Dark current noise is caused by the accumulation of stray electrons in pixels not exposed to light, and reveals itself as random noise in shadow or dark areas of images. The phenomenon is cumulative over time and so becomes more noticeable when exposure periods are very long. Sensors exposed to low-light subjects over periods of hours, such as those used by some astronomers, may therefore be supported by specialized electronic circuits designed to reduce the problem. Many of the better digital cameras utilize sophisticated noise reduction techniques not available in cheap consumer models.
Accumulated dark current noise may, at least in principle, be cleared prior to capturing an image or subtracted from image data at a later stage. The latter technique relies upon using the same exposure settings to record an image while blocking all light from reaching the sensor. The dark current noise, although random in nature, may then be more easily identified and subtracted from the image data. Various physical sources for dark current noise can be identified, including quantum effects and raised sensor temperature. The contributions of all noise sources are of course amplified by selecting a high ISO setting.