Spitzer 3.6 μm surface brightness as the function of τ₂₅₀ derived from Herschel data. The black circles show the median surface brightness in τ₂₅₀ = 0.002 wide bands. The dashed red line is a robust least squares fit to the range τ₂₅₀ = 0.0025–0.0065, with the slope, k, given at the bottom of the frame. The zero point of the I_ν (3.6 μm) axis is arbitrary.