.

Dynamic Range

 

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 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, 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.