To the Editor.
—By decreasing illuminance of the retina with neutral-density filters having a density of 0.5 or more, Klewin and Radius' found a decrease of differential light sensitivity (DLS) when using OCTOPUS Program 32. They conclude that this is due to the fact that the neuro-visual system for the standard background luminance employed in this instrument (4 asb), is working within a range where Weber's law does not hold. They also thought that increasing background luminance from 4 asb to 31.5 asb (typical for the Goldmann perimeter) may shift the operation mode of the eye to a range where Weber's law may hold, and where a superior independence of the DLS from background luminance may therefore be expected.Fankhauser2 has shown that at both 4 asb and 31.5 asb the law of Rose-de Vries'3 (ΔL/L0.5) = constant holds to a fair degree of approximation. (Here AL is
Fankhauser F. Background Illumination and Automated Perimetry. Arch Ophthalmol. 1986;104(8):1126. doi:10.1001/archopht.1986.01050200032023