A theory of visibility on an intensity-modulated display is developed. From it is derived a mathematical formula for visibility on a PPI display, and this formula is confirmed by experimental investigations. It is shown that, under favorable conditions, received signals whose power is 15 db below noise level can be observed. Replacement of a linear detector in the receiver by a square-law detector will, under some conditions, produce a further improvement of 3 db. In the visibility formula all the system variables have been grouped into four parameters, and thus it has been possible to provide nomograms enabling the rapid calculation of the minimum visible signal under any set of conditions.