This is a precisely timed set of images of a bar code. The range was adjusted so that the lowest pixel value is black and the highest pixel value is white. The SNR visibly improves as the exposure time increases. The images are not calibrated, so the dark signal (and noise) increases linearly with exposure time. The decreasingly visible noise would seem to be read noise.

  The depicted relationship between signal and noise only applies with the test signal. Other subjects would require other exposure times to achieve comparable results.