Electrically operated line monitor for tennis

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.

This invention relates to an electrically operated line monitor for tennis 
which incorporates at least one transmitter in a first position. The 
transmitter projects at least one ray to at least one receiver in a second 
position for registering the ray. The monitor also comprises a signalling 
means. 
It is a feature of the game of tennis that it should be known whether the 
ball strikes the ground inside or outside certain parts of the court; 
these certain parts are limited by lines, the outside edges of which are 
the most important and are hereinafter called critical lines. In important 
games, linesmen are positioned so that they can assess where the ball 
strikes the ground. This duty requires a high degree of concentration on 
the part of the linesmen whose decisions are frequently questioned, 
sometimes with consequent upset to the orderly progress of the game; the 
latter is particularly the case when the ball falls near a critical line. 
An electrically operated device for determining the position of the bounce 
of a ball in tennis and other games is known from British Specifications 
1,370,331-2-3. This device incorporates a ball having an electrically 
conducting surface, which, on striking a surface on a court adapted to 
convey an electric current, causes a signal to indicate the position of 
the bounce of the ball. The device also incorporates a single ray to 
detect if a player's foot crosses a line (British Specification No. 
1,370,332). 
Rays for use in safety devices on machines and for burglar alarms and for 
many other uses in detection devices are well known. It is also well known 
to limit a ray by modulation and shutters and optical means so that an 
effective part of a ray will register on a specific receiver. Typical 
object-detecting devices are described in British Specification Nos. 
1,425,896 and 989,041. 
The ball with its electrically conducting surface and the court with its 
special surface proposed in British Specifications 1,370,131-2-3 
constitute requirements which could affect the game of tennis as it now 
stands. The ray proposed in British Specification 1,370,332 for detecting 
whether a player's foot crosses the base line, indicates that the use of 
rays was considered in connection with tennis, but a ray such as that 
proposed would need to reach higher than the height of a tennis ball in 
contact with the ground and moreover, a single ray sited as described in 
British Specification 1,370,332 would, even if its operative part were 
such as to be interrupted by a tennis ball give the same signal for a ball 
which had bounced `in` as for a ball which was about to bounce `out`. 
A tennis ball can travel along approximately the same path as the rays 
which are to be interrupted as distinct from a path approximately at a 
right angle to the rays. For instance, if the rays are to monitor the 
center line on a tennis court, the ball is travelling in approximately the 
same path as the rays monitoring the center line whereas if the rays are 
to monitor the service line the ball is travelling at approximately a 
right angle to the service line. 
Therefore, it is an object of the present invention to provide a device 
which will correctly assess, within very close limits, whether the ball 
strikes the ground inside or outside the correct area of the court when 
the strike is near a critical line. 
Advantages of the invention include the fact that no special ball or 
electrically conducting surface is required, and that the device can be 
used on any first class court and moved from one tournament to another. 
Additionally, by siting rays in a novel manner, proven techniques can be 
used to achieve a result more consistently reliable than hitherto 
SUMMARY 
An electronically 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 substantially complete interruption of its 
effective part, travels over an area of the court in which a ball would 
strike if it were `good`. The said 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 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. 
Further embodiments employ additional rays (referred to as further rays) 
and positioned slightly on either the inside or the outside of a 
perpendicular to the critical line depending on whether `good` or `fault` 
balls are to be monitored. The further rays are spaced progressively 
further from the perpendicular so that a ball in play striking the ground 
in a monitored area will interrupt the passage of at least one of the 
rays, thereby actuating a signal. In one of these embodiments, the first 
ray functions as a master ray to monitor the area just inside the 
perpendicular from the critical line. If the master ray is interrupted by 
the passage of a good ball which subsequently skids into the fault area, 
the master ray prevents any subsequent signal. 
One embodiment of the invention employs a plurality of further rays, each 
ray having the effective cross-section as described above and each 
successive ray being distanced progressively farther from the critical 
line and at substantially the same height above the playing surface of the 
court. The distance between any of the further rays is such that a ball 
striking the ground in the area monitored by the device will interrupt one 
of the rays, thereby actuating the signal. The cross section of a ray may 
conveniently be limited by passing the ray through one or more small 
apertures but alternative methods of obtaining a similar result may be 
used. To reduce the possibility of interference the axes of rays 
travelling in the same direction are spaced no less than the diameter of a 
tennis ball apart at their receivers and preferably adjacent rays travel 
in opposite directions. 
In another embodiment of the invention a transmitter in a first position 
projects a diverging ray which registers on a plurality of receivers in a 
second position, the arrangement being such that the said diverging ray is 
split into a plurality of further rays each having substantially the same 
characteristics as the rays already mentioned. 
According to still another embodiment of the invention, a transmitter in 
the second position also projects a diverging ray which registers on a 
plurality of receivers in the first position. This diverging ray is also 
split into a plurality of further rays, each ray having substantially the 
same characteristics as the rays already mentioned. This combination of 
two transmitters projecting diverging rays from opposite sides of the 
court allows a rectangular area of the court, just outside the critical 
line, to be monitored. 
In another embodiment of the invention, in addition to the effective 
cross-section of the ray being limited at the receiver, it is also limited 
at the transmitter, so that the ray can be directed to register on a 
specific receiver. This is done either by directing the scan of a ray away 
from all but one receiver, by modulation, or by apertures. Other methods 
may be employed. This feature of the invention is useful if an independent 
master ray (hereinafter described) is incorporated. 
