The design, construction and performance of a silicon-tungsten electromagnetic calorimeter built to achieve an experimental luminosity measurement precision at LEP≤0.1% is described. The detector uses homogeneous construction to give full azimuthal acceptance for Bhabha scattering over polar angles from 24 to 58 mrad. Detailed information concerning shower development is obtained from zero-suppressed readout of the 12288 pads of the detector. Trigger decisions are generated from a rapid flash-ADC system using programmable gate arrays.