What happens during the eye tracker calibration

Eye tracker Eye tracking technique

In this section we will discuss the calibration procedure of Tobii Eye Trackers. 

Calibration is the process whereby the geometric characteristics of a subject’s eyes are estimated as the basis for a fully-customized and accurate gaze point calculation.

Before  an  eye  tracking  recording  is  started,  the  user  is  taken  through  a  calibration  procedure. During  this  procedure,  the  eye  tracker  measures  characteristics  of  the  user’s  eyes  and  uses  them together  with  an  internal,  anatomycal  3D  eye  model  to  calculate  the  gaze  data.  This  model includes information about shapes, light refraction and  reflection  properties  of  the  different  parts  of  the  eyes  (e.g.  cornea,  placement  of  the  fovea, etc.).  During  the  calibration  the  user  is  asked  to  look at specific points on the screen, also known as calibration dots. During this period several images of  the  eyes  are  collected  and  analyzed.  The  resulting information is then integrated in the eye model and the gaze point for each image sample is calculated.  When  the  procedure  is  finished  the  quality  of  the  calibration  is  illustrated  by  green lines  of  varying  length.  The  length  of  each  line  represents the offset between each sampled gaze point and the center of the calibration dot. Large  offsets  (long  green  lines)  can  be  caused  by  various  factors  such  as,  the  user  not actually  focusing  on  the  point,  the  user  being  distracted during the calibration or the eye tracker not being set up correctly. However, the user does not have to keep the head completely still during calibration as long as the focus of the user’s eyes is kept  on  the  moving  dots.  During  the  calibration  both  the  light  and  dark  pupil  methods  are  tested to  identify  the  most  suitable  for  the  current  light  conditions and the user’s eye characteristics.

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