Definition: Adjective: (1) ~ exposure, ~ heat: the lowest level of effect or influence which causes a change or phenomenon e.g. value of the exposure which causes a just-detectable change in an emulsion; ~ contrast: that difference in density which is just detectable in normal viewing conditions. (2) ~ level, ~ value: at which a change is allowed or an occurence counted e.g. all pixels values up to threshold value of 128 are set to black, all above are set to white, to give an image with only black or white areas. Noun: (3) Level of response to an effect or influence which just measurably higher than that which produces no response. * For detectors e.g. photo-detectors: the signal level at which the probability of detection is just greater than 50%.

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