We have determined the frequency of the P-3(1)-S-1(0) intercombination
transition of atomic Ca-40 stared in a magneto-optical trap to be nu
= 455 986 240493.95 kHz with an estimated standard uncertainty of 0.43
kHz (delta nu/nu < 10(-12)) using a phase-coherent optical frequency
chain from the Cs atomic clock to the visible. This allows the realiza
tion of the SI-unit meter according to its definition by visible radia
tion with 25-fold reduced uncertainty compared to previous measurement
s.