Schematic illustration of a torus with constant sublimation radius for inclinations 0° < i < 45°, optically thick at 1−2 μm. Zone A (blue) marks hot T ~ 1600 K dust visible at 1−2 μm, zone B (red) cooler T ~ 800 K dust visible at ~4 μm; zone C (shaded black) marks hot dust that is heavily obscured (A_K > 1) in the observer’s line of sight, so that it becomes dim at 1−2 μm, but is clearly visible at ~4 μm. The mean lags for each zone are given.