04-20-2022, 06:11 AM
What is the trackable range?
The trackable range of an eye tracker is the range of eye rotations it can accurately measure, typically expressed in degrees of visual angle. Common issues like poor spatial accuracy and data loss are often caused by the participant's eyes rotating beyond the system's trackable range. Therefore, understanding this range and how to work within it is critical for collecting high-quality data.
Understanding the Trackable Range
The camera's location is fixed, and the Host PC software determines the center of the pupil and the corneal reflection (CR). If an eye rotates beyond the camera's trackable range, these features (especially the CR) may become unstable or untrackable.
The trackable range of the desktop-mounted EyeLink 1000 Plus and EyeLink Portable Duo is 32° horizontally and 25° vertically. As the image below illustrates, this range can be conceived as a rectangle within a larger elliptical area centered on the camera lens. If a participant looks at a point outside this larger area, their eyes have exceeded the system's trackable range. The largest rectangle that fits within the top half of this ellipse subtends 32° x 25° of visual angle. It is critical that your monitor fits entirely within this rectangular space.
Exceeding the trackable range
The most common reasons why participants can end up rotating their eyes beyond the trackable range are:
When an eye rotates beyond the trackable range, a common issue called corneal smearing can occur. This happens when the corneal reflection (CR) moves off the center of the cornea and onto its boundary with the sclera.
Data quality issues from exceeding the trackable range are easily avoided by ensuring an optimal system setup. This is best achieved by following the advice in our Installation and Quick Start guides, available in the Manuals / Documents thread.
The critical steps (for desktop systems mounted beneath the monitor) are :
If for some reason (e.g. 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.
The trackable range of an eye tracker is the range of eye rotations it can accurately measure, typically expressed in degrees of visual angle. Common issues like poor spatial accuracy and data loss are often caused by the participant's eyes rotating beyond the system's trackable range. Therefore, understanding this range and how to work within it is critical for collecting high-quality data.
Understanding the Trackable Range
The camera's location is fixed, and the Host PC software determines the center of the pupil and the corneal reflection (CR). If an eye rotates beyond the camera's trackable range, these features (especially the CR) may become unstable or untrackable.
The trackable range of the desktop-mounted EyeLink 1000 Plus and EyeLink Portable Duo is 32° horizontally and 25° vertically. As the image below illustrates, this range can be conceived as a rectangle within a larger elliptical area centered on the camera lens. If a participant looks at a point outside this larger area, their eyes have exceeded the system's trackable range. The largest rectangle that fits within the top half of this ellipse subtends 32° x 25° of visual angle. It is critical that your monitor fits entirely within this rectangular 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)
When an eye rotates beyond the trackable range, a common issue called corneal smearing can occur. This happens when the corneal reflection (CR) moves off the center of the cornea and onto its boundary with the sclera.
- In the left image, the eye is still tracked, but the CR is distorted and enlarged, shifting its calculated center. This distortion can create asymmetrical calibration models and reduce spatial accuracy.
- In the right image, the CR is no longer tracked at all, resulting in data loss.
Data quality issues from exceeding the trackable range are easily avoided by ensuring an optimal system setup. This is best achieved by following the advice in our Installation and Quick Start guides, available in the Manuals / Documents thread.
- Position the monitor far enough away. As a rule of thumb, the distance from the participant should be at least 1.75 times the monitor's width (e.g., a 40 cm wide monitor should be at least 70 cm away).
- Align the participant's eyes correctly. The participant's eyes should be level with the top quarter (75% of the way up) of the screen. This prevents them from needing to make large upward eye rotations to see the top of the display.
- Set the correct camera height. From the participant's perspective, the camera should be as high as possible without obstructing the view of the bottom of the monitor.
- Check for corneal smearing before calibrating. Ask your participant to look at each of the four corners of the monitor. Watch the eye image on the Host PC for signs of smearing. If it occurs, adjust the setup (e.g., lower the monitor, raise the participant).
If for some reason (e.g. 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.