Surface brightness of scattered light at 1.6 μm (solid lines) and thermal emission at 890 μm (dashed lines) as functions of radius for a grid of models that match the location of the outer ring of HD 131835. The gas disc parameters are selected from Fig. 9 and are shown in each panel title. Different line colours (blue, orange, green) represent a different size distribution exponent q, which changes the brightness ratio of the inner and the outer ring. Black lines and black symbols show the observational constraints on the brightness of the rings in HD 131835. For details, see Section 4.4.