein an emission direction of the laser radiation is controlled according to the rotation of the rotatable portion. 
     In another aspect, a laser based coordinate measuring device comprises a rigid structure rotatable about two substantially orthogonal axes; a laser radiation source disposed off the rigid structure to provide laser radiation; an optical detector disposed off the rigid structure; a retroreflective target disposed remote from the rigid structure; a first optical fiber path optically coupled with the laser radiation source to transmit laser radiation from the laser radiation source to the rigid structure, the first optical fiber path having an end disposed on the rigid structure for emitting the laser radiation to the retroreflective target according to an orientation of the rigid structure and for receiving retroreflected radiation reflected by the retroreflective target; and an optical coupler optically connecting the optical detector with the first optical fiber path to receive the retroreflected radiation. 
     In another aspect, a laser based coordinate measuring device for measuring a position of a remote target comprises a stationary portion having at least a first laser radiation source; a rotatable portion that is rotatable about first and second axes of rotation with respect to the stationary portion; an optical fiber path for optically interconnecting the first laser radiation source with the rotatable portion, wherein a first portion of the optical fiber path is disposed along the first axis and a second portion of the optical fiber path is disposed along the second axis. 
     In another aspect, a laser based coordinate measuring device comprises a structure rotatable about two substantially orthogonal axes; a laser radiation source disposed off the rotatable structure to provide laser radiation; a retroreflective target disposed remote from the rotatable structure, the retroreflective target having a pattern thereon; an optical system for directing the laser radiation from the laser radiation source to the rotatable structure and then to the retroreflective target in accordance with the rotation of the rotatable structure, the retroreflective target reflecting the laser radiation to the rotatable structure; and an orientation camera optically coupled with the reflected laser radiation to determine an orientation of the retroreflective target, the orientation camera including a detector and a lens system that forms an image of the pattern on the detector. 
     In another aspect, a laser based coordinate measuring device comprises a structure rotatable about two substantially orthogonal axes; a laser radiation source disposed off the rotatable structure to provide laser radiation; a retroreflective target disposed remote from the rotatable structure; an optical system for directing the laser radiation from the laser radiation source to the rotatable structure and then to the retroreflective target in accordance with the rotation of the rotatable structure, the retroreflective target reflecting the laser radiation to the rotatable structure; and an orientation camera disposed on the rotatable structure and optically coupled with the reflected laser radiation to determine a three dimensional orientation of the retroreflective target. 
     In another aspect, a laser based coordinate measuring system comprises a structure rotatable about two substantially orthogonal axes; a target disposed remote from the rotatable structure; a locator camera disposed on the rotatable structure for determining an approximate location of the target; and an actuator system to orient the rotatable structure in accordance with the location determined by the locator camera. 
     In another aspect, a laser based method for measuring coordinates of a remote retroreflective target comprises the steps of coupling laser radiation into a first end of an optical fiber path, the optical fiber path having a second end disposed on a rotatable structure; controlling the rotation of the rotatable structure to direct the laser radiation to the remote retroreflective target; coupling a first portion of retroreflected laser radiation with an orientation camera; coupling a second portion of the retroreflected laser radiation with a distance meter; and calculating three positional and three orientational degrees of freedom of the remote retroreflective target. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention. In the drawings: 
         FIG. 1  depicts an embodiment of a laser tracker according to the present invention with a beam-steering mechanism and six degree-of-freedom measurement capability; 
         FIG. 2  depicts in block form the major components of a rigid structure of the laser tracker of  FIG. 1 ; 
         FIG. 3  depicts in block form components of the beam combiner of  FIG. 2 ; 
         FIG. 4  depicts the components of the coupler assembly of  FIG. 3 ; 
         FIG. 5  depicts the components of the interferometer assembly of  FIG. 3 ; 
         FIG. 6  depicts in block form components of the beam expander of  FIG. 2 ; 
         FIG. 7  depicts in block form components of the orientation camera of  FIG. 2  showing the locations of the intermediate and final images; 
         FIGS. 8a and 8b  define the coordinate system for an unrotated cube-corner retroreflector; 
         FIGS. 9a and 9b  show the effect of pitch angle on the retroreflector; 
         FIGS. 10a and 10b  show the effect of yaw angle on the retroreflector; 
         FIGS. 11a and 11b  show the effect of roll angle on the retroreflector; 
         FIG. 12  illustrates the appearance of the image on the photosensitive array within the orientation camera; 
         FIG. 13  depicts the laser tracker of  FIG. 1  where the rigid structure is rotated to enable a wide-field locator camera to simultaneously view plural retroreflector targets; 
         FIG. 14a  is a front view of the locator camera on the rigid structure; 
         FIG. 14b  is a cross sectional view of the locator camera of  FIG. 14a  taken along line  14 b- 14 b; 
         FIGS. 15a-15c  depict the formation of an image on the wide-field locator camera; 
         FIG. 16  depicts a method of routing optical fibers near the two mechanical axes; 
         FIG. 17  depicts a probe assembly of the preferred second embodiment; and 
         FIG. 18  depicts a conventional laser tracker to which an orientation camera has been added for measuring six degrees of freedom. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     The present invention may be implemented as a laser-based coordinate measurement machine, laser tracker, or other suitable system. The present invention provides a new type of beam-steering mechanism; the ability to measure six degrees of freedom rather than just three degrees of freedom; and the ability to locate a plurality of retroreflector targets over a relatively wide field of view. 
     The invention does not require beam-steering optics because the laser light is routed through the laser tracker with optical fibers rather than with beam-steering mirrors or prisms. Laser light is processed, detected, and analyzed by optical and electrical components located for the most part away from the rotating elements within the tracker. One advantage of this approach is that it reduces the size and cost of the laser tracker system. Another advantage is that it improves accuracy and stability of the laser tracker system. The architecture is flexible enough to allow any number of laser beams to be launched without the use of optical beam-steering components. 
     The invention also provides the laser tracker with the ability to measure the six degrees of freedom of a target object which, in an exemplary embodiment, may be a cube-corner retroreflector. A hidden-point probe (capable of measuring points that are obscured from view) can be formed by attaching the target object to one end of a shaft and a probe tip to the other end of the shaft. The target object may also be attached directly to a machine tool or to the end-effector of a robot to more precisely control the movement of the tool or robot. 
     The invention also provides the laser tracker with the ability to determine the location of a plurality of retroreflector targets over a relatively large volume surrounding the tracker. To activate the target-locator feature of the tracker, the rotating portion of the tracker is turned to bring a ring of LED&#39;s surrounding a lens and photosensitive array to face the retroreflector targets. Flashes of light from the LED&#39;s travel to the retroreflectors then return to the tracker, where they pass through the lens onto the photosensitive array. The locations of the spots on the array indicate the angular directions of the targets. 
     Because the invention has the capability of launching multiple laser beams of different types, several modes of distance measurement are possible. One mode of distance measurement uses a laser beam that tracks a retroreflector to indicate either absolute or incremental distance. Another mode of distance measurement uses a laser beam to scan a diffuse surface. Either or both modes of distance measurement may be included in a given coordinate-measuring device. 
