Patent Publication Number: US-8968279-B2

Title: Systems and methods for qualifying and calibrating a beam delivery system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is related to U.S. Pat. Nos. 6,195,164, 6,559,934, 6,666,855; and to U.S. patent application Ser. No. 10/383,445 filed Mar. 6, 2003; Ser. No. 10/085,253 filed Oct. 13, 2003; Ser. No. 11/264,785 filed Oct. 31, 2005; and Ser. No. 10/808,728 filed Mar. 24, 2004, the full disclosures of which are hereby incorporated by reference. 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to methods and systems for qualifying and calibrating beam delivery systems. More specifically, embodiments of the present invention relate to methods and systems for qualifying and calibrating a opthalmological surgery laser beam delivery system based on a laser beam delivery system characteristic. 
     Laser based systems are commonly used in opthalmological surgery on corneal tissues of the eye to correct vision defects. These systems use lasers to achieve a desired change in corneal shape, with the laser removing microscopic layers of stromal tissue from the cornea using a technique generally described as ablative photodecomposition to alter the refractive characteristics of the eye. Laser eye surgery techniques are useful in procedures such as photorefractive keratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), and the like. 
     Laser ablation procedures can reshape or sculpt the shape of the cornea for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and other corneal surface profile defects. In known systems, the laser beam often comprises a series of discrete pulses of laser light energy, with the total shape and amount of tissue being removed being determined by the position, shape, size, and/or number of a pattern of laser energy pulses impinging on the cornea. A variety of algorithms may be used to calculate the pattern of laser pulses used to reshape the cornea so as to correct a refractive error of the eye. 
     Accurate control of the laser beam delivery system is crucial for patient safety and successful vision correction. Accordingly, laser beam delivery systems are qualified and calibrated to ensure control over the positioning and distribution of ablation energy across the cornea so as to minimize undesirable laser system performance, which may result from flawed internal mechanical or optical components, misalignment, and the like. In particular, qualification and calibration of the laser system helps ensure accurate removal of the intended shape and quantity of the corneal tissue so as to provide the desired shape and refractive power modification to the patient&#39;s cornea. Imprecise control of the laser beam may jeopardize the success of the surgery and could cause damage to the patient&#39;s eye. For example, deviation from a desired laser beam path or position may result in tissue ablation at an undesired location on the patient&#39;s cornea which in turn leads to less than ideal corneal sculpting results. As such, it is beneficial to provide precise control over the positioning of the laser beam so as to accurately sculpt the patient&#39;s cornea through laser ablation. 
     In light of the above, it would be desirable to provide improved methods and systems for qualifying and calibrating beam delivery systems on the basis of beam path positioning and other related beam path and beam delivery system characteristics. It would be further desirable if such methods and systems enhanced qualification and calibration accuracy without significantly increasing the overall system cost and complexity. At least some of these objectives will be met by the methods and systems described herein. 
     BRIEF SUMMARY OF THE INVENTION 
     Method and system embodiments are provided for qualifying and calibrating a beam delivery system, such as an excimer laser system for selectively ablating a cornea of a patient&#39;s eye. In particular, improved methods and systems are provided for laser beam positioning using an image capture device such as a microscope camera. Such methods and systems further provide enhanced qualification and calibration accuracy and precision without significantly increasing the overall system cost and complexity and may be applied to a variety of laser systems. Each of the methods described herein may be performed using system computers or modules having any of a wide variety of digital and/or analog data processing hardware and/or software. 
     In a first aspect, an exemplary embodiment includes a method of testing a laser eye surgery system. The method includes imaging a known calibration pattern at an image location with an imaging device, establishing an image scale based on the calibration pattern and the calibration pattern image, imageably altering a series of regions of a test surface with a laser beam of the laser eye surgery system at the imaging location, laterally redirecting the laser beam according to an intended pattern between altering of the regions using a beam delivery system so as to form a test pattern on the test surface, imaging the test pattern at the imaging location with the imaging device, and determining a lateral redirecting characteristic of the beam delivery system based on the image scale, the intended pattern, and the test pattern image. The method may also include qualifying or calibrating the beam delivery system in response to the lateral redirecting characteristic. 
     In some embodiments, the beam delivery system laterally redirects the beam along a first axis from a first region to a second region, and laterally redirects the beam along the first axis from the second region to a third region. The beam delivery system can be calibrated by altering machine readable code of the laser eye surgery system so that a subsequent lateral deflection of the beam between the first region and the second region is determined using a first calibration factor, and so that a subsequent lateral deflection of the beam between the second region and the third region is determined using a second calibration factor, where the second calibration factor is different than the first calibration factor. In related embodiments, the beam delivery system laterally redirects the beam along a second axis a plurality of times, and the beam delivery system is calibrated by altering the machine readable code of the laser eye surgery system so that subsequent lateral deflections of the beam along the second axis are determined using a plurality of different calibration factors associated with different beam locations along the second axis, where the second axis intersecting the first axis. 
     In some embodiments, the beam delivery system laterally redirects the beam along a first axis from a first region to a second region, and laterally redirects the beam along a second axis from the second region to a third region. The beam delivery system can be calibrated by altering machine readable code of the laser eye surgery system so that a subsequent lateral deflection of the beam along the first axis is determined using a first calibration factor, and so that a subsequent lateral deflection of the beam along the second axis is determined using a second calibration factor, where the second calibration factor is different than the first calibration factor. In some embodiments, the beam delivery system laterally redirects the beam along a first test pattern axis of the test pattern from a first region to a second region, and the qualifying or calibrating of the beam includes identifying an offset between the test pattern axis and an intended axis of the intended pattern. Calibrating the beam can include altering machine readable code of the laser eye surgery system so that a subsequent lateral deflection of the beam along the first test pattern axis is determined based on the offset. Qualifying the beam can include enabling use of the laser eye surgery system in response to the offset being below an acceptable tolerance. In some cases, the offset includes an angular offset. 
     In some embodiments, the beam delivery system laterally redirects the beam along a first test pattern vector between the regions of the test pattern, and laterally redirects the beam along a second test pattern vector between the regions of the test pattern. Qualification or calibration of the beam delivery system can include determining offsets between the vectors and intended vectors between regions of the intended pattern. In some embodiments, the method may include calibrating the laser eye system by altering machine readable code of the laser eye surgery system in response to a first lateral beam deflecting characteristic of the beam delivery system, and qualifying the laser eye surgery system by enabling use of the laser eye surgery system in response to a second lateral beam deflecting characteristic of the beam delivery system being within an acceptable threshold tolerance. 
     In another aspect, embodiments include a method of qualifying a laser eye surgery system. The method can include imaging a calibration pattern at an image location with an imaging device, establishing an image scale based on the calibration pattern and the calibration pattern image, imageably altering a test surface with a beam delivery system according to an intended pattern to produce a test pattern on the test surface, imaging the test pattern with the imaging device, and determining a beam delivery system characteristic based on the image scale, the intended pattern, and the test pattern image. The method can also include qualifying the laser eye surgery system if the beam delivery system characteristic satisfies a specified qualification limit. In some aspects, the method can include disqualifying the laser eye surgery system if the beam delivery system characteristic exceeds a specified qualification limit. In some aspects, the method can include calibrating the laser eye surgery system if the beam delivery system characteristic exceeds a specified calibration limit. In a related aspect, the test pattern image can include a test spot image, and determination of the beam delivery system characteristic can be based on a centroid calculated for the test spot image. In some aspects, the beam delivery system characteristic includes a member selected from the group consisting of a scaling calibration, a rotational offset, an axial deflection offset, a pincushion offset, a mirror thickness offset, an alignment offset, a tilt, and a warping factor. In some aspects, the beam delivery system can include a plurality of galvanometer-controlled mirrors. 
     In another aspect, embodiments provide a computer program product for calibrating a beam delivery system of a laser eye surgery system. The computer program product can include code for accepting a calibration pattern image, code for establishing an image scale based on the calibration pattern image, code for accepting an intended pattern, code for accepting a test pattern image, code for determining a beam delivery system characteristic based on the image scale, the intended pattern, and the test pattern image, and code for calibrating the beam delivery system based on the beam delivery system characteristic. In some embodiments, the code for calibrating the beam delivery system includes code for adjusting a signal correlation in the beam delivery system. In related embodiments, the test pattern image includes a test spot image, and the code for determining a beam delivery system characteristic includes a code for calculating a centroid of the test spot image. In some embodiments, the centroid for the test spot image of size is calculated as 
                 C   x     =         1   K     ⁢       ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢       x   k     ⁢     F   ⁡     (     j   ,   k     )                   ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢     F   ⁡     (     j   ,   k     )               ,     
     ⁢       C   y     =         1   K     ⁢       ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢       y   j     ⁢     F   ⁡     (     j   ,   k     )                   ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢     F   ⁡     (     j   ,   k     )               ,         
where {C x ,C y } represents the centroid and F(j,k) represents the test spot image of size J×K.
 
