Eye gaze calibration

Also called Calibration, Gaze calibration, Eye tracker calibration

Teaching the camera this person's eyes by having them follow a few dots. Why it fails, what to change, and when to stop trying and skip it.

What it is

Calibration is the short routine where an eye-gaze system learns how this particular pair of eyes maps to the screen. A dot appears at a few points (usually 1, 2, 5 or 9), the person looks at each, and the software builds a correction for their eye shape, glasses and position. Without it, the pointer is roughly right; with a good one, it is accurate to about a centimetre.

It is the step people fail at, and then conclude "eye gaze doesn't work for him". Usually the calibration didn't work for him. Those are different things.

Who it can help

Every eye gaze user, at set-up and again whenever seating, glasses, screen position or the person's condition changes. A person who was accurate last month and is missing now needs a recalibration before anything else.

It is hardest for people who cannot follow a dot on request: young children, people with a learning disability who do not yet understand the game, people with profound and multiple learning disabilities, and people with nystagmus or a squint whose eyes will not settle. For them a default calibration, a one-point calibration, or a calibration built from watching a moving picture is often the honest best.

How it works

The system needs to see the eyes clearly first: check the track status view, which shows the eyes as two dots in a box, before starting. Then the dots. A 1-point calibration is enough to play; 5 or 9 points give the accuracy needed for a keyboard. Each point wants a second or two of steady looking; systems will accept a point when they have enough samples, so a person who cannot hold still can often pass on the third try.

Common failure causes: the camera cannot see one eye (head turned, hair, a hand), the person is too close or too far (most cameras want 50 to 70 centimetres), reflections from glasses or a window behind the person, the screen tilted so the person looks down at it at an angle, a dot that is too dull to attract the gaze. Good calibration tools let you use a photo of a face or a favourite character as the target, and let you recalibrate a single failed point.

Getting started

Before the next attempt, fix the room: back to the window, screen straight on at eye level, distance measured with a ruler and marked on the mount. Check the track status and get two steady eye dots before you touch the calibration.

Then make it a game, not a test. Use a moving picture or a face as the target, one point first, and stop while it is still fun. Save the calibration under the person's name and the conditions ("morning, with glasses, in the chair") and make a second one for the other conditions. And if after a fortnight of good attempts the person cannot calibrate, use the default profile and big cells. Plenty of people communicate well that way.

Where to get help

Calibration is taught at set-up by the AAC service that provides the device, and the specialised AAC services will revisit it as part of ongoing support. The ACE Centre and CALL Scotland publish troubleshooting guides, and Tobii Dynavox and Smartbox document their own calibration tools.