An apparatus for monitoring one or more parameters of a patient's eye during multiple sessions that are temporarily separated and between which the patient's eye may have moved, said apparatus comprising: a camera for taking one or more images of the eye； a lighting unit for illuminating the eye by a ring-shaped light pattern to generate corneal reflections, said lighting unit being preferably located such that the center of the ring is coaxial with the optical axis of the camera； a module to determine during a first session the location of the corneal reflections in the eye image； a module to determine during said first session based on said determined location of the corneal reflections, at least one additional parameter of the eye and its coordinates in a first coordinate system based on a geometric model representing the eye as a spherical eyeball having a spherically shaped cornea mounted on it； a module for determining during a second session temporarily separated from said first session said location of said corneal reflections of the eye and based on it said additional parameter of the eye and its coordinates in a second coordinate system； a module to determine the movement of the eye in six degrees of freedom between said first and said second session and to determine a coordinate transformation based on it； a module to transform based on said determined eye movement said additional eye parameter and its coordinates from said first coordinate system to said second coordinate system； a module for quantifying and / or visualizing the change of said additional eye parameter between said first and said second sessions based on said additional parameter and its measured coordinates during said second session and said transformed parameter and its measured coordinates during said first session.Un aparato para vigilar uno o más parámetros del ojo de un paciente durante múltiples sesiones que están separadas temporalmente y entre las cuales puede haberse movido el
