What is calibration? In order to know where the participant`s eyes
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1. C prompt on the EyeTracker 2 Have the participant sit comfortably in the chair with his chin on chin rest The chin rest may easily be adjusted by turning the handles on either side to loosen and then tighten once chinrest is in the appropriate spot Note that the handles dials turn in opposite directions from one another 3 Ensure that the participants eyes are visible from the camera view If your participant is shorter or taller than average you may need to physically adjust the camera itself using the grey knob at the neck of the eyetracker Loosen knob to move tracker then tighten knob to ensure tracker stays in the correct place 4 Once you can see both eyes on the EyeTracker Computer use the mouse to click in the center of each eye This should cause a green diamond and red circle to surround each eye more or less symmetrically the clown view These guides let the tracker know where to expect the eye 5 Adjust pupil and corneal reflection CR thresholding as needed PUPIL THRESHOLDING use the up and down arrow keys to change thresholding of the pupil up includes more of the eye in the pupil down decreases the amount of eye considered to be pupil PUPIL number should be in the 80s high 70s low 90s for experiments run in dim to regular lighting CORNEAL REFLECTION THRESHOLDING use the and keys no shift needed to change thresholding of CR CR corneal ref2. again the result should be a green GOOD for all tracked eyes O once the calibration has been performed press o to jump to the actual trials can also be used after re calibrating subject to get back to actual trials ESC when pressed during the drift correct portion of a trial this will jump you back to the calibration set up screen press this if participants are not able to start the next trial because of calibration issues also use if participant takes a break and needs to be recalibrated before re starting the experiment CTL C to exit program without finishing press control and C to cancel out all trials up until the point of cancellation should still be saved in the edf file If pressing a command key doesn t work try pressing ENTER and then letter command See next page for calibration tips 6 29 07 4 Helpful hints for successfully calibrating and running participants MASCARA do not allow participants to wear mascara Seriously put it in the sign up Since the tracker tracks the pupil of the eye the dark part of the eye eyelashes with mascara on them will be counted as pupil and the calibration will not be successful GLASSES are generally OK to wear make sure that the large square yellow reflection off of the glasses does not interfere with the pupil capture or corneal reflection If the reflection on the glasses is getting in the way
3. of the pupil try tilting the glasses slightly forward or backwards on the person s face Also try sitting the participant slightly higher or lower within the camera s view or moving the glasses further up or down on the nose CONTACTS are generally are OK to wear make sure participants eyes are not overly watery If you see more than one corneal reflection try to readjust participant relative to camera s view Sometimes contacts cause eyes to dry out allow participant to rest and try again I ve tried calibrating six times and it won t work What do I do Re threshold pupil and CR If you have re calibrate a participant a few times and are not successful it is possible that you have not entered ideal parameters for the pupil and CR thresholds Go back to the Camera View hit ENTER and ensure that the pupil is indeed filled in with blue and that the yellow CR is visible Ask the participant to move his eyes in sequence to the corners of the computer screen Inspect the Camera View for anomalies as the participant moves his eyes In particular note whether the pupil has static in it which is an indication that you should increase the pupil threshold number Also look to see whether the CR is not visible from a certain position which indicates that you should probably reposition the participant If participant has glasses check for interfering reflections Ask the participant to look from one corner of the screen to
4. What is calibration In order to know where the participant s eyes are fixating on the computer screen you must first teach the computer what the eye looks like when the participant is fixated on known locations on the screen This is what happens during the calibration and validation procedure During a calibration session the participant fixates on 9 13 points on the screen while the eyetracker monitors the eye After calibration is performed the computer then validates the information by re presenting the targets and determining whether its estimation of eye position is indeed close to the known position of the targets If errors are found to be minimal calibration is indicated as successful green bars appear on the EyeTracker Monitor and the trials may start During the experimental trials the computer interpolates between the calibrated landmarks to determine where the eye is fixating on the screen What parts of the eye does the calibration focus on The tracker operates by tracking the pupil of the eye the dark black part of the eye which will appears as filled in with blue in the camera view of the eye as well as the corneal reflection a reflection off of the cornea that appears in yellow on the eye view The relative positions and size of these two landmarks are measured as the participant looks at specific points on the screen during calibration During the rest of the experiment the tracker figures out
5. at is at the same plane as the screen Any vergence movements e g crossing or uncrossing eyes will be treated as the eyes moving on the screen So if you are attempting to show 3d objects while doing eyetracking you may not be able to get accurate location information from the eyetracker In addition if your participant is tired and zoning out or fixating behind the computer screen you may see unanticipated changes in the eye position making calibration and drift correction difficult 6 29 07 2 Lighting The eyetracker assumes that the eye is constantly lit Therefore it assumes that any changes in pupil size occur because of a change in eye location So if you are running a participant in near complete darkness with pupils quite large and suddenly a bright white screen is presented or even a smaller luminant object on the screen the pupil will constrict somewhat and this will be interpreted as an eye movement likely a dip even though the eyes may have remained steady on the screen This is not usually a problem if you are running in dim to normal light but may be an issue if you are running in near darkness For more information on calibrating see Section 2 3 4 in the EyeLink User s Manual found on the stimulus presentation computer at C Program Files SR Research ELCL_UserManual pdf How do I calibrate someone 1 Start the EyeTracker open up EyeLink by pressing computer See from the
6. lection number should be around 230s for experiments run in dim to regular lighting 6 Press C to calibrate or click on Calibrate on the EyeTracker Monitor 7 Once successful press V to validate or click on Validate on the EyeTracker Monitor 8 If validation is successful press O to begin experimental trials 6 29 07 3 In general these commands are used to calibrate and then switch to running trials ENTER switch to camera view of eyes once in this view using the RIGHT and LEFT arrow keys will toggle between right left and full field view of eyes ideally the entire pupil will be blue with no eyelashes as blue usually the tracker automatically calibrates pupil size and corneal reflection CR appropriately if it does not work you can change the thresholds for pupil P and CR C calibrate starts the calibration process on the eyetracker computer display screen you can choose to use the 9 dot calibration pattern or 13 dot pattern 13 dot pattern will take longer but result in finer grained tracking resolution once the initial target appears on screen press space or enter to start make sure result is a green GOOD for all tracked eyes to force the tracker to accept a less than ideal calibration press the FORCE ACCEPT button on the right hand side of the screen V validate essentially a check of the calibration must run after calibration
7. the next while you inspect the Camera View Look to see whether large square yellow reflections off of the glasses lenses are obscuring or interfering with the yellow CR If so try tilting the glasses or adjusting the chin rest height to minimize the interference
8. where the eye must be looking depending on the relative size of the pupil as well as the relative location of the pupil and corneal reflection Anything that disrupts the pupil capture or interferes with the corneal reflection will cause calibration to be very difficult or impossible How many calibration points do I need to get accurate tracking You have the choice of calibrating validating with 3 5 9 or 13 points on the screen The default is generally 9 spots though this can be changed easily From within the Experiment Builder interface navigate to the EL_ CAMERA SETUP node in your experiment In the Properties panel on the bottom left hand side of the screen you will see a Calibration Type property with a set of pull down options Choose HV 9 if you would like the 9 point calibration Choose HV 13 if you would like the 13 point calibration Anecdotal reports from the Dassonville Lab indicate that calibration with 13 points is superior to 9 points and only takes a little longer to run through Alternately you may change the calibration pattern from the EyeTracker monitor by clicking the Set Options button from the main screen and then clicking on the 13 point icon in the Calibration and Validation section upper left hand side of the screen What assumptions does the calibration mechanism make Vergence Calibration is done assuming that the participant is always focusing at screen depth th
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