  A prototype of a luminometer, designed for a future $e^{+}e^{-}$ collider
detector, and consisting at present of a four-plane module, was tested in the
CERN PS accelerator T9 beam. The objective of this beam test was to demonstrate
a multi-plane operation, to study the development of the electromagnetic shower
and to compare it with MC simulations. In addition, the effective
$\textit{Moli\`ere radius}$ of this configuration is extracted.

Instrumentation and Detectors;High Energy Physics - Experiment