Measurement of Torsional Eye Movement Using Terminal Points of Conjunctival Blood Vessels

The angle of torsional eye movement has small amplitude, a high degree of resolution and precision will be required. In the current mainstream method of obtaining the angle of torsional eye movement using an iris pattern, it has been difficult to ensure measurement with a tolerance of 1.0(degree) or less at all times due to changes in the deformation of the iris pattern. We wish to propose a method for high-speed and highprecision measurement of the torsional eye movement by the conjunctival vessel extraction method where attention is paid to the conjunctival vessel end located on the outer edge of the iris as a site impervious to changes in the diameter of the pupil. In the step of evaluation, the vessel end position is measured by visual observation based on the image of torsional eye movement captured from three persons, and the tolerance of this system is calculated. It has been possible to take measurements with a precision of 0.24(degree) or less at a processing speed of 30(fps) or more, even when there is a change in the diameter of the pupil. 1 General Instructions In the field of study using the method of measuring torsional eye movement is employed, sickness from excessive image viewing is evaluated by the analysis of the characteristics of the torsional eye movement with respect to the visual stimulus, and LASIK of eye cornea surgery using measurement of torsional eye movement to control the laser of cutting the cornea. However, the torsional eye movement occurs only within the range of amplitude of about 12(degrees). Thus, Applications using measurement of torsional eye movement require a high degree of resolution and precision. Further, since processing is preferably performed at a real-time processing speed, the processing speed is required to be 30 fps or more. However, Measurement with a tolerance of 1.0(degree) or less is difficult according to conventional method. This is because the iris pattern is deformed when the size of the pupil changes. Although there are the methods being studied to solve this problem[1][2], it is difficult to take measurement with a tolerance of 1.0(degree) or less when there is a greater change in the pupil diameter. Thus, we propose the method of measuring the torsional eye movement by tracking the ”terminal points of conjunctival blood vessels” illustrated in Fig. 1, which were not in the conventional method. Since the terminal points of conjunctival blood vessels are not subjected to the deformation due to the reaction of the pupil, the torsional eye movement can be measured with high precision. It should be noted, however, that not all the terminal points of conjunctival blood vessels can be employed. Only the terminal points of conjunctival blood vessels connected to the cornea can be used (Fig. 2). Further, In our conventional method[3], That method used only one point of a conjunctival blood vessel which manually selected by user. However, there are two or more terminal points of conjunctival blood vessels. To ensure correct measurement of the rotational eye movement, automatic selection of the terminal point not blocked by the noise of an eyelash or others must be made at every measurement. The above-mentioned problems can be solved by our proposed method, where the position of the targeted terminal points of conjunctival blood vessels are obtained by identifying the contour of the iris, and a terminal point not blocked by noise is automatically selected by using the similarity in pattern matching. To evaluate our proposed method, the true value of torsional angle for eye movement using eye images of three persons is obtained, and this value is compared with the torsional angle estimated by this system so that the precision of our method will be obtained, and the adequacy of our method will be justified. 1.1 Test environment A small-sized monochromatic CMOS camera (FireFlyMV by PointGrey Inc.) is attached to the head using a goggle as illustrated. Similarly in this case, two types of illuminations fixed on the spectacles are used to highlight the pupil iris pattern and conjunctival vessel. The infrared LED is applied to inclease the contrast of the pupil and the iris, and the blue LED Figure 1. Relation of conjunctival vessel end position and torsional eye movement. MVA2013 IAPR International Conference on Machine Vision Applications, May 20-23, 2013, Kyoto, JAPAN 4-17