Accuracy criteria from Heisenberg. Top: Flux contrast (δlog₁₀ℱ) adopted during the peak identification for 21 μm (blue squares) and CO(2-1) (black circles) observations, as a function of the average filling factor (ζ). Bottom: Measured t_{fb, 21 μm}/τ as a function of ζ. The gray region indicates the parameter space where high filling factors biase our measurements of the t_{fb, 21 μm} from Kruijssen et al. (2018).