       FIG. 1  shows a perspective, block-diagram view of a laser tracking system according to an embodiment of the invention. The laser tracking system comprises a laser tracker  100  and a probe assembly  180 . The probe assembly comprises target object  185 , adjustable stage  181 , probe shaft  170 , and probe tip  171 . The target object  185  comprises retroreflector  107  and housing  109  comprise target object  185 . Laser tracker  100  emits a laser beam  153  toward cube-corner retroreflector  107  mounted on housing  109 . Housing  109  is attached to adjustable stage  181  that is designed to pivot about axis  182  and lock into place. Adjustable stage  181  is attached to probe shaft  170 , which is attached on the opposing end to probe tip  171 . Probe tip  171  is held in contact with the object  175  under evaluation. With the combination of laser tracker  100 , computer  25 , and probe assembly  180 , it is possible to measure the coordinates of the object  175  under evaluation, even if object  175  is not directly accessible to the laser beam emitted from the tracker. 
     The configuration of the laser tracker of  FIG. 1  will now be described. Laser  102  on stationary base  101  of the laser tracker  100  injects laser light (at least essentially coherent light having one wavelength) into a first end of an optical fiber  111 . The laser of the absolute-distance meter (ADM)  103  injects laser light into a first end of an optical fiber  115  (as shown in  FIG. 3 ) that is contained within optical fiber assembly  112 . Laser  104  injects laser light into a first end of an optical fiber  110 . These optical fibers are routed to rigid structure  190 , at which location laser light is launched out of the second end of optical fibers  110 ,  111 ,  115  (as shown in  FIG. 3 ). If desired, the combined laser light may be conditioned by optical elements within rigid structure  190  and emitted from rigid structure  190  as laser beam  153 . Laser light  153  travels to retroreflector  107 , where it is reflected parallel to laser beam  153 . If laser beam  153  is centered on the vertex of the cube-corner retroreflector, then laser beam  163  will coincide with laser beam  153 . That is, laser beam  163  will retrace the path of laser beam  153 . The laser beam  163  enters rigid structure  190 , where it is conditioned, if desired, and injected back into optical fibers  110 ,  111 , and/or  115 , or otherwise detected and processed as explained below. Rigid structure  190  is rotated by motor  81  with the angle of rotation indicated by angular encoder  91 . Steering platform  195  includes rigid structure  190  and the elements mounted thereto, i.e., motor  81  and angular encoder  91 . Steering platform  195  is turned on base  101  by motor  80 , with the angle of rotation indicated by angular encoder  90 . Rigid structure  190  is therefore supported for rotation about two orthogonal axes on base  101 . 
     ADM  103  measures the absolute distance from laser tracker  100  to retroreflector  107 . This device is capable of measuring the distance to retroreflector  107  in a single shot. Consequently, it can be used to perform rapid point-and-shoot measurements of multiple retroreflector targets. Laser  104  is used in conjunction with optical and electrical elements to measure the incremental distance moved by retroreflector  107 . An example of a device that measures incremental-distance movement is the laser interferometer which measures the number of interference fringes that occur as the retroreflector is moved from a starting position. In a laser interferometer, if an obstruction is placed in the path of the interferometer&#39;s laser beam, all displacement information will be lost. In this circumstance, if a laser-tracker system has only an incremental-distance measurement system and not an absolute-distance measurement system, then the retroreflector must be returned to a reference position and the measurement started anew. Laser  102  is a stand-alone laser and will be discussed in more detail with reference to fiber launch assembly  310  of  FIG. 3 . Any number of laser beams may be sent over optical fibers into rigid structure  190 . 
     Electronics box  140  provides electrical power to motors  80  and  81 , angular encoders  90  and  91 , lasers  102  and  104 , ADM  103 , as well as other electrical components within rigid structure  190 . Electronics box  140  analyzes signals from angular encoders  90  and  91 , from ADM  103 , and from other electrical components to calculate angles and distances from tracker  100  to retroreflector  107 . Electronics box  140  is attached to computer  25 , which provides application software for the advanced analysis of coordinate data. 
     The preferred optical elements within rigid structure  190  are shown in block diagram form in  FIG. 2 . The main functional blocks within rigid structure  190  include beam-combiner block  200 , orientation-camera block  210 , beam-expander block  220 , and locator-camera block  230 . 
     The optical fiber assemblies  110 ,  111 ,  112 , etc. are routed into beam-combiner block  200 , which combines the laser beams and sends them out as a single composite laser beam  250  that includes coherent light at a plurality of separate, discrete wavelengths. The composite laser beam  250  passes through orientation-camera block  210  to become laser beam  251 . Laser beam  251  is beam expanded by beam-expander block  220 , thereby exiting rigid structure  190  as expanded laser beam  153 . The laser beam  153  travels to retroreflector  107  as shown in  FIG. 1  and returns as laser beam  163 . The laser beam  163  retraces the path of the outgoing laser beams  153 ,  251 ,  250  back through beam-expander block  220  and orientation-camera block  210  into beam-combiner block  200 . Electrical lines  41  provide power from electronics box  140  to mechanical and electro-optical devices. Electrical lines  41  also route electrical signals from electro-optical devices to electronics box  140  for analysis. Rigid structure  190  rotates around the center of shaft  270  which is attached to motor  81  as shown in  FIG. 1 . In a typical mode of operation, motor  81  rotates rigid structure  190  so that laser beam  153  points toward retroreflector  107 , thereby causing laser beam  163  to retrace the path of laser beam  153 . In another mode of operation, motor  81  rotates rigid structure  190  until aperture  231  of locator camera  230  is aimed in the general direction of one or more retroreflectors in the surrounding environment. Locator camera  230  determines the approximate location of the retroreflector targets within a wide field of view. 
     Beam-Combiner Block 
       FIG. 3  shows diagrammatically the optical and electro-optical components within a preferred beam-combiner block  200 . The main assemblies within beam-combiner block  200  are first laser-beam fiber launch and pickup assembly  300 , second laser-beam fiber launch and pickup assembly  320 , laser-beam fiber launch assembly  310 , and position-detector assembly  340 . 
     First laser-beam fiber launch and pickup assembly  300  receives light an end of optical fiber  115  which is attached at its opposing end to a laser in the laser and ADM  103  shown in  FIG. 1 . Laser light (at least one essentially coherent light at a first discrete frequency) travels in optical fiber  115  until it reaches coupler assembly  305 . Part of the laser light emerging from coupler assembly  305  is in optical fiber  306 . It travels to fiber termination  301 , at which point it diverges as cone of light  360 . Lens  302  collimates this light as laser beam  361  which passes through beam splitter  314  to become laser beam  365 . Another part of the laser light emerging from coupler assembly  305  travels through an optical fiber  307  to fiber retroreflector  303  and returns through optical fiber  307  into coupler assembly  305  thereby forming a reference path as will be discussed with reference to  FIG. 4 . 
     Laser-beam fiber launch assembly  310  receives light from an end of optical fiber  111  which is attached at its opposing end to laser  102  shown in  FIG. 1 . Laser light travels in optical fiber  111  until it reaches fiber termination  311  where it diverges as cone of light  362 . Lens  312  collimates the light as laser beam  363  which then reflects off mirror  313  to become laser beam  364 . Laser beam  364  reflects off beam splitter  314  to join laser beam  365  from first laser-beam fiber launch and pick-up assembly  300 . Laser beam  365  passes through beam splitter  324  to become laser beam  369 . 
