Imaging System Hardware
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25
Dynamic Range and Detection Limit
The dynamic range of an instrument is the range of signal values over which the
instrument can accurately measure change. For the GenePix scanner, the dynamic
range is 10
4
. Dynamic range is determined primarily by the design of the PMT. In
GenePix, the voltage (gain) applied to the PMT can be adjusted to change the
amplification of the signal as electrons pass through the PMT. The optimum
working range is 500-1000V. If the gain is set below this range, the probability that
each impinging photon will be converted to an electron is diminished, and the
dynamic range will be limited on the low end. Above this range, noise begins to
interfere with accurate signal measurement. Both of these cases will limit the
dynamic range.
The dynamic range of an instrument is often considered in conjunction with its
detection limit. The detection limit is defined as the dye concentration for which
the signal-to-noise ratio = 3. Signal-to-noise ratio is calculated as:
(Signal – Background) / Standard Deviation of Background
Although you may be able to see features below this level, the quantitative accuracy
diminishes significantly.
Chapter 4
