  A light injection system using LEDs and optical fibres was designed for the
calibration and monitoring of the photomultiplier array of the SNO+ experiment
at SNOLAB. Large volume, non-segmented, low-background detectors for rare event
physics, such as the multi-purpose SNO+ experiment, need a calibration system
that allow an accurate and regular measurement of the performance parameters of
their photomultiplier arrays, while minimising the risk of radioactivity
ingress. The design implemented for SNO+ uses a set of optical fibres to inject
light pulses from external LEDs into the detector. The design, fabrication and
installation of this light injection system, as well as the first commissioning
tests, are described in this paper. Monte Carlo simulations were compared with
the commissioning test results, confirming that the system meets the
performance requirements.
