MEASUREMENT OF THE THICKNESS OF FUNDUS LAYERS BY PARTIAL COHERENCE TOMOGRAPHY

In the past few years, a new noninvasive optical ranging technique, partial 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 coherence but short coherence length-15 mu m (full width at half maximum)-emitted by a special super luminescent diode. This technique is extended to obtain measurements not only parallel to the vision axis but at arbitrary horizontal and vertical angles to it. This is achieved by a new instrument, a fully computer controlled scanning partial coherence interferometer. By tilting the laser beam in horizontal and vertical directions, this scanning partial coherence interferometer measures the distance from the anterior corneal surface to different points of the retina, These results are then plotted to form topographic images containing information about the contour and the thickness profile of different retinal structures, e.g., the retinal thickness and the retinal nerve fiber layer thickness, Provided that there is no other strong reflection nearby, the absolute position of these retinal layers (respective to the cornea as a reference surface) can be determined in vivo with a precision of 5 mu m. Furthermore, the intensities of multiple longitudinal scans at different angles between vision axis and measurement direction can be converted into pixel colors and mounted to form a 2-D false color image. These tomograms show the contour and the structure of different retinal layers.