Ionizing radiation strength as a function of the ionization front radius. The diamond marks proplyd 203–504. irradiated mainly by θ² Ori A. For the rest of the targets (dots and squares), the UV source is θ¹ Ori C. Squares mark proplyds for which the I-front measurement has a higher uncertainty. 244–440 is not included in this figure due to the stark difference in size compared to the rest of the sample.