A subject is looking at two disks, one serving as the sample and the other serving as the comparison. Both disks are completely dark. Then, the brightness of the comparison is gradually increased until the subject reports a seeing a difference between the two disks. The value at which the difference is first detected is the absolute threshold for brightness.

Gustav Fechner (1801-1887) considered two thresholds in his analysis of sensation. The first, the absolute threshold, is the lowest intensity at which a stimulus can be detected. Like Gotfried Leibniz (1646-1716) and Johann Herbert (1776-1841) before him, he allowed for negative sensations -- unconscious reactions to stimulus intensities below absolute threshold -- and the possibility that sub-threshold stimuli can cumulate beyond absolute threshold to create a conscious sensation. For Fechner, though, the absolute threshold was of limited usefulness. It was only the lowest level of conscious sensation. His goal was to relate a full range of stimulus intensities to their resultant sensation values. In this regard, he introduced the concept of a differential threshold: the minimal amount of change in a stimulus that can be detected. He assumed the differential threshold for any stimulus within the full range of intensities to be subjectively equal, the equivalent of one jnd.