Note that typical collection efficiency of scintaillation light is often quite low ~ 40..60%, especially in narrow scintillators. Special effort and geometry required to improve it.To estimate amount of light in the photopek of some gamma line one needs detector with known PDE ( photon detection efficiency) and single photoelectron charge. The calibration is usually obtained by other purely optical methods.
In many cases, one can get a at least rough estimation of photoelectric corresponding to the photopeak as the photopeak width often mostly defined by photolectron statistics and relative sigma of the peak width is ~ 1/N^0.5.
but in addition to Hadi's and Nikolay's answers I would like to add, that in scintillation x-ray/gamma detectors the conversion from the optical signal to electrical signals is done via PMTs or SiPMs.
But according to your 'expertise and skills' I think, you know that already.
So you may in more detail specify what do you mean by 'experimental protocol for measuring the light output';
e.g. how to characterize the optical output of the scintillator in terms of a) spectrum and b) life time (in oder to choose the right 'optical detector' and appropriate electronics) ?
You may find this paper useful: JTM de Hasas and P Dorenbos, "Advances in yield calibration of scintillators", IEEE Trans Nucl Sci, 55(3), 1086-1092, 2008.