MEASUREMENT OF THE THICKNESS OF FUNDUS LAYERS BY PARTIAL COHERENCE TOMOGRAPHY

In the past few years, a new noninvasive optical ranging technique, pa rtial coherence interferometry, has been developed to measure various intraocular distances. A dual-beam version of this method offers high longitudinal resolution by using laser light with high spatial coheren ce but short coherence length-15 mu m (full width at half maximum)-emi tted by a special super luminescent diode. This technique is extended to obtain measurements not only parallel to the vision axis but at arb itrary horizontal and vertical angles to it. This is achieved by a new instrument, a fully computer controlled scanning partial coherence in terferometer. By tilting the laser beam in horizontal and vertical dir ections, this scanning partial coherence interferometer measures the d istance from the anterior corneal surface to different points of the r etina, These results are then plotted to form topographic images conta ining information about the contour and the thickness profile of diffe rent retinal structures, e.g., the retinal thickness and the retinal n erve fiber layer thickness, Provided that there is no other strong ref lection nearby, the absolute position of these retinal layers (respect ive to the cornea as a reference surface) can be determined in vivo wi th a precision of 5 mu m. Furthermore, the intensities of multiple lon gitudinal scans at different angles between vision axis and measuremen t direction can be converted into pixel colors and mounted to form a 2 -D false color image. These tomograms show the contour and the structu re of different retinal layers.