It is also useful if the invention is developed so that there are two or 
more transmitters in a first position aligned with the same number of 
receivers in a second position, the arrangement being such that the rays 
connecting the transmitters and receivers are substantially parallel to 
each other. Alternatively there are, in the first position, at least one 
transmitter and at least one receiver, and in a second position, at least 
one receiver and at least one transmitter, the transmitters and receivers 
being aligned so that the rays associated with them are substantially 
parallel to each other. 
The embodiments summarized above are suitable for monitoring an area of a 
court on either one side or the other of the perpendicular to the critical 
line, depending on whether `good` or `fault` balls are to be monitored. If 
a ray is sited within the critical line, i.e. over the area in which the 
ball must bounce if it is a `good` ball, the signal given is preferably 
silent. If a ray is sited outside the critical line, i.e. over the area in 
which the ball would bounce if it were a `fault`, the signal given is 
preferably audible although a silent signal such as a light may also be 
used. The effect is to monitor the area of the court adjacent to the ray 
because the ball will interrupt the reception of the ray by the receiver 
either on approaching the ground to bounce or on leaving the ground after 
bouncing. 
It can occasionally happen that a `good` ball can skid along the ground 
until it interrupts a further ray outside a critical line and thus give a 
false alarm. To cater for this eventually, a further embodiment 
incorporates a preventing device connected with said first ray which is 
then referred to as the master ray, which is provided in similar position 
to said first ray. The master ray is similar to the rays already described 
but it is an independent ray so that if it is interrupted by the passage 
of a ball, instead of actuating an alarm, it immediately prevents any 
alarm which would have been caused by the ball skidding through and 
interrupting any of the further rays which would normally actuate the 
`fault` alarm. This effect is achieved, preferably, by electrically 
preventing any signal from the `fault` rays from reaching the `fault` 
alarm. 
Another embodiment is particularly useful when the ball is travelling along 
approximately the same path as the rays which are to be interrupted as 
distinct from a path approximately at a right angle to the rays. For 
instance, if the rays are to monitor the center line on a tennis court, 
the ball is travelling in approximately the same path as the rays 
monitoring the center line whereas if the rays are to monitor the service 
line the ball is travelling at approximately a right angle to the service 
line. The effect is that because of the trajectory and path of the ball in 
the former case the rays must be closer together. The ray axes may be as 
close as is desired if the rays are projected in opposite directions, but 
this would mean that there would be both a transmitting and receiving 
means at the net which would be unnecessarily clumsy and would necessitate 
extra conductors in the center of the court. Further, when the ball is 
travelling in approximately the same path as the ray monitoring the center 
line, it should be noted that the height of the master ray is preferably 
positioned so that its axis is at about half the height of a tennis ball 
from the ground i.e. 1.25" to 1.5", the further rays being about the same 
height. 
A further embodiment is based on the same principle but also incorporating 
a reverse ray principle. The device is used to simplify setting-up even 
when the rays are monitoring the service line and the ball is, therefore, 
travelling at approximately a right angle to the line being monitored. If 
setting up is done carelessly it is possible for two transmitters in a 
first position to register on one or more receivers in a second position, 
thus causing interference. However, an embodiment of the present invention 
overcomes this problem in that all receivers comprise means for detecting 
an effective part of the ray designed to register on a said receiver. An 
effective part of the said ray is the part of the said ray which registers 
on the said receiver and which causes the said receiver to generate a 
signal when the said ray is interrupted. The cross section of the 
effective part of all said rays throughout their useful length is limited 
so that the said cross section is substantially smaller in any dimension 
than the diameter of a standard tennis ball and so that the highest 
effective part of the said rays above the playing surface of the court is 
less than the height of a standard tennis ball in play making contact with 
the ground. The distance between any rays is such that a standard tennis 
ball in play striking the ground in the area monitored by the device must 
interrupt at least one of the rays. 
All the rays are not necessarily at the same height. For instance, a said 
ray or a master ray monitoring a service or base line would be as low as 
practicable bearing in mind the accuracy of the court, but a said ray or a 
master ray monitoring a line at right angle to the net, for instance a 
side line or center line, would be at approximately half the height of a 
tennis ball. A further ray monitoring outside a service or base line would 
be as high as possible bearing in mind that a ball in play must not be 
able to strike the ground under it without interrupting the ray in passing 
but a further ray monitoring a critical line at a right angle to the net 
should preferably be at approximately half the height of a tennis ball, 
i.e. about the same height as the master ray or said ray at least when 
over its operative area. 
As used herein, the words `near` and `far` referring to the further ray 
receivers designate the position of the said further rays registering on 
the said further receivers in relation to the appropriate service court 
and its position relative to the umpire's chair. The near further ray 
monitors a fault on the far service court and the far further ray monitors 
a fault on the near service court. The associated further ray receiver is 
sited adjacent the master ray receiver in the said second position, the 
said master ray receiver is between the near and far further ray 
receivers. Different characteristics are known by the symbols X and Y. As 
used herein, one example of the manner in which two rays can be said to 
have different characteristics is for the two rays to be of two different 
frequencies, i.e. one ray having a frequency X and another ray having a 
frequency Y. The term `same service ball` or `same ball` refers to one 
delivery, not the same ball during different deliveries. 