     In another aspect, embodiments of the present invention provide a system for calibrating a beam delivery system of a laser eye surgery system. The system can include an input module that accepts an input member selected from the group consisting of a calibration pattern parameter, a calibration pattern image, an intended pattern parameter, a test pattern image, an imaging device position, a calibration pattern position, a test pattern position, and a beam delivery system position. The system can also include a characterization module that determines a beam delivery system characteristic based on the input member; and an output module that generates a calibration for the beam delivery system of the laser eye surgery system based on the beam delivery system characteristic. In some embodiments, the beam delivery system characteristic can include a member selected from the group consisting of a scaling calibration, a rotational offset, an axial deflection offset, a pincushion offset, a mirror thickness offset, an alignment offset, a tilt, and a warping factor. In related embodiments, the calibration can include a signal correlation adjustment. In some embodiments, the test pattern image includes a test spot image, and the characterization module determines the beam delivery system characteristic based on a centroid calculated for the test spot image. In related embodiments, the centroid for the test spot image of size is calculated as 
                 C   x     =         1   K     ⁢       ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢       x   k     ⁢     F   ⁡     (     j   ,   k     )                   ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢     F   ⁡     (     j   ,   k     )               ,     
     ⁢       C   y     =         1   K     ⁢       ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢       y   j     ⁢     F   ⁡     (     j   ,   k     )                   ∑     j   =   1     J     ⁢       ∑     k   =   1     K     ⁢     F   ⁡     (     j   ,   k     )               ,         
where {C x ,C y } represents the centroid and F(j,k) represents the test spot image of size J×K.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B,  1 C,  1 D, and  1 E illustrate exemplary systems for qualifying and calibrating a beam delivery system according to embodiments of the present invention. 
         FIGS. 2A ,  2 B,  2 C,  2 D,  2 E,  2 F,  2 G,  2 H,  2 I,  2 J,  2 K, and  2 L illustrate exemplary techniques for qualifying and calibrating a beam delivery system according to embodiments of the present invention. 
         FIGS. 3A and 3B  illustrate exemplary methods for qualifying and calibrating a beam delivery system according to embodiments of the present invention. 
         FIG. 4  illustrates an exemplary method for determining a beam delivery system characteristic according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Methods and systems are provided for qualifying and calibrating a beam delivery system, such as an excimer laser system for selectively ablating a cornea of a patient&#39;s eye. In particular, improved methods and systems are provided for laser beam positioning using an image capture device, such as a microscope camera, for enhanced qualification and calibration accuracy and precision. By qualifying and calibrating a beam delivery system, a desired corneal ablation treatment can be accurately effected without the beam becoming incident on undesired locations of corneal tissue. Methods and systems may be utilized upon replacement, maintenance, installation, evaluation, or trouble-shooting of any beam delivery system component, e.g., internal mechanical or optical components such as a mirror or an iris, major optical re-alignment of the system, or problems with error generation. In some aspects, these techniques may be useful in periodic maintenance that is performed on a beam system, for example, according to an annual maintenance schedule. These techniques may also be useful during system manufacturing, initial system set-up, or field servicing. As used herein, the term “calibration” encompasses altering or configuring machine readable code or programming instructions for a beam delivery system. The term “qualification” encompasses determining that a system, subsystem, or component is within an acceptable tolerance. 
       FIG. 1A  schematically illustrates an exemplary system  10  embodiment for qualifying and calibrating a beam delivery system  60 . System  10  is particularly useful for qualifying and calibrating a laser ablation system of the type used to ablate a region of the cornea in a surgical procedure, such as an excimer laser used in photorefractive keratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), and the like. System  10  generally includes a beam source  12 , a beam delivery system  60 , a surface such as a photochromic mirror  16 , a calibration pattern  30  disposed at an image location  18 , an imaging device  50 , and a computer  22 . In some embodiments, beam source  12  or mirror  16  may be integral to or otherwise included in beam delivery system  60 , and therefore the qualifying and calibrating techniques described herein may be applicable to these components as well. Calibration pattern  30  can be positioned along an imaging optical path  32  via a hinged support arm or mechanism  34  that allows or controls movement or orientation of calibration pattern  30  in three dimensional space relative to imaging device  50 . In some case, calibration pattern  30  is rigidly fixed relative to imaging device  50  or other components of system  10 . Calibration pattern  30  can be imaged by imaging device  50  to establish an image scale. Calibration pattern  30  can then be removed from imaging optical path  32 . In some cases, an image scale represents a correlation between pixels or sensors of imaging device  50  and distance at image location  18  or measurement plane. Image location  18  or measurement plane often corresponds to a treatment or ablation plane. Imaging device  50  may include one or more pixels or sensors, in combination with one or more mechanical or optical elements such as a lens. It will be appreciated that the depictions herein are for illustrative purposes only and do not necessarily reflect the actual shape, size, or dimensions of system  10 . 
     As illustrated in  FIG. 1B , beam source  12  typically directs a beam  24  through beam delivery system  60  which in turn directs a positioned beam  26  according to a predetermined intended pattern. Positioned beam is reflected from mirror  16  toward a test surface  40 , so as to create a test pattern  42  thereon. Test surface often corresponds to a treatment or ablation plane, and may also correspond to the distance at image location  18  or measurement plane. In some cases, when directed toward test surface  40 , positioned beam  26  travels along an incident beam path  27  which is coaxial or substantially coaxial with imaging optical path  32 . Beam path  27  may also be angularly offset with respect to imaging optical path  32 . As shown in  FIG. 1C , test pattern  42  is imaged by imaging device  50 . Computer  22  can determine a qualification and calibration of beam delivery system  60  based on the image scale, the intended pattern, and the test pattern image, or on any of a variety of variables or parameters as discussed elsewhere herein. In some embodiments, calibration of beam delivery system  60  can involve or provide the basis for scaling or scaling transformations of lateral redirecting characteristics of beam delivery system  60 . In some embodiments, qualification can be determined on the basis of whether a beam delivery system characteristic, such as a lateral redirecting characteristic, meets a specified tolerance. For example, a laser eye surgery system may be enabled for use in response to an offset being below an acceptable tolerance. 
