This paper deals with characteristics of accommodation evoked by perceived depth sensation and the dynamic relationship between accommodation and vergence, applying newly developed optical measurement apparatuses. A total of five subjects looked at three different two-dimensional stimuli and two different three-dimensional stimuli; namely a real image and a stereoscopic image. With regard to the two-dimensional stimuli, a manifest accommodation without any accompanying vergence was found because of an apparent depth sensation even though the target distance was kept constant. With regard to the three-dimensional stimuli, larger accommodation and clear vergence were evoked because of binocular parallax and a stronger depth sensation. As for the stereoscopic image, a manifest overshoot (the accommodation peaked first and receded considerably) was found while the vergence remained constant. On the other hand, the overshoot of accommodation was smaller when subjects were watching the real image. These results reveal that brain depth perception has a higher effect on accommodation than expected. The relationship of accommodation and vergence toward the stereoscopic image suggests a reason why severe visual fatigue is commonly experienced by many viewers using stereoscopic displays. It has also paved the way for the numerical analysis of the oculomotor triad system.

We describe a newly developed three-dimensional visual stimulator (TVS) that can change independently the directions, distances, sizes, luminance, and varieties of two sets of targets for both eyes. It consists of liquid crystal projectors (LCP's) that generate the flexible images of targets, Badal otometers that change target distances without changing the visual angles, and relay-lens systems that change target directions. A special control program is developed for real-time control of six motors and two LCP's in the TVS together with a three-dimensional optometer III that simultaneously measures eye movement, accommodation, pupil diameter, and head movement. The TVS measurement ranges are as follows: distance, 0 to -20 D; direction, ±16° horizontally and ±15° vertically; size, 0-2° visual angle; and luminance, 10(-2)-10(2) cd/m(2). The target images are refreshed at 60 Hz and speeds with which the target makes a smooth change (ramp stimuli) are as follows: distance, 5 D/s; direction, 30°/s, size, 10°/s. A simple application demonstrates the performance.

Stereoscopic display systems have many applications in biomedical engineering. In this study, the influence of visual tasks using stereoscopic displays with relatively short viewing dstance to the eye's accommodation was measured An increase in the raise time of accommochtion response and accommoctation m r was obswved This was significant especially after tasks with stimuli appearing neam than the screen. Appropriate task management such as giving appropriaterest time is necessary in order to avoid eye stress.

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