  A light-emitting diode (LED) pulser for testing the low-rate response of a
photomultiplier tube (PMT) to scintillator-like pulses has been designed,
developed, and implemented. This pulser is intended to simulate 80 ns full
width at half maximum photon pulses over the dynamic range of the PMT, in order
to precisely determine PMT linearity. This particular design has the advantage
that, unlike many LED test rigs, it does not require the use of multiple
calibrated LEDs, making it insensitive to LED gain drifts. Instead, a
finite-difference measurement is made using two LEDs which need not be
calibrated with respect to one another. These measurements give a better than
1% mapping of the response function, allowing for the testing and development
of particularly linear PMT bases.
