Abstract: A device for objectively determining visual acuity includes a viewing screen on which a moving spot of light of diminishing intensity is projected. Infrared eye tracking sensors scan the eyes of the subject and generate a voltage analog signal of the eyes' position. The position signal is compared with a signal representing the position of the moving spot, and the difference is fed to a threshold detector circuit. As the spot becomes too dim to be seen by the subject, the eyes fail to track the spot, and the difference between the two signals surpasses the threshold of the detector circuit. The brightness of the spot when last tracked successfully, which correlates with standard visual acuity measurements, is then displayed as the visual acuity of the subject.