This paper describes the optical and control principles of a newly developed three-dimensional optometer (TDO). The TDO makes it possible to measure simultaneously three basic ocular responses in an actual work situation: accommodation; eye movement; and pupil area. Although the subjects must use a chin rest, they need not fixate their eyes or use any drug for the measurement. Normal lighting is allowed. The TDO can measure accommodation from -12.7 to + 26.6 diopters and 100% of pupil area change when the eye moves within 40 degrees horizontally and 30 degrees vertically. The accuracy of measuring both accommodation and pupil area with the TDO is about the same as that of commerically available apparatus: +/-0.25 diopters and +/-2%, respectively. The error of measuring the angle of the eyes was less than +/-0.5 degrees . Examples of measurements during work with a visual display terminal are presented.