Optical depth at the line formation height τ_h (black line) and the intensity ratio profile r(Δλ), (blue solid line) for Case f10E25d3. We only show the wavelength range at which the emergent intensity is larger than three times the continuum. The blue dashed lines show r(Δλ) which is degraded to the IRIS spectral resolution (0.026 Å). Overplotted are the horizontal line of r = 2 and the vertical line for the position of the line core.