FAQ: How do I tell the system how far the camera is from the screen? Why is this needed?

[Sam]

In order to accurately compute eye velocity and saccade amplitude in degrees of visual angle, the Host PC software needs to know the distance of the participant's eye to the display screen, as well as the screens physical dimensions to accurately compute these measures. Please see our Visual Angle blog post for a deeper discussion of these measures.

When using any of the desktop-based EyeLink systems in head-stabilized mode, you must specify the eye-to-screen distance using the screen_distance parameter. Alternatively, when using any EyeLink system in Remote mode, you must modify the dz parameter in the remote_camera_position command to specify the distance from the camera to the screen. The eye tracker uses this value, along with the eye-to-camera distance it calculates from the size of the Remote Mode target sticker, to determine the total eye-to-screen distance.

It's critical that the values entered into the EyeLink host software are accurate. This allows the system to correctly compute velocity and acceleration, which are used to identify fixations and saccades. Instructions of modifying these values are below"

For the EyeLink 1000 Plus or EyeLink Portable Duo:

• Exit the Host PC to get to the File Manager by pressing CTL+ALT+Q.
  
• Go to the Configuration page by clicking the gear icon in the top left corner of the File Manager
  
• Go to the Screen Settings Wizard (4th icon from the left)
  
• Complete all four steps in the Wizard base on the onscreen directions, making sure to enter the physical measurements in millimeters and the screen resolution in pixel values.
  Note:
  • When using Head Stabilized mode, you step 4 (Camera-to-Screen Distance) will be ignored.
    
  • When using Remote Mode, step 3 (Eye-to-Screen Distance) will be ignored.
    

   

For the EyeLink 1000:

• Reboot the Host PC into Windows.
  
• Go to the EyeLink partition (typically D: or E:)
  
• Navigate to the /ELCL/EXE/ folder within.
  
• Open the FINAL.INI
  
• Copy and modify the values below to reflect your setup:
  
  Code:

      ## screen_phys_coords = <left>, <top>, <right>, <bottom>
    ;; Meaure the distance of the visible part of the display screen edge
    ;; relative to the center of the screen (measured in in millimeters).
    ;; <left>, <top>, <right>, <bottom>:
    ;;    position of display area corners relative to display center
screen_phys_coords = -188.0, 146.0, 188.0, -146.0

    ## screen_pixel_coords = <left> <top> <right> <bottom>
    ;; Sets the gaze-position coordinate system, which is used for all
    ;; calibration target locations and drawing commands.  Usually set
    ;; to correspond to the pixel mapping of the subject display.  
    ;; Issue the calibration_type command after changing this to recompute
    ;; fixation target positions.  
    ;; You should also write a DISPLAY_COORDS message to the start of
    ;; the EDF file to record the display resolution.
    ;;    <left>: X coordinate of left of display area
    ;;    <top>: Y coordinate of top of display area
    ;;    <right>: X coordinate of right of display area
    ;;    <bottom>: Y coordinate of bottom of display area
screen_pixel_coords = 0.0, 0.0, 1024.0, 768.0

    ## screen_distance = <mm to center> | <mm to top> <mm to bottom>
    ;; Used for visual angle and velocity calculations.  
    ;; Providing <mm to top> <mm to bottom> parameters will give better estimates than <mm to center>
    ;; <mm to center> = distance from display center to subject in millimeters.
    ;; <mm to top> = distance from display top to subject in millimeters.
    ;; <mm to bottom> = distance from display bottom to subject in millimeters.
screen_distance = 600 660

    ## remote_camera_position <rh> <rv> <dx> <dy> <dz>
    ;; <rh>: 10; should not be modified
    ;; <rv>: 17; should not be modified
    ;; <dx>: -80; should not be modified
    ;; <dy>:  60; should not be modified
    ;; <dz>: -90; This value represents the distance the screen is behind the camera (should be a negative value).
remote_camera_position 10 17 -80 60 -90
• Save the file.
  
• Reboot the Host in the EyeLink Host software.