GB2292218A

GB2292218A - A method and apparatus for determining the position at which a ball strikes the ground in a game of tennis

Info

Publication number: GB2292218A
Application number: GB9515968A
Authority: GB
Inventors: John Reuben Marshall
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-07-29
Filing date: 1995-07-31
Publication date: 1996-02-14
Anticipated expiration: 2015-07-31
Also published as: GB2292218B; GB9515968D0

Abstract

A method and apparatus for determining, in a game of tennis, the point of contact of a service ball relative to a service line includes the steps of (a) monitoring a field of view along the service line and producing an electronic representation of an upper boundary of the ball as it passes through the field of view, (b) scanning the upper boundary of that representation progressively in the direction of the ball flight and determining at successive spaced positions along the path of the ball the angle of declination of the boundary, and (c) comparing successive values of that angle and noting the position of the ball at which that angle reduces relatively rapidly to a low or even a negative value. A preferred method determines and compares successive horizontal components (Fig. 9, N(H1)...N(H7)) of successive sections of the boundary which have successive equal value vertical components or as an alternative determines and compares successive vertical components (Fig. 10, (NV1)...N(V7)) which have equal horizontal components. The apparatus comprises a collimated light source 30 and an array of photo-detectors 44. <IMAGE>

Description

GAME BALL MONITORING METHOD AND APPARATUS This invention relates to a method and apparatus for monitoring in a ball game the position relative to a reference line at which a game ball strikes a playing surface. The invention has particular reference to the game of tennis, though it is not restricted to that game.

In what follows hereafter, the present invention will be described, for convenience and by way of example only, in the context of a game of tennis.

In the game of tennis, it is important to be able to determine with the greatest accuracy and consistency the position at which a ball on being served first strikes the playing surface relative to the 'service line. That line defines the rearward boundary of the service area in which the served tennis ball must first strike the playing surface. A sevice which results in the ball first striking the playing surface rearward of that service line is a faulty service.

There have been various proposals to provide an apparatus for judging whether a served tennis ball has landed on the playing surface rearward or otherwise of the service line.

For a fuller understanding of the importance of this topic and apparatus, the reader's attention is directed to the following patent specifications which relate to this topic, and which deal at length with the significance of the topic:- US 5,059,944 (CARMONA); US 4,814,986 (SPIELMAN); & EP 0 007 720 (CARLTON ET AL).

However, there still appears to be no commercially available apparatus to reliably assist an umpire in determining whether a service is a good one or a faulty one.

The present invention seeks to provide a different approach to the topic, thereby to overcome the disadvantages of those prior proposals.

According to one aspect of the present invention, a method of determining the position, relative to a reference line, of the first contact of a game ball with a playing surface comprises the following steps: (a) arranging a monitoring means to view along the direction of the reference line a predetermined field adjacent and enveloping the reference line; (b) monitoring the flight of the ball as it passes through that predetermined field adjacent the reference line and producing electronically a representation of at least one of the upper and lower portions of the ball as the ball continues through said field;; (c) scanning a selected one of the boundaries, preferably the uppermost boundary, of that representation progressively in the direction of the ball flight, and at successive spaced positions along the ball flight path determining the angle of declination of the flight path of that selected boundary; and (d) comparing successive values of that angle and noting the ball position relative to the reference line at which the angle reduces relatively rapidly to a relatively low value approaching zero, or even a negative value.

The steps of scanning the representation, determining the declination angle at successive positions, comparing the successive angles, and noting said position may be arranged to take place either after the ball has passed through the monitored field, or otherwise in real time.

The representation is preferabaly staticised in a suitable storage medium, so that those steps may take place at a rate lower than would otherwise be necessary, either during or after the passage of the ball.

Instead of determining the angle of declination at successive spaced positions on the selected boundary and noting substantial changes in that angle, successive changes in the horizontal (or the vertical) component of respective sections of the selected boundary for equal changes in the corresponding vertical (or horizontal) component of said boundary sections can be determined, in which case it is only necessary to compare the successive values of those horizontal (or vertical) component changes, and then to note the ball position relative to the reference line at which the horizontal component changes increase relatively rapidly to a relatively high value (or the vertical component changes reduce relatively rapidly to a relatively low value approaching zero, or even a negative value).

