Abstract:
The present invention is directed to a method and apparatus for assuring that dynamic glare-shielding devices are properly characterized for the optical arrangement of a particular user by using a plurality of calibration points, thereby assuring the maximum effectiveness of such devices

Description:
[0001]     This application claims priority from U.S. Provisional Application No. 60/632,918, by Christopher S. Mullin, for “Method and Apparatus for Calibrating Glare-shielding Sunglasses,” filed Dec. 3, 2004, and is hereby incorporated by reference in its entirety. 
     
    
       [0002]     This application is directed to a method and apparatus for calibrating glare-shielding glasses, sunglasses and similar optical shading devices, and more particularly, to a method and apparatus for assuring that such devices are properly characterized for the optical arrangement of a particular user by using a plurality of calibration points, thereby assuring the maximum effectiveness of such devices.  
       BACKGROUND AND SUMMARY  
       [0003]     To protect the eyes from bright light sources, shading techniques have been applied to optical devices such as eyewear (e.g., glasses), visors, face shields (e.g., pilot helmets) and the like. Such devices may either be made from a shaded material that constantly reduces the amount of light passing therethrough, or from a material that has an alterable shading. Photosensitive lenses change the amount of light transmitted through the entire lens equally, as do some electronically driven lenses and auto-dimming rearview mirrors in cars. The present invention is directed to those devices, referred to herein as glare-shielding glasses where small sections or regions of the device may be selectively darkened (partially or completely) so as to dim glare from a user&#39;s field of view through the device without dimming the rest of the field of view.  
         [0004]     Glare-shielding glasses are known to exist, as demonstrated by several patents, including the following, which are also hereby incorporated by reference in their entirety for their teachings:  
         [0005]     U.S. Pat. No. 4,848,890 to Horn, for a Visor with Sun Point Blocking, which discloses the use of a liquid crystal matrix that is used to block light in response to a photosensor;  
         [0006]     U.S. Pat. No. 5,305,012 to Faris, for an Intelligent Electro-Optical System and Method for Automatic Glare Reduction, which teaches a method and apparatus for selectively reducing the intensity of light rays propagating towards an optical element such as an eye or a camera, including glare caused by intense sources of illumination; and  
         [0007]     U.S. Pat. Nos. 5,671,035 and 5,841,507 to Barnes, for a Light Intensity Reduction Apparatus and Method, which discloses similar light-shielding devices, and the ability of a user to selectively set a light level threshold (user control switches), as well as a user set-up technique.  
         [0008]     Glare-shielding glasses place darkened areas on each lens of the glasses between glare sources and the user&#39;s pupils. A photosensor built into the frames determines the direction of glare sources relative to the glasses, but the glasses must make assumptions about the positions of the user&#39;s pupils. As used in this disclosure, “glare-shield” and glare-shielding “glasses” or “surfaces” will be taken to mean any head-mounted optic through which the user views the scene before them, and includes those with temples that rest on or behind the ears as well as clip-on types that attach to other eyewear and visors that are held in front of the face (e.g., helmet visors, face shields and the like).  
         [0009]     Furthermore, each user will place the glasses on their head in approximately the same place each time, but different users will wear the glasses somewhat differently, depending on the shape of their nose, the distance between their pupils, and other facial features. To shield all of the various positions where a user&#39;s eyes might be situated would require a large amount of the field of view to be blocked or darkened, which would hamper the usefulness of the device. Thus, the glasses need to be calibrated for each user, so that they only shadow the region occupied by one particular user&#39;s pupil positions.  
         [0010]     Disclosed in embodiments herein is a method for calibrating a glare-shield comprising: measuring at least two relationships between a glare source and a glare calibration position; and using the measured relationships, determining the location of a darkened spot on the glare shield in response to a specific location of glare on a glare sensor.  
         [0011]     Also disclosed in embodiments herein is a system for the calibration of glare-shielding glasses worn by a user, comprising: at least one glare source; a darkened region on the glare-shielding glasses; a user&#39;s eye for concurrently perceiving the glare source and the darkened region and adjusting the location of the darkened region relative to the glare source; a controller for determining at least a first and a second relationship between the location of the glare source, the darkened region on the glare-shielding glasses, and the user&#39;s eye; memory for recording the first and second positions of the glare source on a photodetector associated with the glare-shielding glasses when the darkened region occludes the glare source; and said controller further calculating and storing first and second calibration data points, and to calculate a correction factor therefrom and subsequently provide the glare-shielding glasses with the correction factor to adjust the position of the darkened region in response to detection of the glare source location on the photodetector. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic representation of a glare shield shadowing a specific shielded point;  