     Second fiber launch and pickup assembly  320  receives light from an end of optical fiber  110  which is attached at its opposing end to laser  104  shown in  FIG. 1 . Laser light travels in polarization-maintaining (PM) optical fiber  110  until it reaches fiber termination  321  where it diverges as cone of light  366 . Lens  322  collimates the laser light as laser beam  370 . Laser beam  370  passes into interferometer assembly  325  and emerges as laser beam  367 . The laser beam reflects off mirror  323  as laser beam  368  and off beam splitter  324  as a part of laser beam  369 . Laser beam  369  passes through beam splitter  342  to become laser beam  250 . 
       FIG. 2  shows that laser beam  250  passes out of beam-combiner block  200  and continues through the rest of the elements in rigid structure  190 , then travels as laser beam  153  to retroreflector  107  of  FIG. 1  and returns as laser beam  163  to rigid structure  190 . The laser light of beam  163  retraces the path of laser beams  251 ,  250  through the optical elements  220 ,  210  within rigid structure  190 . As shown in  FIG. 3 , when the returning laser beam enters beam-combiner block  200 , some of the returning light reflects off beam splitter  342  as laser beam  374 . Beam splitter  342  reflects a portion of all of the wavelengths of laser light within laser beam  250 . Optical filter  343  blocks all but one wavelength of the light within laser beam  374 , which it transmits as laser beam  373 . Position detector  341  is aligned so that laser beam  373  strikes the center of position detector  341  when laser beam  153  of  FIG. 1  is centered on retroreflector  107 . If laser beam  373  does not strike the center of position detector  341 , an error signal is generated at detector  341 , thereby causing motors  80  and  81  to turn rigid structure  190  to center laser beam  153  on retroreflector  107 . In this way, position detector  341  enables the outgoing laser beam  153  of  FIG. 1  to automatically track a moving retroreflector  107 . Position detector  341  can be any device capable of giving an electrical signal in response to the position of light on a two-dimensional surface. Such a device may include, but is not limited to, a quadrant detector, a lateral-effect detector, a charge-coupled-device (CCD) array, a charge-injection-device (CID) array, or a complementary-metal-oxide-semiconductor (CMOS) array. 
     The number of laser beams launched out of beam-combiner block  200  can be increased or decreased as desired by adding more or fewer beam splitters within beam-combiner block  200 . One way to combine and separate different types of laser beams is on the basis of wavelength. A dichroic beam splitter is a type of beam splitter that can pass particular wavelengths while reflecting other wavelengths. In a specific implementation using dichroic beam splitters, optical fiber  115  may emit laser light at a wavelength of 1550 nm, optical fiber  111  may emit laser light at 690 nm, and optical fiber  110  may emit laser light at 633 nm. Thus, beam splitters  314  and  324  may be dichroic beam splitters with the following characteristics. Beam splitter  314  transmits laser wavelengths longer than 1400 nm, but reflects wavelengths shorter than 1400 nm. Beam splitter  324  transmits wavelengths longer than 660 nm, but reflects wavelengths shorter than 660 nm. In this way, the laser beams are combined as they pass through beam-combiner block  200  on the way out of rigid structure  190 . Similarly, the laser beams are separated on the reverse path through beam-combiner block  200 . Combining and separating the wavelengths with dichroic beam splitters reduces the interaction among the laser beams, thereby preventing measurement errors. Furthermore, the use of dichroic beam splitters reduces power loss that would result from the use of wavelength-insensitive beam splitters. 
     The laser beam sent out of fiber launch assembly  310  may serve a number of purposes. In the specific example shown in  FIGS. 1 and 3 , the laser beam launched from second laser-beam fiber launch and pickup assembly  320  is red (633 nm), thereby providing a visible indication of the direction to which the laser beam is pointing. In the event that the laser beam from second laser-beam fiber launch and pickup assembly  320  is turned off or, not visible, is otherwise not available, the laser beam emitted by laser-beam fiber launch assembly  310  can serve as a visible pointer beam to assist the operator in locating retroreflector targets with the tracker. This same laser beam may be used as a part of a complex system for other purposes such as determining the orientation of a retroreflector target. There are occasions in which it is very useful to have the laser tracker emit multiple laser beams. As noted above, the flexible architecture of the invention allows as few or as many laser beams as desired to be launched. 
       FIG. 4  shows a detailed view of the coupler assembly  305 . Laser light enters coupler assembly  305  on optical fiber  115  and travels to Faraday isolator  420  which allows light to travel in only one direction. Faraday isolator  420  is included to prevent back-reflected laser light from entering and destabilizing the laser found in the ADM  103 . The laser light passes through Faraday isolator  420  and enters optical coupler  401  which, in an exemplary configuration, sends 85% of the optical power to optical coupler  402  and 15% of the optical power to optical coupler  403 . Of the optical power entering coupler  402 , a portion such as one half is sent to low-reflectance termination  412  and the remaining half travels to optical fiber  306 . As shown in  FIGS. 1 and 3 , the laser light in optical fiber  306  is launched from the fiber and travels to retroreflector  107 . The light from the retroreflector retraces its path through the laser tracker and re-enters optical fiber  306 . When light is received via optical coupler  402 , half of the optical power is sent to the Faraday isolator  402  where it is blocked. The remainder is sent to optical fiber  309  and continues to ADM  103  via optical fiber assembly  112 . Of the optical power that is sent from optical coupler  401  to optical coupler  403 , half travels to low-reflectance termination  413 , and the other half travels along optical fiber  307  to fiber retroreflector  303 . The light retraces its path back along optical fiber  307  into coupler  403 . Half of the optical power is sent to coupler  401  where it is sent in equal parts to the low-reflectance termination  411  and the Faraday isolator  420 . The other half of the optical power is sent into optical fiber  308  and continues to ADM  103  via optical fiber assembly  112 . 
     The optical couplers shown in  FIG. 4  split light into two paths in the forward direction and two paths in the reverse direction. A low-reflectance termination is used to absorb the light in one of the four possible paths (two forward paths plus two reverse paths). Another term for a possible path is a “port,” so the couplers shown in  FIG. 4  are examples of four-port couplers having a low-reflectance termination on one of the four ports. An alternative to the type of coupler shown in  FIG. 4  is the optical circulator, which has three ports, rather than four, ports. In other words, in an optical circulator, the laser light travels along one optical-fiber path in the forward direction and branches to a different optical fiber path in the reverse direction. For the purposes of this invention, the term optical coupler is used to encompass both four-port and three-port light splitting devices. In other words, a term fiber-optic coupler (or simply coupler) is understood to include any type of device that splits light in an optical fiber and therefore can be either a four-port coupler or a three-port circulator. 
