This task implements the ruler-based calibration of Muto (2025), which estimates the screen scale (CSS pixels per millimetre) with a procedure designed to be more rigorous than matching a card, and checks the result before it is accepted.

The participant measures three horizontal lines of random length with their own ruler and types each length in whole millimetres. The scale is estimated by least squares from the three pairs of on-screen and measured lengths. A test line of a random physical length between 50 and 100 mm is then drawn at the estimated scale and measured as well. The calibration is accepted when the estimated length of one pixel lies between 0 and 1 mm, the test line is measured within 1 mm, and the residual variance of the three measurements is 1 or less. Otherwise the participant measures a new set of lines. After a successful calibration, an optional check confirms that an area of a size the researcher sets fits in the window.

In simulations, Muto (2025) found an upper bound of 0.007 mm for the standard deviation of the estimated length of one pixel among calibrations that met the criteria. In an online study with 100 participants, 84 met the criteria on the first attempt and 12 on the second, and the procedure took a median of 103.5 s for those who passed at once.

The result is stored in the data and made available to later screens and to the tasks that follow it in the study flow, in the same form as Screen Calibration: Card Matching, so the two can be exchanged and either can precede the Virtual Chinrest: Viewing Distance paradigm.

Screen Calibration: Ruler Measurement