Exposure Time
- Practical Electron Microscopy and Database -
- An Online Book -

http://www.globalsino.com/EM/  



 

This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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An electronic optical imaging sensor consists of photonsensitive regions which are called pixels. Each pixel converts photons into electrons according to its incident light intensity, accumulates the generated electrons (charge collection) according to its well capacity during the exposure time, and transfers the collected electrons via a charge-to-voltage converter according to its bit depth and readout mechanism. Each of these processes, charge generation, collection, and transfer is also accompanied with various types of noise, which affects the quality of formed image. For instance, a dark current noise is caused by thermal effects during the exposure time and generates nonphotonic charges in the wells. The readout noise is added to the readout signal simultaneously while the collected charge is transferred from pixel to the on-chip amplifier and then, is converted into an analog voltage.

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.

In high-magnification TEM imaging, the electron beam must be bright enough to keep exposure times less than 10 - 15 s to minimize the loss in spatial resolution from specimen drift. In low-magnification SEM imaging, the exposure time can be longer since there is much less resolution loss caused by specimen drift.

 

 

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