We determine 37 differential extinctions in 23 gravitational lens galaxies over the range 0 \mathrel { \raise 1.29 pt \hbox { $ < $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } z _ { l } % \mathrel { \raise 1.29 pt \hbox { $ < $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 1 .
Only 7 of the 23 systems have spectral differences consistent with no differential extinction .
The median differential extinction for the optically-selected ( radio-selected ) subsample is \Delta E ( B - V ) = 0.04 ( 0.06 ) mag .
The extinction is patchy and shows no correlation with impact parameter .
The median total extinction of the bluest images is E ( B - V ) = 0.08 mag , although the total extinction distribution is dominated by the uncertainties in the intrinsic colors of quasars .
The directly measured extinction distributions are consistent with the mean extinction estimated by comparing the statistics of quasar and radio lens surveys , thereby confirming the need for extinction corrections when using the statistics of lensed quasars to estimate the cosmological model .
A disjoint subsample of two face-on , radio-selected spiral lenses shows both high differential and total extinctions , but standard dust-to-gas ratios combined with the observed molecular gas column densities overpredict the amount of extinction by factors of 2–5 .
For several systems we can estimate the extinction law , ranging from R _ { V } = 1.5 \pm 0.2 for a z _ { l } = 0.96 elliptical , to R _ { V } = 7.2 \pm 0.1 for a z _ { l } = 0.68 spiral .
For the four radio lenses where we can construct non-parametric extinction curves we find no evidence for gray dust over the IR–UV wavelength range .
The dust can be used to estimate lens redshifts with reasonable accuracy , although we sometimes find two degenerate redshift solutions .