  In this work, we describe the optical properties of the single photoelectron
(SPE) calibration system designed for NectarCAM, a camera proposed for the
Medium Sized Telescopes (MST) of the Cherenkov Telescope Array (CTA). One of
the goals of the SPE system, as integral part of the NectarCAM camera, consists
in measuring with high accuracy the gain of its photo-detection chain. The SPE
system is based on a white painted screen where light pulses are injected
through a fishtail light guide from a dedicated flasher. The screen - placed 15
mm away from the focal plane - is mounted on an XY motorization that allows
movements over all the camera plane. This allows in-situ measurements of the
SPE spectra via a complete scan of the 1855 photo-multiplier tubes (PMTs) of
NectarCAM. This calibration process will enable the reduction of the systematic
uncertainties on the energy reconstruction of $\gamma$-rays coming from distant
astronomical sources and detected by CTA.