         [0013]      FIG. 2  is a schematic of a simple lens or pinhole camera;  
         [0014]      FIG. 3  is a flow chart illustrating general aspects of a method of calibration in accordance with the present invention;  
         [0015]      FIG. 4  is an illustrative example of an aspect of a calibration technique in accordance with the method set forth in  FIG. 3 ;  
         [0016]      FIG. 5  is an illustrative example of another aspect of the calibration technique in accordance with the method set forth in  FIG. 3 ;  
         [0017]      FIGS. 6-8  are illustrative embodiments of calibration systems in accordance with various alternatives of the present invention;  
         [0018]      FIG. 9  is an illustration of the overall calibration set-up when applied to glare-shielding glasses;  
         [0019]      FIG. 10  is an enlarged view of the left lens shown in  FIG. 9 ;  
         [0020]      FIG. 11  is a depiction of one embodiment of glare-shielding glasses to which the described calibration procedure applies;  
         [0021]      FIG. 12  is a depiction of the sensor and a planar glare shield embodiment illustrating the various rotations of the glare shield for which a calibration procedure may account; and  
         [0022]      FIG. 13  is a depiction of the sensor and a non-planar glare shield embodiment illustrating the positional relationship between the calibration position and the glare shield for which a calibration procedure may account. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
         [0024]     The following disclosure is directed to a method for calibrating glare-shielding glasses. Glare-shielding glasses are described in the patents by Horn and Barnes, an example of which is illustrated in  FIG. 11 . In  FIG. 11 , glare-shielding glasses, designated  1110 , selectively reduce the intensity of light reaching the eye from specific portions of the field of view. In a general sense, glare-shielding is accomplished by placing a shield in front of each eye,  960  and  970 , with an addressable matrix of pixels, which can be individually darkened to reduce or eliminate the glare. The shield is most commonly fabricated from liquid crystal materials in a manner similar to a liquid crystal display. When a bright light source, such as the sun, appears in the field of view, one or more pixels between the user&#39;s pupil and the bright light source is activated to reduce the light striking the eye from the bright light source.  
         [0025]     One or more sensors,  200 , in conjunction with control electronics incorporated into the glare-shielding glasses determine which pixels to darken. The sensors are positioned on the glasses such that light from a light source anywhere in the user&#39;s field of view strikes at least one of the sensor pixels. The electronic controls on the glasses are employed to receive signals from the photodetector, and in response to signals from the photodetector, to darken one or more pixels on the glasses. In other words the controls, often in the form of a microcontroller, and so referred to in this document, determines which shield pixels to activate based on which sensor pixels are illuminated by light exceeding a specified threshold. Shield frames,  958  and temples,  1127 , hold the shields  960 ,  970  and sensor  200  in a fixed position relative to the eyes of the user  950 .  
         [0026]     Determining the correct three-dimensional position of the user&#39;s pupil with respect to the shield is critical when determining which shield pixels to activate when a specific sensor pixel is activated. Referring to  FIG. 1 , light which passes through the shield  12  at an angle θ strikes the pupil  11 . The light passes through the shield  12  at a position  14  which depends on the pupil distance, H, from the shield. The dependence on pupil distance increases with the angle of incidence of the light. Although it has been suggested to account for the pupil distance by changing the size of the darkened area on the shield, it will be appreciated that the size required to shield the pupil at different pupil distances, and at all incident angles, is larger than what is required if the pupil distance is determined and accounted for in determining the optimal location of the darkened spot. Furthermore, a larger-than-necessary darkened area also blocks more of the field of view than is necessary, and thus limits the utility of the glare-shielding glasses. The present invention describes a method and apparatus for accurately determining the pupil distance and utilizing the pupil distance when determining which shield pixels to darken.  
         [0027]     The method takes advantage of the geometric similarity between a simple lens camera and the eye-glasses system. The similarity is also illustrated in  FIGS. 1 and 2 .  FIGS. 1 and 2  illustrate the geometric similarity between a simple lens camera and a glare-shielding system.  FIG. 1  is a schematic representation of the glare-shielding system showing one shielded point and one shield.  FIG. 2  is a schematic of a simple lens camera, for example, a pinhole camera. A simple lens camera, exemplified by a pinhole camera, is comprised of a pinhole  1  that allows light rays through only one point in the pinhole plane  5  and a photodetector  2 . The distance A from the point directly under the pinhole to the position  4  at which an incident ray  3  intersects the photodetector depends only on the angle θ of the ray and the distance of the photodetector from the pinhole plane B: 
 