     For absolute-distance measurement, two paths are used: a measurement path and a reference path. Both paths begin at the laser of the ADM  103  and include the optical fiber  115  and the Faraday isolator  420 . In the measurement path, the laser light travels through optical fiber  306 , through rigid structure  190 , to the retroreflector  107  and back, into fibers  306  and  309 , and then into a measurement detector (not shown) in the ADM  103 . In the reference path, the laser light travels through optical fiber  307 , to the fiber retroreflector  303  and back, into fibers  307  and  308 , and then into a reference detector (not shown) in the ADM  103 . The optical fibers  308  and  309  are in the reference and measurement channels, respectively, and are matched in length. They are routed in close proximity to one another so that the local temperatures experienced by each are nearly equal. This commonality of length and temperature has the effect of minimizing the errors caused by temperature-induced changes in the index of refraction of the optical fibers. Without this commonality, a changing temperature might be mistaken for a changing distance to the retroreflector. 
     Many types of ADM are compatible with the fiber delivery beam-steering mechanism depicted in  FIG. 1 . While any suitable type of ADM can be employed, an exemplary type of ADM operates by measuring the phase shift of laser light that is intensity modulated by a sine wave. Thus, the particular type of laser might be a distributed feedback (DFB) semiconductor laser whose optical power is modulated by the direct application of a radio-frequency (RF) electrical signal at a single (sinusoidal) frequency of 3 GHz. For any given distance to the retroreflector  107 , there will be a corresponding difference in the phase of the reference and measurement channels. If a is a constant, f is the frequency of modulation (3 GHz), c is the speed of light (≅3×10 8  m/s), n is the group index of refraction of the air through which the laser light travels (≅1), m is an integer, and φ is the phase difference measured by the ADM, then the distance d from laser tracker  100  to retroreflector  107  is given by the following formula: 
                   d   =     a   +       c     2   ⁢   fn       ⁢       (     m   +     ϕ     2   ⁢   π         )     .                 (   1   )               
The constant a sets the distance scale so that a distance of zero is set at the pivot point through which the laser beam appears to emanate as the laser tracker is turned to different angles. The pivot point is located approximately at the intersection of the laser beam and the center of shaft  270 . The integer m is equal to the number of complete multiples of 2π radians in the phase difference (measurement phase minus reference phase) measured by the ADM. For example, if the frequency of modulation f is 3 GHz, then from Eq. (1) the distance corresponding to a phase difference of 2π radians is approximately 3×10 8 /2(3×10 9 )(1) m=0.05 m. This distance is sometimes referred to as the unambiguous range. If the distance d−a is 1.22 meters, then the number of complete multiples of 2π radians in the phase difference is int(1.22/0.05)=24 and the residual phase shift is approximately φ≅2π(1.22−0.05·24)/0.05=0.8π radians. The most convenient way to determine the integer m is to temporarily reduce the frequency f to a value that is small enough to cover the entire range of interest, but with an accuracy that is large enough to determine the value of m. For example, suppose that the frequency is temporarily reduced to 2.5 MHz. In this case, the unambiguous range is 3×10 8 /2(2.5×10 6 )(1) m=60 m. If the accuracy of the phase measurement is one part in 10 5 , then the position of retroreflector  107  is known to an accuracy of 60·10 −5  m=0.6 mm at any distance up to 60 meters from the tracker. This value is much smaller than the unambiguous range of 50 mm for the higher modulation frequency of 3 GHz. This means that a single measurement of phase difference with the lower modulation frequency is sufficient to determine the integer m in Eq. (1). This technique of reducing the frequency to determine the value of m is of greatest value if it is needed only at the start of a measurement or after the laser beam has stopped tracking the retroreflector  107 . For this to be the case, the phase measurements must be taken rapidly enough to ensure that the retroreflector has not moved over a complete unambiguous range between measurements. For example, if measurements are made 1000 times per second, then the radial speed must not exceed (0.05)(1000)/2=25 meters per second. The human arm is not capable of moving a retroreflector target at a radial speed of greater than about 4 meters per second, so this technique of determining m is feasible under the conditions given above.
 
     The modulated laser light that travels on optical fibers  308  and  309  within optical fiber assembly  112  arrives at optical detectors located within ADM  103 . These optical detectors convert the laser light to electrical signals. For the particular type of ADM described above, electrical components within ADM  103  process the electrical signal to determine the phase of the signal for the measurement and reference paths. 
     As shown in  FIG. 3 , laser light that is launched from optical fiber  110  is collimated by lens  322  to become laser beam  370 . This laser beam travels to interferometer assembly  325 , a detailed view of which is shown in  FIG. 5 . The laser light of beam  370  is linearly polarized at 45 degrees to the plane of the paper in  FIG. 5 . In other words, half of the laser light is polarized in the plane of the paper and half of the light is polarized perpendicular to the plane of the paper, with both polarizations having the same phase. The arrow that is perpendicular to laser beam  370  in  FIG. 5  represents the laser light that is polarized in the plane. The small circle that is centered on laser beam  370  in  FIG. 5  represents the laser light that is polarized perpendicular to the plane of the paper. Laser beam  370  travels to polarizing beam splitter  501 . The portion of laser beam  370  that is polarized perpendicular to the plane of the paper in  FIG. 5  reflects off polarizing beam splitter  501  to become laser beam  510 . The light travels to quarter waveplate  502  having a fast axis oriented at 45 degrees to the plane of the paper in  FIG. 5 . The waveplate converts the polarization state of laser beam  510  from linear to circular. Lens  503  focuses the light onto mirror  504 , which retroreflects the laser beam  510  back on itself. Alternatively, a retroreflector (such as a cube-corner retroreflector) may be substituted for lens  503  and mirror  504 . Lens  503  collimates the retroreflected light, sending it back through quarter waveplate  502 , changing the polarization state of the light from circular to linear, with the direction of the linearly polarized light now in the plane of the paper. This light, which is now p-polarized with respect to the polarizing beam splitter  501 , passes through the beam splitter to become part of laser beam  511 . That portion of laser beam  370  that is in the plane of the paper in  FIG. 5  travels straight through polarizing beam splitter  501  to become laser beam  367 . This light passes through quarter waveplate  367 , whose fast axis is oriented at 45 degrees with respect to the plane of the paper. When laser beam  367  passes through the waveplate, its polarization state changes from linear to circular. The resulting laser beam travels through the optical elements in rigid structure  190 , travels to retroreflector  107 , and travels back through the optical elements in rigid structure  190  to arrive at quarter waveplate  505 . As laser beam  367  travels in the reverse direction through quarter waveplate  505 , its polarization state changes from circular to linear, with the direction of the linearly polarized laser light now in perpendicular to the plane of the paper in  FIG. 5 . (As an alternative to placing quarter waveplate  505  inside interferometer assembly  325 , the waveplate may be placed at some later point along the path of the laser beam.) Laser beam  367 , which is now s-polarized with respect to polarizing beam splitter  501 , reflects off the beam splitter to become part of laser beam  511 . Laser beam  511  comprises of two portions: a reference portion that is polarized in the plane of the paper and a measurement portion that is polarized perpendicular to the plane of the paper. As the retroreflector  107  is moved in a radial direction with respect to laser tracker  100 , the phase difference between these two linearly polarized components will vary. There will be a phase change of 2π radians for each change of one-half wavelength in the radial distance to the retroreflector. Here, the wavelength is that of the laser light in laser beam  370  as seen in the local medium (air) through which the laser light travels. Laser beam  511  travels to processing optics  506 , which uses optical elements such as beamsplitters, waveplates, and optical detectors to provide two electrical signals. One electrical signal is proportional to cos p, and the other electrical signal is proportional to sin p, where p is the phase difference between the two linearly polarized portions of laser beam  511 . The electrical signals are sent to a counter circuit  507  that counts the number of half wavelengths traveled by retroreflector  107 . The product of the wavelength of the light and the number of wavelengths traveled gives the total displacement of retroreflector  107  relative to some starting position. Counter  507  sends electrical signals over electrical line  41  to electronics box  140  for conversion from counts to a radial distance. If laser beam  153  is obstructed from reaching retroreflector  107 , even for a moment, then information on the correct number of counts is lost, and the measurement must be started anew from some reference position whose distance to the tracker has been previously established. The type of interferometer shown in  FIG. 5  is known as a homodyne interferometer because the reference portion and measurement portion that are combined to form laser beam  511  are both at the same wavelength. Alternatively, a heterodyne interferometer in which two different laser wavelengths are mixed together prior to optical detection or other suitable system could be used. 