The respective vertical (or horizontal) components of the selected boundary may lie consecutively, or otherwise overlap one another.

According to a further aspect of the present invention, there is provided an apparatus for carrying out the aforementioned method, which apparatus comprises: (a) monitoring means arranged to view along the direction of the reference line a predetermined field adjacent and enveloping the reference line, thereby to monitor the flight of the ball as it passes through that predetermined field adjacent the reference line and to produce electronically a representation of at least one of the upper and lower portions of the ball as the ball continues through said field; (b) scanning means arranged to scan a selected one of the boundaries (preferably the uppermost boundary) of that representation progressively in the direction of the ball flight, and at successive spaced positions along the ball flight path to determine the angle of declination of the flight path; and (c) comparing means arranged to compare successive values of that angle and to provide a signal noting the ball position relative to the reference line at which the angle reduces relatively rapidly to a relatively low value approaching zero, or even a negative value.

Advantageously, the scanning means is arranged to determine instead successive changes in the horizontal (or the vertical) component of the selected boundary for equal changes in the corresponding vertical (or horizontal) component of the selected boundary, and the comparing means is arranged to provide a signal noting the ball position relative to the reference line at which the horizontal component changes increase relatively rapidly to a relatively high value (or at which the vertical component changes reduce relatively rapidly to a relatively low value approaching zero, or even a negative value.

Other features of the present invention will appear from a reading of the description that follows hereafter and of the claims appended at the end of that description.

One apparatus for use in connection with the game of tennis and embodying the present invention will now be described by way of example and with reference to the accompanying diagrammatic drawings. In those drawings: Figures 1 and 2 show similar side views of a tennis ball at the moment of maximum impact with the playing surface of a tennis court adJacent a service line; Figure 3 shows pictorially and schematically the principal components of the apparatus and their respective relationships and inter-connections; Figure 4 shows a view, in the direction of the arrow IV of Figure 3, of a photo-sensor array forming part of the apparatus; Figure 5 shows the components forming a light source used in the apparatus; Figure 6 shows a more detailed front view of the light sensor array Figure 7 shows the component parts of a single light sensor incorporated in the light sensor array;; Figure 8 shows an electric circuit diagram of a bistable circuit module which incorporates the light sensor of Figure 7; Figures 9 to 11 show various similar views of a tennis ball disposed at its maximum compression position in front of the light sensor array; and Figures 12 and 13 show views similar to those of the Figures 9 to 11, but indicating typical true proportions of a tennis ball and the light sensor array.

In the respective figures, parts which appear in more than one figure or correspond to parts in another figure bear the same reference numeral in all of the respective figures.

Referring now to the drawings, the concept underlying the present method and apparatus will be described and explained with reference to Figure 1. In that Figure, the tennis court playing surface is indicated at reference 10, and a tennis ball at the position of maximum contact with the ground (just before rebound) is indicated at 12. The path of the ball in flight to and from that position is defined by an upper V-shaped boundary line 14 and a lower generally V-shaped boundary line 16. The 'service line' is indicated by the rectangle 20.

The boundary lines 14 and 16 are shown as linear, but in practice on the approach side of the service line they will often be curved progressively downwardly, to an extent dependent on how the ball was served by the serving player. For example, spin applied to the ball in the service will tend to cause the flight path of the ball to be more curved as the service line is approached.

Thus, the angle of declination of the ball's flight path relative to the horizontal increases as the ball approaches the service line. Likewise, the speed at which the ball is served will affect the curvature of the ball trajectory, the curvature being greater for lower ball speeds.

However, regardless of the manner of service and the ball flight path and speed produced thereby, on the approach side of the service line the flight path has generally a positive declination to the horizontal, whilst on the rebound side of the service line, the flight path has at least initially a negative declination (i.e. a positive inclination to the horizontal).

The present invention seeks to determine the position of the ball relative to the service line by determining the point at which the positive angle of declination falls relatively rapidly to a low value approaching zero, or even to a negative value, as the ball makes firm contact with the playing surface. By trial and experiment (and thus calibration of the apparatus) that position can be positively related to the position at which the ball first makes contact with the playing surface.

