  Low dose electron imaging applications such as electron cyro-microscopy are
now benefitting from the improved performance and flexibility of recently
introduced electron imaging detectors in which electrons are directly incident
on backthinned CMOS sensors. There are currently three commercially available
detectors of this type: the Direct Electron DE 20, the FEI Falcon II and the
Gatan K2 Summit. These have different characteristics and so it is important to
compare their imaging properties carefully with a view to optimising how each
is used. Results at 300 keV for both the modulation transfer function (MTF) and
detective quantum efficiency (DQE) are presented. Of these, the DQE is the most
important in the study of radiation sensitive samples where detector
performance is crucial. We find that all three detectors have a better DQE than
film. The K2 Summit has the best DQE at low spatial frequencies but with
increasing spatial frequency its DQE falls below that of the Falcon II.
