Contour plots denoting the ratios observed in the source RM 102 for different values of metallicity and microturbulence, assuming 20% contribution from the bright side of the cloud and 80% from the dark side. The R_UV−opt contours are shown in black, the R_UV contours are shown in red, and the R_opt contours are shown in blue. The position associated with the best χ² for each bin of metallicity is marked with an orange dot.