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The numbers of low surface brightness (LSB) disk galaxies found in recent field surveys (e.g.,\ Schombert et al.\ 1992, AJ, 103, 1107) are too great to be consistent with the notion (Freeman 1970, ApJ, 160, 811) that all large disks have the same central surface brightness. The space density of LSB disks is inferred to be comparable to that of Freeman disks, suggesting a distribution of galaxy surface brightnesses which is roughly flat below a limiting value rather than a delta function.
This has serious consequences for the galaxy luminosity function. Depending on the precise form of the bivariate distribution of galaxy size and surface brightness, it is likely that both $\phi^*$ and $|\alpha|$ are underestimated. This potentially resolves many problems in extragalactic astronomy which imply a greater abundance of objects than provided by the standard luminosity function. For example, the excess number counts at faint magnitudes (e.g.,\ Tyson 1988, AJ, 96, 1; Lilly et al.\ 1991, ApJ, 369, 79) are naturally explained by surface brightness selection effects since observing faint galaxies necessarily involves probing faint isophotal levels. That LSB galaxies have properties which are very similar to those of the excess faint population (very blue colors, low correlation amplitude) is probably not a coincidence.
