Next to the quantum efficiency, resolution (number of bits) and noise figure, this is perhaps the most important CCD specification. Dark count refers to that property of all CCD sensors to generate charge in each pixel on its own with time and depending on the temperature. The lower the temperature of the sensor, the lower the dark count.
Dark count is usually expressed in electrons per unit of time at a given temperature. It can also be expressed as a current per sensor area at a given temperature. The sole reason that CCDs are cooled is to reduce the dark count effect. For example, a Kodak KAF-0402e and KAF-1602e sensor is specified to have a dark count of 50 electrons per second at 25° C. Taking a one-minute exposure at this temperature would generate 3000 electrons of thermally induced charge!
Although the dark count can be subtracted, the dark noise cannot. The dark noise is approximately the square root of the dark count. In this example, the dark noise is about 55 electrons. This could easily degrade any low-light image you attempted to capture over this period of time. These sensors specify a "doubling temperature" of 5-6° C. Cooling the CCD to +10° C would reduce the dark count in a one-minute exposure from 3000 to about 530 electrons. At -5° C this number reduces to about 97. At -20° C it lowers to 17 electrons. These calculations assume a 6° doubling temperature.
It is easy to see why it is necessary to cool the sensor during use. Although corrections can be made for dark contribution in the finished image by subtraction, the error in this process increases with increasing dark count, given some degree of temperature instability and dark noise present in every system. The bottom line is to minimize the amount of dark count in the imaging system.
Not all CCDs exhibit the same dark count characteristics. For example, the Kodak KAF-1301e and KAF-1001e can exhibit 10 to 20 times higher dark count than those cited in the above example. Some back-illuminated sensors generate over 1 electron/sec. dark count at -40° C. These sensors require greater cooling to achieve the same dark counts as those discussed in the previous paragraph. Having lower dark count is always desirable, but there comes a point when other factors such as shot noise or S/N supercede dark count considerations. The design of a camera can include the extreme measure of LN2 cooling to virtually eliminate dark count, but the cost and complexity of this kind of system may not be warranted in most situations. Using hard vacuum with TEC cooling can also greatly enhance cooling performance, but these systems require periodic pumping to restore lost vacuum and can get quite expensive. The key is advancing the appropriate system design and cost to solve the problem at hand without too much excess. Apogee Alta® series systems employ a sealed soft-vacuum chamber with Argon back-fill as a low-maintenance compromise in cost and performance.