1 row
This page lights a thin stripe of rows on an otherwise black screen, so a flicker meter held against the stripe sees only those rows. OLED screens update one row at a time, so a sensor that sees many rows at once blurs the waveform and understates the flicker. Measuring a few adjacent rows avoids that.
A web page can't control brightness. Set it with your device's own controls, and turn off auto-brightness, True Tone, night shift or blue-light filters, and low-power mode (it can change the refresh rate).
Hold the meter against the screen with its sensor directly over the lit stripe, as close as you can get it. Any gap lets light from other rows in.
Start with one row. If the meter's signal is too low, press + to add a row. Between 1 and 8 adjacent rows give essentially the same waveform with more light; stay at 8 or fewer.
One row is one physical pixel row only when the browser zoom is 100% and display scaling is at its native setting.
The page asks your device to stay on while it's open. If the screen still dims or locks, lengthen the auto-lock time.
This display reports a pixel ratio that isn't a whole number. If you changed the browser zoom or the display scaling, set them back to 100% / native. Otherwise the stripe can be off by a fraction of a pixel on some devices.