Any suitable means may be used (a) for determining in known manner the angle of declination of the ball flight path on its approach to the service line, and (b) ascertaining the ball position relative to the service line when there occurs a sudden drop or a reversal in the angle of declination of the flight path.

Downward curvature in the 'approach' flight path does not affect the desired determination, since that only produces an increase in the declination angle as the service line is approached. Such an increase will not affect the determination to be made, in so far as that determination is based not on an increase in the declination angle, but instead a decrease in that angle.

Likewise, any change in the curvature of the ball flight path as the ball rebounds off the playing surface will not affect the desired determination, since the declination on the initial rebound is always of negative value (as compared to that on the approach to the service line).

Instead of determining the angles of declination as such, it is easier and more convenient to determine and then compare the horizontal components of successive sections of the boundary line, which sections all have equal vertical components. This is illustrated in Figure 1, where the points A B C D E F G H on the upper boundary line 14 are all spaced apart in a vertical direction by equal vertical components 24 of the respective boundary line sections AB, BC, CD, DE, EF, FG, GH.

The horizontal components of those sections are all equal as long as the declination of the boundary line remains constant. However, the declination reduces progressively when the ball makes contact with and becomes increasingly compressed against the playing surface. As a consequence the horizontal component of the boundary section beyond G extends indefinately on the rebound side of the service line and so its value is increased suddenly to a very high value. That sudden increase in the horizontal component indicates the lowest position of the ball before rebounding from the playing surface (i.e. the actual rebound or bounce position).

Thus, it will be appreciated that as the bounce position is closely approached, the horizontal component will suddenly increase to a relatively high value, and so indicate that the bounce position has been reached.

Alternatively, the determination can be made by ascertaining and comparing the vertical components of successive sections of the boundary line, which sections all have equal horizontal components. In that case, it will be noted that as the bounce position is closely approached the vertical component will suddenly reduce to a relatively low value, or even a negative value. Such a decrease indicates that the bounce position has been reached.

That alternative is illustrated in the Figure 2, where the points A B C D E F G H on the upper boundary line 14 are all spaced apart in a horizontal direction by equal horizontal components 26 of the respective boundary line sections AB, BC, CD, DE, EF, FG, GH.

The vertical components of those sections are all equal as long as the declination of the boundary line remains constant. However, the declination reduces progressively when the ball makes contact with and becomes increasingly compressed against the playing surface. As a consequence the vertical component of the boundary section beyond G suddenly reduces to a relatively low value. That sudden reduction in the vertical component indicates the lowest position of the ball before rebounding from the playing surface (i.e. the rebound or bounce position).

Thus, it will be appreciated that as the bounce position is closely approached, the vertical component will suddenly reduce to a relatively low value, or even a negative value, and so indicate that the bounce position has been reached.

It will be noted that those determinations of the bounce position have been made using the upper boundary line 14.

However, providing appropriate measures can be taken in practice, there is no reason why corresponding determinations cannot be made using successive sections of the lower boundary line 16.

Those measures include making sure that the playing surface is quite plane at the reference line, and that the lower boundary line 16 can be observed by the relevant monitoring and scanning apparatus right down to the level of the playing surface.

Referring now to the Figures 3 to 6, one apparatus embodying the present invention comprises on one side of a tennis court (not indicated) - at the near end of a service line 28 - a light source 30, and at the other side of the tennis court - at the far end of the service line 28 - a light sensor array 32. Both the light source and sensor array have adjustable feet 34, 36 for enabling proper alignment of the light source and sensor array across the tennis court.

As shown in the Figure 5, the light source comprises a light emitter 38 in the form of a collimated laser, a collimating lens system 40 for projecting a homogeneous, collimated beam of light 42 of uniform light intensity across the tennis court to the sensor array 32. That light beam has a transverse cross section of rectangular shape, and illuminates uniformly the whole of the front face 44 of the sensor array (which front face 44 is also indicated in Figure 6 in alignment with the collimated light beam of Figure 5).

