the system of the present invention has a first pole 1 and second pole 2 . for illustrative purposes only , first pole 1 is on the left in the accompanying figures and second pole 2 is on the right . retractable string or tape 3 is placed between the tops of said first pole 1 and second pole 2 . pole 1 has attached a first light source 4 that emanates a beam 6 through a first beam splitter 8 with a portion of beam 6 directed to a first mirror 5 . pole 2 has attached a second light source 10 that emanates a beam 12 through a second beam splitter 14 with a portion of beam 12 directed to a second mirror 11 . pole 1 has reflective material 13 on its outer surface . pole 2 has reflective material 7 on its outer surface . triangular region 16 is defined by a first triangular side along the edge of pole 1 , a second triangular side along string 3 from pole 1 to pole 2 , and a hypotenuse extending from the base of pole 1 to the top of pole 2 . triangular region 17 is defined by a first triangular side along the edge of pole 2 , a second triangular side extending from the base of pole 1 to the base of pole 2 , and a hypotenuse extending from the base of pole 1 to the top of pole 2 , such that each of regions 16 and 17 share a common hypotenuse . referring to fig1 , a schematic view of the invention is provided illustrating the relative positions of all of the optical elements as well as the physical poles on which , the optical elements are mounted . in fig1 , the “ l ” pole 1 is placed in the desired position on any playing surface , the “ r ” pole 2 is placed relative to pole 1 on the playing surface at any desired position and separated from pole 1 . a retractable string or tape 3 is stretched between the top or a region near the top of pole 1 and the top or a region near the top of pole 2 . string 3 corresponds to the top edge of the virtual net . poles 1 and 2 may be any height , but are typically the height of standard posts supporting a standard tennis net . according to the american tennis federation , the poles are a height of 3 . 5 feet and the net 3 feet high at the center . string 3 may be white or any other color so as to be visible to the participants using the virtual net . a light source 4 , such as a laser , emits a beam of light that is incident on rotating or oscillating mirror 5 , so that the reflected light beam 6 sweeps across a plane corresponding to a portion of the virtual net . beam 6 is retro - reflected from a material 7 that lines the surface of pole 2 , such as retro - reflecting tape , causing beam 6 to be retro - reflected back along the path of incidence so as to reflect from mirror 5 back towards the light source 4 . partially reflecting mirror 8 , also commonly known as a beam splitter , reflects a portion of the retro - reflected beam 6 onto the photodetector 9 . while beam 6 is incident onto any portion of retro - reflecting material 7 , it produces a relatively steady electronic signal level from photodetector 9 . the area swept across by beam 6 is triangular in shape and corresponds to the lower right triangular portion of the complete rectangular virtual net . when a tennis ball passes through this triangular portion , the light incident on photodetector 9 is interrupted . the electronic signal produced by photodetector 9 is markedly reduced , and this signal change may be detected by standard electronic circuitry to actuate and create an audible or visible signal indicating that the ball has not cleared the virtual net . similarly , a light source 10 emits a beam of light that is incident on rotating or oscillating mirror 11 , so that the reflected light beam 12 sweeps across a plane corresponding to a portion of the virtual net . beam 12 is retro - reflected from a material 13 that lines the surface of pole 1 , causing the beam to be retro - reflected back along the path of incidence , then reflected from mirror 11 back towards the light source 10 . partially reflecting mirror 14 reflects a portion of the retro - reflected beam 12 onto the photodetector 15 . the area or region 16 , swept across by beam 12 is triangular in shape and corresponds to the upper left triangular portion of the complete rectangular virtual net . when a tennis ball passes through triangular region 16 , the light incident on photodetector 15 is interrupted , the electronic signal produced by photodetector 15 is markedly reduced , and this signal change may be detected by standard electronic circuitry to actuate the apparatus and create an audible or visible signal . triangular region 17 , covered by the sweep of beam 12 is the complement of triangular region 16 covered by the sweep of beam 6 , and , together , these two beams completely sweep across a rectangular area , establishing a virtual net under the string 3 . fig2 provides a more explicit diagram of the areas covered