     Beam-Expander Block 
     The optical components within beam-expander block  220  of  FIG. 2  are shown in  FIG. 6 . The beam-expander block  220  expands the laser beam as it travels in the forward direction and to contract the laser beam as it travels in the reverse direction. Lens  601  converts collimated laser beam  251  into cone of light  651 . Lens  602  converts cone of light  651  into collimated laser beam  153 . 
     The reason for expanding the laser beam before it leaves rigid structure  190  is to reduce the divergence of the laser beam during propagation. This makes it possible to place retroreflector  107  farther from laser tracker  100  than would otherwise be the case. Alternatively, the beam-expander block  220  could be eliminated by increasing the distance in  FIG. 3  between the fiber terminations  301 ,  311 , and  321  and the corresponding lenses  302 ,  312 , and  322  while increasing the focal lengths of lenses  302 , 312 , and  322  by a corresponding amount. Accordingly, the diameters of laser beams  361 ,  363 , and  370  would be increased, thereby eliminating the need for beam-expander block  220 . The disadvantage of this approach is that it requires that many optical elements (lenses, mirrors, beam expanders, and position detector) be made larger to accommodate the larger beam diameters. By adding beam-expander block  220 , the overall size of beam-combiner block  200  is reduced. 
     Orientation-Camera Block 
     The main elements of orientation-camera block  210  of  FIG. 2  are shown in  FIG. 7 . On the return path from retroreflector  107 , laser beam  251  travels along optical axis  741 . Beam splitter  701  reflects a portion of the beam to a path along optical-axis segments  750 ,  742 ,  743 , and  744 . Eventually, this reflected light arrives at photosensitive array  753 . The complete lens system, which comprises the beam-expander block  220 , afocal lens block  710 , and relay lenses,  721  and  723 , produces an image on the photosensitive array of the pattern of light in the vicinity of the vertex of retroreflector  107 . The beam-expander block  220  and the afocal lens block  710  work together to produce a first intermediate image  751  of this pattern of light. The location of first intermediate image  751  will depend on the distance of retroreflector  107  from the laser tracker. Motorized stage  728  is activated to move lens  721  to an appropriate distance from first intermediate image  751 . Lens  721  forms second intermediate image  752  located past negative lens  723  but inside the back focal point of negative lens  723 . Negative lens  723  converts the second intermediate image into a real image  753  on photosensitive array  725 . 
     The orientation-camera block  210  allows the distance between the tracker and the retroreflector target to be large. For example, a distance of more than thirty meters is possible. The lens systems of the orientation-camera block  210  and beam-expander block  220  have two main functions. First, a magnification that is approximately constant is maintained so that the image will nearly fill the photosensitive array, thereby maintaining high accuracy for large and small distances alike. Second, the adverse effects of diffraction, which may result in lines or other features changing shape or direction during propagation over large distances, are minimized. To maintain constant magnification, afocal lens systems  220  and  710  are used. An afocal lens system is one that converts an incoming ray of light that is parallel to the optical axis into an outgoing ray of light that is also parallel to the optical axis. A succession of afocal lens systems, as represented by the combination of lens systems  220  and  710 , has the property of constant magnification. In other words, the size of first intermediate image  751  is constant, regardless of the distance from retroreflector  107  to the tracker. If first intermediate image  751  is located between lenses  711  and  714 , then it is not possible to place photosensitive array  725  at the location of this intermediate image. Relay lenses  721  and  723  eliminate this problem by converting first intermediate image  751  into image  753  on array  725 . Motorized stage  728  places lens  721  an appropriate distance from first intermediate image  751 . Knowledge of the distance to retroreflector  107 , which is a quantity measured by the tracker, along with knowledge of the focal lengths and positions of the lens elements, is sufficient to determine the correct placement of lens  721 . As is explained below, it is not necessary for the lens system to obtain an exactly prescribed magnification, so motorized stage  728  can be relatively inexpensive. The distance that motorized stage  728  must move will depend on the range of distances to be covered, as well as on the magnification of the lens system. Longitudinal magnification of a lens system varies in proportion to the square of the transverse magnification. As an example, suppose that a 12×12 millimeter area of target object  185  is imaged onto a photosensitive array having an area of 3×3 mm. The required (transverse) magnification for the system will then be 3/12=¼. This could be achieved by making the combined magnification of the afocal lens systems equal to ¼ and the combined magnification of the relay lenses  721  and  723  equal to 1. In this case, however, to cover distances of 1 to 33 meters from the tracker, it would be necessary for motorized stage  728  to have a range of movement of (33−1) m/4 2 =2 m. Such a large range of movement is impractical for most real systems. To solve this problem, the magnification of the afocal lens systems could be reduced, and the reduced magnification could be compensated with the relay lenses. For example, suppose that the afocal lens pairs have a combined magnification of 1/32, while the relay lenses have a combined magnification of 8. In this case, the net magnification is still ¼, but the motorized stage  728  needs to have a range of movement of only (33−1) m/32 2 =31.25 mm. 
     The photosensitive array  725  can be any device capable of returning detailed electrical information about the pattern of light incident on the array. Exemplary photosensitive arrays include the charged-coupled-detector (CCD) array, the charge-injection-device (CID) array, and the complementary-metal-oxide-semiconductor (CMOS) array. Among these, CCD arrays have high performance and small size, but CMOS arrays are often capable of providing high-speed read-out with simpler electrical circuitry. CMOS and CID arrays often have the advantageous feature of random-access read-out of pixel data. 
