Measurement of instantaneous velocity of swimmers or runners is essential for the player to understand the relation between his motion and the resultant performance.
　Feeding back in real time helpful information extracted from velocity data would enable us to develop new methods of training or new skills of play.
　This report describes how to measure the movement of the object optically and how to feedback information to the player. Using a polygon mirror, an optical beam is scanned in the measurement field with a frequency of 360 Hz.
　The player wears a cap on which a special optical reflecting sheet is stuck, which reflects light back exactly to the light source when the beam passes the object in each scan.
　Determination of the time of reflection as exactly as possible has been done to calculate the instantaneous position of the object, using a correlation method and an interpolation technique.
　In the air, measurement noises were proved to be 0.09 mm and 0.56 mm with the measurement distances of 10 m and 20 m, respectively.
　Under still water the comparable accuracy to the one in the air was obtained. Under bubbling whitish water measurement accuracy deteriorated with the increase of depth, but accuracy of 0.9 mm for 10 cm depth and 10 m of measurement distance was obtained.

DESCENTE SPORTS SCIENCE Vol.12/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE