Readout Noise Readout noise is specified both for the CCD sensor and the total system. Noise can be thought of in two ways. First, there is not perfect repeatability each time charge is dumped out of the CCD and digitized. Conversions of the same pixel with the same charge will not always yield exactly the same result from the A/D. The second aspect of noise is the injection of unwanted random signals by the sensor and electronics which ends up getting digitized along with the pixel charge. In addition, every analog to digital conversion circuit will show a distribution or spread about an ideal conversion value. In either case the result is a certain "uncertainty" which is referred to as noise, specified in electrons (e-). For example, a good noise figure for the Kodak KAF-0402e and KAF-1602e is 13e- typical, while a good number for the entire system is 15e- typical. We have measured systems as low as 9e- total noise and routinely see 10-14 e- performance from our scientific systems using these sensors. If we see a system figure of 13 electrons and assume that the sensor is 11 electrons, this implies that the noise in the rest of the electronics is 7 electrons. This is just an estimate, because we do not know for sure what the sensor noise is. The point here is that in this example the sensor is the major contributor to noise. The lower the noise figure of a camera system, the better its ability to harvest a useful signal from a noisy background in low light conditions, assuming of course that it is not significantly sky limited. Also, games can be played with the gain of the system to reduce electonics noise or CCD noise. For example, if a camera system produces 2 counts of electronics noise regardless of the CCD noise, and the gain of the system is 10 electrons/count, then the total noise will be 20 electrons (2*10). However, if the gain is reduced to 2 electrons/count, but now the CCD noise dominates, the total noise could end up at 10 electrons with 5 counts of measured noise. Note that our electronics noise floor is 2 counts, so we no longer are limited by our electronics, but by the CCD! Most applications are not read noise limited, but are limited more by background or dark noise.