On the interpretation of stellar disc observations in terms of diameters

A primitive two-layer model atmosphere may illustrate the formation of different types of brightness distributions on stellar discs. Similar types of distributions are found in realistic atmospheres, some of which are very hard to interpret in terms of a diameter-type quantity. Assigning a disc size to observational data and relating this size to an optical-depth diameter is extremely difficult in these cases. In particular, naive interpretation of disc observations in the light of absorption features in the Wien part of the Planck function and of disc observations of geometrically very extended configurations may yield spurious results.