As will be seen in Figure 6, as well as in Figures 3 and 4, the sensor array 32 comprises a uniform matrix of light sensors 46. In Figure 6, that array is shown as comprising sixteen sensors in each of sixty vertical columns. Those sensors have a pitch in each of the columns and rows of, for example, one milli-metre. As shown in the Figure 7, each sensor 46 comprises a photodiode 48 which is disposed at the output end of an optical fibre 'light pipe' 50. The free, input ends 52 of the light pipes are secured together with optical insulation therebetween to form the light sensitive front face 44 of the sensor array.

As will be seen in Figure 3, the sensor array 32 has an electrical output channel 56 for feeding the output signals of the respective photo-diodes 48 to an electric signal storage array 58 which comprises a plurality of bistable circuit modules 60, one for each of the respective light sensors 46.

As shown in the Figure 8, each bistable circuit module 60 comprises (a) a series electric circuit which includes the associated photo-diode 48 and a load resistor 61, and (b) an output circuit 62 which includes in series a high input impedance amplifying circuit 63. That output circuit 62 supplies (a) an OR gating circuit 64 for gating computer operations, and (b) an electronic bistable device 65. An output circuit 66 of the bistable device delivers an output signal indicative of the state of the bistable device (and hence of the state of illumination of the photo-diode 48) to a tristate device 68. The status of the tristate device 68 may be delivered to a computer data circuit 70 when that device receives a 'read' signal from the computer on an addressable read circuit 72.

The electric signal storage array 58 has an output channel 74 which communicates with a computer (digital processor) 76, (a) to receive address signals from the computer and (b) to transmit to the computer the states of the respective bistable devices 68 in response to the respective address signals delivered by the computer.

The computer has an associated memory 78 into which the states of the respective bistable devices 68 may be transferred for retention there until subsequently cancelled. The computer has also an associated monitor device 80, upon the screen 82 of which the computer can display the respective states of the bistable devices 68, as a series of bright areas on a dark background. Those bright areas are created in response to the absence of light at the respective associated photo-diodes 48. The screen of the monitor is of sufficient size to show the state of all the sensors in the sensor array.

In the absence of any light-obstructing object between the light source 30 and the sensor array 32, all of the light sensors in the array 32 are illuminated uniformly by respective elemental areas of the light beam 42, so that the corresponding bistable circuit modules 60 all indicate that illuminated condition of the sensors, and the whole of the monitor screen remains dark.

When a tennis ball enters the field between the light source 30 and the sensor array 32, the sensors 46 directly behind the tennis ball lie in the shadow of the ball and so receive no light from the light source.

Thus, in turn the associated bistable circuit modules 60 record that new condition in which the sensors disposed behind the ball are not illuminated. In that condition, the monitor screen 82 shows bright illuminated areas corresponding to the un-illuminated sensors. In this way, a bright image of the tennis ball is portrayed on the monitor screen.

In the description that follows hereafter, it will be convenient to refer to a sensor as 'white' when it is illuminated by the light beam 42, and as 'black' when it is in the shadow of the ball and hence un-illuminated.

Thus, a 'white' sensor is a sensor not obscured by the ball, whilst a 'black' sensor is a sensor which is obscured by the ball and hence not illuminated.

In Figure 4, the flight path of the ball is indicated by the upper boundary line 14 and by the lower boundary line 16. The passage of the ball through the light beam thus causes all of the sensors between the upper and lower boundary lines 14 and 16 to become black sensors. All the other sensors remain white sensors.

This condition is recorded in the storage array 58 and also in the memory 78 of the computer 76, so that an image of the flight path of the ball may be displayed on the monitor screen 82.

In response to a command signal emitted when a ball enters between the light source 30 and the sensor array 32, the computer performs the following operations in order to find the point at which the ball has its most compressed condition, and from that position - the likely position of first contact of the ball with the playing surface of the tennis court, so that a determination can then be made as to whether the point of first contact lies within the service area or not.

