On the basis of spectroscopic observations in the lines Fe I 543.5 nm and Na I D2 the main characteristics of simple semi-empirical two-component models of 11 umbrae differing in aize (radii 2"-8") are studied. It is shovm that in large umbrae ru>4") the individual differences in brightness are mainly due to fluctuations in temperature of the dark component, while small umbrae ru<3”) differ in brightness due to the fluctuations in temperature of the dark component as well as the considerable changes in the filling factor (5% ~ 30%). The bright component is cooler than the undisturbed photosphere and in most cases it may be described by the temperature down-scaling factor Δ θ = 0.10.