  Scanning electron microscopy is a powerful tool for nanoscale imaging of
organic and inorganic materials. An important metric for characterizing the
limits of performance of these microscopes is the Detective Quantum Efficiency
(DQE), which measures the fraction of emitted secondary electrons (SEs) that
are detected by the SE detector. However, common techniques for measuring DQE
approximate the SE emission process to be Poisson distributed, which can lead
to incorrect DQE values. In this paper, we introduce a technique for measuring
DQE in which we directly count the mean number of secondary electrons detected
from a sample using image histograms. This technique does not assume Poisson
distribution of SEs and makes it possible to accurately measure DQE for a wider
range of imaging conditions. As a demonstration of our technique, we map the
variation of DQE as a function of working distance in the microscope.