     We will now discuss how the image on the orientation camera can be used to determine the pitch, yaw, and roll angles of retroreflector  107 .  FIGS. 8a and 8b  show an unrotated cube-corner retroreflector. In other words, in  FIGS. 8a and 8b , the roll angle is zero, the yaw angle is zero, and the pitch angle is zero. By definition, the x direction shown in  FIGS. 8a and 8b  is opposite the direction of the laser beam that is sent into the retroreflector. The three perpendicular reflecting surfaces of the cube-corner retroreflector form three lines of intersection. As shown in  FIG. 8a , the x-y plane contains the x axis and one of the lines of intersection. The x-y plane also contains the y axis, which is perpendicular to the x axis and passes through the vertex of the cube corner. The dashed line in  FIG. 8a  is parallel to the y axis and has been included for clarity. In the front view of  FIG. 8b , they and z axes lie in the plane of the paper, while the x axis points out of the paper.  FIGS. 9a and 9b  show the effect of rotating the retroreflector about the −y axis by the pitch angle P, which is 15 degrees in this example. This rotation operation results in new coordinate system: x′, y′, z′, with y=y′.  FIGS. 10a and 10b  show the effect of rotating the retroreflector about the z′ axis by the yaw angle Y, which is 10 degrees in this example. This rotation results in a new coordinate system: x″, y″, z″, with z″=z′.  FIGS. 11a and 11b  show the effect of rotating the retroreflector about the x″ axis by the roll angle R, which is 40 degrees in this example. Note that in  FIG. 11b , the x axis (direction opposite that of the laser beam) still points straight out of the paper. The roll, yaw, and pitch angles are found from a measurement of the three lines of intersection by the orientation camera  210 . The camera detects the three lines of intersection of cube-corner retroreflector  107 . The vertex of cube-corner retroreflector  107 , which is defined as the common point of the three reflecting surfaces, remains centered on the photosensitive array  725  of  FIG. 7 . The electrical signals from photosensitive array  725  may be sent to a local digital-signal processing chip or sent over electrical wires  41  to electronics box  140  of  FIG. 1 . These electrical components determine the slopes of the three lines of intersection. By definition, they and z axes on the surface of photosensitive array  725  point in the horizontal and vertical directions, respectively. The slope of the first (reference) line of intersection is defined as m 1 =Δz 1 /Δy 1 , where Δy 1  and Δz 1  are the horizontal and vertical distances on the surface of photosensitive array  725  from the image of the cube-corner vertex to the image of an arbitrary point on the first line of intersection The slopes of the second and third lines of intersection are defined in a similar manner as m 2 =Δz 2 /Δy 2  and m 3 =Δz 3 /Δy 3 . The three unknown angles, the roll angle R, the yaw angle Y, and the pitch angle P, are found by simultaneously solving the following three equations: 
                       m   1     =         sinPcosY   /     2       -   sinPsinYcosR   +   cosPsinR         sinY   /     2       +   cosYcosR         ,           (   2   )                   m   2     =               sinPcosY   /     2       -     sinPsinYcos   ⁢     (     R   +     120   ⁢   °       )       +               cosPsin   (     R   +     120   ⁢   °       )               sinY   /     2       +     cosYcos   ⁡     (     R   +     120   ⁢   °       )             ,           (   3   )                 m   3     =                 sinPcosY   /     2       -     sinPsinYcos   (     R   +     240   ⁢   °       )     +               cosPsin   (     R   +     240   ⁢   °       )               sinY   /     2       +     cosYcos   ⁡     (     R   +     240   ⁢   °       )           .             (   4   )               
For the example considered here in which R is 40 degrees, Y is 10 degrees, and P is 15 degrees, Eqs. (2)-(4) yield m 1 =0.874, m 2 =0.689, and m 3 =−2.651. As a check of these results, the slope values can also be calculated directly from the y and z values of the lines of  FIG. 11b . These calculations yield m 1 =0.7667/0.8772=0.874, m 2 =0.5528/−0.8026=−0.689, and m 3 =−0.7788/0.2938=−2.651, which match exactly the results obtained from Eqs. (2)-(4).
 
     The visibility of the lines on photosensitive array  725  of  FIG. 7  may be improved by increasing the thickness of the lines of intersection of retroreflector  107  or by coating the lines with a non-reflective material. Thicker lines of intersection on retroreflector  107  will cause the images of the lines seen on the photosensitive array to have higher contrast. However, thicker lines of intersection on retroreflector  107  will not usually result in thicker image lines on photosensitive array  725 . Usually, the thickness of the lines as seen on photosensitive array  725  is determined by the effects of diffraction of the laser light that passes through the clear aperture of laser tracker  100 . The larger the clear aperture (the opening through which the light passes into the tracker), the smaller will be the deleterious effects of diffraction, which included broadening, smearing, and chopping of the image on photosensitive array  725 . The deleterious effects of diffraction are also smaller when the retroreflector  107  is moved closer to the laser tracker  100 . Fortunately, for the system considered here, the smearing effects of diffraction are symmetrical about the lines of intersection, so there is no bias in measuring the slopes of the lines. 
     The appearance of the lines on the image of photosensitive array is shown in  FIG. 12 . The lines on this image that pass through vertex V appear on both sides of the vertex. By comparison,  FIG. 11b  shows that the lines of intersection of the physical cube-corner retroreflector appear on only one side of the vertex. This difference is the result of the symmetry in the paths that light can take when the laser beam is centered on vertex V. For example, if a pencil of light reflects off mirror  1 , then mirror  2 , then mirror  3  of the cube corner, then another pencil of light can reflect off mirror  3 , then mirror  2 , then mirror  1 . If, instead of striking a mirror, the pencil of light strikes a line of intersection, then the light will not reflect back to the tracker and a dark spot will appear on the image of photosensitive array  725 . However, this dark spot would also appear if the light had traveled in the reverse direction before encountering the line of intersection. Hence light entering on either side of vertex V is blocked. It is impossible to tell from the image of the photosensitive array  725  alone which of the three line segments corresponds to which of the three lines of intersection of a cube-corner retroreflector. There are several ways around this problem. The simplest, but least convenient, method for assigning the lines of the image ( FIG. 12 ) to the lines of intersection ( FIG. 11b ) is to have the operator indicate the approximate orientation of probe assembly  180  at the start of a measurement sequence. An approximate orientation is sufficient to determine which of the image line segments corresponds to each of the lines of intersection. Another simple but effective method is to temporarily turn off or reduce the power of the laser beam  153  emitted by laser tracker  100  and, at the same time, to increase the exposure time of photosensitive array  725 . Under these conditions, the photosensitive array will ordinarily be able to make out the features of housing  109  and hence obtain information on the orientation of retroreflector  107 . A third method for assigning the line segments to the corresponding lines of intersection is described in a second embodiment that is discussed later. 
     The method for determining the pitch, roll, and yaw angles as described above provides has two main advantages. First, an essentially constant-magnification camera maintains the accuracy of the measurement for a probe located either near the tracker or far from it. Second, elimination of spurious diffraction effects improves accuracy, which may otherwise change the angles of the lines or dramatically change the appearance of the lines, especially at large distances. 
       FIG. 1  shows that housing  109  can be pivoted about axis  182  mounted on adjustable stage  181  and then locked in place. This allows cube-corner retroreflector  107  to be oriented in any desired direction. This flexibility in the orientation of retroreflector  107  is desirable because it allows probe tip  171  to be placed in slots, holes, and so forth at any given angle. Preferably shaft  182  is aligned with the vertex of retroreflector  107  to simplify calculations to determine the location of probe tip  171 . If the locking mechanism allows a limited number of angular adjustments, each known to a sufficient angular accuracy (perhaps a few are seconds), then measurement may resume as soon as the lock down is complete. If the locking mechanism is not sufficiently precise, then an alternative approach involves adjusting housing  109  to any given orientation and performing a simple compensation routine to determine the angle between retroreflector  107  and probe shaft  170 . Such a compensation routine might include measuring the location of a reference point with a spherically mounted retroreflector target and then measuring the same location with probe assembly  180  tilted to cover a range of pitch, yaw, and roll angles. 
