A large group of football fans comes to the game with colored cards that spell out the name of their team when held up simultaneously. Most of the cards are colored blue (?vacuum = 480 nm). When displayed, the average distance between neighboring cards is 4.0 cm. If the cards are to blur together into solid blocks of color when viewed by a spectator at the other end of the stadium (140 m away), what is the maximum diameter (in mm) of the spectator's pupils?