Referring now to Figure 9, the computer scans down the first column at the entry side of the sensor matrix, from the uppermost sensor down to the first black sensor. On finding that first black sensor, the computer counts down a further three black sensors, and then scans horizontally the black sensors in that same row in the direction of ball flight until the first white sensor is located. The number N(H1) of sensors scanned in that row is noted in a part of the computer memory.

In the next stage, the computer counts down the column of that first white sensor a further three black sensors, and then scans horizontally the black sensors lying in that row to the left until the first white sensor in that row is located. The number N(H2) of sensors scanned in that row is noted in the computer memory, and is then compared with the number N(H1) of the previous stage to ascertain the difference, which in the present case is zero, since the gradient of the upper line 14 has not changed as between the first and second stages.

In the third stage, the computer counts down the column of that last located white sensor a further three black sensors, and then scans horizontally the black sensors lying in that row to the left until the first white sensor in that row is located. The number N(H3) of sensors scanned in that row is noted in the computer memory, and is then compared with the number N(H2) of the previous stage. Again the difference is zero, the gradient of the boundary line 14 not having changed since the previous stage.

Successive stages are completed in like manner until the seventh stage, when the number N(H7) is found to have substantially exceeded the value of the number N(H6), thus indicating that there has been a substantial change in the gradient of the boundary line 14, and that the position of the greatest ball compression has been reached.

The addition of the series of numbers N(H1) to N(H7) indicates a distance of the fully compressed ball position from the starting datum, and the substraction from that distance of the distance of the rearward edge of the service line from the same datum will indicate whether the compressed ball lies straddling the service line, or to one side or the other of that line.

Compensation for the distance between the position of first contact of the ball with the ground and the position of the fully compressed ball can be derived by suitable analysis in the computer having regard to the angle of declination of the flight path as determined in a predetermined number of stages earlier, and the ball speed. Alternatively, that distance may be determined experimentally for a number of different angles of declination of the flight path, and ball speed, and stored in the computer memory.

An audible alarm device 84 may be activated by the computer to give an audible alarm when the first point of contact of the ball with the ground lies rearward of the service line.

Whereas the above process has been described in relation to the upper boundary line 14, providing the sensor array and light beam extend down to the ground level, the process could be carried out on the lower boundary line 16, as likewise illustrated in the Figure 9.

In Figure 10, there is illustrated an alternative method of determining the position of maximum ball compression, and hence the position of ball first contact with the ground relative to the service line.

In this alternative method, the computer first scans down the right hand column of sensors to find the first black sensor, whereupon the computer counts horizontally to the left along the row of that first black sensor a predetermined number of white sensors (i.e. of columns), in this case five columns, and then scans down that column to find the first black sensor. The computer counts the number N(V1) of sensors scanned in that column.

The computer then repeats that process in stage two to find the number N(V2), and then compares that number with the number N(V1) determined in the previous stage. Since the gradient of the boundary line 14 has not changed as between the respective sections of the boundary line 14, the difference of the two numbers is zero.

The same procedure is repeated for successive sections of the boundary line 14 until the difference between one such number and the previous one has fallen by a substantial amount, thus indicating that the angle of declination of the boundary line has fallen to a low value of the order of zero, or even a negative value.

This marks the position of maximum compression of the ball, which position can be compared with the position of the service line, and which position can be compensated for the difference between the positions of maximum compression of the ball and first contact with the ground.

As in the case of Figure 9, the process may be carried out alternatively on the lower boundary line 16, with the same provisoes.

Whereas in the procedures just described with reference to the Figures 9 and 10, the determinations of the respective numbers N(H1) to N(H7) have related to consecutive, non-overlapping sections of the boundary line 14 (or 16), if desired (and it is preferred) the determinations may be made on overlapping sections, thus to provide a 'rolling-over' procedure which is capable of giving a more refined determination, resulting in greater accuracy in the determination of ball position.

If desired the procedure described above with reference to the Figure 9 (or that of Figure 10) may be supplemented by carrying out the the same procedures from both the right and left hand ends of the sensor array in opposite directions, either simultaneously or consecutively, to arrive at complementary determinations, which will either confirm one another, or otherwise provide by an average of the respective determinations the position of maximum compression of the ball.