As disclosed herein, one embodiment of an electrically operated device for 
monitoring an area near a critical line on a tennis court comprises first 
transmitting means; first receiving means; first signalling means; second 
receiving means; and second signalling means the first transmitting means 
is situated substantially in a first position, said first transmitting 
means comprising at least two transmitters for projecting at least two 
tennis ball monitoring rays; one of which is a said first ray being 
projected over a `good` area of the court and one of which is a further 
ray being projected in substantially the same direction as the said first 
ray but over a fault area of the court; the said first ray being 
substantially parallel to and in practice slightly distanced from a 
perpendicular from the said critical line, said first ray being the 
nearest `good` ball monitoring ray to said critical line and projected 
from the said first position, the said further ray also being projected 
from the said first position; the said first ray being so positioned that 
a ball in play falling across the court from the direction of the net and 
which strikes the said critical line must interrupt the said first ray. 
The first receiving means is situated substantially in said second 
position and comprises at least one receiver for receiving the said first 
ray projected by said first transmitting means. 
The first signalling means is connected to said first receiving means for 
generating a `good` signal when a standard tennis ball completely 
interrupts the reception of the effective part of said first ray at said 
first receiving means. 
The second receiving means is situated in said second position for 
receiving at least one further tennis ball monitoring ray, said second 
receiving means comprising a receiver for receiving the at least one 
further tennis ball monitoring ray. 
The second signalling means is connected to said second receiving means for 
providing an indication when a standard tennis ball completely interrupts 
the reception of the effective part of said further tennis ball monitoring 
ray at the said further ray receiver. 
The projected characteristics of the said first ray are different from the 
projected characteristics of the said further ray, both rays being 
projected from transmitters in the said first position and the difference 
between the said characteristics being such that the said first ray, 
although registering on the said further ray receiver will not provide an 
indication of said registering on said further ray receiver and such that 
the said further ray, although registering on the said first ray receiver 
will not provide any indication of said registering on said first ray 
receiver. 
In one form, to monitor the center line on a tennis court, the device 
comprises at least a master transmitter in a first position at the net 
projecting along the center line of the tennis court a master ray of 
characteristics X to a master ray receiver activated only by 
characteristics X in a second position at the back of the run-back area of 
the court. A near further ray transmitter is in the said first position 
for projecting a near further ray of characteristics across the far court 
and close to the center line to a near further ray receiver activated only 
by characteristics Y in the said second position. A far further ray 
transmitter is in the said first position for projecting a far further ray 
of characteristics Y across the near court and close to the center line to 
a far further ray receiver activated only by characteristics Y in the said 
second position. A signalling means indicates when the said master ray is 
substantially completely interrupted by a tennis ball, the said signalling 
means generates only a `good` signal. A near further ray signalling means 
indicates when the said near further ray is interrupted by a tennis ball. 
A far further ray signalling means indicates when the said far further ray 
is interrupted by a tennis ball. Prevention means associated with the said 
master ray signalling means prevents any signal being caused by the 
interruption of any said further ray by a same ball which also interrupts 
the said master ray. 
The switch means controls any signal caused by the interruption of any said 
ray. An indicator indicates the appropriate signal caused by the 
interruption of any said ray and allowed by the switch. A signal is 
generated by the interruption of the reception of the said near further 
ray by the said near further ray signalling means. A signal is generated 
by the interruption of the reception of the said far further ray by the 
said far further ray receiver if said signal is not prevented by said 
prevention means associated with said master ray signalling means. The 
switch means has a position during the service in which the said far court 
is the `good` service court for allowing an indicator to indicate if the 
said master ray has been interrupted and for allowing a different 
indicator to indicate that only the said far further ray which monitors a 
fault when a `good` service should strike in the `far` court has been 
interrupted by the same service ball and preventing any signal indicating 
that the said near further ray which monitors a fault when a good service 
should strike in the near court has been interrupted. The switch means has 
a position during the service in which the said near court is the `good` 
service court for allowing an indicator to indicate if the said master ray 
has been interrupted and for allowing a different indicator to indicate 
that only the said near further ray has been interrupted by the same 
service ball and preventing any signal indicating that the said far 
further ray has been interrupted. Preferably there is a repose position in 
the switch means which prevents any indicator indicating any signal caused 
by the interruption of any ray. 
The interruption of the master ray prevents any signal caused by the 
interruption of any further ray by the same service ball. The said master 
ray projected by said master ray transmitter is projected with 
characteristics `X` and received by said master ray receiver which is 
activated only by characteristics X. The said diverging ray projected by 
the said further ray transmitter is projected with characteristics `Y` and 
received only by the said near further ray receiver and the said far 
further ray receiver, the said near further ray receiver and the said far 
further ray receiver being activated only by characteristics Y. 
In another embodiment of the invention, to monitor, for instance, a service 
line, a master ray transmitter in a first position well outside the court, 
projects a master ray to a master ray receiver in a second position well 
outside the opposite side of the court. The master ray runs substantially 
parallel to the service line and, in practice, slightly distanced in from 
the perpendicular to the critical line which is the outside edge of the 
service line. The master ray is projected with characteristics X and the 
master ray receiver will only be activated by characteristics X. 
