Abstract: An electrically operated line monitor for tennis wherein different characteristics are used for rays which are projected from the same first position to the same second position. In one embodiment of the invention, to monitor the center line (104) on a tennis court, a master ray (111) is projected down the center line (104) and has different characteristics to two other rays (114, 117) projected on either side of the master ray (111) and over the service courts (108, 107) adjacent to the center line (104). In another form of the invention, a ray (128) which can be a master ray is projected just inside a critical line from a first position to a second position, a diverging ray (133) having the same characteristics as the master ray is projected from the second position to the first position and another diverging ray (135) having different characteristics to the two other rays is projected from the first position to the second position; this prevents interference due to careless setting-up.

Abstract: An electrically operated line monitor for tennis which uses one or more rays substantially smaller in effective cross-section than a tennis ball to monitor areas of a tennis court adjacent critical lines thereof. The rays pass over the playing surface of the court at a height lower than the height of a tennis ball. In various embodiments, a first ray which actuates a `good` signal on essentially complete interruption of its effective part travels over an area of the court in which a ball would strike if it were "good". The first ray travels essentially parallel to and in practice slightly distanced from a perpendicular from said critical line and is so positioned that a ball in play falling across the said `good` area towards the said critical line and which just completely interrupts the reception of the effective part of the said first ray must afterwards strike the said critical line. The said first ray is the nearest `good` ray to the perpendicular from the said critical line.