     Computer or programmable processor  22  generally includes a processor, random access memory, tangible medium for storing instructions, a display, and/or other storage media such as hard or floppy drives. Processor  22  may include (or interface with) a conventional PC system including the standard user interface devices such as a keyboard, a display monitor, and the like. Processor  22  may include an input device such as a magnetic or optical disk drive, an internet connection, or the like. Such input devices can be used to download a computer executable code from a tangible storage media embodying any of the methods described herein. Tangible storage media may take the form of a floppy disk, an optical disk, a data tape, a volatile or non-volatile memory, RAM, or the like, and the processor may include the memory boards and other standard components of modern computer systems for storing and executing this code. Although tangible storage media will often be used directly in cooperation with a input device of processor  22 , the storage media may also be remotely operatively coupled with processor by means of network connections such as the internet, and by wireless methods such as infrared, Bluetooth, or the like. 
     Relatedly, each of the calculations or operations described herein may be performed using computer  22 , which may be a stand-along general purpose computer, or the like, having hardware, software, and/or firmware. The various method steps may be performed by modules, and the modules may comprise any of a wide variety of digital and/or analog data processing hardware and/or software arranged to perform the method steps described herein. The modules optionally comprising data processing hardware adapted to perform one or more of these steps by having appropriate machine programming code associated therewith, the modules for two or more steps (or portions of two or more steps) being integrated into a single processor board or separated into different processor boards in any of a wide variety of integrated and/or distributed processing architectures. These methods and systems will often employ a tangible media embodying machine-readable code with instructions for performing the method steps described herein. Suitable tangible media may comprise a memory (including a volatile memory and/or a non-volatile memory), a storage media (such as a magnetic recording on a floppy disk, a hard disk, a tape, or the like; on an optical memory such as a CD, a CD-R/W, a CD-ROM, a DVD, or the like; or any other digital or analog storage media), or the like. In some embodiments, code may be downloaded from a communication modality such as the Internet, and stored as hardware, firmware, or software, or the like. 
     Beam source  12  may include, but is not limited to, an excimer laser such as an argon-fluoride excimer laser producing laser energy with a wavelength of about 193 nm. Alternative lasers may include solid state lasers, such as frequency multiplied solid state lasers, flash-lamp and diode pumped solid state lasers, and the like. Exemplary solid state lasers include ultraviolet solid state lasers producing wavelengths of approximately 188-240 nm such as those disclosed in U.S. Pat. Nos. 5,144,630, and 5,742,626; and in Borsutzky et al., “ Tunable UV Radiation at Short Wavelengths  (188-240  nm )  Generated by Sum - Frequency Mixing in Lithium Borate” Appl. Phys. B,  52, 380-384 (1991), the full disclosures of which are incorporated herein by reference. A variety of alternative lasers might also be used, such as infrared or femtosecond lasers. For example, a pulsed solid state laser emitting infrared light energy may be used as described in U.S. Pat. Nos. 6,090,102 and 5,782,822, the full disclosures of which are incorporated herein by reference. The laser energy generally comprises a beam formed as a series of discrete laser pulses, and the pulses may be separated into a plurality of beamlets as described in U.S. Pat. No. 6,331,177, the full disclosure of which is incorporated herein by reference. Further exemplary beam systems and methods are described in U.S. Pat. Nos. 4,665,913; 4,669,466; 4,732,148; 4,770,172; 4,773,414; 5,163,934; and 5,556,395, the disclosures of which are hereby incorporated by reference in their entireties for all purposes. In an exemplary embodiment, a VISX STAR Excimer Laser System™, commercially available from VISX, Incorporated of Santa Clara, Calif., may be used for the ablation. This system can produce an output of 193.0 nm, operates at a frequency of 6.0 Hz and can be adjusted to deliver uniform fluence of 160.0 millijoules/cm 2  with a 6.0 mm diameter ablation zone. Other laser systems suitable for use may include the T-PRK™ scanning and tracking laser from Autonomous Technologies Corporation, the SVS Apex™ laser from Summit Technology Inc., the Keracor™ 117 scanning laser system from Chiron Vision, or the like. In addition to the beam types described above, it is appreciated that any of a variety of energy streams or radiation beams such as ultraviolet, gamma, and x-ray beams may be used. 
     In some embodiments, imaging device  50  can be exemplified by a microscope camera. In a related embodiment, imaging device  50  can include a camera having an image sensor such as a charge-couple device (CCD) or a complimentary metal oxide semiconductor (CMOS) digital image sensor. Relatedly, imaging device  50  may include an infrared sensitive CCD. It is appreciated that in some embodiments, system  10  may include more than one imaging device  50 . Imaging device  50  may make use of at least a portion of the optics of a beam delivery system, such as with an on-axis or near on-axis viewing arrangement integrated into a microscope. In some cases, imaging device  50  may be entirely separate and/or off axis, optionally using off-axis tracking cameras. Imaging device  50  may also be coupled with or include a video system so as to enable a system operator to observe various steps of the qualification and calibration procedures. Some examples of imaging device  50  are described in U.S. Pat. Nos. 6,251,101; 6,322,216; and 6,562,026; and in U.S. Patent Publication No. 2005/0094262, the entire disclosures of which are hereby incorporated by reference for all purposes. 
       FIG. 1D  schematically illustrates an embodiment of beam delivery system  160 . A beam  102  is generated from a suitable beam source  104 , such as an argon fluoride (ArF) excimer laser beam source for generating a laser beam in the far ultraviolet range with a wavelength of about 193 nm. Beam  102  is directed toward a beam splitter  106 . A portion of beam  102  is reflected onto an energy detector  108 , while the remaining portion is transmitted through beam splitter  106 . Reflective beam splitter  106  may include a transmitting plate of partially absorbing material to attenuate the beam. Transmitted beam  102  is reflected by an adjustable mirror  110  that is used to align the path of the beam. In alternate embodiments, a direction of the beam path may be controlled with adjustable prisms. Beam  102  reflects from the mirror  110  onto a rotating temporal beam integrator  112  that rotates a path of the beam. Another type of temporal beam integrator may be used to rotate the beam. 
