tokens: − K )/ E ( J − K ); Indebetouw et al. 2005 ; Jiang et al. 2006 ; Zasowski et al. 2009 ; Xue et al. 2016 ) and absolute (right, A ( λ )/ A ( K ); Rieke Lebofsky 1985 ; Rieke et al. 1989 ; Indebetouw et al. 2005 ; Chiar Tielens 2006 ; Fritz et al. 2011 ; Wang Chen 2019 ; Hensley Draine 2020 ) measurements. Real variations are seen in the relative measurements indicating different MIR extinctions between the diffuse and dense ISM. The variations in the absolute measurements are likely due to a combination of real variations and uncertainties in deriving the total column normalizations. Note that the Lutz ( 1999 ) work presented preliminary results that were finalized in Fritz et al. ( 2011 ). In the case of the E ( λ − K )/ E ( J − K ) measurements, these differences are likely due to the relative contribution between diffuse and dense material in the sight lines studied. This can be seen from the Zasowski et al. ( 2009 ) measurements that encompass the full range of values with higher extinctions seen toward the Galactic Center ( l = 10–15) and lower extinctions well away from the Galactic Center ( l = 90). Zasowski et al. ( 2009 ) interpret this as less dense material in the l = 90 case compared to the l = 10–15 case, and this would be consistent with much of the previous work probing denser material given their biases toward high dust column sight lines. In the case of the A ( λ )/ A ( K ) measurements, some of the differences can be due to uncertainties in deriving the total column. The absolute level for the Chiar Tielens ( 2006 ) curves was set by fiat to that of Indebetouw et al. ( 2005 ) and Lutz ( 1999 ) and, hence, their overall high MIR extinction may just be due to this normalization. Similarly, Hensley Draine ( 2020 ) used Fitzpatrick et al. ( 2019 ) to set the absolute level of the Cyg OB2 12 extinction. However, the differences between the Rieke et al. ( 1989 ) and Fritz et al. ( 2011 ) results are enigmatic. The Br α /Br γ ratio for the former reference agrees well with the measurements by Wade et al. ( 1987 ); between these references, the ratio is measured for eight sight lines (one twice) and the (weighted) average Br α /Br γ ratio is 16 ± 1. Some of the flux in the beams used may originate from luminous early-type stars, which typically have a Br γ equivalent width of order 6 × 10 −4 μ m (Hanson et al. 1996 ). The result could be that the ratio is overestimated by about 5%. Fritz et al. ( 2011 ) determined a ratio value of 11.6 ± 2 from the ISO spectrum (for Br α ) and a ground-based spectral image (for Br γ ). The difference in these two measurement approaches suggests that the best value is toward the large end of the ones permitted by Fritz et al. ( 2011 ) and that the Galactic Center extinction in Figure 10 probably lies between that shown for Fritz et al. ( 2011 ) and ours (and the point from Rieke et al. ( 1989 ). Only Rieke Lebofsky ( 1985 ), Rieke et al. ( 1989 ), Chiar Tielens ( 2006 ), and Hensley Draine ( 2020 )
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