All experiments observing showers light use telescopes equipped with
pixellised photodetectors. Monte-Carlo (MC) simulations of the apparatus
operation in various situations (background light, shower energy, proximity of
tracks...) are mandatory, but never enter into detector details like pulse
shape, dead-time, or charge space effects which are finally responsible for the
data quality. An apparatus where each pixel receives light from individual 370
nm UV LEDs through silica fibers is being built. The LEDs receive voltage
through DACs, which get their input (which pixel, at what time, which
amplitude) from a shower plus noise generator code. The typical time constant
of a shower being one /mus (300 m for light), the pulses are one /mus wide.
This is rather long compared to the intrinsic time constant (around 10 ns) of
the light detectors, hence, these see "constant light" changing every /mus.
This is where important loading effects which are not included in MC code can
be observed. The fibers illuminate the pixels through a diffuser, and each
fiber illuminates only one pixel. The number of equipped pixels is such that it
englobes a full shower (much less than the full focal surface). Finally, this
equipment can be used also to calibrate the pixels