  A pair of compact Silicon-Tungsten calorimeters was operated in the OPAL
experiment at LEP to measure the integrated luminosity from detection of Bhabha
electrons scattered at angles between 25 and 58 mrad from the beam line. In the
eight years from 1993 to 2000 the detector worked first at the Z mass peak and
then at center of mass energies up to 209 GeV. The fine radial and longitudinal
segmentation (2.5mm x 1X0) allowed the radial position of electron and photon
showers to be measured with a resolution of 130-170 microns and a residual
radial bias as small as 7 microns. Reducing the bias in the definition of the
inner acceptance radius was the key element in obtaining an experimental
systematic error on the integrated luminosity of only 3.4 10^-4. The
performance of the detector at both LEP-I and LEP-II is reviewed. Energy
resolution, sensitivity to overlapping electromagnetic showers and sensitivity
to minimum ionizing particles are discussed.
