Page 287 - Fundamentals of Light Microscopy and Electronic Imaging

270 DIGITAL CCD MICROSCOPY

Binning

Binning is the combining or pooling together of photoelectrons of adjacent pixels on the
CCD to form electronic superpixels (Fig. 14-9). The pooling of photoelectrons occurs in
the serial register of the CCD during readout. A superpixel that is 2 2 pixels contains
the combined photoelectron content of 4 physical pixels, but is processed by the camera
and amplifier as a single pixel. Binning reduces spatial resolution, but it offers the fol-
lowing advantages:

• Faster acquisition of image sequences (if the rate of acquisition is limited by cam-
era processing speed)
• Smaller size of image files on the computer
• Shorter exposure time to obtain the same image brightness (a major benefit for live
cell imaging)
• Improved S/N ratio for the same exposure time

Gain

Increasing the electronic gain reduces the number of photoelectrons that are assigned
per gray level, allowing a given signal to fill a larger range of gray levels (Fig. 14-10).
For example, for a gain setting of 10 electrons per gray level, 1000 electrons corre-
sponds to 100 gray levels. If the gain is increased 4-fold, there are now 2.5 electrons per
gray level, and 4000 gray levels are obtained. Note the difference between gain adjust-
ment with a digital CCD camera and gain adjustment using a PMT or a vidicon tube,

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1 × 1 Binning 2 × 2 Binning

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3 × 3 Binning

Figure 14-9
Binning. Photoelectrons contained in the unit pixel elements of the CCD are combined
during CCD readout to form electronic superpixels. Binning is set by the user in software and
is controlled by the timing pulses that drive the pixel parcels during the time of readout in the
serial register on the CCD. Since photoelectrons are pooled, camera exposures can be
shortened in proportion to the number of pixels included in a superpixel. Binning also
reduces the image file size on the computer and allows more rapid frame rates, but spatial
resolution is reduced compared to that available in an unbinned image. (Sketch from Roper
Scientific, Inc. with permission)