C.8.26.4.1.2 Foveal Sensitivity

For Foveal Sensitivity (0024,0087), the terminology commonly used for perimetric measurements is employed, with reference to visual thresholds designated by a decibel (dB) scale. The decibel scale for conventional perimetry is based on the maximum stimulus luminance that can be superimposed on a uniform background, and it is given a value of 0 dB of sensitivity. Sensitivity is the inverse of threshold (sensitivity = 1/threshold) and vice versa. In other disciplines, visual threshold is usually defined by the probability of seeing a difference in luminance, brightness, hue or saturation, or a difference in some other attribute exhibited by a given set of stimuli. As such, a threshold is usually described in terms of the minimum amount of luminance, contrast, hue or other attribute that can be minimally detected. However, many sources that refer to perimetry have described threshold in terms of decibel (dB) values rather than attributing the dB measures to sensitivity. It would be advisable to check each individual device to determine how dB is defined.