 A=B *tan (θ). 
 
         [0028]     Continuing with  FIG. 1 , there is depicted a glare-shielding system that seeks to shield one point in space  11  (such as a user&#39;s pupil) from incident light rays  3  using a transmissive display or glare shield  12  placed between the shielded point and the glare source. In a glasses application, the pupil would be in position  11 , and one lens of the glasses would be in position  12 . The distance G from the point on the shield closest to the shielded point to the “intersection position”  14 , at which an incident ray  3  intersects the shield, depends only on the angle θ of the ray and the “pupil distance” of the shield from the shielded point (H): 
 
 G=H *tan (θ). 
 
         [0029]     The relationship between the respective systems depicted in  FIGS. 1 and 2  arises because the shield is analogous to the photodetector, and the shielded point is analogous to the pinhole, since all rays of interest pass through that point.  
         [0030]     For each incident glaring light ray  3 , the glare-shielding system needs to determine G when it detects a glare source at A. The system then darkens the shield segment at G in order to shield the shield point from the glare source. Clearly, 
 
 G=H *tan (θ)= H*A/B =( H/B )* A.    Eq. 1 
 
         [0031]     Note that the complex trigonometric dependence on the incident angle has dropped out of the equation because of the analogy between the camera and the glareshield. Note also that the rays can be skewed (i.e. not in the plane of the figure, thereby introducing a second dimension needing to be calibrated), while the geometric relationships still hold true. This implies that the plane of the figure can be taken to be coincident with either the row or column direction of the photodetector. It also implies that the calibration techniques described herein can be carried out in two dimensions. The parameter B will be fixed by the manufacture of the pinhole camera, but the parameter H will vary from user to user, depending on the geometry of the glasses relative to their facial features and the locations of their eyes.  
         [0032]     It is often the case that only positions relative to the edge of the sensor array F, and to the edge of the glare shield K, are known. Clearly, 
 
 F=A+D , and 
 
 K=G+J,  
 
 where D is the position of the sensor directly under the pinhole relative to the edge of the sensor, and J is the position on the glare shield closest to the shielded point, relative to the edge of the glare shield. Equation 1 still stipulates that 
 
 K =( H/B )* F+J −( H/B )* D.  
 
 The quantity J−(H/B)*D is composed of constants, and will be designated Q. To correctly place a darkened spot at a position K on the glare shield when a glare source is observed at a position F on the sensor array, the two quantities (H/B) and Q must be determined. 
 
