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

Abstract

Purpose: To evaluate microscopically whether the print quality and accuracy of sizing of Landolt ring near vision charts are adequate for the calibration of reading charts.

Methods: Near vision charts with Landolt rings from Oculus GmbH (C-Test; Wetzlar, Germany), Precision Vision (Woodstock, IL) and the RADNER Charts were examined, as well as custom-made Landolt rings optimized for print quality. Microscopic investigations and measurements were performed by using a Huvitz HSZ 600 stereomicroscope (Nikon NIS Elements software) to evaluate the height of the Landolt rings, the thickness of the lines, and the width of the openings. The deviations from the mathematically correct values, which were calculated as given in the EN/ISO 8596 and by the International Council of Ophthalmology (ICO), were analyzed (calculated for a test distance of 40 cm).

Results: All the near vision charts showed notable deficiencies in print quality and aberrations from the nominal values in the height, thickness of the lines, and width of the openings. The openings were too narrow, whereas the height and thickness of the lines were larger than the nominal values. Even the openings of Landolt rings optimized for print quality were not always within an acceptable 5% tolerance and need further improvement.

Conclusion: This study reports inaccuracies in the heights, thicknesses of the lines, and widths of the openings of Landolt rings in all the near vision charts investigated. The extent of these inaccuracies excludes such near vision charts as reference tests for the calibration of reading charts. The x-height in relation to the visual angle still seems to be the most reliable method for standardizing the print sizes for reading charts.

Keywords: Landolt rings; Near vision charts; Near visual acuity; Print quality; Reading acuity; Sizes of optotypes.

Fig. 1

C-Test Landolt ring test (left column) and Landolt rings of the near vision chart from the RADNER Charts (right column); original magnification, ×40. Photographs of Landolt rings taken of print sizes ranging from 0.1 to − 0.2 logMAR. Nominal sizes are given between the columns. The C-Test shows deficiencies in print quality and aberrations in the optotype heights, thicknesses of the lines, and widths of the openings. The shape of the Landolt rings and endings that configure the opening of the Landolt rings are distorted. The lines are blurred and frayed. The openings are too narrow; the heights and thicknesses of the lines are > 5% compared with the nominal values. The Landolt rings of the near vision charts from the RADNER charts are more accurate. The edges of the lines are sharper, and the endings that form the openings are in parallel. The heights are within the 5% tolerance, but the widths of the openings and lines lie outside that tolerance

Fig. 2

Landolt ring near vision chart from Precision Vision (left column) and Landolt rings graphically optimized for print quality (right column; RADNER optimized); original magnification, ×40. Photographs of Landolt rings taken of print sizes ranging from 0.1 to − 0.3 logMAR. Nominal sizes are given between the columns. The Landolt ring near vision charts of Precision Vision show deficiencies in print quality and aberrations in the optotype heights, thicknesses of the lines, and widths of the openings. The endings that configure the opening of the Landolt rings are parallel from 0.0 to 0.1 logMAR, but the lines are blurred. The heights of the Landolt rings are within the 5% tolerance at all sizes. The openings are narrower, and the widths of the lines are wider than the nominal values (outside 5%). The Landolt rings optimized for printed quality show the best print quality. The edges of the lines are sharp, and the endings at the openings are clearly parallel. The Landolt rings closely match the nominal values in height and line width. Only two openings are outside the 5% tolerance, i.e., − 0.1 and 0.1 logMAR