A further ray transmitter in the second position projects a diverging ray 
of characteristics X which crosses the court just outside the service line 
and is received by three further ray receivers in the first position, 
these three further ray receivers will only be activated by 
characteristics X. Yet another further ray transmitter in the first 
position projects another diverging ray of characteristics Y which crosses 
the court progressively farther from the service line. This ray is 
received by three further ray receivers in the second position. These 
receivers will only be activated by characteristics Y. The effect of using 
a diverging ray being received by two or more receivers is to split the 
ray into three parts, each part of which monitors an additional area of 
the court. If the master ray is interrupted, because the master ray is a 
prevention ray, any signal which would have been caused by the 
interruption of any further ray by the same ball will be prevented. If a 
further ray is interrupted by a ball which does not interrupt the master 
ray, the interruption of the further ray will activate a fault signal. 
If the receivers in the second position had all been activated by the same 
characteristics and the device had been carelessly set up so that the 
master ray transmitter had registered on the further ray receiver or vice 
versa, the interruption of a ray could have failed to cause a signal. With 
the present arrangement so long as the receivers are all activated, only 
correct signals can be given. 
In one embodiment of the invention, the said first ray transmitter projects 
a first diverging ray which registers on a said first ray receiver and on 
at least one said further ray receiver, both the said first ray receiver 
and the said further ray receiver comprising means for detecting different 
characteristics in the said first diverging ray as projected thus 
splitting the said first ray into at least two parts one part becoming the 
said first ray part and the other part becoming the said further ray part. 
If the said first ray part is interrupted such interruption causes the 
generation of a `good` signal and may be made to prevent any signal being 
given by the said further ray part; if the said further ray part is alone 
interrupted, such interruption causes the generation of f a `fault` 
signal. If prevention means is provided, the said first ray part becomes a 
master ray.

DETAILED DESCRIPTION OF THE DRAWINGS 
In FIGS. 1 and 3 the prefix X identifies the ray nearest a perpendicular 
from a critical line. In FIGS. 4 and 5 the prefix X identifies the ray 
other than the master ray which is nearest a perpendicular from a critical 
line. 
Referring now to FIG. 1, the outside of a typical box 1 housing one form of 
the invention is shown in relation to the cross-section of four infra red 
further rays 3.sup.1, a first ray X3.sup.1, and a service line 24.sup.2 
-24.sup.3 the outside edge 24.sup.3 being a critical line. The axes of the 
further rays are about 4.5 centimeters and that of the first ray X3.sup.1 
is about 2 centimeters above the play surface of the court and the highest 
effective part of the rays is less than the height of a typical tennis ball 
27.sup.1 when it is making contact with the ground. The nearest of the rays 
3.sup.1 to the perpendicular P from the critical line 24.sup.3 is about 15 
centimeters from the perpendicular P and inside it, the first ray X3.sup.1 
is 5 centimeters from the perpendicular P, that is, the rays are over that 
part of the court in which the ball would bounce if it were `good`. The 
dotted lines 4 and 5 represent the path of the center of balls 27 and 
27.sup.1 striking the ground within the area monitored by the rays 
3.sup.1. The ball 27 passes through a ray 3.sup.1 after striking the 
ground whilst the ball 27.sup.1 on path 5 passes through a ray as it 
approaches the ground. The cross-section of any of the rays is 
substantially smaller in any dimension than the diameter of typical tennis 
balls 27 and 27.sup.1. A ball 27.sup.1 passing across a ray interrupts the 
reception of the ray by its receiver even though the path 5 does not pass 
through the axis of a ray. The distance between the axes of any rays 
having their receivers in the same position is, at their receivers, 
greater than the diameter of a tennis ball and, in this instance, is 
illustrated as about 15 centimeters. When the passage of a ray is 
completely interrupted by a ball, the device gives a signal. This device 
would monitor an area of the court in which a ball would strike if it were 
`good`. 
Referring now to FIG. 2, this shows three forms of the invention in 
position to monitor various critical lines on a tennis court. Boxes 8 and 
9 contain transmitters and receivers arranged in the currently preferred 
form of the invention. This device monitors, in this instance, the service 
line; further explanation will be given with reference to FIG. 5. 
Boxes 12 and 13 contain transmitters and receivers which provide several 
parallel rays which, in this instance, monitor an area just inside a 
critical line of the doubles court. In this case, a signal would indicate 
that the ball was `good`. 
Referring now to FIG. 3, a box 12.sup.1 houses two transmitters 7.sup.1 and 
two receivers 6.sup.4 ; a box 13.sup.1 houses two receivers 6.sup.5 and two 
transmitters 7. Incorporated in box 12.sup.1 are two screens 14 spaced 
about 30 centimeters apart and in box 13.sup.1 are two screens 15 also 
about 30 centimeters apart. In the screens and aligned with their 
respective transmitters and receivers are small apertures 17 in box 
12.sup.1 and similar apertures 16 in box 13.sup.1 ; the apertures about 
1.5 centimeters in diameter. These apertures limit the spread of the rays 
3.sup.1 at the transmitters and the cross-section of the rays 3.sup.1 at 
the receivers. This device corresponds with that shown at 12 and 13 on 
FIG. 2. 
Referring now to FIG. 4, box 8.sup.1 houses a transmitter 18 which limits 
the ray 3.sup.2 which it projects so that it registers only on receiver 19 
in box 9.sup.1. Box 8.sup.1 also houses three receivers 20 on which a 
diverging ray 3.sup.3 from transmitter 21 in box 9.sup.1 registers. Box 
8.sup.1 also houses a transmitter 22 which projects a diverging ray 
3.sup.4 which registers on the three receivers 23 housed in box 9.sup.1. 