 K =( H/B )* F+Q    Eq. 2 
 
         [0033]     To determine the parameters (H/B) and Q for one shield and one shielded point (for example, one glasses lens and one pupil), the following calibration procedure is performed. The general steps of the method are depicted in the flowchart in  FIG. 3 , and refer to the schematic diagrams of  FIGS. 4 and 5 . More specifically,  FIG. 4  depicts elements of the first six steps of the calibration procedure, and  FIG. 5  depicts aspects of the remaining steps of the calibration procedure. Together, the steps determine how to shield the position  130 , termed the calibration point or position. If the user turns their eye  420  towards the glare source in each step, the calibration point will be at the center of the eyeball, denoted by the cross-hair at  130 . It may be desirable to generally shield a position, termed the shielded point  140 , which is offset from the calibration point.  
         [0034]     For example, position  140  at the front of the eye,  420 , may be shielded by adjusting the calibration parameters using the distance E. One embodiment of the calibration method includes the following:  
         [0035]     1) At S 310  an environment is created with a first single glare source present at position  100   a . The glare source(s) referenced herein could be the sun, an incandescent lamp, light emitting diodes (LEDs), infrared emitters (IREs), or other artificial light sources. Infrared emitters behave in a similar manner to light emitting diodes, but they emit electromagnetic energy in the infrared portion of the spectrum. IREs have an advantage in that their emissions are not visible to the user, but they are detectable to a silicon-based sensor, the most likely construction of the glare sensor. If made powerful enough, the IREs&#39; emissions would serve as a glare source for calibration without being blinding to the user. It is contemplated, in one embodiment of a system for carrying out the calibration method, that an LED could be co-located or adjacent to an IRE at glare source location  100   a  as an indication of the glare source position for the user in the calibration procedure.  
         [0036]     2) The calibration sequence is initiated in the glare-shielding system at S 314 .  
         [0037]     3) At S 318 , a darkened spot  110   a  is displayed on a shield  410 .  
         [0038]     4) The system is altered by adjusting the relative location of the darkened spot  110   a  with respect to the glare source location  100   a  until the darkened spot lies between the glare source and the desired calibration point  130 , as depicted by S 322 . The light ray  101  from the glare source to the calibration point  130  will then be passing through the darkened spot  110   a . If the user is determining the best calibration position, then it will be advantageous for the user to be looking at the glare source at the time, as depicted in  FIG. 4 , since the human visual system has far better resolution at the center of the visual field than at the edge. It will be appreciated that various methods may be used to make the relative adjustment, including for example, (a) moving the darkened spot, (b) moving the position of the glare source, and/or (c) moving the position of the wearer&#39;s head on which the shield is mounted, as well as combinations of one or more of the preceding alternatives.  
         [0039]     5) At S 326 , a signal is given to the glare-shielding system that a first calibration point is in position and the position can be measured and recorded. The signal could be generated in response to the user pressing a button on the system, coding a signal in the glare source, or sending a signal to a separate detector on the system (for instance, an infra-red detector, as is used in TV remote controls).  
         [0040]     6) As part of the measurement step S 326 , the system also determines the position F 1  of the glare source on the photodetector, and the position K 1  of the darkened spot on the shield. In determining F 1 , it is possible that several sensor pixels on the photodetector will be excited above a threshold level, as would be the case when the glare source&#39;s light falls on a boundary between adjacent pixels. Therefore, the average position of the excited pixels should in general be calculated. It is also possible that reflections or stray light from a source, different from the intended glare source, could concurrently excite sensor pixels above the threshold, which would lead to an erroneous calculation of the average glare position. To preclude such a miscalculation, one calibration method contemplates a determination of the spread in pixel positions calculated by summing the square of the pixel positions and subtracting the square of the average pixel position. The result equals the square of the spread in pixel positions, and if the result exceeds a predetermined value, the calibration point is discarded and the calibration procedure abandoned or returns to step S 318 .  
         [0041]     7) At S 330 , a darkened spot  110   b  is displayed at a second position on the shield  410 .  
         [0042]     8) The system is altered at S 334 , again adjusting the relative position of the darkened spot with respect to the glare source as described above, until the darkened spot lies between the glare source and the desired calibration point. The light ray  102  from the glare source  100   b  to the calibration point  130  will then be passing through the darkened spot  110   b.    
         [0043]     9) At S 334 , a signal is given to the glare-shielding system that second calibration point is complete.  
         [0044]     10) The system determines, at S 338 , the position F 2  of the glare source on the photodetector and the position K 2  of the darkened spot on the shield. The validation step described relative to S 326  can again be performed to assure a valid calibration result.  
         [0045]     11) Next, at S 342 , the quantity (H/B) can then be calculated as H/B=(K 1 -K 2 )/(F 1 -F 2 ).  
         [0046]     12) Once H/B is determined, the parameter Q can be calculated as Q=K 1 −(H/B)*F 1  at S 346 .  
         [0047]     13) Since the sensor and glare shield are both two dimensional, with the second dimension perpendicular to the plane of  FIG. 2 , the positions determined in the steps described above will be understood to have two coordinates (the second coordinate omitted for purposes of simplicity). In the following discussion unprimed values used S 342  and S 346  refer to the first coordinate (in the plane of  FIGS. 4 and 5 ), and primed values (F 1 ′, K 1 ′, F 2 ′, and K 2 ′) to refer to the second coordinate perpendicular to the plane of  FIGS. 4 and 5 . A calibration of the parameter Q′ must also be made, and this can be done from the data already taken in steps S 326  and S 338 , since the parameter H/B (for which two calibration points are required) is already known from S 342 . The parameter Q′ can be calculated as 
 
