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Last modified: 2006-11-08

The DFKI Evaluation Center for Language Technology

 

Tips and Tricks for Mobile Eye Tracking


 
 

 
 

Here, you find a survey of the main problems experienced and their potential solutions based on our experience with the ASL Model 501 mobile eye tracker gained in the studies performed.

To help you understand the tips and tricks, this is a picture of the mobile eye tracker's head mounted optics we are using and it's labelled components:

Mobile eye tracker's head mounted optics

  • In general:
    • The calibration environment is optimized during the procedure by minimizing excessive overhead light, sunlight, and/or spot lighting resulting in better pupil and corneal reflections.
  • Preparation:
    • Preparation should include a few pre-studies done with various people to understand:
      • Where the best lighting conditions exist
      • What is the best subject positions are with the mobile system(s) to be studied
      • What is the best way to secure the user for the calibration process
    • It is a good idea to evaluate the study environment at least daily while performing studies over longer time spans. This is because conditions may change daily in the environment.
  • Calibration:
    • Calibration surface:
      • It is best to use a neutral, non-glare surface
      • Should be located in a position covering the subject's primary field of view in the desired scene plane
      • Should only be a bit larger than the actual scene of interest
      • Should normally be marked with nine fixed points covering most of the scene camera (see picture above) image
        • e.g.
          Calibration surface
      • When placing the calibration screen, it should be located in a similar position as the actual scene plane in order to retain accurate calibration
      • Perform the calibration quickly, for the reason that subjects can become fatigued during the process
        • Quick calibrations minimize the likeliness that the subject moves during the calibration process
      • When the calibration is complete, the calibration surface may be removed or replaced with the desired (mobile) system.
    • Use of different-sized screens:
      • According to the size of the actual scene of interest
        • For example, when the scene of interest is the size of a PocketPC, then the calibration screen should be a little smaller than an A4 sheet of paper
    • Stool:
      • During calibration, restrict physical movement by holding the head/body of the subject as much as possible immobile
        • We found it extremely beneficial to help the subject accomplish this by having him/her sitting on a stool
    • Chin rest:
      • It is extremely important to find an adequate method of temporary head stabilization during both the set target procedure and calibration data entry procedure for accurate data recording results
        • To ensure the subject does not move, it is important to stabilize him/her through the use of a chin rest
          • We use the Cheekrest offered by ASL (right hand side)
    • Tripod:
      • To fasten the Cheekrest, we use a tripod with adjustable height
      • It can also help the subject to stand still by leaning slightly on the Tripod (an alternative to the stool)
  • Video recordings:
    • Of the scene camera image including the crosshair during the experiment for later analysis
      • We use Camtel's USB TV Genie
    • Use an encoder (e.g. avi2vcd) to turn your larger avi files into smaller mpeg files
  • Data analysis:
    • Event tables :
      • Each event (see below) was logged systematically according to begin and end time in a table
      • An event is defined as an action the user is performing.
        • For example, an event can be defined as walking or walking again if the user has previously stopped walking.
        • Example event table (click on the image to enlarge it)
        • Event table of an event-driven person
      • This data is investigated to understand the user behaviour in the context of the developing PERT diagrams
    • PERT diagrams:
      • Program Evaluation Review Technique (PERT) diagrams or Critical Path diagrams
        • Example PERT Diagram
          PERT diagram "The Challenge of Multitasking"