1. Introduction 1

1.1 The ‘band gap’ in silicon 2

2. Principles of CCD detector operation 3

2.1 Direct detection 3

2.2 Electron energy conversion into light 4

2.3 Optical coupling: lens or fibre optics? 6

2.4 Readout speed and comparison with film 8

3. Practical considerations for electron microscopic applications 9

3.1 Sources of noise 9

3.1.1 Dark current noise 9

3.1.2 Readout noise 9

3.1.3 Spurious events due to X-rays or cosmic rays 10

3.2 Efficiency of detection 11

3.3 Spatial resolution and modulation transfer function 12

3.4 Interface to electron microscope 14

3.5 Electron diffraction applications 15

4. Prospects for high-resolution imaging with CCD detectors 18

5. Alternative technologies for electronic detection 23

5.1 Image plates 23

5.2 Hybrid pixel detectors 24

6. References 26

During the past decade charge-coupled device (CCD) detectors have increasingly become the preferred choice of medium for recording data in the electron microscope. The CCD detector itself can be likened to a new type of television camera with superior properties, which makes it an ideal detector for recording very low exposure images. The success of CCD detectors for electron microscopy, however, also relies on a number of other factors, which include its fast response, low noise electronics, the ease of interfacing them to the electron microscope, and the improvements in computing that have made possible the storage and processing of large images.

CCD detectors have already begun to be routinely used in a number of important biological applications such as tomography of cellular organelles (reviewed by Baumeister, 1999), where the resolution requirements are relatively modest. However, in most high- resolution microscopic applications, especially where the goal of the microscopy is to obtain structural information at near-atomic resolution, photographic film has continued to remain the medium of choice. With the increasing interest and demand for high-throughput structure determination of important macromolecular assemblies, it is clearly important to have tools for electronic data collection that bypass the slow and tedious process of processing images recorded on photographic film.

In this review, we present an analysis of the potential of CCD-based detectors to fully replace photographic film for high-resolution electron crystallographic applications.