 Q′=K   1 ′−( H/B )* F   1 ′. 
 
         [0048]     Alternatively, it may be more accurate to repeat steps S 318 -S 346  using darkened spots displaced along the second dimension in order to determine Q′ and an independent measurement of (H/B). It may also be more convenient to perform both calibrations at once by displacing the second darkened spot (in S 330 ) along both the first and second dimensions from the position in S 318  and arrive at two independent measurements (one in each dimension) from the two calibration points.  
         [0049]     It will also be noted that the rays portrayed in  FIGS. 4 and 5  need not be in the plane of the figure in order to accomplish the calibration. They merely need to be separated sufficiently such that F 1  is not equal to F 2 , or else the ratio H/B calculated in S 338  is undefined.  
         [0050]     14) If an additional shield requires calibration, as determined at S 354 , the calibration process repeats S 318  through S 350  on the additional shield. For example, if steps S 318  through S 350  were applied to the shield protecting the right eye of the user, they would need to be repeated using the shield protecting the left eye of the user. Some simplifying assumptions might simplify the second iteration. For instance, it might be assumed that the pupil distance from the shield is equal in the two cases, in which case a single calibration point would be sufficient to calibrate the second shield.  
         [0051]     In some embodiments, an extra procedure may be performed prior to the above calibration wherein the size of the darkened spot is adjusted according to the environment and the user so that the user can most accurately alter the system to shield the shielded point with the darkened spot. For example, a user&#39;s pupil size expands in a darkened room, so a calibration performed in a darkened room may require a larger darkened spot than a calibration performed in a brightly lit room.  
         [0052]     Referring also to  FIGS. 9 and 10 , the overall calibration technique is depicted in  FIG. 9 , and  FIG. 10  depicts a close-up view of a portion of  FIG. 9 . Two glare positions ( 100   a ,  100   b ) and four calibration rays ( 101 ,  901 ,  102 ,  902 ) are shown in  FIG. 9 , but it will be appreciated that they do not occur simultaneously. The user  950  wearing the glare shielding glasses  1110  faces an area where only a single glare source is present at a time. A first glare source  100   a  is turned on and a first shield  960  is calibrated with a first darkened spot  110   a . The relative positions are adjusted until the ray  101  from the glare source to the calibration point (depicted as circle  1011   a ) is intercepted by the darkened spot  110   a .  FIG. 10  shows the darkened spot  110   a  coincident with the glare location  1010   a  where the ray passes through the shield. After measuring the position of the first glare source on the sensor (e.g., S 326  in  FIG. 3 ), a second darkened spot  110   b  is displayed on the shield for calibration with a second glare source position  100   b .  FIG. 10  also shows the darkened spot  110   b —in this case incorrectly positioned with respect to the location  1010   b , where the ray passes through the shield. The relative positions are adjusted until the ray  102  from the glare source to the calibration point is intercepted by the darkened spot  110   b . The position of the glare source on the sensor is then measured.  
         [0053]     The process is repeated for the second shield  970 . A first darkened spot is aligned with the ray  901  from a first position of the glare source  100   a , and then a second darkened spot is aligned with the ray  902  from a second position of the glare source  100   b . It should be noted that one aspect of the present invention also contemplates the change in relative glare source location from  100   a  to  100   b  due to the user modifying the position of the head and associated glare-shield sufficiently so as to provide two distinct glare source positions. Accordingly, a single, stationary glare source may suffice for the calibration. The glare sources are shown separated to emphasize the difference in positions of the two calibration points with respect to the user.  
         [0054]     Several alternative embodiments are envisioned for this invention, depending on the means of signaling the glare-shielding system in steps S 314 , S 326  and S 338 . In one embodiment, depicted for example in  FIG. 6 , a push-button contact interfaced with the microcontroller may be used,. Referring to  FIG. 6 , once the darkened spot  110   b  on the shield  410  intercepts the ray  102  traveling from the glare source  100   b  to the calibration point  130 , a signal is given to the microcontroller  210  by pressing the button  220 . The microcontroller then determines the position of the glare source by recording the ray  104  incident on the glare detector  200 . Since the microcontroller positioned the darkened spot  110   b  on the shield, it has all the information required to do a calibration.  