In a modification of the method of determining that position of the ball described with reference to Figure 9, the value of the number N(H6), for example, may be ascertained in the corresponding procedures of that Figure as carried out from both ends of the array.

Those numbers in conjunction with the numbers of sensors counted down in the corresponding vertical components of the respective right and left hand boundary line sections determine the respective gradients of those right and left sections of the boundary line 14. From those gradients, the computer can compute the position of intersection of those gradients as extended towards the position of maximum compression of the ball. That intersection will lie on the line defining the maximum compression position of the ball.

Figure 11 shows the flight path of a ball which descends on to the playing surface at a relatively steep angle (i.e. at a high declination angle). In that case, the sensors in the triangular area 86 above the ball as defined by the respective right and left hand parts of the upper boundary line 14 will be triggered to the black state on the first descent of the ball, so that on the rebound of the ball those sensors will already be in the black state unless special precautions are taken to enable the left hand part of the boundary line 14 to be recorded. To this end, the bistable circuit modules 60 are arranged to be triggerable back to their former white state on being triggered a second time, that is, on the rebound of the ball. Hence, in a monitor display of this type of steep decent of the service ball the sensors in that triangular area 86 will be seen as a black area superposed on the illuminated display of the rest of the flight path of the ball. The cross-over point at the top of the triangular area lies over and hence denoteds the position of maximum compression of the ball.

By scanning both the right and left hand portions of the upper boundary line 14 oppositely from both ends of the sensor array and determining the location and gradients of the right and left hand parts of the upper boundary line 14, their point of intersection can be determined as denoting the position of maximum compression of the ball.

Figures 12 and 13 show in proper proportions the size of a tennis ball in relation to the sensor array which is illustrated in Figure 6, for the respective shallow and steep descents of the service ball.

Whereas in the apparatus described above, the light source has emitted light in the visible part of the spectrum, other sorts of energy radiation may be used with appropriate forms of radiation sensor.

The apparatus described above has the merit that there is no need to carefully align each light sensor with its own individual light source. All of the sensors respond to the common collimated light beam, 80 that the problems of alignment of the light source and sensor array is minimised.

It is important to the success of the present invention that the light sensor array is illuminated by a single homogeneous collimated light beam. A matrix of small, individual light sources, for illuminating the respective light sensors of the array would not suffice, due to the relatively greater dispersion of light beams emitted from such small light sources.

Such relatively greater dispersion of light beams from small individual light sources would mean that each light sensor would be affeeted by light from other ones of the light sources besides its own particular light source.

Hence, such a matrix of small light sources would not produce a sufficiently well defined representation of a tennis ball passing between the light source 30 and the light sensor array 32.

Claims

1. A method of determining the position, relative to a reference line, of the first contact of a game ball with a playing surface comprising the following steps: (a) arranging a monitoring means to view along the direction of the reference line a predetermined field adjacent and enveloping the reference line; (b) monitoring the flight of the ball as it passes through that predetermined field adjacent the reference line and producing electronically a representation of at least one of the upper and lower portions of the ball as the ball continues through said field; (c) scanning a selected one of the boundaries, preferably the uppermost boundary, of that representation progressively in the direction of the ball flight, and at successive spaced positions along the ball flight path determining the angle of declination of the flight path of that selected boundary; and (d) comparing successive values of that angle and noting the ball position relative to the reference line at which the angle reduces relatively rapidly to a relatively low value approaching zero, or even a negative value.

2. A method according to claim 1, wherein the steps of scanning the representation, determining the declination angle at successive positions, comparing the successive angles, and noting said position are arranged to take place after the ball has passed through the monitored field, 3. A method according to claim 1, wherein the steps of scanning the representation, determining the declination angle at successive positions, comparing the successive angles, and noting said position are arranged to take place in real time whilst the ball is passing through the monitored field.

4. A method according to claim 1, wherein the representation is staticised in a suitable storage medium, and the respective scanning, determining, comparing and noting steps take place, either during or after the actual passage of the ball through the monitored field, at a rate lower than would otherwise be necessary.

