Analysis of emission line ratios along the eastern (left) and western (right) filament. In the top three panels the emission in Hα, Hβ, and [O III] λ5007 is shown, now aligned such that the x-axis runs along the direction of the filament (Δl) and that the y-axis runs across the filament (Δw); the correpsonding areas have also been outlined in the bottom left panel of Fig. 1. The two bottom panels show the Hβ/Hα and [O III]/Hα ratio in bins of 1.2″ along Δl. These ratios are obtained after integrating the emission in each line along Δw. The 1-σ errors on the ratios have been computed with Eq. (2). For Hβ/Hα the Case-A recombination expectations from Storey & Hummer (1995), are indicated as dashed lines for three different temperatures (T = {1, 2, 3}×10⁴ K in green, orange, and red); cf. Fig. 11. For Hβ/Hα the corresponding conventional Balmer decrement values are measured on the right ordinate.