Optimising slit-lamp biomicroscope fluorescein imaging filters

Fluorescein dye plays a pivotal role in ophthalmic care as a diagnostic agent due to its fluorescent properties when exposed to blue light. This study examined the spectral profile of blue and yellow filters within commercial slit lamps with a view to determining whether these are optimal.

The spectral transmission of the white light, blue filtered light(s) and inbuilt yellow filters (where available) of 16 models of slit lamp biomicroscope from 11 different manufacturers were measured with a spectrophotometer. To confirm the peak wavelength required to excite sodium fluorescein dye, the intensity of fluorescence was measured in vitro on exposure to blue light of narrow wavelength bands between 400 and 490 nm. Finally, the effect of altering the blue light, from a Cobalt filter profile (broad ranged with a peak around 450 nm) to one centred at approximately 490 nm, was demonstrated on-eye at 1-min intervals post-fluorescein dye instillation.

Most slit lamp blue lights had a similar spectral profile to Cobalt blue, with a peak wavelength of between 443 and 463 nm. The spectral width of the blue light band at 50% of its height was significantly greater with a tungsten (63 to 80 nm) than a LED source (20 to 40 nm; p < 0.001). Yellow filters had a 50% cut-off of 512.3 to 524.5 (with a 16.0 to 33.3 nm range), higher than the optimal ∼505 nm. Optimising blue and yellow filters improved fluorescein visibility by 1.8 ± 0.3× (p < 0.05).

Despite the suboptimal transmission of Cobalt blue filters for exciting fluorescein molecules, their use prevails in the majority of slit lamps, and the yellow filters lack a sharp spectral cut-off. Adopting new "fluoro-" enhanced slit-lamp illumination filters would address shortcomings of existing filters.