You could do it optically with a quad cell or tetralateral.  For these
optical components check with Hamamatsu or UDT.
Justin D. Mansell                | "If your opponent strikes with fire,
Ginzton Laboratory               |         counter with water."
Stanford, CA 94305               | "To injure an opponent is to injure
tel: 650-725-2284                |          yourself."
fax: 650-723-2666                |              -Morihei Ueshiba
http://www.stanford.edu/~jmansell|