2005ApJ...619..931I

We determine and tabulate A_[λ]/A_K, the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25µm≤λ≤8.0µm along two lines of sight: l=42° and 284°. The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49, as well as a ``field'' region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3-8 µm by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A_[λ]-A_K)/(A_J-A_K) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g., A_J/A_K, is required to calculate A_[λ]/A_Kfrom those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sight lines and fit the reddening as a function of magnitude (distance) to determine A_Jkpc^–1, A_Kkpc^–1, and A_J/A_K. Our values of A_[λ]/A_Kshow a flattening across the 3-8 µm wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center.