  We examine the wavelength dependence of flux ratios for six gravitationally
lensed quasars using K and L' images obtained at the Gemini North 8m telescope.
We select lenses with source redshifts z_s < 2.8 so that K-band images probe
rest-frame optical emission from accretion disks, while L'-band images probe
rest-frame near-infrared flux emitted (in part) from the more extended
surrounding torus. Since the observations correspond to different source sizes,
the K and L' flux ratios are sensitive to structure on different scales and may
be useful for studying small-structure in the lens galaxies. Four of the six
lenses show differences between K and L' flux ratios. In HE 0435$-1223, SDSS
0246-0825, and HE 2149-2745 the differences may be attributable to known
microlensing and/or intrinsic variability. In SDSS 0806+2006 the wavelength
dependence is not easily attributed to known variations, and may indicate the
presence of substructure. By contrast, in Q0142-100 and SBS 0909+523 the K and
L' flux ratios are consistent within the uncertainties. We discuss the utility
of the current data for studying chromatic effects related to microlensing,
dust extinction, and dark matter substructure.

Cosmology and Nongalactic Astrophysics