  Observations of the emission at radio, millimeter, sub-millimeter, and
infrared wavelengths in the center of the solar disk validate the
auto-consistence of semi-empirical models of the chromosphere. Theoretically,
these models must reproduce the emission at the solar limb. In this work, we
tested both the VALC and the C7 semi-empirical models by computing their
emission spectrum in the frequency range from 2 GHz to 10 THz, at solar limb
altitudes. We calculate the Sun's theoretical radii as well as their limb
brightening. Non-Local Thermodynamic Equilibrium (NLTE) was computed for
hydrogen, electron density, and H-. In order to solve the radiative transfer
equation a 3D geometry was employed to determine the ray paths and
Bremsstrahlung, H-, and inverse Bremsstrahlung opacity sources were integrated
in the optical depth. We compared the computed solar radii with high resolution
observations at the limb obtained by Clark (1994). We found that there are
differences between observed and computed solar radii of 12000 km at 20 GHz,
5000 km at 100 GHz, and 1000 km at 3 THz for both semi-empirical models. A
difference of 8000 km in the solar radii was found comparing our results
against heights obtained from H{\alpha} observations of spicules-off at the
solar limb. We conclude that the solar radii can not be reproduced by VALC and
C7 semi-empirical models at radio - infrared wavelengths. Therefore, the
structures in the high chromosphere provides a better measurement of the solar
radii and their limb brightening as shown in previous investigations.