     Beam  115  travels to the spatial integration plane at which a variable diameter aperture  116  is disposed. In some embodiments, aperture  116  is a circular aperture. The spatial integration plane is disposed near the focal point of the positive lens  114 . An apertured beam  120  emerges from the variable aperture  116 . The variable aperture  116  may be a variable diameter iris, optionally combined with a variable width slit (not shown) used to tailor the shape and size profile of the beam  115  to a particular application, such as an opthalmological surgery procedure. The apertured beam  120  is directed onto an imaging lens  122 , which may be a biconvex singlet lens with a focal length of about 125 mm. In some surgical embodiments, the beam  126  emerging from the imaging lens  122  is reflected by a mirror/beam splitter  130  onto the surgical plane  132 , and the apex of the cornea of the patient is typically positioned at or near the surgical plane  132 . Imaging lens  122  may be moved transverse to the beam to offset the imaged beam in order to scan the imaged beam about the surgical treatment plane  132 . A treatment energy detector  136  senses the transmitted portion of the beam energy at the mirror/beam splitter  130 . A beam splitter  138 , a microscope objective lens  140 , and the imaging device  150  form part of the observation optics. The beam splitter may be coupled to the imaging device  150  to assist in iris calibration as well as for viewing and recording of the surgical procedure. A heads-up display may also be inserted in the optical path  134  of the microscope objective lens  140  to provide an additional observational capability. Other ancillary components of beam delivery system  160  such as the movable mechanical components driven by an astigmatism motor and an astigmatism angle motor, are not shown to avoid prolixity. 
     In another embodiment as schematically illustrated by  FIG. 1E , beam delivery system  160 ′ may include beam source  104 ′, mirrors  117 ′, spatial and temporal integrators  112 ′, a variable aperture  116 ′, and a scanning mechanism  122 ′. In some embodiments, scanning mechanism  122 ′ can be configured to selectively deflect beam  120 ′ laterally across the corneal surface of eye E in the X-Y plane. In some cases, scanning mechanism  122 ′ may laterally deflect beam  120 ′ in response to movement of eye E. Although certain aspects of  FIGS. 1D and 1E  are described in terms of a surgical application, it is appreciated that beam delivery systems  160  and  160 ′ may represent a component of a non-surgical system as well. In some embodiments, electromechanical transducers such as galvanometers can be used to control movement of one or more components of beam delivery system  160  and  160 ′. For example, as depicted in  FIG. 1E , a galvanometer  119 ′ can be used to produce rotary movement in a deflection mirror  117 ′ or other beam directing element. In some embodiments, galvanometers  119 ′ can be used to adjust mirror  117 ′ positioning so as to provide alignment of beam according to or relative to an intended pattern. Similarly, a computer controller may scan a beam by pivoting mirrors  117 ′ using galvanometric motors  119 ′, or any of a variety of other scanning mechanisms. 
       FIG. 2A  illustrates an exemplary embodiment of an imaging device  250  and a calibration pattern  230 . Calibration pattern  230  typically has known spatial dimensions and a known positional and angular orientation, where such orientation is often known relative to imaging device  250 . For example, calibration pattern  230  may be disposed at a known image location, and may be imaged in any of a variety of positions and planes. In some cases, calibration pattern  230  may include a cross-hair or other feature to evaluate the perpendicularity of imaging device  250 . Such features may be useful in calibrating the axis of imaging device  250  or otherwise evaluating rotational offsets. Any of a variety of trackers can also be used to evaluate XY or torsional movement or orientation. In the embodiment shown here, calibration pattern  230  includes a pattern of 9 (3×3) calibration spots. It may be desirable to set the illumination to a certain level prior to capturing any images with imaging device  250 . Calibration pattern  230  may include a reticle or other optical element having lines, grids, spots, or other patterns. For example, calibration pattern  230  may include a pattern of 16 (4×4) spots, of 25 (5×5) spots, and the like. In some embodiments, calibration pattern  230  may include a series of marks  231  such as one or more circular chrome layers, each having a 10 mm or other known diameter, disposed on a surface  233  such as a glass or crystal plate. The captured image of the calibration pattern can represent, for example, the vertical and horizontal dimensions of each mark  231 , and as described elsewhere herein, images of a test pattern may be rescaled according to the relative dimensions of the image of calibration pattern  230 . Relatedly, the use of the image of calibration pattern  230  allows the various attributes of imaging device  250  to be quantified. Imaging device  250  attributes may include, for example, magnification, three dimensional position, angular orientation, and the like. In some embodiments, where calibration pattern  230  is disposed at an image location  235  or measurement plane, imaging device  250  can acquire an image of calibration pattern  230  so as to establish a link or association between imaging device pixels or sensors and the distance between imaging device  250  and image location  235 . In some cases, calibration pattern  230  may be used to establish a calibration factor based on pixels per unit distance. Relatedly, imaging device  250  will often not have a fixed relationship between pixel counts or size and a size or scale of an object being imaged at the image location, so that, for example, a 0.1 mm 2  object in the center of the viewing field may correspond to and be imaged onto 9 (3×3) pixels in the center of an image sensor of imaging device  250 , and may correspond to only 4 (2×2) pixels in one peripheral corner of the viewing field. Further, imaging device  250  may be responsible for some distortions of the calibration pattern image, which may also vary across the viewing field. Hence, the image of calibration pattern  230  may be fit to a modification algorithm to account for such imaging device distortion. In some embodiments, an operator may move calibration pattern  230  to a desired plane so that a sharp image may be obtained for calibration purposes. 
       FIG. 2B  illustrates an embodiment of an intended pattern  270  which beam delivery system  260  is configured to create on a test surface  240 . It is appreciated that in many embodiments intended pattern  270  is not actually imageable or visible on test surface  240 . In this sense, intended pattern  270  may represent a pattern which beam delivery system  260  is configured to create. In some embodiments, intended pattern  270  may correspond to a set of beam placement or positional parameters of beam delivery system  260 , and may reflect intended dimensions or offsets.  FIG. 2C  illustrates an embodiment of an imageably altered test pattern  280  produced on test surface  240  according to intended pattern  270 . In some embodiments, creation of test pattern  280  involves setting beam direction parameters of beam delivery system  260  according to intended pattern  270 , directing a beam toward test surface  240  along beam path  226   a  to create an individual test spot  282   a , adjusting beam direction parameters of beam delivery system  260  according to intended pattern  270 , directing the beam toward test surface  240  along beam path  226   b  to create an individual test spot  282   b , adjusting beam direction parameters of beam delivery system  260  according to intended pattern  270 , directing a beam toward test surface  240  along beam path  226   c  to create an individual test spot  282   c , and so on, so as to create test pattern  280 . In some embodiments, procedures such as these may involve optics of beam delivery system  260  scanning the beam over tissue of an eye according to instructions from a computer, which may be encoded to correlate with intended pattern  270 . 