5. A method according to any preceding claim, wherein instead of determining the angle of declination at successive spaced positions on the selected boundary and noting substantial changes in that angle, successive changes in the horizontal component of respective sections of the selected boundary for equal changes in the corresponding vertical component of said boundary sections are determined and the successive values of those horizontal component changes are compared thereby to note the ball position relative to the reference line at which the horizontal component changes increase relatively rapidly to a relatively high value.

6. A method according to claim 5, wherein the respective vertical components of the selected boundary lie consecutively.

7. A method according to claim 5, wherein the respective vertical components of the selected boundary overlap one another.

8. A method according to any one of the claims 1 to 4, wherein instead of determining the angle of declination at successive spaced positions on the selected boundary and noting substantial changes in that angle, successive changes in the vertical component of respective sections of the selected boundary for equal changes in the corresponding horizontal component of said boundary sections are determined and the successive values of those vertical component changes are compared thereby to note the ball position relative to the reference line at which the vertical component changes reduce relatively rapidly to a relatively low value approaching zero, or even a negative value.

9. A method according to claim 8, wherein the respective horizontal components of the selected boundary lie consecutively.

10. A method according to claim 8, wherein the respective horizontal components of the selected boundary overlap one another.

11. An apparatus for carrying out the method of claim 1, which apparatus comprises: (a) monitoring means arranged to view along the direction of the reference line a predetermined field adjacent and enveloping the reference line, thereby to monitor the flight of the ball as it passes through that predetermined field adjacent the reference line and to produce electronically a representation of at least one of the upper and lower portions of the ball as the ball continues through said field; (b) scanning means arranged to scan a selected one of the boundaries (preferably the uppermost boundary) of that representation progressively in the direction of the ball flight, and at successive spaced positions along the ball flight path to determine the angle of declination of the flight path; and (c) comparing means arranged to compare successive values of that angle and to provide a signal noting the ball position relative to the reference line at which the angle reduces relatively rapidly to a relatively low value approaching zero, or even a negative value.

12. An apparatus according to claim 11, wherein the scanning means is arranged instead to determine for respective sections of the selected boundary successive changes in the horizontal component of the respective boundary sections for equal changes in the corresponding vertical component of the respective boundary sections, and the comparing means is arranged to provide a signal noting the ball position relative to the reference line at which the horizontal component changes increase relatively rapidly to a relatively high value.

13. An apparatus according to claim 12, wherein the respective vertical components of the selected boundary sections lie consecutively.

14. An apparatus according to claim 12, wherein the respective vertical components of the selected boundary sections overlap one another.

15. An apparatus according to claim 11, wherein the scanning means is arranged instead to determine for respective sections of the selected boundary successive changes in the vertical component of the selected boundary sections for equal changes in the corresponding horizontal component of the respective boundary sections, and the comparing means is arranged to provide a signal noting the ball position relative to the reference line at which the vertical component changes reduce relatively rapidly to a relatively low value approaching zero, or even a negative value.

16. An apparatus according to claim 15, wherein the respective horizontal components of the selected boundary sections lie consecutively.

17. An apparatus according to claim 15, wherein the respective horizontal components of the selected boundary sections overlap one another.

18. Apparatus according to any one of the claims 11 to 17, wherein the monitoring means comprises means for emitting a single homogeneous collimated beam of radiation through said field, a plurality of radiation sensing means for sensing the respective radiations exiting from respective individual areas of said field and providing in response thereto respective electrical output signals which vary in response to the radiation incident upon them, and electrical storage and display means for storing and displaying the respective output signals of the radiation sensing means thereby to form said representation.

19. A method substantially as hereinbefore described with reference and as illustrated by the accompanying diagrammatic drawings.

20. An apparatus substantially as hereinbefore described with reference and as illustrated by the accompanying diagrammatic drawings.

21. A method comprising any novel and inventive combination of steps described in the aforegoing description, other than a combination as claimed in any one of the claims 1 to 10 and 19.

22. An apparatus comprising any novel and inventive combination of features described in the aforegoing description, other than a combination as claimed in any one of the claims 11 to 18 and 20.

