An apparatus comprising: a camera for taking one or more images of the eye； anillumination unit for illuminating the eye by a ring-shaped light pattern togeneratecorneal reflections； a module for determining during a first session thelocation of thecorneal reflections in the image； a module for determining during said firstsessionbased on said determined location of the corneal reflections, at least onefurtherparameter of the eye and its coordinates in a first coordinate system based onageometrical model representing the eye； a module for determining during asecondsession, temporally spaced apart from the first session the location of thecornealreflections of the eye and based thereon the further eye parameter and itscoordinates in a second coordinate system； a module for determining the eyemotionin six degrees of freedom between the first and said second session and fordetermining a coordinate transformation based thereon； a module fortransformingbased on the determined eye motion, the further eye parameter and itscoordinatesfrom the first coordinate system into the second coordinate system； a moduleforquantifying and/or visualizing the change of the further eye parameter betweenthefirst and said second session based on the further parameter and itscoordinatesmeasured during the second session and the transformed parameter and itscoordinates measured during the first session.