     As beam moves from beam path  226   a  to beam path  226   b , the beam can be described as being laterally redirected along an axis  227  from a first region to a second region, and as beam moves from beam path  226   b  to beam path  226   c , the beam can be described as being laterally redirected along axis  227  from the second region to a third region. In some embodiments, axis  227  may correspond to a test pattern axis. In  FIG. 2C , test pattern  280  is shown as deviating from intended pattern  270 . Typically, this deviation is due to one or more beam system characteristics of beam delivery system  260 , which may include alignment parameters or optical parameters, such as a lateral redirecting characteristic. In some embodiments, alignment parameters may include rotational offset, axial deflection offset, tilt, or other warping factors associated with beam delivery system  260 . In related embodiments, optical parameters may include mirror thickness offset or pincushion effect. In some embodiments, such beam system characteristics can be referred to as beam positional or placement parameters. 
     Test surface  240  may be constructed on any of a variety of materials, including, for example, a silkscreen or luminescent material. Individual marks  282  or test spots may be characterized by a permanent change in color, a luminescent glow, a disrupted surface characteristic, and the like. For example, a luminescent material may include a piece of glass, crystal, or polymer that is optically activated, such as chromium doped, and has a relatively long luminescent lifetime. Images may be recorded after each beam pulse, wherein the luminescence of mark  282  may have decayed before the next beam pulse is directed onto the luminescent surface. Other types of test surface materials include photosensitive materials, photoreactive materials, photographic materials, Zapit paper, polymers that change color based on temperature, and polymethylmethacrylate materials. Individual marks  282  may include an ablation, a permanent change in color, a luminescent glow, and the like. In some embodiments, test surface  240  includes a photosensitive material, and marks  282  include a permanent change in color, such as a white spot on a black background or vice versa, or a luminescent glow. In some embodiments, test surface  240  includes a photoreactive material, a polymethylmethacrylate material, or other VISX calibration material, available from VISX, Incorporated of Santa Clara, Calif. For example, marks  282  may be ablated on a polymethylmethacrylate material with a laser beam. 
     As seen in  FIG. 2D , imaging device  250  can be used to image test pattern  280 . Beam delivery system  260  can then be qualified or calibrated, in response to the test pattern image. In some embodiments, with reference to  FIG. 2C , beam delivery system  260  may laterally redirect the beam along an axis  228  a plurality of times. Beam delivery system  260  can be calibrated by altering machine readable code of the beam system so that subsequent lateral deflections of the beam along axis  228   a  are determined using a plurality of different calibration factors associated with different beam locations along axis  228 . In some embodiments, axis  228   a  may intersect axis  227 . In related embodiments, beam delivery system  260  may laterally redirect the beam along axis  227  from a first region to a second region, and laterally redirect the beam along axis  228   b  from the second region to a third region. Beam delivery system  260  can be calibrated by altering machine readable code of the laser eye surgery system so that subsequent lateral deflection of the beam along axis  227  is determined using a first calibration factor, and so that subsequent lateral deflection of the beam along axis  228   b  is determined using a second calibration factor. The second calibration factor may be different than the first calibration factor. In another embodiment, beam delivery system  260  can laterally redirect the beam along a test pattern axis  227  of test pattern  280  from a first region to a second region, and the qualifying or calibrating of the beam can be based on an offset between test pattern axis  227  and an intended axis  229  of intended pattern  270 . In some cases, the offset is an angular offset. In a further related embodiment, beam delivery system  260  laterally redirects the beam along a first test pattern vector between the regions of test pattern  280 , and laterally redirects the beam along a second test pattern vector between the regions of test pattern  280 . Qualification or calibration of beam delivery system  260  can be based on offsets between the vectors and intended vectors between regions of intended pattern  270 . In still another related embodiment, a laser eye system can be calibrated by altering machine readable code of the laser eye surgery system in response to a first lateral beam deflecting characteristic of beam delivery system  260 , and qualifying the laser eye surgery system by enabling use of the laser eye surgery system in response to a second lateral beam deflecting characteristic of beam delivery system  260  being within an acceptable threshold tolerance. 
     As seen in  FIG. 2E , in some embodiments, calibration pattern  230  may be imaged at or near a beam focus plane  201  or at or near a treatment plane  203 . Either of these planes, or any plane substantially near or between these planes, may be considered as a measurement plane. In some embodiments, the plane at which calibration pattern  230  is imaged, the plane at which test pattern  280  is created, and the plane at which test pattern  280  is imaged, may all be at the same plane. In some cases, treatment plane  203  may be a few millimeters or some other desired distance away from beam focus plane  201 . As seen in  FIG. 2F , in some embodiments, a beam can be oriented toward a beam focus plane  201  so as to produce test pattern  280  at beam focus plane  201 . Optionally, test pattern  280  may be produced at treatment plane  203 . As seen in  FIG. 2G , test pattern  280  may be imaged by imaging device  250  at beam focus plane  201  or at treatment plane  203 . In such cases, the captured image may be slightly out of focus if imaging device  250  is oriented toward treatment plane  203 . In some cases, creation of test pattern  280 , and imaging of calibration pattern  230  or test pattern  280  can be automatically implemented without operator intervention. 
     A fitting routine can accurately and precisely estimate the center position of beam. In some embodiments, calibration pattern  230  is imaged prior to directing the beam onto test surface  240 , and test pattern  280  parameters may be calculated as the qualification and calibration procedure advances. 
     Image Processing 
     The location of an individual test spot image in a test pattern image can be described using centroid detection. In some embodiments, a center of mass for an image F(j,k) of size J×K can be represented with the following formula. 
     
