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CHAPTER 2 : FLUORESCENCE IMAGING SYSTEMS
Sensitivity and linearity
CCD arrays are sensitive to light, temperature, and high-energy radiation.
Dark current from thermal energy, cosmic rays, and the preamplifier
causes system noise that can have a profound effect on instrument
performance. Cooling of the CCD significantly reduces noise levels and
improves both sensitivity and linearity of the system. For example,
active thermal cooling to -50 °C improves the linear response of a CCD
three- to five-fold. Combining charges from adjacent pixels during
acquisition can also enhance sensitivity, although image resolution may
suffer.
Dynamic range
The dynamic range of a CCD is defined as the ratio of the full saturation
charge to the noise level. CCD cameras typically have a dynamic range of
up to 105. An imaging system with a 15 × 15 µm pixel has a 225 µm2
area and a saturation level of about 180 000. If the system noise level is
10, then the dynamic range is the ratio of 180 000:10 or 18 000:1, thus
demonstrating how system noise can limit the dynamic range.
Amersham Pharmacia Biotech imaging systems
Amersham Pharmacia Biotech offers a variety of imaging instrumentation,
including laser scanning and CCD-based systems. A brief description of
each instrument is given in Table 1. For more information, please visit
www.apbiotech.com.
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