PROC. SPIE, 3591, 63-70 (1999) Pupil-meter and tracking system based in a fast image processing algorithm Ignacio Miró *1,2 Norberto López-Gil 1 and Pablo Artal 1 1 Laboratorio de Óptica, Universidad de Murcia. Campus del Espinardo (Edificio C), 30071 Murcia, Spain. 2 Dep. Ingeniería Electrónica, E.P.S. de Alcoy, Universidad Politécnica de Valencia. Plaza de Ferrándiz y Carbonell, 1 - 03801 Alcoy, Spain ABSTRACT A low cost pupil-meter and tracking apparatus was developed. It is based in digital analysis of images of the anterior eye to measure the diameter of the eye pupil and to track the position of its geometrical center at video rate. The system consists in an array of infrared LEDs to illuminate the eye and a CCD video camera that captures the images of the pupil to be grabbed in a computer. A fast algorithm analyses the digital images (512x512 pixels) in real time (25 images per second) providing the pupil diameter and the location of the geometrical center within that temporal rate. The algorithm was implemented to run under DOS, Win 3.x or Win95 and is compatible with any frame-grabber. The reliability of the apparatus was tested in different subjects for the particular problems of measuring the pupil diameter during accommodation and tracking the eye position during fixation. Under the measuring range, the system is robust to moderate lateral and axial movements of the subject. The spatial resolution is 0.02 mm and the linearity in measuring the pupil diameter is better than 1% in the complete useful range. The apparatus was adapted to operate without interfering either the visual task or possible simultaneous measurements. Keywords: pupilometry, digital image processing, eye tracking. 1. INTRODUCTION The size of the human pupil is an important parameter in different studies in Vision and Ophthalmology as well as in other areas such as Psychology, Psychiatry, Publicity, etc. The pupil controls the amount of light reaching the retina behaving as a negative feedback servomechanism 1 by decreasing the pupil size when the irradiance that reaches the eye increases, and vice versa. From an optical point of view, the pupil plays an important role on the retinal image quality due to the relation between the monochromatic aberrations and the spatial cut-off frequency transferred with the pupil radius. Most of current pupil-meters are based on the Green and Maaseidvaag closed-circuit television version 2 , incorporating new characteristics and improving the performances 3-5 . In this paper we present a new version of a pupil-meter. It is a relatively low-cost system operating in real time (25 images per second) under MS Windows and providing both, the pupil size, and the position of the geometrical pupil center. 2. MATERIAL AND METHODS The pupil-meter is based on the real time digital image processing of the images of the pupil. The system consists basically on an array of infrared LED’s (iLED) for pupil illumination, and a b/n CCD video-camera (Sony XC 75-CE), with manual gain mode. The images captured by the camera (768(H) x 494(V) pixels) are transferred to a frame grabber (Matrox Magic). Near infrared light (950 nm) was used for illumination because it does not affect the pupil size. Another advantage on using this wavelength is that the iris presents a higher reflection factor so increasing the contrast of the images.