Possible limits of calibrating reading charts with the Landolt ring: a microscopic study

Area of Science:

• Ophthalmology
• Optometry
• Vision Science

Background:

• Accurate near vision charts are essential for reliable eye examinations and the calibration of reading charts.
• Landolt rings are a standard optotype used in visual acuity testing.

Purpose of the Study:

• To microscopically assess the print quality and dimensional accuracy of Landolt ring near vision charts.
• To determine if these charts are suitable for calibrating reading charts.

Main Methods:

• Microscopic examination of Landolt ring charts from multiple manufacturers (Oculus, Precision Vision, RADNER) and custom-made charts.
• Measurement of Landolt ring height, line thickness, and opening width using a stereomicroscope.
• Analysis of deviations from nominal values based on EN/ISO 8596 and International Council of Ophthalmology (ICO) standards.

Main Results:

• All tested near vision charts displayed print quality deficiencies and dimensional aberrations.
• Landolt ring openings were consistently narrower than specified, while line thickness and height were larger.
• Even optimized Landolt rings did not always meet the acceptable 5% tolerance for opening width.

Conclusions:

• Inaccuracies in Landolt ring dimensions render current near vision charts unsuitable for calibrating reading charts.
• The study highlights the inadequacy of existing charts for precise visual acuity standardization.
• Standardizing print size using x-height relative to visual angle remains the most reliable method.