The rays 3.sup.3 and 3.sup.4 are infra red rays which are coupled so that 
if the reception of any of them is interrupted, a signal is given. This 
signal will preferably be an alarm if the device is sited in relation to a 
critical line as will be indicated in the description for FIG. 5 and the 
ray 3.sup.2 has not been interrupted. The ray 3.sup.2 is a master ray and 
is independent of the rays 3.sup.3 and 3.sup.4 except that, if the 
reception of the master ray 3.sup.2 is substantially completely 
interrupted it immediately gives a signal which prevents any signal given 
by the rays 3.sup.3 and 3.sup.4 caused by the passage of the same ball. 
This may be done, for instance, by electrically stopping the supply to the 
alarm normally actuated by the interruption of any of the rays 3.sup.3 and 
3.sup.4. Adjusting screws 28.sup.1 may be provided. 
Referring now to FIG. 5, the playing surface of a court 6.sup.1 -6.sup.2 
-6.sup.3 is shown in relation to a cross-section XX of the rays 3.sup.2, 
3.sup.3, and 3.sup.4 shown in plan in FIG. 4 and a typical service line 
24-24.sup.1 the edge 24 of which is a critical line. The net is in the 
direction of the arrow B. The axes of the rays 3.sup.3 and 3.sup.4 are 
about 4.5 centimeters above the surface of the court, the axis of ray 
3.sup.2 is about 2 centimeters above the surface of the court, and the 
highest part of any ray above the surface of the court is less than the 
height of a tennis ball making contact with the ground. The device is 
sited so that the first ray 3.sup.2 runs parallel to and adjacent to and 
slightly distanced in (about 8 centimeters) from the perpendicular P.sup.1 
from the critical line 24. The nearest of the further rays 3.sup.3 to the 
critical line 24 likewise runs parallel to and adjacent to the 
perpendicular P.sup.1 from the critical line 24 but is about 15 
centimeters outside the perpendicular P.sup.1, that is, on the opposite 
side of the perpendicular P.sup.1 to the master ray 3.sup.2. The other 
further rays 3.sup.3 and 3.sup.4 are spaced progressively from the 
perpendicular P.sup.1. If a ball and its path 25 from a service strikes 
the ground between 6.sup.3 and 6.sup.2 and then interrupts the ray 
3.sup.2, a light confirms that it is a `good` ball. If a ball and its path 
28 from a service strikes the ground between 6.sup.2 and 6.sup.1 it will 
interrupt at least one of the rays 3.sup.3 or 3.sup.4 thus actuating an 
alarm. If a ball and its path 26 from a service substantially completely 
interrupts the ray 3.sup.2 and then strikes the ground between 6.sup.3 and 
6.sup.2 or strikes the critical line 24 and subsequently skids into the 
area 6.sup.2 -6.sup.1 interruptiing any of the rays 3.sup.3 or 3.sup.4 the 
prior interruption of the ray 3.sup.2 electrically prevents any supply from 
actuating any alarm and a light confirms that the ball was `good`. 
The rays currently used are infra red or gallium arsenide based but any 
other rays which achieve similar results may be used. The reaction of a 
receiver to the interruption of its ray must be very fast and it is an 
advantage if a delay is incorporated so that although the signal is 
instantly actuated by the interruption of a ray, the signal itself is 
prolonged after the interruption of the ray has ceased. The transmitters 
are conveniently housed in weatherproof boxes which are designed and sited 
so that the axes of the further rays will preferably cross the playing 
surface of the court at a height of about 4.5 centimeters, the first or 
master ray is preferably lower. It is currently preferred that the 
greatest dimension of the effective cross-section of a typical ray be less 
than 1.5 centimeters. In the embodiments of FIGS. 1, 5 and 6A, for example, 
the axis of the nearest of the `fault` or further rays to the perpendicular 
from the critical line being monitored is preferably sited about 15 
centimeters outside the perpendicular. The further `fault` rays are 
distanced progressively further from the critical line. The distance 
between the axes of any rays transmitted in the same direction and 
parallel to each other should not be less than the diameter of a tennis 
ball. If the rays diverge from the transmitter, the distance between the 
axes of neighboring rays at the receiver end should not be less than the 
diameter of a tennis ball. A distance of 15 centimeters separation has 
proven satisfactory in some embodiments. It is currently preferred that 
the axis of the master ray should be between 5 and 10 centimeters inside 
the perpendicular from the critical line being monitored. The authority 
governing the game will almost certainly lay down the position in the 
light of experience and the perfection or otherwise of the court being 
used. 
FIGS. 6A and 6B illustrate two embodiments of the invention with their 
respective rays shown in positional relationship. In both FIGS. 6A and 6B, 
P.sup.1 represents the perpendicular from the critical line 24 and the 
arrow B indicates the direction of the net. Accordingly, the area of the 
court to the left of the perpendicular P.sup.1 is the area of the court in 
which a `good` ball would strike as it first approaches the court to 
bounce, while the area of the court to the right of the perpendicular is 
the area of the court in which a `fault` ball would strike. 
The embodiment of FIG. 6A is substantially the same as the embodiment of 
FIG. 5 discussed above. Ray 3.sup.2 and further rays 3.sup.3 of the FIG. 
