Figure 4.
Zero-corrected gray levels (GLs) versus the fluorescence of a calibrated pattern AFpattern at different sensitivities, S, (as indicated) for the HRA2 and the Spectralis. Inset: an image of the calibrated pattern. Dashed lines: point at which nonlinearity reached 5% at different sensitivities. Pixel-sized flashes of colored light appear slightly below the 5% level. For high exposures, the GLs saturate at a level of 255 − GL0. The range of AFpattern was chosen to correspond roughly with measurements previously obtained by fluorometry: normal subjects (AFpattern = 200–400), patients with AMD (AFpattern = 300–500), Stargardt's disease (AFpattern = 500–1200), and Best disease (AFpattern ≈ 1600).1,13–15 Dark arrow: the equivalent exposure for the internal reference currently used in the study. For example and for the HRA2, if we set a lower limit for GL − GL0 to 25 (mainly for contrast), then the reference should be adequate to cover a large range of fundus AF levels ranging from an equivalent of 1600 (using S = 89, GLF − GL0 ≈ 110, and GLR − GL0 ≈ 25 at the limit) to an equivalent of 100 (using S = 96, GLF − GL0 ≈ 25 at the limit, and GLR − GL0 = 165).