  Photon detectors like the ALICE Calorimeters are vulnerable to both optical
and electrical crosstalk between detector channels belonging to the same
particle event.
  In order to quantify the crosstalk generated by a single channel on its
neighbours, the standard method used so far are calibrated electrical pulses in
order to quantify the crosstalk between individual frontend preamplifier
channels.
  However due to grounding imperfections, cross-couplings via bias voltage
supplies and imperfect electromagnetic shielding, this method introduces biases
and is not precise.
  A new approach is the use of emulated scintillation light over optical fibre
bundles in order to effectively generate scintillation light in clustered
channels.
  This method is based on generation of quasi time-coherent light with similar
time profiles as scintillation light.
  Theoretically, and based on linear superposition, pulsing a single channel
with calibrated light would be sufficient to measure the crosstalk in the
surrounding channels.
  However, non-linear effects in real detector frontends with energy
measurements over cluster sizes like 3x3 can add up to significant errors.
  Non-linearity is for example due to preamplifier supply voltage drop in the
distribution and GND return lines, in particular if power is provided over long
cables.
  This can generate locally reduced or increased signal amplitudes depending on
cumulative signal intensity.
  The presented octal light ALED~(Analogue LED) pulser prototype emulates
scintillation light with individually tunable light amplitudes in 8 channels.
  It can be used for pulsing single channels, or for clustered channels with a
predefined amplitude profile.
  Unbiased single channel crosstalk and non-linear side-effects can be
determined for single and multichannel situations.