6A embodiment are positioned substantially identically as in FIG. 5. Ray 
3.sup.2 of FIG. 6A, which functions as the master ray described above, is 
also known as "the first ray" for purposes of equating the position of ray 
3.sup.2 with a ray 4.sup.2 of the embodiment of FIG. 6B. 
As in the embodiment of FIG. 5, ray 3.sup.2 must be substantially 
completely interrupted in order for interruption thereof to prevent any 
subsequent signal occasioned by the interruption (be it partial or 
complete) of any of the rays 3.sup.3. 
The embodiment of FIG. 6B includes a first ray 4.sup.2 and a plurality of 
further rays 4.sup.3. The first ray 4.sup.2 is positioned substantially 
identically to the first ray 3.sup.2 of FIG. 6A: ray 4.sup.2 runs 
substantially parallel to the critical line 24 and is over an area of the 
court which a ball in play would strike if it were a `good` ball; the ray 
4.sup.2 is in practice slightly distanced from the perpendicular P.sup.1 
from the critical line 24 and is so positioned that a ball falling from 
over the `good` area towards the critical line must, if it just completely 
interrupts the said ray 4.sup.2, strike the critical line 24. The line A' 
in FIG. 7 shows just complete interruption in respect of the ray 3.sup.2 
for a ball falling from over the `good` area. 
The further rays 4.sup.3 of FIG. 6B travel over the `good` area of the 
court at distances progressively farther from said critical line and are 
distanced apart from one another so that a ball in play striking the 
ground in the area monitored interrupts one of the further rays. 
Interruption (either partial or complete) of one of the further rays 
4.sup.3 causes the generation of a signal indicative of the fact that the 
ball is `good`. In this regard, if any rays are transmitted in the same 
direction and are parallel to each other the distance between these rays 
at their receivers should not be less than the diameter of a tennis ball. 
If the further rays 4.sup.3 were to be diverging rays (although not 
illustrated as such in FIG. 6B but easily understood in conjunction with 
FIG. 4), the distance between the axes of neighboring rays at the receiver 
end should not be less than the diameter of a tennis ball. Unlike ray 
3.sup.2 of FIG. 6A, however, ray 4.sup.2 of FIG. 6B does not function as a 
master ray. In this respect, substantially complete interruption of ray 
4.sup.2 indicates that the ball is `good` and generates a signal 
accordingly. 
It is to be noted that in none of the foregoing embodiments is an area of a 
tennis court adjacent a critical line monitored by an effective ball 
monitoring ray that passes directly over the critical line. In this 
regard, in each embodiment the nearest effective ball monitoring ray to 
the critical line is distanced from the perpendicular from the critical 
line. In FIG. 1, for example, the nearest effective ball monitoring ray to 
the perpendicular P to the critical line 24.sup.2 is the ray X3.sup.1. In 
FIG. 6A the nearest effective ball monitoring ray to the perpendicular 
P.sup.1 to the critical line 24 is the first ray 3.sup.2, which functions 
as a master ray. In FIG. 6B the nearest effective ball monitoring ray to 
the perpendicular P.sup.1 to the critical line 24 is the first ray 
4.sup.2, which does not function as a master ray. The term "effective ball 
monitoring ray" as used herein denotes a ray which is used to determine 
whether a ball is a `good` ball or a `fault` ball. According to the 
embodiments of the invention a first ray or master ray is not positioned 
directly over a critical line because it is not possible to make an 
absolutely flat court. The final decision on this point rests with the 
authority governing the game who may decide that the said first ray or 
master ray is the critical line. 
The figures just given are currently considered suitable for monitoring the 
service line for which the device is primarily intended; if it were desired 
to monitor an area in which the ball was likely to fall more 
perpendicularly, the rays would be closer together. A device not required 
as the game progresses is suspended, for instance, by cutting off the 
supply to the audible `fault` alarm after a `good` service. The boxes 
housing the transmitters and receivers may be made adjustable to assist 
alignment of the rays. Reflectors may be incorporated to change the 
direction of the rays so that, for instance, the rays are projected from 
and received on the same side of the court. The siting of the rays, 
particularly the `fault` ray nearest to its critical line and an 
associated master ray, is very important. If the `fault` ray nearest to 
its critical line is too far from the perpendicular from that critical 
line, a ball could strike the ground outside the critical line and then 
bounce over the nearest `fault` ray, thus giving no alarm. If the master 
ray is too far in from the perpendicular from its critical line, a ball 
could pass over the master ray without interrupting it, then strike the 
critical line and, after bouncing, interrupt a `fault` ray and thus 
actuate an alarm without cause. 
Referring now to FIG. 7, this is an enlarged view of the area around the 
ray 3.sup.2 in FIG. 6A, to show the definite dividing line caused by 
complete interruption. If the ball falls lower than the line A.sup.1 the 
ball must be `good`. If the ball falls with the bottom of the ball just 
completely interrupting the effective area of ray 3.sup.2 along the line 
A.sup.1 the ball just strikes the critical line and a `good` signal is 
given. If ray 3.sup.2 is a master ray, any further signals are prevented. 
With partial interruption, for instance, the bottom of the ball falling 
along the line B.sup.1 the ball is a `fault`. Ray 3.sup.2 gives no signal 
and the further or `fault` rays give a `fault` signal. The ray 3.sup.2 is 
not directly on the critical line because in practice the court is never 
exactly flat. 