     It is possible to replace the described cube-corner retroreflector  107 , which is made of three reflecting mirrors, with a cube-corner retroreflector prism formed of solid glass. Each type of retroreflector has advantages. For example, the cube-corner retroreflector that uses mirrors (also known as a hollow-core cube-corner retroreflector) is more accurate because it is not prone to transverse and radial offset errors and because it has no glass/air interface to cause unwanted optical reflections. The solid glass cube-corner retroreflector has a wider field of view and is usually less expensive. Equations (2), (3), and (4) can be readily modified to account for a solid-glass, rather than a hollow-core, cube-corner retroreflector. 
     Locator-Camera Block 
     The locator-camera block  230  of  FIG. 2  allows laser tracker  100  to quickly determine the approximate location of multiple retroreflectors within a wide field of view. The locator camera is shown in greater detail in  FIGS. 14a and 14b . As shown in  FIG. 2 , locator-camera block  230  is placed to one side of rigid structure  190  and has an aperture at  231 , which might be considered the “top” side of rigid structure  190 . When rigid structure  190  is rotated about the center of shaft  270 , locator-camerablock  230  faces the retroreflectors in the region of interest. Locator-camera block  230  then emits cone of light  1320  as shown in  FIG. 13 . This light reflects off retroreflectors  107 ,  1311 ,  1312 , and  1313  shown in  FIG. 13 . Here, retroreflector  107  represents a target of interest and retroreflectors  1311 ,  1312 , and  1313  represent a number of other targets. The corresponding reflected light bundles  1357 ,  1351 ,  1352 , and  1353  enter rigid structure  190 . The light entering locator-camera block  230  falls onto a photosensitive array  1404  in  FIG. 14b , and the pattern is analyzed to determine the approximate location of the targets in the region of interest. 
       FIGS. 14a and 14b  depict an example locator-camera arrangement. A plurality of identical light sources  1401  is provided in a ring surrounding a lens  1402 . The individual light sources emit overlapping cones of essentially incoherent light  1440  that collectively constitute the cone of light  1320  in  FIG. 13 . Each of the retroreflectors  107 ,  1311 - 1313  reflects some of the light from the cone of light  1320  back to the locator-camera block  230  as the bundles of light  1351 - 1353  or  1357 . The bundle of light  1357  is shown in  FIG. 14b . Lens  1402  focuses the bundle  1357  down to a spot on the surface of photosensitive array  1404 . The photosensitive array  1404  is separated from the front principal plane,  1403 , of lens  1402  by the focal length f of the lens. 
     Electrical wires  41  provide power from electronics box  140  to light emitters  1401  and photosensitive array  1404 . Electrical wires  41  also transmit the pixel data from photosensitive array  1404  to electronics box  140  for analysis. Electronics box  41  analyzes the pattern of light on photosensitive array  1404  to determine the location of central point  1452  on photosensitive array  1404 . Electronics box  140  also performs this analysis of the pattern formed by the other bundles of light returned by the retroreflectors. In other words, reflected light bundles  1357 ,  1351 ,  1352 , and  1353  are focused by lens  1402  into patterns on photosensitive array  1404 . Electronics box analyzes these patterns to determine the central point of each pattern. From the location of the central points, the approximate angular direction to each of the retroreflectors can be determined. 
     Suppose that the retroreflector of interest is retroreflector  107 . Once the information from the locator camera has been used to determine the approximate direction to retroreflector  107 , motors  80  and  81  are activated to turn rigid structure  190  until laser beam  153  points in the approximate direction of retroreflector  107 . The tracker then begins a search pattern, in which the direction of laser beam  153  is changed in a systematic fashion. For example, the laser beam might be steered along a spiral pattern. When the laser beam intersects the target, position detector  341  of  FIG. 3  senses the reflected light. The signals from position detector  341  provide enough information to enable motors  80  and  81  to point rigid structure  190  directly to the center of retroreflector  107 . 
       FIG. 15a  shows rays of light emitted by light emitter  1401  located above lens  1402 . Ray of light  1520  travels to vertex V of retroreflector  107 . Reflected light  1521  is sent directly back to light emitter  1401 . It does not enter lens  1402  or appear as a spot of light on photosensitive array  1404 . Ray of light  1530  is sent to the bottom of retroreflector  107  and emerges as reflected light  1532 . It also misses lens  1402  and photosensitive array  1404 . 
       FIG. 15b  shows additional rays of light from light emitter  1401  located above lens  1402 . Light emitter  1401  sends ray of light  1540  to a location above vertex V on retroreflector  107 . This ray emerges as reflected ray  1541 , which passes near the top of lens  1402 , is bent into ray  1542 , and arrives at photosensitive array  1404  near central point  1563 . Light emitter  1401  sends ray of light  1550  to the top of retroreflector  107 . This ray emerges as reflected ray  1552 , which travels to lens  1402 , is bent into ray  1553 , and arrives at photosensitive array  1404  near central point  1563 . As the distance from light emitter  1401  to retroreflector  107  increases, rays  1541  and  1552  become nearly parallel, and the spot of light about point  1563  gets smaller and smaller. 
       FIG. 15c  shows rays of light from light emitter  1401  located below lens  1402 . The rays of light in the bottom diagram are mirror images of the rays in the middle diagram. If N is the number of pixels in photosensitive array  1404 , W is the width of photosensitive array  1404 , D is the diameter of lens  1402 , and h is the distance from the edge of lens  1402  to light emitters  1401 , the number of pixels between central points  1563  and  1593  will, in most cases, be less than [2N (D+h)/L] arctan(W/2f). For example, if N=5 12, D=25 mm, h=5 mm, L=3 m, W=13 mm, and f=10 mm, the number of pixels between central points  1563  and  1593  will be less than six. Since light emitters  1401  are arranged in a circle, the image will be symmetrical, somewhat blurry, and about six pixels across. For retroreflectors further than 3 meters away, as most will be, the pattern of dots will be smaller. Electrical signals are sent from photosensitive array  1404  through electrical wire  41  to electronics box  140 . Electronics box  140  analyzes the intensity of light in the pixels to obtain the best estimate of the center of the pattern produced by each retroreflector. 
     Routing of Optical Fibers 
       FIG. 16  shows a configuration of a fiber launch laser tracker  1600  for providing advantageous routing of optical fibers. Here, optical fibers are preferably routed close to the two mechanical axes  1670  and  1671 . The routing of the optical fibers in the system of  FIG. 16  has many advantages. For example, a large angular field of view of the tracker can be obtained. Also, bending or kinking of the optical fibers is prevented, thereby preserving measurement accuracy. Laser beam  1653 , which is launched from rigid plate  1630 , travels to retroreflector  107  as shown in  FIG. 1  and returns as laser beam  1663 . Laser light from optical fiber  1611  is collimated by lens  1613 , reflected by mirror  1615 , and transmitted through beam splitter  1614 . Laser light from optical fiber  1610  is collimated by lens  1612  and reflected off beamsplitter  1614 . Any number of laser beams can be combined into a common path to form outgoing laser beam  1653 . Returning laser light may pass through a number of elements as previously discussed and omitted in  FIG. 16  for clarity. For example, beam splitters for the position detector and the orientation camera may be employed in accordance with the specific application. The returning laser light may reflect off beam splitter  1614  and pass through lens  1612  to be coupled into optical fiber  1610 . Alternatively, the laser light may pass into another device located on rigid plate  1630  for processing as previously discussed, for example, with reference to the absolute distance meter and the interferometer. Similarly, the returning laser light may reflect off mirror  1615  and couple back into optical fiber  1611 . Alternatively, this laser light may travel to another device on rigid plate  1630  for processing. 