         [0055]     Referring to  FIG. 7 , which depicts an alternative embodiment, once the darkened spot  110   b  on the shield  410  intercepts the ray  102  traveling from the glare source  100   b  to the calibration point  130 , a signal is given to a controller  230  of the glare source. This may be done, for example, by pressing the button  222 . The glare source controller turns the glare source on and off in a sequence which is received by the glare sensor  200  and recognized by the shield microcontroller  210 . The microcontroller then determines the position of the glare source by recording the ray  104  incident on the glare detector (photodetector)  200 , and storing the calibration data in a memory. This system has the clear advantage of reducing the number of components in the glare shield system itself.  
         [0056]     To distinguish a flashing calibration light source from a flashing natural light source, it will be advantageous to use a glare detection sensor or photodetector  200  suitable for producing an output that directly reflects whether a glare source is present anywhere in the field of view—in other words a “glare present” signal. Such an output would eliminate the necessity for reading out and interpreting the entire frame of data from the sensor  200 . Modulation of the glare source at thousands or even millions of times per second can then be reflected in the “glare present” signal. In one embodiment, the glare detector is responsive to, and the glare source can generate, a pulsing light signal in the 100-1,000,000 cycles per second range, although it will be appreciated that a range greater than that perceptible by a human eye (e.g., 100 cycles per second) may be preferable. Since such rapid oscillation is practically impossible in a natural glare situation, it can be used to initiate and sustain communication with the glare-shielding system. The communication could be used to initiate the calibration sequence in S 314 , signal the glare-shielding system in S 326  and S 338 , and ensure that the system remains in calibration mode in all the other steps of the process. The artificial glare sources believed most amenable to fast modulation are the light emitting diodes and the infrared emitters.  
         [0057]     Yet another embodiment is illustrated in  FIG. 8 . Referring to  FIG. 8 , signals are transmitted to the microcontroller  210  through a separate signal receiver  250 . Signals are transmitted by a signal transmitter  240  that is activated, for example, in response to a pushbutton switch  221 . The form of the signal,  242 , may be in one of various wavelengths, including radiofrequency, microwave, infrared, or any other means of creating a unique and timely signal. The embodiment depicted in  FIG. 8  has the advantages of simplicity in the glare shield system and the signaling system without the moving parts on the glare shield system necessitated by a pushbutton, such as that depicted in  FIG. 6 .  
         [0058]     Those familiar with optical devices will appreciate that the alteration of the system in steps S 322  and S 334  could be accomplished in any of several ways, some of which have been previously noted. For example, the user could turn their head until the darkened spot lies between the calibration point and the glare source, where the turned “orientation” is monitored by the location of the glare on the photodetector (e.g.,  2 ,  200 ) of the pinhole device. Alternatively, the user&#39;s head and glasses could remain fixed, and glare source could be moved or manipulated until the darkened spot lies between the calibration point and the glare source, again using the photodetector to monitor the location of the glare source passing through the pinhole. In these first two alternatives, the position of the spot is fixed and therefore known for use in the calibration. In another embodiment, the user could interact with the glare shield system to move the darkened spot on the glare shield until the spot lies between the calibration point and the glare source, where both the position of the spot and the position of the glare source on the photodetector are employed in the calibration. Allowing the user to move their head to position the darkened spot over the glare source may be the most useful of these methods because it only requires a single, fixed-position glare source, and no extra communication with the glare-shield system. In this embodiment, the procedure would be as follows: 
        1) An environment is created with a single glare source present.     2) The calibration sequence is initiated in the glare-shielding system.     3) A darkened spot is displayed at a fixed position, K 1 , on the shield.     4) While looking at the glare source, the user turns and tilts their head to position the darkened spot over the glare source.     5) A signal is given to the glare-shielding system that a first calibration point is in position.     6) The system determines the position F 1  of the glare source on the photodetector.     7) A darkened spot is displayed at a second fixed position, K 2 , on the shield.     8) The user turns and tilts their head to position the darkened spot over the glare source when the user is looking at the glare source.     9) A signal is given to the glare-shielding system that a second calibration point is in position.     10) The system determines the position F 2  of the glare source on the photodetector.        
 
