Abstract:
The invention is directed to a device and a method for the contactless determination of the actual gaze direction of the human eye. They are applied in examinations of eye movements, in psychophysiological examinations of attentiveness to the environment (e.g., cockpit design), in the design and marketing fields, e.g., advertising, and for determining ROIs (regions of interest) in two-dimensional and three-dimensional space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of International Application No. PCT/DE2005/001657, filed Sep. 19, 2005 and German Application No. 10 2004 046 617.3, filed Sep. 22, 2004, the complete disclosures of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    a) Field of the Invention 
         [0003]    The invention is directed to a device and a method for the contactless determination of the actual gaze direction of the human eye. They are applied in examinations of eye movements, in psychophysiological examinations of attentiveness to the environment (e.g., cockpit design), in the design and marketing fields, e.g., advertising, and for determining ROIs (regions of interest) in two-dimensional and three-dimensional space. 
         [0004]    b) Description of the Related Art 
         [0005]    The prior art discloses various devices and methods by which eye gaze direction and gaze point can be determined in a contactless manner. 
         [0006]    Corneal reflection method: In this method, the eye is illuminated by one or more infrared light sources so as not to impair vision. The light sources generate a reflection on the cornea which is detected by a camera and evaluated. The position of the reflection point in relation to anatomical features of the eye and those that can be detected by the camera characterizes the eye gaze direction. However, the variability of the parameters of the human eye requires an individual calibration for every eye under examination. 
         [0007]    Purkinje eye tracker: These eye trackers make use of camera-assisted evaluation of the light reflected back at the interfaces of the eye from an illumination device whose light impinges on the eye. These Purkinje images, as they are called, occur as a corneal reflection on the front of the cornea (first Purkinje image), on the back of the cornea (second Purkinje image), on the front of the lens (third Purkinje image) and on the back of the lens (fourth Purkinje image). The brightness of the reflections decreases sharply in order. Established devices based on this principle require extremely elaborate image processing and are very expensive. 
         [0008]    Search coil method: A contact lens containing thin wire coils is placed on the eye with these wire coils making contact on the outer side. The head of the subject is situated in orthogonal magnetic fields in a time-division multiplexing arrangement. In accordance with the law of induction, an induced voltage is detected for every spatial position of the contact lens synchronous to the magnetic field pulsing. This method is disadvantageous because of the elaborate measurement technique and the cost of the contact lens which holds only about 3 to 5 measurements. In addition, this is a contact method. The contact of the lens is a subjective annoyance to the subject. 
         [0009]    Limbus tracking: In this method, reflection light barrier arrangements are placed close to the eye and are oriented to the limbus (margin between the cornea and the sclera). The optical sensors detect the intensity of the reflected light. A shift in the position of the corneal-scleral junction in relation to the sensors and, therefore, the gaze direction can be determined from the differences in intensity. The disadvantage consists in the weak signal of the measurement arrangement which, in addition, sharply limits the visual field which is unacceptable for ophthalmologic examinations. 
         [0010]    EOG derivation: From the perspective of field theory, the eye forms an electric dipole between the cornea and the fundus. Electrodes fitted to the eye detect the projection of a movement of this dipole related to eye movement. Typical electric potential curves are approximately linearly proportional to the amplitude of the eye movement. The disadvantage consists in the strong drift of the electrode voltage which is always present and which, above all, prevents detection of static or gradually changing gaze directions. Further, variability between individuals with respect to the dependency of gaze direction on amplitude requires patient-specific calibration. This problem is compounded in that relatively strong potentials of the surrounding musculature are superimposed on the detected signal as interference. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0011]    It is the primary object of the invention to provide a device and a method which makes possible a contactless determination of the gaze vector of the human eye without calibrating for every subject. 
         [0012]    According to the invention, this object is met in a device for the contactless determination of eye gaze direction in that two cameras are provided, each of which generates images of the human eye simultaneously from different directions, in that the two cameras are connected to an image processing system, and in that at least the spatial coordinates of the cameras and their distance from the eye are stored in the image processing system. 
         [0013]    Further, the object of the invention is met through a method for the contactless determination of eye gaze direction in that the eye of a subject is imaged by at least two cameras from at least two different spatial directions, and in that the gaze direction is determined by means of morphological features of an eye which can be evaluated in the image and the spatial coordinates of the cameras and at least their distance from the eye which are stored in the image processing system. When the geometry of the measurement arrangement is known, the gaze point can be determined from the starting point at the eye and from the determined gaze vector. The head need not be fixated, nor must the system be calibrated—as in conventional eye tracking—by correlating a plurality of gaze points and eye positions. The construction is not positioned immediately in front of the eye, but rather can be situated at a sufficient distance from the eye so as not to impair the required visual field (the visible space at a distance of at least 30 cm). The visual field can be further expanded by the arrangement of optical devices such as mirrors, since the photographic systems can now be arranged outside of the visual field. The principle can be applied wherever a fast determination of the actual gaze direction is necessary without impairing the visual field and the well-being of the subject. 
         [0014]    The invention will be described more fully in the following with reference to embodiment examples and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the drawings: 
           [0016]      FIG. 1  shows a basic measuring arrangement of the device; 
           [0017]      FIG. 2  is a schematic illustration of the measurement principle; and 
           [0018]      FIG. 3  shows another illustration of the measurement principle. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to  FIG. 1 , the device comprises two cameras, each camera having its essential parts, the receiver surfaces  1   a  and  1   b  with their imaging optics  2   a  and  2   b  arranged in front. The cameras are located within a spatial reference system (coordinate system). The eye  2  is photographed from at least two spatial directions with simultaneous image recordings. The shape of the pupil  4  and the position on the receiver surfaces  1   a  and  1   b  are determined from the images and mathematically described. As can also be seen from  FIG. 1 , the cameras are connected to an image processing system  5 . The surface normal  6  of the respective receiver surface  1   a  or  1   b  and the gaze direction vector  7 , which is defined as the vector of the tangential surface of the pupil  4 , enclose an angle α ( FIG. 2 ). The pupil  4 , which is round per se, is imaged as an ellipse  8  through this angle α. The ellipse  8  is characterized by its semimajor axis a and its semiminor axis b. The semimajor axis a corresponds exactly to the radius R of the pupil  4 . Further, the distance D (intersection of the axes of the ellipse with the center point of incidence on the pupil  4 ) is known and is stored in the image processing system  5 . The goal is to determine the virtual point  9  from the quantities which are known beforehand and from the measured quantities. The virtual point  9  is the intersection, formed by the straight line of the gaze direction and the projection plane  10 , that is given by the receiver surface  1   a  ( FIG. 2 ). Of course, there is a second virtual point—that of the intersection through the same straight line of the gaze direction and the projection plane—that is formed by receiver surface  1   b.  The two virtual points need not necessarily coincide. As can be seen from  FIG. 3 , the determination of the two virtual points can show that they do not lie on a straight line. The gaze direction is then defined by the mean straight line. The simple mathematical equations 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     = 
                     
