Upper panel: comparison of the R SBP of I Zw 18 (filled circles), with a synthetic SBP (labeled core+env1) that is due to the superposition of two exponential components of differing central surface brightness μ₀ and exponential scale length α. The first one, labeled core (open circles) approximates the stellar component that is confined to and dominates within the inner (R^⋆  ≤  6′′) HSB part of the observed SBP. The second component (env1; dotted line intersecting the abscisca at μ ~ 22 mag/◻″) is a linear fit to the nebular LSB envelope. A Sérsic model to the composite core+env1 SBP (orange solid-line curve) yields a Sérsic exponent η ≈ 2, close to the best-fitting value of η ≈ 2.2 for the observed SBP. The synthetic SBP labeled core+env2 (red solid-line curve) is computed by superposing on the core an exponential nebular envelope of equal luminosity but twice as large α as the observed envelope env1. A Sérsic fit to core+env2 (thin curve) yields an η ≈ 5. Lower panel: residuals between the synthetic SBPs core+env1 (thin curve) and core+env2 (thick curve) and their Sérsic fits when the latter are computed down to a surface brightness level μ_lim of 26.5 and 29 mag/◻″ (solid and dotted curves, respectively).