         [0069]     11) The ratio H/B can then be calculated as H/B=(K 1 -K 2 )/(F 1 -F 2 ).  
         [0070]     12) The parameter Q can be calculated as Q=K 1 −(H/B)*(F 1 -D′).  
         [0071]     13) The parameter Q′ can be calculated as Q′=K 1 ′−(H/B)*(F 1 ′-D′).  
         [0072]     14) If an additional shield requires calibration, repeat steps  3  through  13  on the additional shield.  
         [0073]     In the above procedure, the user looks at the glare source being blocked by the calibration spot, which is advantageous because the user has the best resolution in seeing in the direction he is looking, and is thus able to position the calibration spot most accurately. However, this implies that the user is turning their eyes between the two calibration points. As a result, the point in space that is calibrated to be shaded is at the center of the user&#39;s eyeball, as illustrated by location  130  in  FIGS. 4 and 5 . It will often be desirable to position the shielded point at the front of the eyeball, as illustrated by location  140 , so that the user&#39;s pupil is shaded when the user is looking forward. To accomplish this adjustment in calibration, the parameters must be adjusted to account for the smaller distance between the front of the eye and the shield, as compared with the center of the eye and the shield. The shielded position is closer to the shielding array by a distance E, the radius of the eyeball. For example, a typical eyeball radius for adults is 11 mm. The adjustment can be performed if the values B and D are known for the sensor from the manufacturing process or from a separate calibration procedure of the sensor. The parameters are modified as follows: 
 
(H/B)→(H/B)−(E/B) 
 
Q→Q+(E/B)*D 
 
         [0074]     These equations also clarify the adjustment required to account for pupil movements if a pupil-tracking device is employed. Pupil-tracking determines the position of the pupil in real time, so that the shielded position could change according to the pupil position. Without pupil tracking, an approximate position of the pupil is assumed, and the darkened spot may be made large enough to cover a predetermined set of possible pupil positions. Determining the actual pupil position would enable the glare-shielding device to further minimize the area of the darkened spot on the shield and therefore decrease the interference with the desired field of view. Moving the eye will primarily change the value of J, and will also slightly change the value of H. To a first approximation, only the value Q would need to be altered, and (H/B) could be adjusted if an even more exact correction is required.  
         [0075]     In the case of glare-shielding glasses, some slight departures from the above-described ideal geometry are anticipated. Using coordinate axes defined by the sensor, there are three axes about which the shield (e.g., glasses lens) could be rotated with respect to the sensor, as shown in  FIG. 12 .  FIG. 12  depicts the sensor  200  and one shield  960 . Borrowing nomenclature from the field of aviation, the three rotations can be referred to as pitch (depicted as  1210 , a rotation about the x-axis), yaw (depicted as  1220 , a rotation about the y-axis), and roll (depicted as  1230 , a rotation about the z-axis). Yaw may be the most likely departure to arise because in most glasses, the two lenses shielding the pupil may be swept back, whereas a single sensor placed above the bridge of the nose would face directly forward, thus creating a small yaw angle between the sensor plane and the shielding plane. A pitch angle could also arise from styles of glasses where the lenses are angled slightly downward instead of directly forward. Such styles are fairly common, since the shape of the human face allows a view downward to steeper angles than it does upwards, on account of the eyelids and eyebrows. Finally, roll could occur in some styles of glasses, and it could also occur during manufacturing if the sensor is not placed with an exact horizontal alignment. Thus, an embodiment of the calibration methods set forth herein would further account for all three types of rotation.  
         [0076]     From a mathematical point of view, pitch and yaw create the same type of transformation along two different axes. Both rotations break the symmetry between the sensor and the shielding system, so that the trigonometric relationship expressed in Equation 1 does not simplify into a linear relationship. Instead, if L is the angle of the shield&#39;s yaw rotation about the y-axis, J is the distance from the edge of the shield to the shielded point parallel to the sensor plane, H is the distance from the edge of the shield to the shielded point perpendicular to the sensor plane, and K is the distance along the shield, then light incident at an angle θ intersects the shield at a position  
             K   =       J   -       H   *     ⁢     tan   ⁡     (   C   )                 cos   ⁡     (   L   )       *     ⁡     [     1   -         tan   ⁡     (   L   )       *     ⁢     tan   ⁡     (   C   )           ]                 Eq   .           ⁢   3             
 
