The stimulus. No blue disc is drawn.
A real disc, tinted by t. Adjust until the two look alike.

Tint t = 0.000

Match
Flicker (reference only)

Flicker can trigger seizures in people with photosensitive epilepsy. It stays off until you turn it on. Rates above about 30 Hz cannot be shown cleanly on a 60 Hz screen.

Stimulus