FAQ: How can I convert pupil size to mm?

[SR-SUPPORT]

Journals generally prefer pupil size data to be reported in mm (to facilitate comparisons across studies from different labs). EyeLink systems return pupil size as an arbitrary unit, which is based on the number of pixels on the eye tracking camera which are thresholded (e.g. coloured dark blue) and are considered part of the pupil. As such, these arbitrary units reflect a number of factors (in addition to the actual size of the pupil) such as the distance of the camera from the eye, and the threshold level chosen. For these reasons we recommend that researchers who are interested in pupil size data should not use Remote Mode for data collection. In Remote Mode the distance of the eye to the camera can change as the participant is free to move their head (so the pupil may appear smaller or larger to the camera as the head moves further away / closer). In addition the pupil threshold is continually being auto-adjusted by the Host software in order to maintain stable tracking, which can again result in changes in reported pupil size that may not reflect actual physical changes.

If you have recorded (or plan to record) pupil data in Head-Fixed mode, you can convert the arbitrary pixel units to mm by performing a "pupil size calibration". If you keep the camera at the same distance from the head-support for each participant then this only needs to be performed once.

The pupil size calibration can be performed with an artificial eye, or by printing out a black circle of known diameter that will serve as an artificial eye. For detailed instructions on using an Artificial Pupil, please watch the video tutorial linked below:

• Converting Pupil Size Data into Millimeter
  

Instructions for using a printed artificial eye are below:

Pupil Calibration using a Printed Artificial Eye

1. Print out a black dot of a known size (say 8 mm) on a piece of paper using a laser printer. This will serve as an artificial pupil.
   
2. Attach the artificial pupil to the front of the head support so that it is at the same depth as the test participant's eye itself
   
3. Add the lines below to FINAL.INI in the /elcl/exe folder of the Host PC (See below for how to enable Pupil-Only recording)
   
4. Record data for a few seconds. You can either run a trial or two of a pre-existing experiment and then press CTRL+C (Window) or Cmd+C (Mac) on the display PC to quit the experiment early or do a manual recording on the Host PC. To do a manual recording click the Output / Record button, then click Open File, type a name for the EDF file (such as PUPIL) press Enter, then press the RECORD button, wait for a couple of seconds and then press Stop Recording. Then press the Close File button. Back on the Display PC you can use the Get File utility (Start -> All Programs -> SR Research -> Utilities -> Get File) to copy the EDF file from the Host PC onto the Display PC.
   
   Once the file is recorded, you can open it in Data Viewer (or convert to ASCII) and determine the average pupil size reported in arbitrary units for the printed pupil with a known mm diameter. With these two pieces of information you can create a scaling factor that you can apply to your experimental data. The math involved will depend on whether the EDF file recorded pupil diameter or pupil area (selected from the Set Options screen of the Host PC or from the script you run on your Display PC). A simple linear interpolation can be use if the EDF file is recorded in pupil diameter. While if the file is recorded in pupil area calculate the square root all pupil area data before applying the linear interpolation.
   
   For example, if pupil diameter was recorded:
   
   If the artificial eye has a diameter of 8 mm and the eye tracker is reporting 1260 at the distance you are using. And during an experiment the eye tracker reports a diameter of 1000 then the participant's pupil size is 6.35 mm. Because 8/1260 = x/1000.
   
   For example, if pupil area was recorded:
   
   If the artificial eye has a diameter of 8 mm and the eye tracker is reporting 7070 at the distance you are using. And during an experiment the eye tracker reports a diameter of 6000 then the participant's pupil size is 7.37 mm. Because [8/sqrt(7070)] = [x/sqrt(6000)].
   

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To enable Pupil-Only recording: add the following commands to the FINAL.INI file in the "ELCL\EXE" directory of the Host PC. This enables a 'PUPIL" button on the Camera Setup screen that can be selected instead of PUPIL-CR. Enabling the pupil only mode will allow the system to track a printed black dot (instead ofrequiring both pupil and CR signal). Be sure to remove these lines from FINAL.INI once you have finished the pupil calibration process.

Code:

################################################## ###

## force_corneal_reflection = <Value>
;; Hides "Pupil" mode button on Camera Setup screen
;; Pupil Only mode should only be used in EyeLink 1000 when participants head is completely fixed.
;; Default Value: OFF
force_corneal_reflection = OFF

## allow_pupil_without_cr = <switch>
;; Allows pupil without a CR nearby to be detected
;; in pupil search (after pupil loss or on startup).
;; This command is overridden in P-CR mode.
allow_pupil_without_cr = ON

## elcl_hold_if_no_corneal = <switch>
;; If true, eye window is frozen until both pupil and CR are present.
;; Default Value: OFF
elcl_hold_if_no_corneal = OFF

## elcl_search_if_no_corneal = <switch>
;; If corneal missing for long period, assumes false target and searches for pupil/CR candidate.
;; Default Value: OFF
elcl_search_if_no_corneal = OFF

## elcl_use_pcr_matching = <switch>
;; Selects enhanced pupil-CR matching during pupil identification.
;; If used, pupil and CR are selected as best matching pair.
;; This can be used even if Cr is not being used for tracking.
;; Default Value: ON
elcl_use_pcr_matching = OFF

## Portable Duo users please uncomment the following line (remove the #)
#corneal_mode = OFF

################################################## ###