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Stanley Smith Stevens was an American experimental psychologist perhaps best known in the world of color science for introducing the psychophysical power law and for collecting data on brightness perception as a function of adaptation. He was even more well known in the general field of experimental psychology and psychophysics for many accomplishments in acoustical psychophysics, his important publications, and founding of the Psycho-Acoustic Laboratory at Harvard University.
Among Stevens’ written contributions to the field are the extensive, and immensely praised, Handbook of Experimental Psychology [1], that provides much information remaining useful more than half a century later and the book Psychophysics [2], that was published posthumously after final editing by his wife. It, too, remains a useful reference on the subject. Among the topics described is the hierarchy of mathematical scales (nominal, ordinal, interval, and ratio) in psychophysics, something else developed by Stevens.
Stevens was an undergraduate student at the University of Utah and then transferred to Stanford University where he completed his degree in an undetermined field since the variety of courses he took was so extensive. He then was accepted at the Harvard Medical School, but chose to enroll in Harvard’s School of Education to avoid the medical school’s $50 fee and organic chemistry requirement and still have access to Harvard’s resources. At Harvard, he was transformed by a course on perception taught by E. G. Boring, became Boring’s unpaid research assistant, and earned a Ph.D. in Philosophy by the end of his second year there. After some more studies in physiology and physics, he was appointed an instructor in the Psychology Department, where he remained until his death. In 1962, Harvard University agreed to appoint Stevens as a professor of Psychophysics in honor of the field founded by G. T. Fechner.
One of Stevens’ most notable papers was titled “To honor Fechner and repeal his law,” published in Science in 1961 [3]. In it, Stevens summarized psychophysical scaling data for a variety of perceptual stimuli (e.g., brightness of light, loudness of sound, hotness of heat, pain of electric shock) and illustrated that they all could not possibly follow the logarithmic relationship between stimulus and perception predicted by Fechner’s Law. Instead, all of his data could be well described by power functions with various exponents, which depend on the type of perception being scaled. This new relationship is known as the psychophysical power law or Stevens’ power law. One technique used to establish the power law that Stevens developed is known as cross-modal matching. In these experiments, observers match perceived magnitude of one stimulus (e.g., brightness of light) with another (e.g., loudness of sound).
75.1 Stevens Effect
Stevens’ work on brightness is well known in color science and led to the description of a color appearance phenomenon as the Stevens Effect. This work, published in 1963 by Stevens (no relation) and Stevens, examined the effects of light and dark adaptation on the perception of brightness [4]. Scales of brightness were obtained using magnitude estimations (Steven’s hallmark) and showed that the contrast of brightnesses increases with increasing adapting luminance (exponent in the power increases). Therefore, as Stevens showed, when the level of lighting increases, dark colors look darker and light colors lighter.