The Freeman law is a statement in astronomy which says that disk galaxies have the same surface brightness, Σ at the center. It was described in 1970 by Ken Freeman.
The Freeman law was confirmed in year 2010 for a sample of 30000 Sloan Digital Sky Survey galaxy images and it can be used with the Tully-Fisher relation to determine the luminosity and therefore distance of an observed galaxy.
However more recently it has been argued that the Freeman law is an effect of selection bias.
References
Freeman law Wikipedia(Text) CC BY-SA