The radiological diagnostic process is composed of the three major phases, psychophysical, psychological and nosological. An apparent improvement in image quality in the psychophysical phase does not necessarily imply an increased diagnostic performance. This may be true for the general diagnostic processes, but may not for the caries diagnosis, because psychophysical phase is of most significance in such special and relatively simplified task. In this article the processes to correlate perception to approximal caries diagnosis are reviewed using the Perceptibility Curve (PC) tests and Receiver Operating Characteristic (ROC) curve tests. The PC test was developed to represent the psychophysical property of the radiographic imaging system. Since physical properties are shown to be closely correlated with psychophysical properties, it is possible to theoretically calculate psychophysical properties of the radiographic systems from their physical properties. In a similar manner, observers' low contrast detectability in the psychophysical phase can be predicted from some physical parameters of the radiographic system. Observers' low contrast detectability is also correlated with the diagnostic performance obtained from ROC curve in the task of approximal caries diagnosis. Thus, considerably high correlation between psychophysical properties and diagnostic accuracy indicates close relationship between perception and approximal caries diagnosis. It implies that an improvement in the physical image quality leads to increased diagnostic performance to some extent in the approximal caries diagnosis.
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