A compact fine-grained calorimeter for luminosity measurement at a linear collider

Based on a paper published in 2019 by the FCAL Collaboration, this talk is giving an update of the Collaborations effort to design prototype of highly compact calorimeter to instrument the very forward region of a detector at future $e^+e^-$ colliders. A luminometer prototype, based on sub-millimeter thick detector planes, is tested with an electron-beam of energy 1-5 GeV. The effective Moliere radius of the prototype comprising eight detector planes was measured to be (8.1 +/- 0.1 (stat.) +/- 0.3 (syst.))mm, and the result is well reproduced by the Monte Carlo simulation.