Abstract: A device (1) for initiating an entoptic phenomenon perceivable by a patient, a system with such a device (1) as well as a data processing unit (12) providing the patient with specific information relating to the use of the device as well as a functional status of his/her retinal vessels are described. The device (1) described is characterized in that a housing (4) with a contact surface (6) is provided, which can be placed on the patient's head (8) in the region of the eye (7) in such a way that light emitted by the lighting unit (5) falls at least partially onto the retina (2), wherein the lighting unit (5) is configured to emit light at different times along various beam paths in such a way that the emitted light strikes the retina (2) at various points when the contact surface (6) is in place.