<P> A tennis ball is a ball designed for the sport of tennis . Tennis balls are fluorescent yellow at major sporting events, but in recreational play can be virtually any color . Tennis balls are covered in a fibrous felt which modifies their aerodynamic properties, and each has a white curvilinear oval covering it . </P> <P> Modern tennis balls must conform to certain criteria for size, weight, deformation, and bounce criteria to be approved for regulation play . The International Tennis Federation (ITF) defines the official diameter as 6.54--6.86 cm (2.57--2.70 inches). Balls must have masses in the range 56.0--59.4 g (1.98--2.10 ounces). Yellow and white are the only colors approved by the ITF, and most balls produced are a fluorescent yellow known as "optic yellow", first introduced in 1972 following research demonstrating they were more visible on television . </P> <P> Tennis balls are filled with air and are surfaced by a uniform felt - covered rubber compound . The felt delays flow separation in the boundary layer which reduces aerodynamic drag and gives the ball better flight properties . Often the balls will have a number on them in addition to the brand name . This helps distinguish one set of balls from another of the same brand on an adjacent court . </P>

When did tennis balls changed from white to yellow