Normalized number of pixels in the Hα image within a given surface brightness interval (﹩d\,\mathrm{log}\,\,( \Sigma ) =\mathrm{log}\,\,( 1.25) ﹩) vs. the relative surface brightness (scaled by the mean surface brightness in the galaxy ﹩\overline{\Sigma }﹩). The surface brightness plotted starts at minimum values of 30 pc cm⁻⁶ for NGC 6946 and IC 342 and 15 pc cm⁻⁶ for the other galaxies, and it increases by a factor of 1.25 at each successive step. At each interval of surface brightness the number of pixels is counted and normalized by total number of pixels with surface brightnesses greater than the minimum values given above. The solid line is the model prediction for high surface brightness gas based on the typical luminosity function [﹩N( L) \propto L^{-2}﹩] and exponential radial brightness profiles of H II regions. The normalization of this function along the y‐axis is arbitrary. See the text for details.