Ionization front radius versus the projected separation from θ¹ Ori C. Squares mark proplyds for which the I-front measurement has a higher uncertainty. The radii of the ionization fronts increase with larger projected distances from the UV source. Due to noisy subcubes, measurements were not taken for proplyd 167–325 (closest to the UV source) in the bottom panels. For the same reason, the panel of [O III] 5007 Å excludes the radii of several targets.