Aph162.DigitalImaging - A brief primer on digital imaging...

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A brief primer on digital imaging (by DW) : 4 sections: 1) basics of CCDs. 2) glossary of terms. 3) how to image. 4) software help The basics of CCDs (charge coupled devices): In Aph162, we use CCDs to capture an image. How does a CCD work? We can think of a CCD as an array of pixels, each of which can hold a set number of electrons; the electrons are generated when a photon arrives at the pixel. Since a CCD is an array of pixels, we can form an image. That being said, there are a few basic parameters we need to take into consideration when forming our image. First of all is noise: the detectors in this lab are not single-photon imaging devices since the inherent (statistical) noise of the system limits the signal to noise ratio. Additional sources of noise arise when we convert this inherently analog signal to digital form (read noise). However, if we integrate over long times (or increase the brightness of our samples), we can usually reduce statistical noise, but not read noise, since the A to D conversion takes place after we have formed our image. After exposing an image on the CCD for some time, the pixels are traditionally (as of late 1990’s, at least) shifted in one direction to a register on the edge of the chip, before being read out one pixel at a time (see figure). This should give you an idea of what we need to do in order to form a nice image. Caption: An image is exposed onto the CCD. After exposure, all the pixels are shifted in the direction of
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Aph162.DigitalImaging - A brief primer on digital imaging...

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