by the sweeps of beams 6 and 12 . region 16 is the triangular region covered by the sweep of beam 6 , while region 17 is the triangular region covered by the sweep of beam 12 . referring to fig3 a and 3b , beam splitting mirror 5 ( along with the light source and photodetector contained in pole “ l ”) is described in more detail . fig3 a depicts a partially - reflecting beam splitter 18 consisting of a plate of material ( e . g . glass or plastic ) that is transparent to the wavelength of the light source . the beam from the light source 4 , located in one pole , is incident on the first surface 19 of splitter 18 , which has an anti - reflection ( ar ) coating ; hence no light is reflected from this surface . the beam enters splitter 18 and is transmitted to the second surface 20 , which has a coating that reflects 50 % and transmits 50 % of the light . the reflected portion of the beam comes back out through the first surface and is blocked by a light absorbing baffle 21 , so that it will not produce any spurious signal on the detector . the transmitted portion of the beam then follows the path 6 depicted in fig1 . following retro - reflection from material lining the other pole , the returning beam is incident on mirror 5 , which reflects it to the second surface 20 of the beam splitter 18 , which reflects 50 % of this beam to the photodetector 9 referring to fig3 b , an enhancement of retro - reflected light detection is illustrated . in fig3 b , a structure is provided for splitting off , focusing , and sensing the light returned from the mirror 5 . instead of a partially - reflecting beam splitter , an arc - shaped reflector 22 with a central opening 23 serves the function of permitting the entire narrow incident beam from the light source 4 to pass through the opening 23 . this transmitted beam follows the path depicted in fig1 . following retro - reflection from the material lining the other pole , the returning beam , which has widened significantly in cross section , is reflected from mirror 5 onto arc - shaped reflector 22 , which reflects all of this returning beam , except for the small portion that passes through the opening 23 . due to the concave shape of reflector 22 , the reflected light is converged onto photodetector 9 . it will be apparent to one skilled in the art , in the light of the principles set forth , that many substitutions and refinements will be readily apparent to one skilled in the art that encompass alternative embodiments . the retro - reflecting material may be tape or some other combination of glass or plastic beads that retro - reflect a significant portion of light incident upon the material . since this material is retro - reflecting , it is not critical that the poles 1 and 2 be positioned at exact rotation angles around vertical axis of the poles . it is only necessary that beam 6 strike retro - reflecting material 7 somewhere along the length ( height ) and width of this material on pole 2 , and it is only necessary that beam 12 strike retro - reflecting material 13 somewhere along the length ( height ) and width of this material on pole 1 . the power of the laser beams emitted by light sources 4 and 10 , while low enough to satisfy the bureau of radiological health requirement that the beam is not dangerous to the human eye even when aimed directing into the human eye , is still strong enough to easily register a significant and measurement photo - signal on photo - detectors 9 and 15 . further these beams may scatter significantly from dust and water vapor droplets to be visible to participants when the game is played under low ambient light levels , making it possible to play this game in the evening without additional peripheral lighting . while the invention has been described in its preferred form or embodiment with some degree of particularity , it is understood that this description has been given only by way of example and that numerous changes in the details of construction , fabrication , and use , including the combination and arrangement of parts , may be made without departing from the spirit and scope of the invention .
the present invention is an apparatus to provide a virtual tennis net that can be placed on any convenient surface having two movable poles each having a light source dependent thereon ; at least two scanning mirrors ; at least two retroflectors ; at least two beam splitters ; at least two photo - detectors ; and an audible or visible signal device that is actuated in response to interruptions of light falling on said photo - detectors such that when a the light beam is interrupted by a tennis ball , the beam is momentarily blocked and the signal to the photo - detector is interrupted , providing a signal that is used to trigger a light or sound signal that indicates the ball did not pass over the virtual net .