Screen Calibration: Card Matching

A stimulus defined in pixels has a different physical size on every participant's screen, because displays differ in pixel density and the browser does not report the physical size of the screen. This task measures the screen scale, the number of CSS pixels per millimetre, with the card-matching procedure that forms the first step of the virtual chinrest of Li, Joo, Yeatman and Reinecke (2020).

The participant holds a card in the standard bank-card format (ID-1, 85.60 × 53.98 mm) against the screen and moves a slider until an on-screen rectangle with the card's proportions and rounded corners is exactly as large as the card. The width of the rectangle in CSS pixels divided by 85.6 gives the screen scale. Any card of the same size can be used, and a participant without a card can use a ruler instead.

The result is stored in the data and made available to every later screen and to the tasks that follow it in the study flow, so stimuli can be sized in millimetres with calc(30 * var(--mm)). To size stimuli in degrees of visual angle, the viewing distance is also needed; the Virtual Chinrest: Viewing Distance paradigm measures it. For a procedure designed to be more rigorous, with a built-in accuracy check, see Screen Calibration: Ruler Measurement (Muto, 2025).