Background Illumination and Automated Perimetry

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Abstract
To the Editor. —Drs Klewin and Radius1 present an excellent practical application of the Weber-Fechner fraction in "Background Illumination and Automated Perimetry." They used neutral density filters in 31 normal volunteers to decrease retinal illumination during perimetry with the Octopus 2000. Significant visual field defects were produced with progressive neutral filter density. They conclude that a "low level of background illumination... may allow relatively minor reduction in light transmission by ocular media to produce significant changes in the recorded level of threshold sensitivity during visual field evaluation." They base this conclusion on the fact that both cataracts and neutral density filters decrease visual acuity. However, the mechanism of decreasing visual function with neutral density filters is not analogous to poor visual acuity from cataracts. Neutral density filters affect visual function in several different ways. At normal levels of illumination, the greatest effect on visual acuity is from decreasing the

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