Using 0.5 ps pulses of 5.9 eV light to excite electron-hole concentrations varied up to 2x1020 e-h/cm3 corresponding to energy deposition within electron tracks, we measure dipole-dipole quenching rate constants K2 in SrI2 and CsI. We previously reported determination of K2 directly from the time dependence of quenched STE luminescence in CsI. The nonlinear quenching rate decreases rapidly within a few tens of picoseconds as the host excitation density drops below the Förster threshold. In the present work, we measure the dependence of integrated light yield on excitation density in the activated scintillators SrI2:Eu2+ and CsI:Tl+. The “z-scan” method of yield vs. irradiance is applicable to a wider range of materials, e.g. when the quenching population is not the main light-emitting population. Furthermore, because of using an integrating sphere and photomultiplier for light detection, the signal-to-noise is substantially better than the time-resolved method using a streak camera. As a result, both 2nd and 3rd orders of quenching (dipole-dipole and Auger) can be distinguished. Detailed comparison of SrI2 and CsI is of fundamental importance to help understand why SrI2 achieves substantially better proportionality than CsI in scintillator applications. The laser measurements, in contrast to scintillation, allow evaluating the rate constants of nonlinear quenching in a population which has small enough spatial gradient to suppress the effect of carrier diffusion.