  Slice energy spread is one of the key parameters in free electron laser
optimizations, but its accurate measurement is not straightforward. Two recent
studies from high energy ($>$100 MeV) photoinjectors at SwissFEL and European
XFEL have reported much higher slice energy spread than expected at their XFEL
working points (200 - 250 pC). In this paper, a new method for measuring slice
energy spread at a lower beam energy ($\sim$20 MeV) is proposed and
demonstrated at the PhotoInjector Test facility at DESY Zeuthen (PITZ), and the
results for 250 pC and 500 pC are much lower than those measured at high energy
injectors.