       
         
           
             
               
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     A priori knowledge of the number and location of the individual spots of intended pattern  270  can be used to automate centroid detection of multiple test spot images within a single test pattern image. An individual test spot image centroid can be represented as C N ={C x , C y }. In some embodiments, test spots  282  can be disposed toward the outer extremes of imaging device&#39;s field of view, thus enhancing the ability describe the performance of beam delivery system  260 . 
     Scaling Calibration 
     As noted above, imaging device  250  can acquire an image of calibration pattern  230  so as to establish a link or association between imaging device pixels and the distance at an image location or measurement plane. Test pattern  280  can be created at the same plane, and Measurement Scale Factors M X  and M Y  can be computed by averaging the horizontal and vertical spot distances and then normalizing them by their intended displacements. These scale factors can be used to adjust Calibration Scale Factors S CX  and S CY  according to the following formula: 
               [           S   ex   i               S   ey   i           ]     =       [           M   x         0           0         M   y           ]     ⁡     [           S   ex   0               S   ey   0           ]             
where S cx   o  and S cy   o  represent an intended design specification, and S cx   i  and S cy   i  represent an updated calibrated scale. In some embodiments, this formula can relate beam deflection in units of distance to that of motor counts for controlling various components, such as galvanometers, of beam delivery system  260 .
 
