Learn how to perform a calibration and calibration validation with Tobii Pro Lab.
The human eye shows a natural individual variation in its shape and geometry. For example, the exact location of the fovea, the area responsible for the part of the visual field that we perceive in focus and full color, varies from individual to individual. Tobii Pro Eye Trackers, use an individual calibration procedure to optimize its gaze estimation algorithms (i.e. the 3D eye model) and account for this variation. The end result of this optimization is a fully-customized and more accurate gaze point calculation. Additionally, some Tobii Pro eye trackers use the calibration procedure to select the detection mode (e.g. dark or bright pupil tracking) that provides the most accurate data.
Figure 1. The geometry of the visual axis of the eye, optical axis, fovea, center of the pupil (CP) and gaze point. One of the objectives of the calibration is to finetune the geometry to get as close as possible to the true location of the fovea the participant.
If you choose to report the data quality using the data collected to optimize the 3D eye model, you lose predictive power when estimating the accuracy and precision error of the recording. This is because the 3D eye model is being fine-tuned to that particular data set data and is likely to report better performance than when a new data set is used, which is the case when you perform a recording. As a consequence, it is best practice to test the new configuration of the 3D eye model with a new data set (i.e. validation targets). These values can then be used in your report as an estimate of the data quality.
Now that you have a basic understanding of why you need to calibrate the eye tracker, let's see how you can set up and perform a calibration in Pro Lab.
*If you disable the presentation of the validation targets, after calibration, Pro Lab will display both a diagram and a table with the numerical results of the calibration instead. These results represent the residual error after the 3D eye model has been optimized. Thus, you will only be able to use this information as a first step to help you troubleshoot tracking issues and perform a new calibration or decide whether to exclude test participants with sub-optimal results.
In this section, we have collected a few practical tips and tricks to enable you to optimize your calibration procedure and run a successful calibration.