In this paper, we analyze the mechanism of the probe doped layer measurement to study the excitation efficiency distribution across the phosphor in thin film electroluminescent devices. The characteristics of excitation efficiency and its distributionacross the phosphor are measured in low voltage driven thin film electroluminescent devices fabricated in our laboratory. The experimental results prove that the excitation efficiency across the phosphor of the device is not homogeneous, and its variation depends on the applied voltage. We infer that the low-voltage-driven thin film electroluminescent mechanism is attributed to inhomgeneous distribution of electric field in the phosphor.