FIG. 8 represents a plan view of a half tennis court on which is shown a 
net 101; umpire's chair 102; run back area 103; a center line 104; part of 
the service line 105; part of base line 106; part of a far service court 
107; part of a near service court 108; a said first position 109; and a 
said second position 110. Associated with the said first position 109 is a 
master ray 111 of characteristics X projected from master ray transmitter 
112 in the said first position 109 and received by master ray receiver 113 
in the said second position 110; far further ray 114 of characteristics Y 
projected from far further ray transmitter 115 in first position 109 and 
received by far further ray receiver 116 in said second position 110; near 
further ray 117 of characteristics Y projected from near further ray 
transmitter 118 in the said first position 109 and received by near 
further ray receiver 119 in said second position 110. As used herein, one 
example of the manner in which two rays can be said to have different 
characteristics is for the two rays to be of two different frequencies, 
i.e. one ray having a frequency X and another ray having a frequency Y. 
When the far service court 107 is the court in which a `good` service 
should strike, the center line 104 is part of that court and the edge 104A 
of the center line 104 which is adjacent the near service court 108 is the 
critical line being monitored by the device. In this case the far further 
ray 114 is at least partially and in this case wholly projected over the 
near court 108; if the ball strikes in this court without striking any 
part of the far court 107 including any part of the center line the ball 
is a fault. If, in striking near court 108 the ball also completely 
interrupts far further ray 114 without interrupting master ray 111 the 
interruption of the reception of the far further ray 114 on the far 
further ray receiver 116 will generate a signal activating a sound device 
to indicate that the ball is a fault. The switch 122, placed, for instance 
on the umpire's chair and operated by the umpire will have been pushed by 
the umpire into the `far court` position and will allow the signal to 
reach the light or sound device in use. 
If, however, the far further ray 114 and the master ray 111 had both been 
completely interrupted, the interruption of the master ray 111 would have 
electrically prevented any signal caused by the interruption of far 
further ray 114. It could be that far further ray 114 was, at the same 
time registering on master receiver 113 but because further ray 114 
operates on characteristics Y and master ray 111 operates on 
characteristics X, such registering will have no effect. It could also 
happen that the ball, instead of interrupting far further ray 114 had 
instead interrupted only near further ray 117. In this case the ball would 
be `good` but, when the umpire pushed the switch 122 into the `far court` 
position the movement of the switch would have made an electrical 
connection to allow a desired signal caused by the interruption of far 
further ray 114 to reach the light or sound device but, at the same time 
the same movement would prevent for instance, by mechanical means, e.g. 
breaking the connection, any signal caused by the interruption of near 
further ray 117. 
When the near service court 108 is the court in which a `good` service 
should strike, the center line 104 is part of that court and the edge 104B 
of the center line 104 which is adjacent the far service court 107 is the 
critical line being monitored by the device. In this case the near further 
ray 117 is at least partially and, in this case, wholly projected over the 
far court 107; if the ball strikes in this court without striking any part 
of the near court 108 including any part of the center line, the ball is a 
fault. If, in striking far court 107 the ball also completely interrupts 
near further ray 117 without interrupting master ray 111, the interruption 
of the reception of the near further ray 117 on the near further ray 
receiver 119 will generate a signal activating a sound device to indicate 
that the ball is a fault. The switch 122 will, before the service 
commences, have been pushed by the umpire into the `near court` position 
and will allow the signal to reach the light and sound device in use. It 
will be understood that it is a convention in tennis that only `faults` 
are called so that the present invention uses a sound device for faults. 
If, however, the near further ray 117 and the master ray 111 had been 
completely interrupted, the interruption of the master ray 111 would have 
electrically prevented any signal caused by the interruption 111 of the 
near further ray 117. If desired, the interruption of master ray 111 will 
also cause a `good` signal, e.g. a light. It could be that near further 
ray 117, at the same time as registering on near further receiver 119 was 
also registering on master receiver 113 but because near further ray 117 
operates on characteristics Y and master ray 111 operates on 
characteristics X, such registering on master receiver 113 will have no 
effect. It could also happen that the ball, instead of interrupting near 
further ray 117 had instead interrupted only far further ray 114. In this 
case the ball would be `good` and well within the `good` court but, when 
the umpire had pushed the switch 22 into the `near court` position the 
movement of the switch would have made an electrical connection to allow a 
desired signal caused by the interruption of the near further ray 117 to 
reach the light or sound signalling device but, at the same time, the same 
movement would prevent, for instance by breaking the connection, any signal 
caused by the interruption of the far further ray 114. 
FIG. 9 shows a diagrammatic cross section showing the position of the rays 
relative to the center line in FIG. 1 with the ball superimposed. The line 
123 represents the playing surface of the court. FIG. 9 also shows the 
center line 104 with its two edges 104A and 104B; the master ray 111 of 
characteristics X, the far further ray 114 of characteristics Y and the 
near further ray 117 also of frequency Y. Because the ball is travelling 
along approximately the same path as the rays which are to be interrupted, 
the rays must be substantially closer than the diameter of a tennis ball, 
otherwise it would be possible for a ball to strike within the area 
monitored by the device without interrupting at least one of the rays. A 
standard tennis ball 120 to which this specification applies, is about 2 
1/2 inches in diameter and, assuming the effective area of the rays to be 
1/2 inch diameter in cross-section, the distance between the axes of the 
master ray and the further rays is, in this instance, just under 2 inches. 
