title: Ophthalmic technology for the clinician: direct ophthalmoscopy revisited.
abstract: The history and basic optical principles of the direct ophthalmoscope are reviewed, and new developments, including the halogen-tungsten bulb, are explored. To counter the problem of trying to discern retinal details through an immature cataract or an edematous cornea, a system has been designed that sends a thin light annulus along the outer edge of a dilated pupil, avoiding the middle of the cornea or lens. Recent studies suggest that the direct ophthalmoscope with a standard bulb at design voltage can safely be used steadily for only about a minute and a half, and occasional pausing during prolonged viewing is recommended.
mesh_terms: D004869:Equipment Safety; D008029:Lighting; D019721:Ophthalmoscopes