  This paper reports on the behaviour of Silicon Photomultiplier (SiPM)
detectors under continuous light. Usually, the bias circuit of a SiPM has a
resistor connected in series to it, which protects the sensor from drawing too
high current. This resistor introduces a voltage drop when a SiPM draws a
steady current, when illuminated by constant light. This reduces the actual
SiPM bias and then its sensitivity to light. As a matter of fact, this effect
changes all relevant SiPM features, both electrical (i.e. breakdown voltage,
gain, pulse amplitude, dark count rate and optical crosstalk) and optical (i.e.
photon detection efficiency). To correctly operate such devices, it is then
fundamental to calibrate them under various illumination levels.
  In this work, we focus on the large area ($\sim$1~cm$^{2}$) hexagonal SiPM
S10943-2832(X) produced by Hamamatsu HPK for the camera of a gamma-ray
telescope with 4 m-diameter mirror, called the SST-1M. We characterize this
device under light rates raging from 3~MHz up to 5~GHz of photons per sensor at
room temperature (T = 25 $^{\circ}$C). From these studies, a model is developed
in order to derive the parameters needed to correct for the voltage drop
effect. This model can be applied for instance in the analysis of the data
acquired by the camera to correct for the effect. The experimental results are
also compared with a toy Monte Carlo simulation and finally, a solution is
proposed to compensate for the voltage drop.