     Rotational Offsets 
     One approach to establish rotational orientation involves calibrating imaging device  250  in angular space using a reference marking from a calibration pattern  230  such as a precision reticle or target. The rotational offset measured in test pattern  280  can then be compared to the known rotational offset of imaging device  250 . If desired, rotational transformations can be performed. In some embodiments, a limit can be specified for the magnitude of difference between rotational offsets to disqualify a defective scanning system. 
     Axial Deflection Offset Error 
     If beam delivery system  260  uses reflecting surfaces to steer the beam, then offsets in deflection from the true axis of rotation may introduce nonlinearities or other complex direction effects. In some embodiments, such an error may be a function of the distance between the true axis and the reflecting surface (T) and the alignment of the entry beam with respect to the axis of rotation (K). The error may be represented by the following formula. 
     
       
         
           
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     Similar approaches are discussed by Gerald F. Marshall in  Optical Scanning , page 561 (1991). If beam delivery system  260  operates within a narrow angular range, then the errors introduced due to surface offset may be extremely linear and may be accounted for by this calibration method directly, obtaining as much accuracy as imaging device  250  resolution provides. Optionally, the above relationship can be used to determine if the alignment of the beam onto the turning mirrors is outside a specified amount An exemplary embodiment of a axial deflection offset error is depicted in  FIG. 2H , which shows a test pattern  280   h  axially offset from an intended pattern  270   h  on a test surface  240   h.    
       FIGS. 2I and 2J  depict a comparison between a linear offset and a nonlinear offset. An intended pattern  270   i  and a corresponding test pattern  280   i  are shown on a test surface  240   i  in  FIG. 2I . The relationship between intended pattern  270   i  and test pattern  280   i  can be illustrated graphically by  FIG. 2J . The horizontal axis (X i ) represents lateral displacement in the intended pattern along the x direction, when viewing test surface  240   i . The vertical axis (X t ) represents lateral displacement in the test pattern along the x direction, when viewing test surface  240   i . The solid curved line NL represents the nonlinear relationship of intended pattern  270   i  and test pattern  280   i  as shown in  FIG. 2I . The dashed straight line L represents a hypothetical linear relationship between an intended pattern and a test pattern. 
     Pincushion Error 
     When the scanning operation is separated by axis, deflections propagated from the first mirror to the second can introduce a one dimensional distortion in the scanning direction corresponding to the first axis onto the imaging plane. The change of a large-offset intended position from the first mirror (Y i ) as the second mirror angle changes (θ x ) can be represented relatively by the following formula 
             ɛ   =           Y     i   ⁢           ⁢   θ   ⁢           ⁢   x       -     Y     i   ⁢           ⁢   0           2   ⁢           ⁢     Y     i   ⁢           ⁢   0           =       1   -     cos   ⁢           ⁢     θ   x           2   ⁢     (     1   +     e   /   d       )     ⁢   cos   ⁢           ⁢     θ   x                 
where e represents the spacing between the mirrors and d represents the distance from the last mirror surface to the image plane. Similar approaches are discussed by Gerald F. Marshall in  Optical Scanning , page 568 (1991). In some embodiments, mirror angle changes may be sufficiently small such that pincushion effects may be ignored. Correcting for this pincushion error may in some cases be limited by the resolution of the imaging device or measurement camera.  FIGS. 2K and 2L  illustrate embodiments pincushion offsets, showing intended patterns  270   k  and  270   l , and test patterns  280   k  and  280   l , on test surfaces  240   k  and  240   l , respectively.
 
     Checking for Alignment Errors 
     As described above, rotational alignment errors can be checked against a fixed target imaged by the imaging device or camera. For tilt errors to be detected, it is helpful to know that the plane of investigation for imaging the test spots (e.g. the surface of the material ablated) is in the correct position. Linear trends in the measured distances of the spots from left-to-right or top-to-bottom during the Scaling Calibration can be used to detect whether the scanner system is tilted. Other errors in the alignment of the optics may be detectable by the amount of error not accounted for in the linear scaling transformation. In addition to tilt, other warping factors may be measurable that may indicate artifacts such as spherical aberration. 
     TABLE 1 illustrates an exemplary qualification and calibration matrix. A beam delivery system may be qualified or calibrated based on certain observed or calculated beam delivery system characteristics, such as a lateral redirecting characteristic. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Scaling 
                 Rotation 
                 Axial Deflection 
                 Pincushion 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Qualification 
                 disqualify system if 
                 disqualify system 
                 disqualify system 
                 disqualify system 
               
               
                   
                 scale exceeds 
                 if rotation 
                 if deflection 
                 if pincushion 
               
               
                   
                 certain limits 
                 exceeds certain 
                 exceeds certain 
                 effect exceeds 
               
               
                   
                   
                 limits 
                 limits 
                 certain limits 
               
               
                 Calibration 
                 calibrate system if 
                 calibrate system 
                 calibrate system 
                 calibrate system if 
               
               
                   
                 scale is within 
                 if rotation is 
                 if deflection is 
                 pincushion effect 
               
               
                   
                 specified limits 
                 within specified 
                 within specified 
                 is within specified 
               
               
                   
                   
                 limits 
                 limits 
                 limits 
               
               
                   
               
            
           
         
       
     
