Performing a calibration and validation in 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.   

Performing a calibration

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.

To set up the calibration procedure:

1. Open a screen project type.
2. Click on the Design tab. All new projects start with a Calibration element on the Timeline.
3. Click the Calibration element to open the calibration properties on the right-side panel.
4. Toggle the Validate calibration selector to enable/disable* the presentation of the validation targets. 
5. Select the background color.
6. Select the number of calibration targets: 3, 5 or 9. The number of validation targets is fixed and equal to 4.
7. Choose the calibration type: Timed (the points are displayed automatically for a pre-set period of time) or Manual (the operator decides when to collect the data, by pressing a key on the keyboard). 
8. Choose the type of calibration target: either a Point, default target, or a custom Video type of target (you can choose among the existing videos or import your own).
9. Once you have set up your calibration and study design you will be ready to perform your first recording.

*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.  

To start a calibration in a screen based project in Pro Lab:

1. Open your screen based project.
2. Open the Record tab.
3. Click the Start Recording button on the presentation screen.
4. Adjust the position of the participant using the track box feedback (read Calibration positioning tips below) and provide the calibration instructions to the participant.
5. Click Start calibration. 
6. After the targets have been displayed and the data has been collected, the calibration and validation results are shown on the screen. Decide whether:
   • Continue the recording and proceed to the stimulus presentation, click Continue. 
   • Deal with calibration problems in specific target locations, by selecting the targets and then select Recalibrate n points to recalibrate the participant. 
   • Cancel the recording, press Escape (or Shift + Esc if that was selected). 

To review and export your calibration and validation results:

1. Select a participant’s row in Project Overview.
2. Click the down arrow to see more information. If there is a calibration or validation result, you’ll see the estimated accuracy and precision values. These results represent the total average results across all targets. 
3. To save the image, click the Save image to file button at the bottom right and navigate to where you want to save the .png file.
4. To export all the results, click on Export Calibration data on the heading of the recording list (Top-right corner). The export file contains the data of all recordings in the recordings list. The values in the export file are average values for the whole screen.

Best Practices

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.

Calibration design

• For good performance, choose a calibration background that matches the color and brightness of your stimuli.
• Use more calibration targets if you can, but remember that there is a trade-off between test participants focus on the procedure and the number of targets.
• Don't disable the validation targets.

Participant positioning

• Before the calibration and recording, make sure the participant is in a comfortable posture they can maintain for the duration of the recording.
• The participants should be facing the front of the eye tracker and screen.
• The eyes of your participant should be in the center of the track box. The eyes should be in the center of the track status feedback window. The participant should be at the optimal distance to the eye tracker (Please check your eye tracker user manual for the optimal distance recommendation).
• Make sure the participant can comfortably read the text on the screen, if any, and reach the keyboard and mouse.

Calibration feedback

• Determine the size of your stimuli items and areas of interest (AOIs) in pixels. Pay particular attention to the size of the smallest items, AOIs, and the space between them.
• After calibration, diagrams will be presented showing error vectors for each target location (and eye if enabled), and numerical estimation of accuracy, precision and recorded valid samples.
• Combine the information from the two previous bullet points to decide if you should: a) begin recording using the current calibration or b) adjust the eye tracker or participant and then recalibrate
• If it is a pilot and the offset is large (for the study), then consider redesigning the stimuli and place the items and related AOIs further apart.
• Dealing with calibration problems:
  • Missing calibration in a specific target location: Click the missing target location on the illustration and recalibrate.
  • Large offsets (Accuracy): Tell participant not to guess where the target is but rather just follow it.
  • High noise (Precision): Clean glasses and eye tracker, open eyes wider, pin back hair.