Use of diffusing filters for artificially reducing visual acuity when testing equipment and procedures
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When evaluating ophthalmological devices and procedures, for instance those for visual electrophysiology, it is often desirable to perform tests with reduced acuity. Doing this with individuals with actual visual impairments has a number of disadvantages, such as considerable recruitment efforts, especially when a specific acuity range is targeted, and little control about the actual perceptual characteristics of the impairment, which are normally not fully known. Lenses with positive diopters or blurring filters that are placed in front of the eyes of visually normal observers promise a simple solution to the problem. However, defocus results in considerable spurious resolution, and previous studies suggest that the frequently used Bangerter occluders are not optimal for the purpose. The present study therefore reviews a number of other options and tests a selection of filters with respect to their effect on acuity and contrast sensitivity with the aim of identifying filters that primarily degrade acuity while mostly sparing contrast sensitivity.
First, we screened several filters for potential usefulness. The Freiburg Acuity and Contrast Test was then used to measure visual acuity and contrast sensitivity with a subset of three filters (Luminit LSD 0.5° and 1°, and LEE 420) and, for comparison, with a Bangerter occluder with a nominal acuity grade of 0.1. A qualitative comparison of the filters’ effect on the checkerboard-reversal VEP was also performed.
With both Luminit filters, variability in acuity across participants was relatively small, and at least with the 0.5° version, contrast sensitivity was relativity little affected. The LEE filter and the Bangerter occluder resulted in more variability and, compared to the effect on acuity, a relatively strong reduction in contrast sensitivity. Comparing the Luminit 0.5° and 1° filters, the reduction of acuity was not proportional to physical stimulus degradation. The effect on VEP responses was consistent with the psychophysical data.
The Luminit filters, which have a Gaussian light diffusion profile, appear to be a good choice for artificial reduction of acuity.
KeywordsVisual acuity Contrast sensitivity Occlusion Blur Observer method Artificial degradation
We thank Verena Gauggel for their competent assistance with data acquisition, Patrick Weisert for making the holders that allowed for inserting the filters into regular trial frames (Fig. 2), and Gottfried Martin for support with the micrographs of the filters (Fig. 1). We are also grateful to the volunteers who participated in the study. Optrovision, Munich, Germany, a distributor of Luminit diffusers, provided free-of-charge samples.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional research committee (Ethik-Kommission der Albert-Ludwigs-Universität Freiburg, application number 622/14) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. This article does not contain any studies with animals performed by any of the authors.
Informed consent was obtained from all individual participants included in the study.
Data are available on request from the authors.
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