     The direction of laser beam  1653  is determined by the orientation of rigid plate  1630 , which in turn is determined by the angle of rotation of the zenith mechanical axis  1671  and the azimuth mechanical axis  1670 . The zenith motor  1681  rotates the zenith axis  1671 , and the azimuth motor  1680  rotates the azimuth axis  1670 . Zenith angular encoder  1691  and the azimuth angular encoder  1690  measure the zenith and azimuth angles. Bearings  1681  and  1680  are also attached to the zenith and azimuth axes. The outside of zenith bearings  162   1 , zenith angular encoder  1691 , and zenith motor  1681  are attached to the azimuth structural frame (not shown). The azimuth structural frame turns with the azimuth axis. Consequently, the zenith axis rotates within the azimuth structural frame. The outside of azimuth bearings  1620 , azimuth angular encoder  1690 , and azimuth motor  1680  are attached to stationary structural frame (not shown). The stationary structural frame is stationary with respect to the surroundings to which the tracker is mounted. Consequently, the azimuth axis rotates within the stationary structural frame. 
     Optical fibers  1610  and  1611  are incorporated into optical fiber assembly  1605 . Optical fiber assembly  1605  passes through zenith axis  1671  and azimuth axis  1670 . Lasers within optoelectronic module  1606  (which, like the azimuth motor  1680 , is stationary) inject laser light into optical fibers  1610  and  1611 . Optoelectronic module  1606  may also contain optical detectors and electronics to determine the distance to retroreflector  107  or to a diffuse surface under investigation. The optical fiber assembly  1605  travels from optoelectronic module  1606  to the underside of azimuth axis  1670 . It is attached to the stationary structural frame near point A shown in  FIG. 16 . At point A, the fiber is stationary with respect to the rotating azimuth axis. At the other end of the azimuth axis, optical fiber assembly  1605  is attached to the azimuth structural frame near point B, which rotates along with azimuth axis  1670 . Since one end of the fiber is fixed and the other end of the fiber is rotating with respect to the rotation of the azimuth axis, the optical fiber will experience a torsional twist. In most cases, a gentle twist of this sort will not degrade measurement accuracy. Optical fiber assembly  1605  is routed to the zenith axis, where it is attached to the azimuth structural frame near point C. At point C, the fiber is stationary with respect to the rotating zenith axis. At the other end of the zenith axis, fiber assembly  1605  is attached near point D, which rotates along with the zenith axis  1671 . 
     Optical fiber assembly  1605  is routed through the two mechanical axes. The fiber assembly is stationary at one end of each axis. At the other end, the fiber assembly rotates along with the axis. This produces a torsional twist, which is acceptable in most situations. A slightly different method of routing optical fiber assembly  1605  near the two mechanical axes may be preferable in some cases. In this method, the optical fibers are placed in coils to the outside of the mechanical axes, with the end of the optical fiber attached at one end to a point that is stationary relative to the mechanical axis and at the other end attached to a point that moves with the mechanical axis. Here, the diameter of the coils will change slightly as the axis is rotated. In most cases, this small change in the radius of the coiled fiber assembly will not adversely affect measurement accuracy. By heat treating fiber assemblies, it is possible to make low-cost cables that naturally coil into the desired geometry, thereby simplifying production and increasing reliability. 
     Second Embodiment 
     The second embodiment of the invention is generally similar to that shown in  FIG. 1  except for the probe assembly  180  is replaced by probe assembly  1780 , as shown in  FIG. 17 . Probe assembly  1780  contains a single small retroreflector  1708  to the side of retroreflector  107 . Retroreflector  1708  is approximately aligned with the first line of intersection. At the start of the measurement, the photosensitive array  725  of  FIG. 7  displays a pattern similar to that of  FIG. 12 , with the details of the pattern dependant on the pitch, yaw, and roll angles of retroreflector  107 . As explained previously, at the start of the measurement, it is not possible to tell which line segments correspond to each of the three lines of intersection. To resolve this ambiguity, the tracker performs a search in which it directs laser beam  153  in succession to each of the six possible locations of retroreflector  1708 . A flash of light on position detector  341  of  FIG. 3  indicates that the first line of intersection has been identified. Also on probe assembly  1780 , two thin wires  1711  and  1712  have been stretched across the top of retroreflector  107 . Additional thin wires or alternative shapes may also be used. These wires provide redundant information for determining the pitch, yaw, and roll angles for those cases in which accuracy is more important than measurement speed. 
     Probe assemblies  180  and  1780  can be used in either a scanning mode or a trigger mode. In the scanning mode, probe tip  171 , shown in  FIGS. 1 and 17 , is moved across the surface of the object under evaluation  175  while data is continually collected at a high rate. In the trigger mode, probe  180  or  1780  is moved successively to the points of interest. When the probe is properly positioned, the operator triggers the measurement by performing an action such as pressing a button or issuing a voice command. 
     Either target object  185  in the first preferred embodiment or target object  1785  in the second preferred embodiment can be detached from adjustable stage  181  and probe shaft  170 , then attached to the end effector of a robot arm. Alternatively, the target object can be attached to a machine tool such as a drilling or milling machine. The tracker sends a laser beam to the target object to determine the six degrees of freedom of the drill or mill. The information provided by the tracker on the six degrees of freedom of target object  185  or  1785  can be used in a control loop to precisely direct the machine tool or robot end effector to the desired locations. If the tracker measures the six degrees of freedom fast enough, real-time control of machine tools and robots is possible. 
     Third Embodiment 
     The third embodiment of the invention provides a laser tracker  1800  as shown in  FIG. 18  that uses a steering reflector  1804  within gimbal mount  95  to direct laser beam  1853  to retroreflector  107 . Laser  1802  emits laser light that is sent to retroreflector  107 . Optical block  1806  contains beam expander  220  and any other optical beam-conditioning elements that may be required. Laser light returning from retroreflector  107  is sent to distance-measuring device  1814 , which may be either an absolute-distance meter or an incremental-distance meter. Part of the returning laser light is also reflected off beam splitter  1809  to position detector  341 . The beam splitter  701  reflects a portion of laser beam  54  into orientation-camera subsystem  1810 . Orientation camera subsystem  1810  comprises afocal lens block  710  and relay/array block  720 , also shown in  FIG. 7 . The optical elements within blocks  1806  and  1810  of  FIG. 18  are substantially equivalent to the optical elements within blocks  220  and  210  of  FIGS. 6 and 7 . In effect, an orientation camera comprising elements  701  and  1810  is embedded within laser tracker  1800 . This orientation camera is equivalent to the orientation camera  210  of  FIG. 2  and can therefore be used to measure the six degrees of freedom of target object  185 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the laser-based coordinate measuring device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.