Landolt Ring Inaccuracies Exclude Their Use In Calibrating Reading Charts

Commercially-available near vision charts often show inaccuracies in print quality along with the height, line thickness, and the size of the openings of the Landolt rings, according to research published in Eye and Vision.

Although it has been the standard optotype for more than 100 years, the Landolt ring’s use for calibrating reading charts is questionable due to variations in construction. Researchers conducted an investigation to determine whether near vision charts with Landolt rings have the size and print quality necessary for valid, reliable calibration.

The researchers analyzed 2 commercially-available near vision charts with Landolt rings, custom-made Landolt rings printed with offset printing, and a Radner reading chart and completed microscopic measurements to determine print quality, Landolt ring height, line thickness, and width of the rings’ openings.

According to the report, all charts deviated from EN/ISO (European International Organization for Standardization) 8596 and International Council of Ophthalmology (ICO) standards for print quality. Overall, Landolt ring openings were too narrow and the height and thickness of the lines were larger compared with nominal values.

The custom-made, offset-printed chart showed the best print quality with line edges that were sharp and parallel endings at the ring openings. 

Landolt rings in the Radner reading charts had more accurate print quality, sharper lines, parallel openings, and heights within the 5% tolerance. Openings and thicknesses were better compared with those of the commercially-available test printed with offset printing. 

“[N]ear vision charts with Landolt rings do not achieve a level of quality sufficient to meet the premises for calibration,” the study authors state. “Since the x-height related to the visual angle represents an already well-recognized standard that has been demonstrated in many studies to produce reproducible and reliable results for reading acuity and other reading parameters, it seems to be obvious to retain this definition as the standard for reading charts in Latin script.”

Investigators did not report any study limitations. 

Disclosure: One study author declared an affiliation with biotech, pharmaceutical, and/or clinical research organizations. Please see the original reference for a full list of authors’ disclosures.