Disk scalelengths normalized to R_opt as a function of Hubble type T. The top panel shows the individual 3.6 μm scalelengths R_d3.6, the middle the individual 250 μm scalelengths R_d250, and the bottom their ratio. In each panel, the large circles with error bars report the means over a small range of Hubble types as indicated by the horizontal error bars; the vertical error bars are the standard deviations. The horizontal dashed lines in the top two panels show the mean scalelength obtained by averaging over spirals only (1 ≤ T ≤ 5); the dotted lines show the range from 0.25 to 0.45. In the bottom panel, the dotted line shows a scalelength ratio of unity.