[en] This paper has tried to show how an internal noise source (darklight and threshold jitter) would tend to explain experimental data concerning the visual detection of noise-limited signal. The interesting conclusions can be drawn that the internal noise sets the upper limit to the utility of data processing techniques designed to reduce image noise. Moreover, there should be instances where contrast enhancement techniques may be far more useful to the human observer than corresponding reductions in noise amplitude, especially at high count rates. Then too, the limitations imposed on the human observer by an internal noise source, may point towards the need for additional methods (e.g. computer/microdensitometer) of interpretating images of high photon density so that the highest possible signal to noise ratio might be obtained