Amendments to the claims have been filed as follows 1. A method of determining the position, relative to a reference line, of the first contact of a game ball with a playing surface comprising the following steps: (a) arranging a monitoring means to view along the direction of the reference line a predetermined field adJacent and enveloping the reference line; (b) monitoring the flight of the ball as it passes through that predetermined field adjacent the reference line and producing electronically a representation of at least one of the upper and lower portions of the ball as the ball continues through said field;; (c) scanning a selected one of the boundaries, preferably the uppermost boundary, of that representation progressively in the direction of the ball flight, and at successive spaced positions along the ball flight path determining the angle of declination of the flight path of that selected boundary; and (d) comparing successive values of that angle and noting the ball position relative to the reference line at which the angle reduces relatively rapidly to a relatively low value approaching zero, or even a negative value.

2. A method according to claim 1, wherein the steps of scanning the representation, determining the declination angle at successive positions, comparing the successive angles, and noting said position are arranged to take place after the ball has passed through the monitored field,

3. A method according to claim 1, wherein the steps of scanning the representation, determining the declination angle at successive positions, comparing the successive angles, and noting said position are arranged to take place in real time whilst the ball is passing through the monitored field.

8. A method according to any one of the claims 1 to 4, wherein instead of determining the angle of declination at successive spaced positions on the selected boundary and noting substantial changes in that angle, successive changes in the vertical component of respective sections of the selected boundary for equal changes in the corresponding horizontal component of said boundary sect ions are determined and the successive values of those vertical component changes are compared thereby to note the ball position relative to the reference line at which the vertical component changes reduce relatively rapidly to a relatively low value approaching zero, or even a negative value.

12. An apparatus according to claim 11, wherein thescanning means is arranged instead to determine for respective sections of the selected boundary successive changes in the horizontal component of the respective boundary sections for equal changes in the corresponding vertical component of the respective boundary sections, and the comparing means is arranged to provide a signal noting the ball position relative to the reference line at which the horizontal component changes increase relatively rapidly to a relatively high value.

16. An apparatus according to claim 15, wherein the respective horizontal components of the selected boundary sections lte consecutively.

23. A method of determining the position, relative to a reference line extending between near and far ends, or the first contact or a game ball with a playing surface carrying said reference line, which method comprises the following steps: (a) rrom said near end of the reference line, directing a single homogeous collimated beam of radiation along said reference line so as to irradiate a predetermined field at said far end of the reference line lying adjacent the reference line, bald radiation beam having a transverse cross section such as will be materially but not wholly interrupted by the passage of the game ball through the beam; (b) at the rar end of the reference line, receiving at closely spaced locetions in said field respective separate elements of said radiation beam;; (c) converting the respective received elements of beam radiation into corresponding electrical signals; (d) storing the respective electrical signals in respective signal storage locations of a storage means for retention temporarily tllereln; (e) energizing a visual display means in dependence upon said stored electrical signals 60 as to display on a visual display screen thereof a representation of the respective radiation elements received at said far end of the reference line, thereby also to represent on the screen the game ball, and/or the flight path of the ball, during or after the passage of the ball through the collimated radiation beam.

24. Apparatus for determining the position, relative to a reference line extend between near and far ends, of the first contact of a game ball with a playing surface carrying said reference line, which apparatus comprises: (a) disposed at said near end of the reference line, s radiation emitting meant r. directing a single homogeous collimated beam of radiation along said reference line so as to irradiate a predetermlned field at said far end of the reference line lying adjacent the reference line, said radiation beam having a transverse cross section such as will be materially but not wholly interrupted by the passage of the game ball through the beam;; (b) disposed at said far end of the reference line, e radiation receiving means for receiving at closely spaced locatione in said field respective radiation elements of said radiation beam; (c) converting means for converting the respective received elements of beam radiation into respective corresponding electrical signals; (d) a signal storage meane for storing the respective electrical signals in respective signal storage locations of the storage means for retention temporarily therein; (e) a visual display means; and (f) means for energising the visual display means in dependence upon said stored electrical signals so as to display on a visual display screen thereof a representation of the respective radiation elements received at said far end of tlle reference line, thereby also to represent on the screen the gatne ball, and/or the flight path of the ball, during or after tho passage of the ball through the collimated radiation beam.