Measurement of rotational eye movement under blue light irradiation by tracking conjunctival blood vessel ends

Measurement of rotational eye movement gives suitable physiological indicators for car sickness, space sickness, visually-induced motion sickness (VIMS), dizziness, and sickness feeling. However, to get certain types of physiological indicators, a measurement system is required, which enables rotational eye movement, of which dynamic range is narrow, to be measured at high-accuracy and high-resolution. Nevertheless, among the conventional methods, the most-frequently used one for finding an eyeball rotational angle based on the contrast in the iris pattern has a disadvantage that it is difficult to measure rotational eye movement constantly within an error range of 1.0 degree. It is because the position and shape of the iris pattern vary with changes in pupil diameter depending on the conditions of ambient light. To solve this problem, turning our attention to the “conjunctival blood vessel ends” located on the periphery of the iris, which are not affected by the changes in pupil diameter, we developed a new method for measuring rotational eye movement at high speed and high accuracy by enhancing the contrast in the image of the vessels on the white of the eyeball under blue light irradiation. This paper proposes the above-mentioned new method to enable a measuring system, which is not subject to the changes in pupil diameter and eyelid closure, and has been compactly-designed in terms of both software and hardware. In our evaluation experiment, the images of rotational eye movements were captured in three subjects, and data on the vessel end positions were obtained by both visual measurement and this system to evaluate the estimated error and processing speed. The result suggests that our proposed system is capable of measuring rotational eye movement with an average of estimated errors equal to or lower than 0.24 degrees and at a processing speed equal to or higher than 78 fps, even if the pupil diameter varies widely.