Another technical problem prevents us from seeing all the way into the darkest regions of the scene. The sensor is sensitive to a wide range of radiation, not just the visible kind. This other radiation is all around us and is also emitted by components of the sensor itself. In the illustration the extra radiation is represented by condensation and leaking pipes. Each time we measure how full the bucket is, part of the sampled data is extra. Its mostly random, and we dont know how much of our measurement is this extra radiation. To compensate, the chip has rows of sensors with opaque covers. It measures the signal in these dark buckets to determine the average and maximum amount of stray radiation during the capture. This dark current measurement defines a level beyond which we cant be sure of our data. In the vocabulary of signal processing we call this the noise floor.