| 摘要 |
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.
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