Standardization
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 65.41-68.58 mm (2.575-2.700 inches). Balls must weigh between 56.0 g and 59.4 g (1.975-2.095 ounces). Yellow and white are the only colors approved by the United States Tennis Association (USTA) and ITF, and most balls produced are fluorescent yellow (known as "optic yellow") the color first being introduced in 1972 following research demonstrating they were more visible on (colour) television. The man credited with introducing the yellow ball in a tournament is Mike Davies Tennis Hall of Fame 2012 Tennis Hall of Fame Inductees]. In 2007, company Whitetennisballs.com brought white balls back to the mainstream market. Tennis balls are filled with air and are surfaced by a uniform felt-covered rubber compound, although some manufacturers have produced balls filled with small polystyrene balls . 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.
Tennis balls begin to lose their bounce as soon as the tennis ball can is opened and can be tested to determine their bounce. A ball is tested for bounce by dropping it from a height of 254 cm (100 inches) onto concrete; a bounce between 135 and 147 cm (53-58 inches) is acceptable (if taking place at sea-level and 20°C / 68°F with relative humidity of 60%; high-altitude balls have different characteristics when tested at sea-level). Modern regulation tennis balls are kept under pressure (approximately two atmospheres) until initially used.
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