         [0077]     If L is known as a manufacturing parameter, then measuring the value of K for two different values of θ would determine the calibration values for J and H. Equation 3 could then be used to determine the correct shield spot position during normal operation.  
         [0078]     Using Equation 3 as the determining equation may require significant computational effort, so an alternative method would be to approximate the relationship as a quadratic one, 
 
 K=M+N*F+P*F*F    Eq. 4 
 
 and determine the three constants M, N, and P, from three calibration measurements instead of two as described above. As in Equation 2, F is the position of a glare source image on the sensor. With three pairs of values for K and F, the three constants can be solved for using linear algebra. The constants can be related back to J and H through a Taylor series expansion of Equation 3. To adjust the shielded position to a place different from the calibration position, as was done, for example, to shield the front of the eyeball instead of the center of the eyeball, the constants would be adjusted according to their relationship with J and H. 
 
         [0079]     A pitch rotation about the x-axis creates a similar set of relationships for similar parameters defined regarding glare source displacements along the y-axis. The calibration for pitch and yaw could be performed separately and sequentially by first displaying three darkened calibration spots displaced along one axis and then along the other. However, it will be appreciated by one skilled in the art that the two calibration operations can be performed simultaneously by displaying a total of three darkened calibration spots displaced diagonally on the shield, and solving for Equations 3 or 4 along the two axes independently.  
         [0080]     For a roll, or a rotation about the z-axis, the symmetry between the glare shield and the sensor is maintained, but the x- and y-axis relationships become mixed. For the small amounts of roll expected in manufacturing variations, the mixing parameters are small, and only a linear term is required. Additional calibration points are not required because the mixing will appear from the relationships between x- and y-axis measurements in the calibration steps described above. For example, if the two calibration spots used in the flowchart in  FIG. 3  (at S 318 , S 330 ) are separated only in the x-direction, but the measured glare positions on the sensor differ in the y-direction, then that difference is directly related to the amount of mixing between x- and y-axes, and to the amount of roll between the sensor and the shield.  
         [0081]     A more sophisticated shield may have a slight curvature in one or two dimensions to reduce distortion of the viewed scene. Today&#39;s commercial glasses, for example, use a spherical base shape, so they are curved in both the x- and y-directions. A cross-section of a curved shield is depicted in  FIG. 13 . The shield  960  has a center of curvature  1220  and a radius R. The front of the shield  1210  is defined as the position at which the tangent to the shield is parallel to the sensor plane. The calibration point at the center of the eye  420  is located at a position displaced from the front of the shield by H in the z-direction and J in a perpendicular direction. The mathematical relationship between the appropriate darkened position on the shield and the glare image position on the sensor is more complicated than in the cases described so far, but can still be approximated by the quadratic relationship of Equation 4. Furthermore, the parameters can be adjusted for a shift in glare shielding position away from the glare calibration position.  
         [0082]     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.