Studies in Photocoagulation

THE photocoagulating beam was studied in the first article of this series. This second article is devoted to the observation system, the essential requirement of which is proper illumination of the fundus. Taking into account that the coagulating beam is already directed towards the patient's eye, it seems obvious that it should also be used for illuminating the fundus. In fact, this has been the system of fundus illumination in all photocoagulators described so far. It should be remembered, however, that an attenuating diaphragm is located on the path of the coagulating beam. It serves to limit the light intensity admitted into the eye below the threshold of coagulation. The attenuated beam is composed of parallel rays focused by the optics of the eye into a small image on the retina. This image is the target at which the coagulating beam is aimed. Such a system leaves only a little scattered light for illumination of the fundus around the small retinal image of the source. It follows that the observer is obliged to look for the area to be coagulated in the fundus much as if looking for an object in the dark sky with a searchlight. Another possibility is for the observer to look into the fundus with an ophthalmoscope and to memorize the configuration of the fundus. This facilitates the pointing of the coagulating beam in the desired direction, but the technique is tiring and time-consuming, as it often requires repeated switches from a regular ophthalmoscope to the photocoagulator, and vice versa. The ideal observation system in the photocoagulator is that which would give a stereoscopic view of the fundus with a wide field. The conditions to be fulfilled in order to permit such a performance will now be investigated. Since it is impractical to use the coagulating beam for fundus illumination, a second beam, suitably shaped, must be introduced into the patient's eye, for illumination purposes. Instead of being collimated, it must be focused in the pupillary plane of the patient's eye. The illumination of the fundus must cover the area located around the image created by the attenuated coagulating beam. Consequently, the axis of the illuminating beam must coincide with the axis of the coagulating beam, at least at their entrance into the patient's eye. One solution consists in locating the source of the illuminating beam at a calculated distance behind the source of the coagulating beam. In this case, the two beams will reach the pupil of the patient's eye through the optical system of the coagulating beam. For this purpose, the illuminating source may be formed by a concave