Shows optotypes at the correct angular size for your screen and test distance. Calibrate once (saved on this device), set the distance, and step through the lines. Screening aid only — not a substitute for a calibrated clinical chart.
Method 1 (recommended): hold a credit card (or any ID-1 card) against the box and drag the slider until the box matches the card's width, then save.
Method 2: enter your screen's diagonal size (requires 100% browser zoom and full-resolution display scaling — less reliable than Method 1).
Optotypes follow the standard 5×5-unit construction with stroke width one-fifth of letter height (Sloan 1959; NAS-NRC 1980); a 20/20 optotype subtends 5 minutes of arc. Letter spacing is one letter width per line (ETDRS convention).