  The light output produced by light ions (Z<=4) in CsI(Tl) crystals is studied
over a wide range of detected energies (E<=300 MeV). Energy-light calibration
data sets are obtained with the 10 cm crystals in the recently upgraded
High-Resolution Array (HiRA10). We use proton recoil data from 40,48Ca + CH2 at
28 MeV/u, 56.6 MeV/u, 39 MeV/u and 139.8 MeV/u and data from a dedicated
experiment with direct low-energy beams. We also use the punch through points
of p, d, and t particles from 40,48Ca + 58,64Ni, 112,124Sn collisions reactions
at 139.8 MeV/u. Non-linearities, arising in particular from Tl doping and light
collection efficiency in the CsI crystals, are found to significantly affect
the light output and therefore the calibration of the detector response for
light charged particles, especially the hydrogen isotopes. A new empirical
parametrization of the hydrogen light output, L(E,Z=1,A), is proposed to
account for the observed effects. Results are found to be consistent for all 48
CsI(Tl) crystals in a cluster of 12 HiRA10 telescopes.