The height of the axes of all the rays above the court surface is about 
1.125 inches - this allows for a slight flattening of the ball when it 
strikes the court. 
According to the rules of tennis, the center line from its edge 104B to its 
edge 104A is 2 inches wide; a ball G which just interrupts the master ray 
111 on the center line of the court will, therefore, clearly strike the 
outside edge 104A of the center line 104 which, if the near court 108 
(FIG. 8) were the `good` court, the edge 104A would be the critical line 
and the ball G would be good. Although the ball G had also interrupted the 
near further ray 117 no fault signal would be given. If, however, the ball 
F had interrupted the near further ray 117 without interrupting the master 
ray 111, the ball F would not strike the edge of the critical line 1O4A and 
the ball G would be a fault. A fault signal would be given because the 
master ray 111 bad not been interrupted. 
The tennis authorities may decide that when the ball G flattened on 
striking the ground less tolerance should be allowed to the server, in 
which case the master ray 111 would diverge and would be received on twin 
master ray receivers 113A and 113D shown on FIG. 10 which is a partial 
view of the pertinent part of FIG. 8. 
FIG. 11 shows a diagrammatic cross section of a diverging ray which is 
registering on receivers 113A and 113B. The effect is to give the effect 
of monitoring a slightly wider center line 104C. The master ray 111 is now 
split into two parts, 111A and 111B. Both parts will act as a master ray. 
Further rays 114A and 117A can now be farther apart. 
Referring now to FIG. 12, a box 126 in a first position houses a 
transmitter 127 which projects a ray 128 of characteristics X which 
registers on receiver 129 in box 130 in a second position. Box 130 also 
houses transmitter 132 which projects a further ray, a diverging ray 133 
of characteristics X from the said second position to register on three 
receivers 131 in box 126 in the said first position. As ray 128 and ray 
133 are projected in opposite directions, the transmitter of ray 128 being 
in the first position and the receivers of ray 133 being in the same first 
position, no interference in the reception of the rays is possible. Box 
126 in the said first position also houses a transmitter 134 which 
projects a further ray, also a diverging ray 135 of characteristics Y 
which registers on three receivers 136, housed in box 130 in the second 
position. The receivers 136, being in the same said second position as 
receiver 129, and ray 128 being projected in the same direction as ray 
133, because ray 128 is of characteristics X which has no effect on the 
receivers 136 of ray 135 which are activated by characteristics Y and, 
similarly, ray 135 can have no effect on receiver 129, ray 135 being of 
characteristics Y and receiver 129 only being activated by characteristics 
X. 
The arrangement just described and shown in FIG. 12, is, if ray 128 is a 
said first or master ray, suitable for monitoring the service line and the 
area just outside it. In this case, ray 128 will be a `prevention` ray and, 
if it is interrupted, will prevent any signal being caused by the 
interruption further ray, i.e. the fault rays 133 and 135. If the device 
is monitoring a critical line and the area inside it, the ray 128 will be 
a said first ray and the further rays will be projected over a `good` area 
of the court. 
If a critical line parallel to the net is being monitored the master ray or 
the said first ray is so positioned that a standard tennis ball in play 
falling across the area of the court in which it would strike if it were a 
`good` ball and falling towards the said critical line, and which just 
completely interrupts the effective part of the said first ray, must 
afterwards strike the critical line. 
FIG. 13 shows another form of the device shown in FIG. 8, but instead of 
there being two separate further ray transmitters projecting further rays 
114 and 117 there is a single further ray transmitter 115A sited directly 
above master ray transmitter 112 in the said first position; said further 
ray 114C is a diverging ray which is split into two parts as it registers 
on near further ray receiver 119 and far further ray receiver 116. It will 
be remembered that services in first class tennis usually fall on the outer 
end of the center service line. 
Referring now to FIG. 14, a transmitter 136 projects a diverging ray 137 
which is made up on an outer ineffective area 139 and an inner effective 
area 138 which registers on two receivers 141 and 141X with their lenses 
140 and 140X. The distance between the transmitter 136 and the receivers 
141 is of the order of 70 feet (about 21 meters) so the drawing is not to 
scale. 
Referring now to FIG. 15, the receiving end of the ray 138 is, at the 
distance, substantially parallel. The receiver 141A and its lens 140A are 
approximately full size. The ball 142 is substantially completely 
interrupting the direct passage of the rays on to the receiver, as 
distinct from partial interruption which would be much less definite. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various alterations in form and detail may 
be made therein without departing from the spirit and scope of the 
invention, for instance, when the device is monitoring critical lines at 
right angles to the net the device is more accurate if the said first ray 
is so positioned that a ball in play falling across the court from the 
direction of the net and which strikes the said critical line must also 
substantially completely interrupt the said first ray. The receiver is 
activated when the effective part of the ray is substantially completely 
interrupted. 
Referring to FIG. 8, the arrangement already described applies to the use 
of rays having different characteristics. If it is desired to use rays 
having the same characteristics the direction of projection of the said 
master ray 111 would be reversed so that the said first position for only 
the said master ray 111 would be at 110 and the said second position would 
be at 109. The master ray transmitter would be at 110 and the said master 
ray receiver would be at 109. No interference would then be possible but 
the arrangements at the net would not be so neat. The operation of the 
device will readily be understood from the description already given for 
FIG. 8.