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Editor,—Goldmann’s applanation tonometry is generally performed using cobalt blue filter and fluorescein in order to obtain accurate localisation of the apex of the tear meniscus.1 2 The peak transmission value of the cobalt blue filter BG12 on the Haag–Streit slit lamp 900 BM is 0.80 at 400 nm, whereas that of the red-free filter BG39 is 0.965 at 490 nm. The peak absorption value of fluorescein in dilute aqueous solution at physiological tear pH is also at 490 nm.3 Greater intensity of fluorescence and better visibility of the tear menisci could therefore, be obtained by the use of red-free filter. We designed a study to compare intraocular pressure (IOP) measurements obtained using red-free light with those taken with the blue light.
Fifty six consecutive follow up glaucoma patients attending ophthalmic clinic during February 1998 were the subjects for the study. The order of testing of the two eyes and the order of use of the filters were determined by random permuted block method. After instillation of 4% lignocaine and 0.25% fluorescein with polyvinylpyrrolidone, and with slit lamp illumination at 7.5 V, both eyes were applanated at the same sitting using both cobalt blue and red-free illumination in succession. Three readings were taken for each illumination and the average was used for statistical purpose. The mean value of IOP of 112 eyes obtained using red-free light was 17.19 (SD 5.14) mm Hg whereas using blue light it was 17.17 (6.44) mm Hg. On two tailed pairedt test analysis at the 5% level of significance, the difference is not significant.
The red-free filter does not diminish the overall light intensity as much as the blue filter. Consequently, the ocular structures are seen more clearly in the background during the procedure. At the same time the tear menisci are seen brightly fluorescent as a result of both to greater overall intensity and more appropriate wavelength of the light. Red-free light applanation tonometry, therefore, achieves optimal visualisation of the tear menisci and accurate estimation of IOP.
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