Time-series observations at high spatial resolution have shown that SiO masers form in rings around the parent star, which move and vary in intensity as the star pulsates. Many features of these observations were reproduced by a model which used inner envelope computations by George Bowen. I introduce an improved model, in which the dust radiation field in the circumstellar envelope is computed much more accurately, and I investigate the role played by the position of the infra-red photosphere in the SiO pumping bands. These photospheric radii vary considerably with wavelength, as well as with the pulsational cycle of the star. The physical conditions for the new model are based on computations by Michael Scholz.