Dynamic range refers to intrascene performance
(i.e., the ability to quantitatively detect very
dim and very bright parts of a single image). Because
the smallest measurable intensity varies between
applications and experimental conditions, CCD manufacturers
have adopted a definition for specifying dynamic
range that is independent of how the camera is used.
This definition is defined mathematically as:
linear full well (electrons)/read
and is therefore a dimensionless number. The
linear full well
is a specific measure of pixel well capacity (see
blooming). With a high-performance CCD camera,
the read noise (i.e., the noise associated with
a single readout event) is minimized to yield the
largest dynamic range possible.
As a specific example, consider a Sony® ICX285,
which has a full well capacity of 16,000 electrons.
At a typical readout rate of 10 MHz, the read noise
is 4.5 e-. The dynamic range of this CCD is
therefore 16,000:4.5. In order to take full
advantage of this dynamic range, cameras
incorporating Sony® ICX285 devices usually utilize a
12-bit A/D converter (4096 gray levels).
To extend dynamic range beyond the 12 bits given
in the previous example, a camera with a lower read
noise or a CCD or EMCCD with a larger full well
capacity is required. Full well capacity is related
to pixel size. For instance, the e2v CCD97 EMCCD
chip (pixel size 16.0 µm), has a capacity of 200 ke-
and a read noise of 6.5 e- rms at 1.25 MHz for the
non EM port. The dynamic range is thus 30,000:1.