Eye Tracking Technology - Cerebral Palsy Alliance

TECHNOTALKVOLUME 20 - ISSUE 1 - APRIL 2011HOW DOES IT WORK?The most common techniques used to track eye movements are called Pupil Centre Corneal Reflection (PCCR) or dark pupilinfrared illumination. These techniques are non-intrusive and both options use a light source to illuminate the eye enabling aspecial camera to track the movements and positions of the eye. The illumination is near infrared and therefore unnoticeableto the user but creates patterns on the cornea and/or pupil of the eye that can be tracked by image sensors that are usedto capture images of the eyes and the infrared patterns. A computer then uses advanced image processing algorithmsand a physiological 3D model of the eye to calculate the position of the eye in space and then estimate the point of gaze.Adapted From www.userfirst.comMOUNTING AND POSITIONINGMounting of any device is important, but for eye tracking, it is essential. In order to maintainaccuracy of a system, it needs to be held still.In order to enable a user to access all eye tracking systems that are currently available, the screenof the system needs to be parallel to the plane of the user’s face and at eye level. Furthermore, thesystem needs to be generally 50-60cm from the user’s face. For clients who are able to sit uprightand can access a desk, this means that mounting is fairly straightforward as the system can be setup on a desk stand or desk clamp mounting arm.For clients who need their eye tracking system to be portable, there are a number of significantchallenges for mounting. Firstly, the weight and required positioning of the device means that eyetracking systems CANNOT be mounted on paediatric manual wheelchairs, strollers or any form ofwalking or standing frame. Care should be taken and stability testing should be carried out whenattaching eye tracking systems to adult manual wheelchairs and paediatric powered wheelchairsas there is a chance that the weight and position of the device when mounted to the chair couldcompromise the balance and structure of the wheelchair. Careful attention needs to be paid toweight limits of the wheelchair also.Even when the device can be securely attached to a wheelchair, the stability is tested and the clientis able to use the device when it is attached to the wheelchair, careful monitoring needs to becarried out on the mounting as significant strain can be placed on the mounting system over time.TASC has known of several bolts in eye tracking mounting systems that have needed to be replacedafter a few months of use as they were at risk of failing.For clients who want or need to access eye tracking in bed, when reclined or in a tilt-in-space posture, positioning a device canbe difficult (diagrams from ECOPoint Manual). For example, if a user is lying in bed, then the device needs to be mounted directlyabove their head and (depending on the device) 40cm-60cm away from their face with the device tilted so that the screen facesdirectly down. This position may not be able to be achieved with desk mounts or standard rolling or over-bed mounts.SUMMARYDue to the complexity of eye tracking systems and the considerations around calibration, mounting/positioning, portability,language system, accuracy, lighting issues, selection methods, and user skill, it recommended that assessment and trial of anyeye tracking system is completed with the support of a person who is experienced in dealing with such a system. TASC canprovide support to therapists in NSW and ACT who are interested in looking at eye tracking with their clients either remotely andconsultatively or through specialist assessment.CEREBRALPALSY.ORG.AU5