Dynamic Range

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 noise (electrons)

and is therefore a dimensionless number. The linear full well is a specific measure of pixel well capacity (see saturation and 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). It is important that the cameras readout and signal-processing electronics be optimized so that low read noise is maintained, otherwise the dynamic range will be compromised.

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. In commercial cameras, this is usually coupled to a 14 or 16-bit A/D converter.