  A Large Size air Cherenkov Telescope (LST) prototype, proposed for the
Cherenkov Telescope Array (CTA), is under construction at the Canary Island of
La Palma (Spain) this year. The LST camera, which comprises an array of about
500 photomultipliers (PMTs), requires a precise and regular calibration over a
large dynamic range, up to $10^3$ photo-electrons (pe's), for each PMT. We
present a system built to provide the optical calibration of the camera
consisting of a pulsed laser (355 nm wavelength, 400 ps pulse width), a set of
filters to guarantee a large dynamic range of photons on the sensors, and a
diffusing sphere to uniformly spread the laser light, with flat fielding within
3%, over the camera focal plane 28 m away. The prototype of the system
developed at INFN is hermetically closed and filled with dry air to make the
system completely isolated from the external environment. In the paper we
present the results of the tests for the evaluation of the photon density at
the camera plane, the system isolation from the environment, and the shape of
the signal as detected by the PMTs. The description of the communication of the
system with the rest of detector is also given.
