FAQ: How can I convert pupil size to mm?

[SR-SUPPORT]

For publication, some journals require pupil size data to be reported in millimeters (mm) to allow for comparisons across studies. EyeLink systems record pupil size in arbitrary units, which are based on the number of thresholded pixels that make up the pupil area in the EyeLink camera image. These units are influenced by factors like camera distance and the tracking threshold (which is automatically adjusted when operating in remote mode), not just the true size of the pupil. For these reasons, we recommend against using Remote Mode for any research focused on pupil size as they may introduce confounds that are unrelated to actual physiological changes in pupil diameter.

You can reliably convert the arbitrary units for head-fixed data to millimeters by performing a one-time pupil size calibration.

This is done using an artificial eye or a printed black circle of a known diameter. If you keep the camera at the same distance for all participants, this calibration only needs to be performed once.

For detailed, step-by-step instructions on this procedure, please watch our video tutorial:

• Converting Pupil Size Data into Millimeter
  

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How to Calibrate Pupil Size with an Artificial Eye
You can convert the eye tracker's arbitrary pupil units to millimeters by performing a simple calibration. This guide explains how to do this using a printed black dot as an artificial pupil.

1. Using a laser printer, print a black dot of a known size (e.g., 8 mm diameter) on a piece of paper. This will serve as your artificial eye.
   
2. Attach the paper to the head support so the dot is at the same distance from the camera as a participant's eye would be.
   
3. Configure your Host PC for pupil-only recording if necessary (see below).
   
4. Next, record a short EDF file of the eye tracker measuring the artificial eye. You can do this in two ways:
   • Run a short trial: Start a normal experiment and quit after a few seconds of recording.
     Note: You can exit an Experiment Builder task at any point using CTL+C (Windows) or CMD+C (macOS) if necessary.
     
   • Manual Recording: On the Host PC, use the "Output / Record" screen to manually start, record for a few seconds, and stop a new EDF file.
     
5. Open the recorded EDF file in Data Viewer (or convert it to ASCII) and find the average pupil size value reported by the eye tracker. The calculation you use next depends on whether your experiment recorded pupil diameter or pupil area.
   • If you recorded Pupil Diameter:
     The relationship is a simple linear ratio.
     (Known Diameter in mm) / (Recorded Diameter in units) = (Actual Pupil Diameter in mm) / (Experimental Pupil Diameter in units)
     
     For example, if your 8 mm dot records as 1260 units, and a participant's pupil records as 1000 units, their actual pupil size is 6.35 mm.
     
     Code:

     8 / 1260 = x / 1000  ->  
x = (8 * 1000) / 1260 = 6.35
     
     
   • If you recorded Pupil Area:
     You must first take the square root of the area values to convert them to a linear scale comparable to diameter.
     (Known Diameter in mm) / sqrt(Recorded Area in units) = (Actual Pupil Diameter in mm) / sqrt(Experimental Pupil Area in units)
     
     For example, if your 8 mm dot records as 7070 area units, and a participant's pupil records as 6000 area units, their actual pupil size is 7.37 mm.
     
     Code:

     8 / sqrt(7070) = x / sqrt(6000)  ->  
x = (8 * sqrt(6000)) / sqrt(7070) = 7.37

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Host PC Settings for Pupil-Only Calibration
To track a printed artificial eye (a black dot), you must temporarily enable Pupil-Only tracking mode on your Host PC. This allows the system to track a "pupil" without requiring a corneal reflection (CR) signal.

Important: These changes are for calibration only. Before editing, consider making a backup of your FINAL.INI file. You must remove these lines after you have finished the calibration process to restore normal PUPIL-CR tracking.

Instructions for the EyeLink 1000, EyeLink 1000 Plus and EyeLink Portable Duo:

• Exit the Host software:
  • EyeLink1000 Plus and EyeLink Portable Duo - Press CTL+ALT+Q to exit to the file manager
    
  • EyeLink1000 - Reboot the PC to Windows and navigate to the EyeLink drive (typically D: or E)
    
• Navigate to the /elcl/exe/ directory.
  
• Open the FINAL.INI file.
  
• Add the following lines of code to the end of the file:
  • EyeLink 1000 and EyeLink 1000 Plus:
    
    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

################################################## ###
  • EyeLink Portable Duo:
    
    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

################################################## ###
• Save the file
  
• Restart the Host PC for the changes to take effect