- Dynamic range refers to intrascene performance; that is, 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
linear full well (electrons) / read noise
- 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, cooled camera, the read noise
(the noise associated with a single readout event) is therefore minimized
to yield the largest dynamic range possible.
- As a specific example, consider a Kodak 1400 CCD, which has a full-well
capacity of 45,000 electrons. At a typical readout rate of 1 MHz, the read
noise is 11e-. The dynamic range of this chip is therefore 45,000:11, or 4,091:1.
In order to take full advantage of this dynamic range, cameras incorporating
Kodak 1400 chips usually utilize a 12-bit A/D converter (4096 gray levels).
It is important that the camera's 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 with a larger full well capacity
is required. Full well capacity is related to pixel size. For instance, the
Thomson 7895 has a capacity of 375,000e- and a read noise of 5e- rms at 40
kHz. The dynamic range is thus 75,000:1. In our cameras this is usually coupled
to a 16-bit A/D converter (65,536 gray levels).
- As a general rule, camera cost increases with increasing dynamic range,
so dynamic range requirements should be considered very carefully when selecting
a camera. Note that in an uncooled video camera the read noise is much higher,
and the true dynamic range is usually 8 bits or lower.