Top panel: optical depth τ (computed for the wavelength λ = 1630 nm) of the β Lyr A disk, which is observed approximately edge-on (that is from the top). The coordinate z thus corresponds to the line of sight, while x (and y) to the sky plane. The center is empty because the gainer is a non-transparent object. In this case, the inner-rim density of the wedge power-law model is set low, ρ₀ = 10⁻⁹ g cm⁻³, to demonstrate a separation from the outer rim. The dashed line shows the physical position of the photosphere (τ = 2/3). The lines-of-sight grid exhibits “steps” due to the limited spatial resolution of our model (1 R_⊙), even though the integration of the radiation transfer (the contribution function) is internally performed on a much finer grid. Bottom panel: corresponding photospheric temperature T_phot of the disk, which varies along with the radial temperature profile T(R).