FAQ: What is the "trackable range" and why is it important?
What is the trackable range?
The trackable range of any eye tracker refers to the range of eye rotations that it can accurately track. Typically this range is given in degrees of visual angle. Some of the most common issues with poor quality eye tracking data (particularly poor spatial accuracy and data loss) can be traced to situations in which the eyes being tracked are rotating beyond the system's trackable range - so understanding what the trackable range is, and how to ensure that you are not exceeding it, is critically important.

Understanding the Trackable Range
The camera location is fixed, and the Host PC software is trying to determine the locations of the center of the pupil and corneal reflection (CR). If the eye rotates beyond the trackable range of the camera, one or other of these (typically the CR) could become unstable or even untrackable. 

The trackable range of the Desktop mounted EyeLink 1000 Plus and EyeLink Portable Duo is 32 degrees horizontally and 25 degrees verticially. As the image below illustrates, the entire trackable range of eye rotations can be thought of as an elliptical area (the blue circle) centered on the camera lens. In other words if the participant rotates their eyes to view something outside of the blue circle, they will have rotated their eyes beyond the trackable range of the camera. The camera sits below the (rectangular) monitor, and a rectangle that subtends 32 x 25 degrees of visual angle is the largest rectangle that can fit into the top half of the circle. It is critical that your Display PC monitor fits within this space.


Exceeding the trackable range
The most common reasons why participants can end up rotating their eyes beyond the trackable range are:
  • The monitor is too close
  • The camera is too low / the monitor is too high (or participant too low)
The consequences of these scenarios for the trackable range are illustrated below. As you can see, in each case, in order to fixate the top corners of the monitor, the participant will be forced to rotate their eyes beyond the trackable range (e.g. outside the blue circle).


When the eye rotates beyond the trackable range, one very common issue is "corneal smearing". Essentially the corneal reflection begins to fall off the cornea itself and move onto the boundary between the cornea and sclera, or even onto the sclera itself. The images below illustrate the issue:


In the image on the left, note that the eye is still being tracked, but the CR has become distorted and enlarged, and its "center" is shifted. This can create asymmetrical calibration models and cause issues for spatial accuracy. In the image on the right the CR is not being tracked at all - resulting in data loss.

How to avoid exceeding the trackable range

Data quality issues due to exceeding the system's trackable range can be avoided very easily, simply be ensuring that your system has been set up optimally. This is best acheived by following the advice we provide in the Installation and Quick Start guides (which can be downloaded from the Manuals / Documents thread.

The critical steps (for desktop systems mounted beneath the monitor) are :
  • Make sure the monitor is sufficiently far away: As a rule of thumb it needs to be at a distance that is at least 1.75 times its width (so a 40cm wide monitor would need to be at least 70cm away). The following app on our website is a great way to check that your monitor is at the correct distance.
    Calibration Check
  • Make sure that the participants eyes are aligned with the top 25% of the screen: This ensures that the participant is not forced to rotate their eyes upwards, away from the camera, in order to view the top of the screen
  • Make sure that the camera is at the correct height: Place yourself in the position of the participant (eg seated in front of the Display PC's Monitor / in the head-support). Look at the bottom of the monitor. There should be no visible gap between the bottom of the monitor and the top of the camera. In other words the camera should be positioned as high as it possibly can be without blocking the view of the bottom of the monitor. This arrangement increases the likelihood that the monitor falls within the trackable range (as illustrated by the blue circle in the illustrations above)
  • Check for corneal smearing before attempting to calibrate: Ask your participant to look in turn at  each of the four corners of the monitor. Check the eye thumbnail images on the Host PC for corneal smearing (see images above / below). If smearing occurs adjust the set up (lower monitor / raise participant etc).


Our Quick Start Guides contain images that illustrate the optimal arrangement between participant, camera and screen for Desktop settings:


If for some reason (eg physical limitations) you are unable to place your monitor at a sufficient distance to ensure that it fits within the trackable range of the camera, you can shrink the calibrated area by following the instructions in this FAQ:

Can I change the location of the calibration targets during the calibration routine?

Please note that your experimental stimuli should only be presented within the calibrated area, so if you reduce the size of the calibrated area, make sure you adjust the size / location of  your stimuli so that they are only ever presented within this reduced area.