Practical Electron Microscopy and Database

The heart of a digital imaging camera is a detector (also called electronic imaging sensor), e.g. CCD, acting as a transducer of emanating light to a measurable electrical signal. The signal of CCD’s output represents the spatial distribution of light intensity falling onto the imaging sensor. The CCD devices can be categorized by full-frame transfer and interline transfer depending on the readout mechanism. Charge-couple device (CCD) has the advantage of a good electron-to-counts conversion efficiency, good spatial resolution, and often high dynamic range (≥16 bit).

The electrons generated by photons during the exposure time are collected in the wells in the sensors of CCD camera. The maximum number of electrons that a pixel can hold is determined by the full-well capacity and plays a dominant role in characterizing the CCD’s dynamic range. If the number of generated charges exceeds full-well capacity of a pixel during the exposure time, the extra charges will flow to the neighbor cells, meaning if these wells are saturated the extra charges will flow to the adjacent pixels. This process is called smearing or blooming, which induces fully saturated streaks on the image, destroying pixel information. For most CCD, the full-well capacity of active pixels is the same as the maximum charge transfer capacity of corresponding storage registers.

Note that the attenuation in the CCD camera also limits the phase contrast transfer function in TEMs.