  On October $10^{\rm th}$ 2018 started the commissioning of the first Large
Size Telescope (LST) prototype at the Cherenkov Telescope Array (CTA) northern
site at the Observatorio del Roque de los Muchachos, Canary Island of La Palma
(Spain). For a precise event energy reconstruction, an LST camera requires a
uniform and constant calibration over a large dynamic range, up to $10^{4}$
photo-electrons (p.e.), for each camera photomultiplier tube (PMT). This paper
describes the performance of the LST-1 camera calibration system (named
CaliBox) in the first commissioning period and provides preliminary results of
measurements of the light flat field
