We present PLANET observations of OGLE-1999-BUL-23 , a binary-lens microlensing event towards the Galactic bulge . PLANET observations in the I and V bands cover the event from just before the first caustic crossing until the end of the event . In particular , a densely-sampled second caustic crossing enables us to derive the linear limb-darkening coefficients of the source star ; c _ { V } = 0.786 ^ { +0.080 } _ { -0.078 } and c _ { I } = 0.632 ^ { +0.047 } _ { -0.037 } . Combined analysis of the light curve and the color-magnitude diagram suggests that the source star is a G/K subgiant in the Galactic bulge ( T _ { eff } \simeq 4800 K ) . The resulting linear limb-darkening coefficient of the source is consistent with theoretical predictions , although it is likely that non-linearity of the stellar surface brightness profile complicates the interpretation , especially for the I band . The global light curve fit to the data indicates that the event is due to a binary lens of a mass ratio q \simeq 0.39 and a projected separation d \simeq 2.42 . The lens/source relative proper motion is ( 22.8 \pm 1.5 ) { km } { s } ^ { -1 } { kpc } ^ { -1 } , typical of bulge/bulge or bulge/disk events .