                       tan 
                        
                       
                           
                       
                        
                       α 
                       * 
                       D 
                     
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       tan 
                        
                       
                           
                       
                        
                       α 
                     
                     = 
                     
                       
                         
                           
                             a 
                             2 
                           
                           - 
                           
                             b 
                             2 
                           
                         
                       
                       b 
                     
                   
                    
                   
                     
 
                   
                    
                   give 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   r 
                   = 
                   
                     
                       
                         
                           
                             a 
                             2 
                           
                           - 
                           
                             b 
                             2 
                           
                         
                       
                       b 
                     
                     * 
                     
                       D 
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0020]    Since the spatial coordinates of the receiver surface  1   a  are stored in the image processing system  5 , the spatial coordinates of the virtual point  9  which characterizes the desired gaze direction can be determined. 
         [0021]    An embodiment form of the method will be described in more detail in the following. In the first step, the eye  3  is partially or completely imaged on the image receivers  1   a  and  1   b  by the imaging optics  2   a  and  2   b  arranged in front. The images are first binarized and the binarization threshold of the gray level distribution is dynamically adapted. The pupil  4  is classified from the binary images and described mathematically approximately as an ellipse. Based on a known algorithm, the two semiaxes a and b, the center point and the angle α are calculated. These parameters depend upon the horizontal and vertical visual angles θ and φ of the eye and upon the dimensions of the pupil and its position in space. The greater semiaxis a is also the diameter of the pupil  4 . 
         [0022]    Another possibility for realizing the method consists in determining the virtual point by backward projection of characteristic points of the pupil periphery or of points of known position on the pupil from the image to the origin as in trigonometry. It is also possible to arrive at the eye gaze direction by making characteristic diagrams of characteristic curves of b/a−θ−φ and α−θ−φ and determining the intersection of curves of determined parameters. 
         [0023]    Instead of the cameras being oriented directly to the eye, the imaging can also be carried out indirectly by means of optical devices which impair the visual field to a much lesser degree. 
         [0024]    Investigations of the human eye have shown that the geometric gaze direction vector does not always match the real gaze direction, so that a systematic error can occur. However, the angular deviation is always constant for every subject so that this deviating angle can be included as a correction angle after determining the geometric gaze direction vector. Finally, it should be noted that, within limits, a movement of the head is not critical if it is ensured that 60% of the pupil is still imaged on the receiver surfaces. 
         [0025]    While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention. 
       REFERENCE NUMBERS  
       [0000]    
       
           1   a  receiver surface 
           1   b  receiver surface 
           2   a  imaging optics 
           2   b  imaging optics 
           3  eye 
           4  pupil 
           5  image processing system 
           6  surface normal 
           7  gaze direction vector 
           8  ellipse 
           9  virtual point 
           10  projection plane 
         a semimajor axis 
         b semiminor axis 
         R radius of the pupil 
         r distance between the center point of the ellipse and the virtual point 
         D distance 
         α angle between 6 and 7 
         φ vertical angle of view 
         θ horizontal angle of view