     It is appreciated that some embodiments may avoid disqualification by coupling certain calibration axes (e.g. x axis and y axis), so that angular errors or offsets result in adjustments to both axes. 
       FIG. 3A  illustrates an exemplary method for testing a laser eye surgery system. The method can include imaging a known calibration pattern at an image location with an imaging device, and establishing an image scale based on the calibration pattern and the calibration pattern image as depicted in step  310   a . The method also typically includes imageably altering a series of regions of a test surface with a laser beam of the laser eye surgery system at the imaging location, as depicted in step  320   a , and laterally redirecting the laser beam according to an intended pattern between altering of the regions using a beam delivery system so as to form a test pattern on the test surface, as depicted in step  330   a . In step  340   a , the test pattern is imaged at the imaging location with the imaging device. In step  350   a , a lateral redirecting characteristic of the beam delivery system is determined based on the image scale, the intended pattern, and the test pattern image. The beam delivery system can be qualified or calibrated in response to the lateral redirecting characteristic, as shown in step  360   a.    
       FIG. 3B  illustrates an exemplary method  300   b  for qualifying and calibrating a beam system. A beam system typically includes components such as a beam source, a beam delivery system, or both. The method can include establishing an image scale by imaging a calibration pattern at an image location, as depicted by step  310   b . Step  320   b  involves imageably altering a test surface with a beam system according to an intended pattern to produce a test pattern on the test surface. In step  330   b , the test pattern is imaged with an imaging device. Step  340   b  includes determining a beam system characteristic based on the image scale, the intended pattern, and the test pattern image. Step  350   b  is characterized by the decision of determining if the beam system characteristic is within specified qualification limits. If the beam system characteristic is not within specified qualification limits, the beam system may be disqualified based on the beam system characteristic, as shown in step  360   b . For example, a rotation effect of the beam delivery system may be too excessive. If the beam system characteristic is within specified qualification limits, the beam system may be qualified based on the beam system characteristic, as shown in step  370   b . After the beam system is qualified, a decision is by determining if the beam system characteristic is within specified calibration limits, as shown in step  380   b . Step  390   b  illustrates the outcome when the beam system characteristic is within specified calibration limits, and typically involves proceeding with operation of the beam system. In some embodiments, this can involve treating a patient with the beam system. For example, upon qualification or calibration, a patient&#39;s cornea may be ablated to correct a variety of vision defects, including myopia, hyperopia, astigmatism, and other corneal surface profile defects. 
     Step  395   b  illustrates the outcome when the beam system characteristic is not within specified calibration limits, and typically involves adjusting certain components of the beam system. Such adjustments can be system-specific. Relatedly, such adjustments can be made so as to individually and independently alter the position of each of the test spots of a subsequent test pattern created by the beam delivery system. After the beam system has been calibrated, it can be tested again, beginning with step  320   b . In some cases, it may be possible to calibrate and then use the system, without retesting. 
     It is appreciated that some embodiments may include both the qualification and calibration steps, some embodiments may include only the qualification steps, and some embodiments may include only the calibration steps. Calibration or adjustment may involve changing a drive signal for an actuator, for example a galvanometer which controls placement of a beam delivery system mirror. Such drive calibrations may also be variable drive calibrations. In some aspects, a calibration may not involve a change in beam delivery system hardware, but may involve an adjustment in a signal correlation. 
     Analysis of the test pattern may be automated using the systems described herein. In some laser ablation embodiments, the computer may indicate whether the beam delivery system is sufficiently accurately calibrated to perform any ablation, or to perform a particular photorefractive resculpting. The computer system may optionally adjust the ablation algorithm based on the actual position of the test pattern, either automatically or with manual input, to avoid or attenuate an unwanted beam delivery system characteristic, for example. Hence, the system can provide a feedback mechanism to enhance the accuracy of the change in corneal shape effected by a laser. 
     In some cases, a given beam system characteristic may vary in direct proportion to adjustment of a beam delivery system component. For example, adjustment of the position or rotation of a mirror of the beam delivery system may result in a corresponding change in the positioned beam trajectory. In other aspects, a given beam system characteristic may vary in a non-linear fashion in response to adjustment of a beam delivery system component. Beam delivery system performance data can be collected so as to produce interpolation curves, fitting curves, charts, look-up tables, and other similar means for determining or representing a relationship between a beam system characteristic or positional parameter and a beam delivery system configuration. For example, a look-up table may be created based on an intended pattern and a test pattern, and standard interpolation routines may be used between discrete table entries. Such curves or charts may be useful in the qualification and calibration techniques described herein. 
     A drift of the beam delivery system may be determined by monitoring a variance in a test pattern. It will be appreciated that drifts may be dependent upon several factors, such as the manner in which the system is used between measurements, the particular set of system parameters, changes in environmental conditions such as temperature, and the like. Embodiments of the present invention can also be applied to judge the stability of the beam delivery system. For example, the test surface may include a luminescent plate. After each beam pulse, an image is captured while the plate is still emitting light. Images are then analyzed. Positions of test spots can be calculated and plotted on x and y axes so that the plot provides a map of where the beam pulses landed. This plot can then be used to determine any systematic movement of the laser beam with time. Alternatively, the data can be used to determine parameters such as the statistical variations in x and y positions. In some embodiments, a beam delivery system may be qualified or calibrated on the basis of drift. 
     Referring back to  FIG. 1D , a number of the optical elements in beam delivery system  60  may be rotated along the beam delivery path, as described in detail in U.S. Pat. No. 6,816,316, the disclosure of which is hereby incorporated by reference, to distribute any distortion caused by imperfections of the optical elements. In one embodiment, the lens  114  is rotated around its axis. In other embodiments, the beam splitter  106  may be moved along its plane; the mirror  110  may be moved along its plane; the diffractive optic  113  may be moved in its plane, and the mirror/beam splitter  130  may be moved along its plane. Although the path of the beam is stable with respect to movement of an optical element, minor deviations in the position of the optical center about the axis of rotation may occur, and such deviations may induce a slight wobble in the path of the laser beam as the optical element rotates. Advantageously, the techniques described herein may also be utilized to identify a rotation-induced laser induced wobble from a plurality of marks. Analysis of images of the marks may help account for these small deviations due to rotation of the optical element. 
       FIG. 4  illustrates an exemplary method  400  for determining a beam delivery system characteristic  490  by inputting a variety of variables or measurements. For example, the beam delivery system characteristic can be based on one or more of a calibration pattern parameter  410 , a calibration pattern image  420 , an intended pattern parameter  430 , a test pattern image  440 , a position of an imaging device  450 , a position of a calibration pattern  46 , a position of a test pattern  40 , and a position of a beam delivery system  480 . It is understood that the position of each of these components can be considered in terms of a location, an angular disposition, or any other orientation in three dimensional space. 
     It will be appreciated that the presently disclosed qualification and calibration systems and methods will find use in a variety of different laser systems, including scanning lasers and large area laser ablation systems. Examples include the VISX STAR™, STAR S2™, STAR S3™, STAR S4™ Excimer Laser Systems™, and laser systems employing wavefront technologies, all of which are commercially available from VISX, Incorporated of Santa Clara, Calif. Other laser systems include those available from Alcon Summit, Autonomous Technologies Corp., Bausch &amp; Lomb, Chiron Vision, LaserSight, Nidek Co., Ltd., Zeiss Meditec, Schwind, Wavelight Technologies, and a variety of other companies. 
     The techniques described herein may be used to analyze a variety of radiation beams such as ultraviolet, gamma, and x-ray beams, may be used with a wide variety of ablation planning protocols or algorithms, and may provide input to such algorithms which can enhance their accuracy. A variety of parameters, variables, factors, and the like can be incorporated into the exemplary method steps or system modules. While the specific embodiments have been described in some detail, by way of example and for clarity of understanding, a variety of adaptations, changes, and modifications will be obvious to those of skill in the art. Treatments that may benefit from the invention include intraocular lenses, contact lenses, spectacles and other surgical methods in addition to refractive laser corneal surgery. Therefore, although certain exemplary embodiments and methods have been described in some detail, for clarity of understanding and by way of example, it will be apparent from the foregoing disclosure to those skilled in the art that variations, modifications, changes, and adaptations of such embodiments and methods may be made without departing from the true spirit and scope of the invention. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims.