Red-free versus cobalt blue illumination in fluorescein diagnostic staining of the external ocular surface
Aim: To investigate the usefulness of the red-free filter of the Haag-Streit slit lamp 900 in fluorescein diagnostic staining of the external ocular surface.
Patients and methods: One author examined 200 consecutive patients (371 eyes) requiring diagnostic staining of the external surface with fluorescein dye. The evaluation was carried out using the Haag-Streit slit lamp 900 BM. Both red-free and cobalt blue filters were used in succession after the saline-moistened fluorescein strip was applied to the conjuctival sac. The initial filter used for each patient was randomly determined. The appearances of the tear film and of the areas of dye uptake were noted, as was the ease with which the associ- ated lesions and the ocular structures were visualized.
Results: The red-free filter was found to be superior to the cobalt blue filter in all clinical situations except for the assessment of tear film break-up time. The superiority of the red-free illumination in fluorescein diagnostic staining of the external ocular surface is due to its ability to produce intense fluorescence without obscuring the details of the associated anterior ocular structures.Conclusion: The use of the red-free filter in place of the traditionally used cobalt blue filter greatly enhances the diagnostic capability of fluorescein staining of the external ocular surface and deserves to be used more widely.
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